Mechanical Properties and Microstructure of Alkali-Activated Cements with Granulated Blast Furnace Slag, Fly Ash and Desert Sand

Abstract

In this study, desert sand was used as supplementary materials in alkali-activated cements (AAC) with granulated blast furnace slag (GBFS) and fly ash (FA). For the first time, a systematic investigation was conducted on the effects of various treatment methods and contents of desert sand on the strength and microstructure of AAC. This study also analyzed the X-ray diffractometer (XRD), Scanning Electron Microscopy-Energy Dispersive X-ray Microanalysis (SEM-EDX), Mercury Intrusion Porosimetry (MIP), pH values, and Fourier-transform infrared spectroscopy (FT-IR) properties of AAC pastes containing differently treated desert sand to uncover the mechanisms by which these treatments and dosages influence mechanical properties of AAC. Untreated desert sand (DS), temperature-treated desert sand (DS-T), and ground desert sand for two different durations (20 mins and 30 mins) all exhibited some pozzolanic activity but primarily acted as fillers in the AAC pastes. Among the samples, DS-T demonstrated the highest pozzolanic activity, though it was still less than that of fly ash (FA). The optimal dosage for the modified desert sands was determined to be 10%. However, The optimal dosage of different modified desert sands is 10%. The flexural strength of DS-G30-10 reaches 6.62 MPa and the compressive strength reaches 72.3 MPa, showing the best comprehensive mechanical properties.

1. Introduction

The use of calcium carbonate and coal in the production of cement clinker results in substantial CO₂ emissions. The combined production of Portland cement with cement clinker and supplementary materials accounts for approximately 5% of the global total CO₂ emissions, with clinker CO₂ emissions comprising 90–98% of the total CO₂ emissions from Portland cement [1,2]. Reducing the use of clinker and the consumption of calcium carbonate and coal in clinker production is the most effective way to decrease CO₂ emissions from Portland cement [3,4,5,6,7,8]. Alkali-activated cement are regarded as environmentally friendly alternatives to Portland cement. These cement consist of cementitious materials and activators, have a simple production process, and primarily utilize raw materials with cementitious activity, such as the solid wastes from the metallurgical industry like GBFS and steel slag, and FA from the thermal power industry [9,10].

Desert areas worldwide currently cover approximately 36 million square kilometers, accounting for approximately 25% of the global land area(Figure 1). China, a country with significant desert regions, has a total desert area of approximately 1.73 million square kilometers, representing approximately 18% of its land area. Furthermore, desertification is still ongoing [11,12]. Desert sand is regular in shape and smooth in surface [13], with smaller particle sizes and a lower fineness modulus compared than river sand. DS can partially replace sand in concrete and mortar. The main oxide component of desert sand is SiO₂, along with other components such as Al₂O₃, CaO, MgO, and Fe₂O₃, as well as certain amounts of Na₂O and K₂O [14,15,16,17]. Desert sand has a relatively high alkali content and exhibits pozzolanic activity [18,19,20,21,22]. When used as supplementary materials, DS acts similarly to inert supplementary materials like limestone powder. The mechanisms of limestone powder’s action primarily include the micro-filling effect, nucleation effect, dilution effect, and weak chemical effect [23,24,25,26]. Quartz is generally considered inert, interacting with cement hydration mainly through physical interactions, such as the dilution of cement particles, nucleation of cement hydrates, and space filling [27]. Additionally, some studies have shown that due to changes in the crystal structure and reduction in particle size of quartz powders after mechanical grinding [28,29]. Quartz powder has a certain degree of cementitious activity and can react with calcium hydroxide to produce hydration products similar to those of Portland Cement [30]. When DS is used as supplementary materials for Ordinary Portland Cement (OPC), it is found that DS not only acts as an inert filler but also exhibits some cementitious activity [31,32,33]. Using sodium hydroxide and sodium sulfate to modify DS, the alkaline modification facilitates the dissolution of SiO₂ in DS, resulting in improved mechanical properties for OPC when modified with DS [34]. Mechanically ground desert sand powder exhibits some pozzolanic activity, and an appropriate amount of desert sand powder can improve the workability and mechanical properties of concrete [19]. Furthermore, the resources of river sand and natural stone are gradually decreasing, so Tran et al. [35] replaced natural sand with steel slag and studied the fluidity, slump and strength of 3d printed mortar. In previous studies, desert sand was only subjected to screening and simple grinding treatment, and used as a supplementary material for Ordinary Portland Cement [31,32,33,34]. There have been no systematic studies reported on the effects of different treatment methods and dosages of desert sand on the mechanical properties and microstructure of cementitious materials. The grading of desert sand (DS) is poor [11,13,14], which limits its added value when used as an aggregate in concrete and mortar. Furthermore, research on the use of desert sand as an inert supplementary material for alkali-activated cementitious materials is scarce. This article systematically investigates the effects of different treatment methods and content of DS on the mechanical properties and microstructural performance of AAC with GBFS and FA.

In recent decades, many scholars have carried out extensive research in the field of alkali-activated binders. When 20% steel slag and 6% alkali content were used, the compressive strength of the specimens was relatively superior. As water usage decreased, the compressive strength at the same age increased [37]. However, with the increase in the amount of fly ash, the compressive strength of Ordinary Portland Cement (OPC) decreased while its plasticity increased [38]. The mechanical properties and frost resistance of alkali-activated calcined coal gangue (CACG) and GBFS cements were better than those of Ordinary Portland Cement, with GBFS contributing positively to both mechanical properties and frost resistance [39]. Ma et al. found that as FA content increased, the compressive strength of AAC with GBFS and FA decreased. The incorporation of MgO promoted the hydration reaction of GBFS cements and improved their pore structure and microstructure [40]. In alkali-activated GBFS cements, the addition of calcium hydroxide increased the compressive strength of the cementitious materials while decreasing their flowability, whereas the addition of CaSO₄∙2H₂O reduced both compressive strength and flowability [41]. Srinivasamurthy et al. [42] observed that under the same age but different curing conditions, AAC with FA and GBFS exhibited an inverse relationship between compressive strength and flexural strength. Water glass-activated GGBS binders contain various gels, including CASH, NASH, and CSH [43]. Due to the loss of Na⁺ and OH⁻ in unsealed cement paste, the pH value of sealed FA-GBFS cement paste is higher than that of unsealed cement paste at the same age [44]. In alkali-activated cement, the concentrations of Ca²⁺, Si⁴⁺, Al³⁺, and Mg²⁺ correlate with the solution pH; higher pH levels lead to increased concentrations of Al³⁺ and Si⁴⁺ and decreased concentrations of Ca²⁺ and Mg²⁺. A pH greater than 11.5 favors the hydration reaction of alkali-activated GBFS cements [45]. Using flue gas residues, air pollution control residues, and water glass to prepare composite activators for AAC with GBFS and FA, the primary hydration products in the paste were N-A-S-H, C-N-A-S-H, and C-A-S-H [46,47]. Compared to standard curing, the types of hydration products in AAC with GBFS and FA under low-temperature curing conditions remained consistent; however, the quantity of hydration products in AAC varied, resulting in an increase in harmful pores and a decrease in the compressive strength of the mortar [48]. Previous research on AAC has mainly focused on the use of cementitious solid waste from the metallurgical and thermal power industries. There is relatively little research on inert supplementary materials in AAC. The application of inert supplementary materials in AAC can improve workability, hydration properties, and mechanical performance [25,44], and can also reduce the cost of AAC to some extent.

In current research, desert sand is primarily used as fine aggregate in concrete and mortar. However, due to its small particle size and poor gradation, desert sand needs to be used in combination with manufactured sand or river sand. An alternative approach worth considering is using desert sand as supplementary materials in cementitious materials. This could reduce the reliance on natural supplementary materials while also providing a new perspective for the high-value and large-scale application of desert sand. Research on desert sand as a supplementary material is still relatively scarce. To gain a deeper understanding of its impact on the mechanical properties of cementitious materials, this study modifies desert sand at a temperature of 500 °C and employs grinding for durations of 20 min and 30 min, respectively. The untreated desert sand (DS), thermally treated desert sand (DS-T), 20 min ground desert sand (DS-G20), and 30 min ground desert sand (DS-G30) were incorporated into the cementitious materials at substitution rates of 5%, 10%, 15%, and 20%, this study for the first time systematically explores the effects of treatment methods and dosage of desert sand on the strength of AAC with GBFS and FA. Building on the research foundation of AAC with GBFS and FA, this study systematically investigates the application of desert sand as a supplementary material in AAC. It compares the effects of desert sand on AAC with those of Ordinary Portland Cement. This study examines the impact of desert sand on the compressive strength and flexural strength of AAC at different curing ages. Additionally, various tests, including SEM, FT-IR, MIP, XRD, and pH measurements, were conducted on AAC with varying amounts of desert sand at 28 days of age. The research results comprehensively analyze the effects of modification methods and substitution levels of desert sand on the mechanical properties of AAC, by combining multiple sets of test data, this study reveals the mechanisms by which these modification methods and substitution levels influence AAC.

2. Materials and Methods

2.1. Materials

Hebei Construction Group Co., Ltd. (Baoding, China) provided GBFS, FA and OPC (OPC 42.5). OPC 42.5 was used as a reference. The sodium silicate of analytical grade is Na₂SiO₃∙9H₂O, and the sodium silicate was purchased from Tianjin Huashi Chemical Reagent Co., Ltd. (Tianjing, China). The water used for the experiment is municipal tap water from the laboratory. Desert sand (DS) was sourced from the Tengger Desert, specifically located Alxa Left Banner.

The desert sand was subjected to thermal and grinding treatments. Thermal modification was performed using a muffle furnace at a constant temperature of 500 °C for 2 h, with the thermally treated desert sand abbreviated as DS-T. The desert sand was also treated using a ball mill, with grinding times of 20 min and 30 min, and the two types of ground desert sand are abbreviated as DS-G20 and DS-G30, respectively. As shown in Figure 2. DS and DS-T have smooth surfaces with few angular edges [12], while DS-G20 and DS-G30 exhibit rougher surfaces with more angular edges, with DS-G30 having the smallest particle size [18].

The main chemical composition of the raw material powders was analyzed using X-ray fluorescence (XRF) with a PANalytical Axios Instrument (Almelo, The Netherlands), and the results of the chemical compositions are presented in Table 1. The particle size distribution of the raw material powders was measured using a BT-9300S laser particle size analyzer (China). The density and specific surface the raw material powders area are shown in Table 2, the particle size distribution of the raw materials is shown in Figure 3.

2.2. Mixture Proportions and Specimen Preparation

The mass ratio of Na₂O to the powder is 3%, and the water to cement ratio is 0.4. A total of 18 mix proportions were designed, with their numbering and specific mix ratios detailed in Table 3. Among them, 6 groups of samples were prepared for each mix ratio, 3 test blocks in each group of samples, 5 groups of samples were used for 3 d, 7 d, 14 d, 21 d, 28 d strength tests, and one group of samples was used for test characterization.

Dissolve the specified water and sodium silicate in the mix design in advance, ensuring that the resting time at room temperature does not exceed 4 h. Pour the weighed powder into the mixing pot, then add the sodium silicate solution and stir for 4 min. Pour the mixed slurry into the 40 mm × 40 mm × 160 mm three-part mold. After curing at room temperature for one day, the specimens were demolded and then subjected to further curing under conditions of 20 ± 2 °C temperature and a relative humidity of no less than 95%.

2.3. Methods

2.3.1. Mechanical Properties Test

The compressive strength and flexural strength of the paste specimens were tested in accordance with GB/T 17671 [49]. The testing ages are 3 day, 7 day, 14 day, 21 day, and 28 days, respectively.

2.3.2. MIP and SEM with EDX

At 28 days of curing, we used a cutting machine to cut the center of a 40 mm × 40 mm × 160 mm specimen and selected a representative amount of non-destructive hardened paste. We soaked the slurry samples in anhydrous ethanol to terminate the hydration process, then vacuum dry to constant weight. Finally, we used the Micromeritics AutoPore V 9620 automatic mercury porosimeter to test the pore structure of the samples.

At 28 days of curing, we used a cutting machine to cut the center of a 40 mm × 40 mm × 160 mm specimen and selected a representative amount of non-destructive hardened paste. Then, the selected amount of non-destructive hardened paste was soaked in anhydrous ethanol to terminate hydration. The paste samples were vacuum dried and sputter-coated with gold for 120 s to increase conductivity. The apparent morphology of the paste samples and the EDX data were captured using the Czech TESCAN MIRA LMS.

2.3.3. pH Values Test

At 28 days of curing ages, a small amount of the hardened paste was soaked in anhydrous ethanol to terminate hydration, then vacuum dried and ground. A portion of the ground sample was sieved using a 200-mesh square hole sieve (with a mesh size of 0.075 mm). Weigh approximately 1 g of the sieved sample and place it in a beaker. Add 10 g of deionized water. Stir the mixture with a magnetic stirrer for 10 min, then let it stand for 2 h. Afterwards, measure the pH of the supernatant using a pHS-2C pH meter.

2.3.4. Characterization

At 28 days of curing ages, small amounts of the hardened paste from 40 mm × 40 mm × 160 mm specimens were removed and broken into small pieces. These pieces were then soaked in anhydrous ethanol for at least 48 h to terminate hydration. Following this, the samples were dried in a 40 °C vacuum drying oven for at least 48 h. The dried samples were then ground and passed through an 80 μm sieve.

Using a Rigaku Ultima IV X-ray diffractometer (Tokyo, Japan) with a Cu target, we tested the types of hydration products in the sample. The operating voltage was set at 40 V, and the operating current at 30 mA. The scanning speed was 2°/min, with a scanning angle range of 10°–80° (2θ).

Using a Thermo Fisher Scientific Nicolet iS20 FT-IR spectrometer (Thermo Fisher, Waltham, MA, USA), we analyzed the functional groups of the hydration products in AAC. The analysis was performed over a frequency range of 4000–400 cm⁻¹, using a resolution of 4.0 and 128 scans.

3. Results

3.1. Compressive Strength and Flexural Strength

3.1.1. Compressive and Flexural Strength on the Same Curing Day

According to Figure 4, with the exception of DS-20 and DS-T-20, the 3-day compressive strength of OPC is generally lower than that of DS-0 and AAC containing various modified desert sands. The 3-day flexural strength of OPC is also generally lower than that of AAC with untreated or temperature-treated desert sand, but it is higher than that of AAC with ground-treated desert sand. The 3-day compressive strength of DS-0 is lower than that of DS-5, DS-T-5, DS-G30-5, DS-G30-10, and DS-G30-15, while its 3-day flexural strength is lower than that of DS-T-5, DS-T-10, DS-T-15, and DS-20. The desert sand in DS-5 and DS-T-5 may primarily serve as a skeleton filler, and the desert sand ground for 30 min may optimize the particle size distribution of AAC, thereby reducing shrinkage caused by hydration within the first 3 days. Research results indicate that the compressive strength of alkali-activated cement containing quartz sand is superior to that of mortar containing standard sand [50]. Very fine particles of desert sand in OPC exhibit heterogeneous nucleation and pozzolanic effects [20], and ground quartz possesses pozzolanic properties that can react with Ca(OH)₂ to form hydration products [28].

In AAC containing DS, as the DS content increases, the 3-day compressive strength of AAC decreases, while the 3-day flexural strength gradually increases. The 3-day compressive strength of DS-5 is the highest, and the 3-day flexural strength of DS-20 is the highest. In temperature-modified desert sand AAC, as the DS-T content increases, both the 3-day compressive strength and flexural strength of AAC decrease. The filling and skeleton effects of DS and DS-T allow AAC with low content of DS and DS-T to achieve strength comparable to that of DS-0. Additionally, due to the thermal activation of DS-T, DS-T may participate in the hydration reaction of AAC, leading to higher 3-day flexural strength in AAC containing DS-T. Research shows that the high Al₂O₃ content in DS allows fine particles of DS to participate in the hydration reaction of Ordinary Portland Cement, similar to the hydration reaction of calcium sulfoaluminate cement [31]. The 3-day compressive strength and flexural strength of the AAC with DS-G30 are both superior to those of AAC with DS-G20.

As shown in Figure 5, except for DS-T-5, the 7-day compressive strength of DS-0 is higher than that of both AAC and OPC [51], with DS-0 exhibiting the highest 7-day flexural strength (8 MPa). As the content of DS and DS-T increases, the compressive strength of AAC containing these materials tends to decrease, while the 7-day flexural strength is highest in the AAC containing 5% DS, DS-T, DS-G20, and DS-G30. After 3 days, the hydration rate of AAC and OPC slows down [33,52], and with the increasing incorporation of desert sand treated by different modified methods, the formation of hydration products in AAC decreases, leading to a reduction in its strength. The compressive strength of DS-T-5 (58 MPa) is the highest, likely due to the skeleton-filling effect of DS-T and the thermal activation, which may enable some fine particles of DS-T to potentially participate in the hydration reaction of AAC [31].

As shown in Figure 6, the compressive strength of OPC (51 MPa) at 14 days lower than that of AAC [51], while its 14-day flexural strength (9 MPa) is higher than that of AAC. The 14-day compressive strength of DS-0 (70 MPa) is higher than both OPC and AAC containing different types of desert sand. After 7 days, the hydration products in AAC containing different types of desert sand are relatively few [31], which is why DS-0 exhibits a higher strength compared to AAC with desert sand. The 14-day flexural strength of DS-0 is similar to that of AAC containing different types of desert sand. As the desert sand content increases, the 14-day compressive strength of AAC first increases and then decreases, with DS-10 exhibiting the highest 14-day compressive strength (64 MPa). The 14-day flexural strength of AAC containing DS varies slightly, averaging approximately 6 MPa. The increase in DS-T content in AAC leads to a decrease in both compressive strength and flexural strength. As the content of DS-G20 and DS-G30 increases, the 14-day compressive strength of AAC first increases and then decreases. The smaller particle size of DS-G20 and DS-G30 fills the pores in AAC caused by shrinkage from the hydration reaction. Due to the high content, the 14-day compressive strengths of DS-G20-15, DS-G20-20, DS-G30-15, and DS-G30-20 are higher than those of DS-15, DS-20, DS-T-15, and DS-T-20, possibly because DS-G20 and DS-G30 exhibit some cementitious activity [31].

As shown in Figure 7, the 21-day compressive strength of OPC (52 MPa) is lower than that of AAC [51], while its 14-day flexural strength (8 MPa) is higher than that of AAC. The 21-day compressive strength of DS-0 is higher than that of AAC containing different types of desert sand. However, except for DS-G20-5, the 21-day flexural strength of DS-0 is lower than that of AAC containing different types of desert sand. The incorporation of desert sand plays a skeletal filling role, which somewhat inhibits the shrinkage of AAC, resulting in a higher 21-day flexural strength for AAC containing different types of desert sand. As the DS-T content increases, both the 21-day compressive and flexural strengths of AAC decrease. With increasing DS and DS-G30 content, the 21-day compressive strength of AAC first increases and then decreases, with DS-10 and DS-G30-10 showing the highest strength values of 66 MPa and 70 MPa, respectively. The 21-day compressive strengths of DS-G30-10, DS-G30-15, and DS-G30-20 are higher than those of AAC with the same content of desert sand.

As shown in Figure 8, the 28-day compressive strength of OPC is lower than that of AAC [51,53], while the 28-day flexural strength of OPC is higher than that of AAC. The 28-day compressive strength of DS-0 is higher than both OPC and AAC [33,51]. In the AAC with DS-T system, DS-T-5 exhibits the highest 28-day compressive strength (71 MPa). With increasing DS, DS-G20, and DS-G30 content, the 28-day compressive strength of AAC first increases and then decreases, with DS-10, DS-G20-10, and DS-G30-10 showing the highest 28-day compressive strengths of 69 MPa, 69 MPa, and 72 MPa, respectively. Except for DS-G20-20, the 28-day compressive strength of AAC containing the same quantities of DS-T, DS-G20, and DS-G30 is superior to that of AAC containing the same amount of DS. Compared to DS-10, the XRD data for 28-day AAC show that the quartz diffraction peaks corresponding to DS-T-10, DS-G20-10, and DS-G30-10 are weaker, indicating that the thermal and grinding treatments of desert sand enhanced the pozzolanic effect of the sand. Relevant studies have shown that under curing temperatures of 20–40 °C, waste glass powders exhibit a pozzolanic effect in OPC paste [54]. When waste concrete powders are thermally treated as supplementary materials in OPC, those treated at 400 °C and 700 °C show higher activity. Similarly, recycled cements treated at 600–800 °C exhibit higher strength and lower porosity within the paste [55]. Ground desert sand and fine desert sand particles provide both skeletal filling and pozzolanic effects in OPC [32,33].

3.1.2. Compressive and Flexural Strength with Different Modification Methods of Desert Sand

As shown in Figure 9 and Figure 10, from 14 to 28 days, the compressive strength of OPC is lower than that of AAC mixed with DS and DS-T, while its flexural strength is higher than that of AAC with DS and DS-T. From 14 to 28 days, the compressive strength of DS-0 is higher than that of AAC mixed with DS and DS-T. There is a noticeable strength retrogression in the flexural strength of OPC, DS-0, DS-5, and DS-T-5 [56],which may be due to microcracks caused by shrinkage during the hydration process of the cementitious materials. This phenomenon is observable in the 28-day SEM image of DS-T-20, resulting in a reduction in flexural strength over time. Relevant studies have shown that alkali-activated cementitious materials can exhibit strength retrogression in compressive strength [42,57]. For AAC mixed with DS and DS-T, the flexural strength reaches its peak at 3–7 days, the compressive strength shows significant growth from 3 to 14 days, and the compressive strength growth slows down from 14 to 28 days.

As shown in Figure 11 and Figure 12, from 14 to 28 days, the compressive strength of OPC is lower than that of AAC mixed with DS-G20 and DS-G30, while the flexural strength of AAC mixed with DS-G20 and DS-G30 is lower than that of OPC. There is a noticeable strength retrogression in the flexural strength of OPC, DS-0, DS-G20-5, and DS-G30-5, similar to the retrogression observed in AAC mixed with DS and DS-T-5, with all mixes containing 5% desert sand. From 14 to 28 days, the compressive strength of DS-0 is higher than that of AAC mixed with DS-G20 and DS-G30. For AAC mixed with DS-G20 and DS-G30, the flexural strength peaks at 3–7 days, the compressive strength shows significant growth from 3 to 14 days, and then the compressive strength growth slows down from 14 to 28 days.

Analyzing the strength data for OPC and AAC from 3 to 28 days, the compressive strength of OPC increases rapidly from 3 to 7 days, but slows down from 7 to 28 days, while the flexural strength initially increases and then decreases over this period. Unlike OPC, the compressive strength of DS-O and AAC mixed with different types of desert sand shows significant growth from 3 to 14 days and then levels off from 14 to 28 days. The flexural strength peaks at 3–7 days, with little change from 7 to 14 days (except for DS-0, DS-5, DS-T-5, DS-G20-5, and DS-G30-5). From 14 to 28 days, the compressive strength of AAC mixed with different types of desert sand is lower than that of DS-0. Although different types of desert sand exhibit some pozzolanic effect, they primarily serve as skeletal fillers in the AAC paste, with limited impact on the strength growth of AAC over time as the hydration products are relatively few [18,31,32,33,54]. The shrinkage during the curing process of AAC paste causes internal microcracks, which are detrimental to the flexural strength of AAC. As the curing period progresses and hydration products increase, these microcracks grow and become interconnected, as observed in the 28-day SEM images of ACC, leading to strength retrogression in the flexural strength of DS-0 and AAC mixed with 5% of different types of desert sand, with DS-0 showing the greatest retrogression. From 3 to 7 days, the compressive strength of OPC and AAC mixed with different types of desert sand does not differ significantly, but from 14 to 28 days, the compressive strength of OPC is lower than that of DS-0 and AAC mixed with different types of desert sand. Comparing the compressive and flexural strength development trends of AAC mixed with different types of desert sand from 3 to 28 days, it can be concluded that the mechanical properties of AAC are better when DS, DS-T, DS-G20, and DS-G30 are added at 10%.

3.2. Pore Structure and Scanning Electron Microscopy with EDX

Figure 13 presents the pore size distribution in 28-day-old pastes of DS-5, DS-T-5, DS-G20-10, and DS-G30-10. According to relevant literatures and pore size characteristics, the pore structures in cementitious materials can be classified into gel pores (<0.01 μm), medium capillary pores (0.01–0.05 μm), large capillary pores (0.05–1 μm), and macropores (>1 μm). Considering the influence of pore distribution on the properties of cementitious materials, such as mechanical property, crack, and durability, pores within the 0–1000 nm range are further categorized as harmless pores (<10 nm), less harmful pores (10–50 nm), and harmful pores (>50 nm) [58,59,60].

As shown in Figure 13b, the DS-T-5 paste exhibits the highest cumulative volume proportion of pores smaller than 100 μm. Compared to DS-5, DS-T demonstrates a more pronounced pozzolanic effect, which enhances the early hydration of AAC, leading to increased microcracking due to hydration shrinkage and, consequently, a higher cumulative pore volume. The DS-G20-10 paste has the lowest cumulative volume proportion of pores smaller than 100 μm, yet the cumulative volume of harmful pores (0.05–100 μm) is comparable to that of DS-T, which accounts for the similar 28-day compressive strengths observed in DS-T-5 and DS-G20-10 (Figure 8a). The DS-G30-10 paste shows the highest cumulative volume of gel pores (<0.01 μm) and the lowest cumulative volume of harmful pores (0.05–100 μm), resulting in the highest 28-day compressive strength (Figure 8a). When comparing the cumulative pore volumes between DS-G20-10 and DS-G30-10 pastes, DS-G30-10 has the lowest cumulative volume and proportion of harmful pores and the highest cumulative volume and proportion of gel pores. This suggests that the particle gradation of DS-G30 is superior to that of DS-G20, contributing not only to the skeleton filling and pozzolanic effects in AAC but also to the optimization of pore distribution and proportion.

Based on the SEM images of DS-T and DS-G30 (Figure 2e,g) and the particle size distribution (Figure 3), the positions of DS-T and DS-G30 in Figure 14 are identifiable. DS-T is embedded within the AAC matrix and is surrounded by fewer cracks [56]. The matrix of DS-G30-20 is denser than that of DS-T-20, and DS-G30 is firmly embedded in the 28-day AAC matrix with fewer surrounding cracks [31]. This suggests that the filling effect of DS-G30 inhibits the propagation of cracks in the AAC.

Based on the particle shape and size of FA observed in the SEM images of raw materials (Figure 2b), the positions of FA in the 28-day SEM images of the DS-T-20 and DS-G30-20 pastes (Figure 15) are identifiable. The distribution and morphology of FA reveal that the paste contains incompletely reacted FA, whose pozzolanic activity is lower than that of GBFS. FA participates in the hydration reaction slowly within the AAC matrix. In the 28-day AAC paste, the FA appears rough and is covered with hydration products [61,62]. In Figure 15b, the larger FA particles are tightly bonded with the paste in the interfacial transition zone. Compared to the FA observed in the SEM images of the AAC paste, the different modified desert sands embedded in the AAC paste have relatively smooth surfaces (Figure 14) [56], indicating that the activity of desert sand is lower than that of FA, and it primarily serves a filler and provides skeletal support.

Figure 16 shows the SEM images of the 28-day AAC paste at 10,000× magnification. By referencing the SEM images of raw materials (Figure 2a) and relevant literature on hydration products [63,64,65], the distribution of GBFS and the types of hydration products in Figure 16 can be identified. It can be observed that there is incompletely reacted GBFS in both DS-T-5 and DS-G30-20, along with the presence of dispersed calcite and calcium hydroxide (CH) in the AAC matrix. Cracks formed by hydration shrinkage are distributed around the DS-T-5 region, and these cracks are through-cracks, which lead to a regression in the flexural strength development of DS-T-5. Due to the high content of DS-G30, the DS-G30-20 sample has more pores, and the large amount of desert sand in DS-G30-20 results in fewer hydration products. Consequently, the 28-day compressive strength and flexural strength of DS-G30-20 are lower than those of DS-G30-5, DS-G30-10, and DS-G30-15.

Figure 17a,b show the 28-day paste SEM images and corresponding EDX results for DS-T-5 and DS-G30-10 [56]. The DS-T-5 paste exhibits more cracks compared to DS-G30-10. At the same magnification, the DS-G30-10 paste surface shows a higher amount of hydration products such as calcite and CH. The content of Ca, Al, Si, and O in DS-T-5 and DS-G30-10 are not significantly different, with Ca/Si and Al/Si ratios are similarly close. However, the Ca and Al content in DS-T-5 is higher than in DS-G30-10, likely because the GBFS contains much higher amounts of CaO and Al₂O₃ compared to desert sand (as shown in Table 1). Additionally, the desert sand content in DS-G30-10 is higher than in DS-T-5, leading to a higher Ca/Si and Al/Si ratio in DS-T-5 compared to DS-G30-10 [56,66]. The Na/Si ratio, however, shows a significant difference, possibly due to the localized concentration of the sodium silicate solution during the mixing process with the reactive powders in DS-T-5. This may have resulted in a higher Na content in the 28-day local paste of DS-T-5 compared to DS-G30-10.

3.3. Fouier-Transform Infrared Spectroscopy

As shown in Figure 18, FT-IR can be used to characterize the chemical bonds formed between reactants, with reaction products being identified by their corresponding absorption bands. A weak peak in the range of 1625–1657 cm⁻¹ is related to the bending vibration of O-H bonds, while the broad band between 3445 and 3474 cm⁻¹ is attributed to the stretching vibration of O-H [67,68].Peaks at approximately 1600 cm⁻¹ and 3400 cm⁻¹ indicate the presence of chemically bound water, which is associated with the formation of Calcium-Silicate-Hydrate (C-S-H). The pronounced peaks at these positions suggest a higher content of C-S-H in the paste, contributing to greater paste strength. The vibration peaks of the Si-O-T bond (where T stands for Al or Si) fall within the range of 370–1300 cm⁻¹ [41,69], with the peak at 960 cm⁻¹ corresponding to the asymmetric stretching vibration of Si-O, and the peak at approximately 450 cm⁻¹ representing the bending vibration of Si-O-Si. These peaks are associated with the hydration product C-S-H. Additionally, the peak in the range of 1400–1450 cm⁻¹ is due to the stretching vibration of C-O-C, which corresponds to the presence of calcium carbonates [70].

After curing the AAC pastes for 28 days, the hydration process was terminated by immersion in anhydrous ethanol, followed by vacuum drying. The samples were then ground and analyzed using X-ray diffraction (XRD) to identify the hydration products. As shown in Figure 19, by comparing the diffraction patterns with PDF standard cards and referencing existing literature, the presence of four minerals was confirmed: mullite-like, C-S-H, calcite, zeolite, and quartz. In the DS-0 sample, the intensity of the diffraction peak at 2θ = 29.5° increased, indicating a higher content of C-S-H gel and a reduced amount of N-A-S-H gel, which contributed to the higher compressive strength of DS-0 [56,71,72].Compared to DS-10, the diffraction peaks at 2θ = 26.6° were lower for DS-T-10, DS-G20-10, and DS-G30-10, with the DS-T-10 peak being the smallest. This suggests that the high-temperature treatment of desert sand caused phase changes. DS-T, DS-G20, and DS-G30 exhibit pozzolanic effects, dissolving in the alkaline environment and participating in the hydration reactions, promoting the formation of C(N)-A-S-H gel [54,56]. As a result, the 28-day compressive strength of AAC containing DS-T, DS-G20, and DS-G30 is higher than that of AAC containing DS.

In an alkaline environment, the activator disrupts the silicate network structure of GBFS, leading to the hydration of GBFS and the formation of various zeolite-like products such as sodalite, natrolite, and tobermorite [73]. Li et al. observed hydration products such as hydrotalcite, C-(N)-A-S-H, calcite, mullite, and quartz in samples of AAC with GBFS and FA dried at different temperatures [74]. Rafeet et al. also prepared FA-GBFS cements, identifying hydration products such as mullite, C-A-S-H, hydrotalcite, and quartz in pastes with varying GBFS content [71]. Luan et al. used flue gas desulfurization gypsum and water glass to prepare a composite activator for producing AAC with GBFS and FA. The main hydration products of AAC were calcium silicate, calcite, mullite, ettringite, and quartz [64].

3.4. pH Values

In this study, the pH values of pore solutions extracted from 28-day AAC paste powders were determined using a solid–liquid extraction method, as shown in Figure 7. In AAC with GBFS, the pH values of the pore solution correlated with the degree of GBFS activation. OH⁻ ions can disrupt the aluminosilicate network in the GBFS glass phase, and pH values greater than 11.5 indicates a high level of activation of GBFS [45]. Most of the Na⁺ ions are captured or absorbed by the C-(A)-S-H gel [75]. Due to the low cementitious activity of fly ash, under wet curing conditions, the loss of Na⁺ and OH⁻ leads to a decrease in the pore solution pH as the fly ash content increases within the same curing period [44].

As shown in Figure 20, the 28-day pH values of AAC are approximately 12. The pH value for DS-0 is 12.03, while the pH values for DS-T-5, DS-G30-5, and DS-G30-10 are below 12. In the same modified desert sand AAC, the 28-day pH values initially increase and then decrease as the desert sand content increases. Specifically, for the modified desert sand AAC with 10% and 15% sand content, the 28-day pH values are relatively the highest.

The relevant research works [76,77] simulated the effects of pH on the nucleation process, nanostructure, and pore size structure of alkali-activated aluminosilicate gel, finding that pH plays a critical role in the dissolution, polycondensation, and reorganization processes of geopolymer materials. During the dissolution stage, in a system with a pH of 13, the number of silicate monomers is higher than that of aluminates. In the polycondensation process, systems with pH values of 11 and 12 yield more gel particles. Once the gel reorganization is complete, in a system with a pH of 13, the number of monomers that did not participate in polycondensation is higher, resulting in larger pore diameters. In this study, the pH of AAC pastes with different modified desert sands at 28 days ranged between 11.5 and 12.5. The hydration process of water glass-activated cement with GBFS and FA is similar to that of alkali-activated kaolin binding materials, exhibiting both dissolution and polycondensation stages [78].

4. Discussion

4.1. Compressive and Flexural Strengths of the Optimized Mixes

As shown in Figure 21, the compressive strength growth trend of DS-0 is consistent with that of AAC samples containing different types of desert sand. From 14 to 28 days, the compressive strength of DS-G30-10 is higher than that of DS-10, DS-T-10, and DS-G20-10. From 7 to 28 days, the compressive strength growth trends of DS-10, DS-T-10, and DS-G20-10 are similar, with only slight differences in strength at the same age. From 3 to 28 days, the flexural strength of DS-0 shows a significant decrease, and from 21 to 28 days, the flexural strength of DS-0 is lower than that of the AAC samples containing different types of desert sand. The variation in flexural strength from 7 to 28 days is minimal for DS-10, DS-T-10, DS-G20-10, and DS-G30-10. During the same period, the flexural strength of DS-10 and DS-G30-10 is higher than that of DS-T-10 and DS-G20-10, with DS-10 having a higher flexural strength than DS-G20-10. Comparing the compressive and flexural strengths of the four mixes at different ages, it is evident that DS-T has higher reactivity than DS, which promotes the hydration reaction of ACC, leading to a peak in flexural strength for DS-T-10 at 3 days. The particle size distribution and pozzolanic effect of DS-G30 are superior to those of DS-G20, resulting in the accumulation of more harmful pores in DS-G20-10, which causes its flexural strength at the same age to be lower than that of DS-10, DS-T-10, and DS-G30-10. Additionally, the compressive strength of DS-G20-10 at the same age is lower than that of DS-G30-10. In summary, among the four mixes, DS-G30-10 exhibits the best mechanical properties.

4.2. Compressive and Flexural Strengths Developments of the Optimized Mixes Compared with DS-0 and OPC

As shown in Figure 22,comparing the compressive strength of DS-0 from 3 to 28 days, the compressive strength ratio of DS-10, DS-T-10, and DS-G20-10 is approximately 90%, indicating that the 28-day compressive activity index of DS, DS-T, and DS-G20 in AAC with a 10% substitution is approximately 90%. Additionally, this compressive activity index can reach approximately 90% as early as 3 days. For DS-G30-10, the compressive strength ratio ranges from 96% to 101% between 3 and 28 days, suggesting that the 28-day compressive activity index of DS-G30 in AAC at a 10% substitution is 96%. This indicates that DS-G30 exhibits a pozzolanic effect, and its superior particle size distribution improves the pore structure of AAC.

Comparing the flexural strength of DS-0 from 3 to 28 days, the flexural strength ratios of DS-10, DS-T-10, DS-G20-10, and DS-G30-10 generally show an increasing trend, with the 28-day flexural strength ratios ranging from 109% to 130%. This suggests that the addition of different types of desert sand inhibits the shrinkage during AAC hydration, and as the curing period increases, there is no significant regression in AAC’s flexural strength. Additionally, the flexural strength from 14 to 28 days is greater than that of DS-0. From 3 to 21 days, the flexural strength ratio of DS-T-10 is higher than that of DS-10, DS-G20-10, and DS-G30-10, with the highest flexural strength ratio for DS-T-10 at 3 days being 112. This indicates that thermally treated desert sand has a strong pozzolanic effect, promoting early hydration of AAC. The highest flexural strength ratio at 28 days is observed in DS-G30-10, reaching 130%, and the flexural strength ratio of DS-G30-10 from 3 to 28 days is consistently higher than that of DS-G20-10, indicating that the particle size distribution of DS-G30 is superior to that of DS-G20.

5. Conclusions

This study systematically investigates for the first time the effects of different treatment methods and contents of desert sand on the mechanical properties and microstructure of AAC with GBFS and FA. Desert sand was treated at high temperatures and subjected to two different grinding durations, with content of 5%, 10%, 15%, and 20%, respectively. OPC and AAC without desert sand (DS-0) were used as control groups. The effects of different treatment methods and content of desert sand on the compressive strength and flexural strength of AAC from 3 to 28 days were investigated. Additionally, this study examined the types of hydration products, pore structure, pH values, and other properties of AAC containing different types of desert sand. The main findings are as follows:

• During the 3–7 day period, as the content of DS and DS-T increases, the compressive strength of AAC initially increased and then decreased. Similarly, from 14 to 28 days, with the increasing content of DS, DS-T, DS-G20, and DS-G30, the compressive strength of AAC first increased and then decreased. Except for DS-T-20, the 3-day and 28-day flexural strength of AAC with DS and DS-T showed little difference. For AAC with DS-G20 and DS-G30, the flexural strength from 7 to 28 days also showed minimal difference, indicating that the skeletal filling effect of DS and DS-T during the early stages of standard curing contributed to the increase in AAC’s flexural strength. At 28 days, the compressive strength of AAC with different types of desert sand was higher than that of OPC but lower than that of DS-0. Meanwhile, the flexural strength of AAC with different types of desert sand and DS-0 was lower than that of OPC. Between 3 and 28 days, the flexural strength of OPC, DS-0, and DS-T-5 showed a significant decline. The thermal treatment of DS-T promoted the hydration reaction of AAC but also led to the formation of more cracks in the AAC matrix, causing a slight decrease in the flexural strength of DS-T-10 and DS-T-15 from 3 to 28 days. In contrast, the skeletal filling effect of DS, DS-G20, and DS-G30 helps suppress the decline in AAC’s flexural strength.

• Based on the SEM images of 28-day AAC paste, DS-T and DS-G30 were embedded within the matrix, primarily serving as skeletal fillers. The bond between DS-G30 and the AAC matrix was denser compared to that of DS-T. According to the XRD data of the 28-day AAC paste, the quartz diffraction peaks corresponding to DS-T-10, DS-G20-10, and DS-G30-10 were lower than those of DS-10, indicating that the thermally treated DS and ground DS exhibited enhanced pozzolanic effects. The MIP data of the 28-day AAC paste showed that DS-G20 and DS-G30 reduce the cumulative pore volume of the AAC, with DS-G30 demonstrating the most significant improvement. The comparative analysis of the early strength of AAC with low content of desert sand found that DS-T exhibited the highest pozzolanic effect.

• Comparing the development trends of compressive and flexural strength from 3 to 28 days for AAC with different types of desert sand, it was found that AAC pastes at a 10% re addition level of DS, DS-T, DS-G20, and DS-G30 exhibit superior mechanical properties. Among them, DS-G30-10 demonstrated the best overall performance, with its 28-day compressive strength, flexural strength, and pore structure outperforming those of DS-10, DS-T-10, and DS-G20-10. Thus, AAC with a 10% addition of DS-G30 exhibited the most optimal comprehensive properties.

• Desert sand can be effectively used as supplementary materials in AAC. The two grinding modification of desert sand prove to be superior to thermal modification. Ground desert sand helps to mitigate the decline in AAC’s flexural strength. Furthermore, the particle size distribution and pozzolanic effect of DS-G30 are better than those of DS-G20. The optimal addition level for both DS-G20 and DS-G30 is determined to be 10%. DS has a larger particle size, which could negatively impact the workability of concrete and mortar if used directly as supplementary materials for AAC and OPC. DS-T, which is treated at high temperatures, has a higher cost and also a larger particle size, making AAC with DS-T more expensive and less convenient for market application and promotion.

6. Future Works

Future research will focus on exploring the effects of different treatments of desert sand on the hydration heat release and drying shrinkage of AAC. This includes investigating the hydration mechanisms of AAC with different treatments of desert sand, and examining the relationship between the drying shrinkage and strength development of AAC with differently treated desert sands. Further studies will also be conducted on the durability performance of AAC with different treatments of desert sand, including carbonation, freeze–thaw resistance, and long-term strength, to provide data support for the use of AAC with different treatments of desert sand in mortar and concrete applications.

Author Contributions

Conceptualization, Y.L., K.N. and Y.W.; methodology, X.Y., Y.L. and Z.F.; software, X.Y. and Z.F.; validation, B.L., Y.Z. and S.L.; formal analysis, Y.L. and X.Y.; investigation, X.O. and G.W.; resources, Z.Y., T.Z. and D.L.; data curation, S.Y., H.Y. and X.Y.; writing—original draft preparation, Y.L., X.Y. and W.T.; writing—review and editing, Y.L., K.N. and R.G.; visualization, Z.F. and Y.L.; supervision, Y.W., K.N. and R.G.; project administration, Y.L. and D.X.; funding acquisition, Y.W. and K.N. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

Authors Zhiyu Yao and Tian Zhao were employed by the company Hebei Construction Group Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Desert sand distribution in the world [36].

Figure 2. Morphology of materials: (a) GBFS; (b) FA; (c) OPC; (d) DS; (e) DS-T; (f) DS-G20; (g) DS-G30.

Figure 2. Morphology of materials: (a) GBFS; (b) FA; (c) OPC; (d) DS; (e) DS-T; (f) DS-G20; (g) DS-G30.

Figure 3. Particle size distribution: (a) cumulative volume; (b) particle volume.

Figure 4. Strength of different mixes at 3 days: (a) 3 d compressive strength; (b) 3 d flexural strength.

Figure 5. Strength of different mixes at 7 days: (a) 7 d compressive strength; (b) 7 d flexural strength.

Figure 6. Strength of different mixes at 14 days: (a) 14 d compressive strength; (b) 14 d flexural strength.

Figure 7. Strength of different mixes at 21 days: (a) 21 d compressive strength; (b) 21 d flexural strength.

Figure 8. Strength of different mixes at 28 days: (a) 28 d compressive strength; (b) 28 d flexural strength.

Figure 9. Strength of DS mixes from 3 to 28 days: (a) 28 d compressive strength; (b) 28 d flexural strength.

Figure 10. Strength of DS-T mixes from 3 to 28 days: (a) 28 d compressive strength; (b) 28 d flexural strength.

Figure 11. Strength of DS-G20 mixes from 3 to 28 days: (a) 28 d compressive strength; (b) 28 d flexural strength.

Figure 12. Strength of DS-G30 mixes from 3 to 28 days: (a) 28 d compressive strength; (b) 28 d flexural strength.

Figure 13. Results of mercury intrusion porosimetry: (a) cumulative pore size distribution; (b) cumulative pore volume of different types of pores; (c) the proportion of cumulative pore volume of different types of pores.

Figure 14. SEM images of desert sand in 28 d AAC pastes: (a) DS-T-20; (b) DS-G30-20.

Figure 15. SEM images of FA in 28 d AAC pastes: (a) DS-T-20; (b) DS-G30-20.

Figure 16. SEM images of GBFS in 28 d AAC pastes: (a) DS-T-5; (b) DS-G30-20.

Figure 17. SEM images with EDX: (a) DS-T-5; (b) DS-G30-10; (c) elemental comparison.

Figure 18. FT-IR spectra of AAC pastes with different modified 10 additives of DS at 28 days.

Figure 19. XRD patterns of AAC pastes at 28 days.

Figure 20. pH values of 28-day AAC.

Figure 21. Compressive and flexural strengths of the optimized mixes: (a) compressive strength; (b) flexural strength.

Figure 22. Comparison of compressive and flexural strengths with the optimized mixes and DS-0: (a) compressive strength comparison; (b) flexural strength comparison.

Table 1

Main composite of materials.

	SiO2	Al2O3	Fe2O3	CaO	TiO2	K2O	SO3	MgO	P2O3	Na2O
GBFS	29.43	14.38	0.9	44.50	1.61	0.35	2.34	5.49	0.03	0.28
FA	48.70	35.90	5.06	3.88	0.7	1.36	0.80	0.61	0.50	0.38
OPC	16.20	5.05	2.61	65.85	0.78	1.25	3.07	3.42	1.19	0.24
DS	84.10	8.27	1.67	079	0.24	2.71	0.02	0.78	0.06	1.17

Table 2

Density and specific surface area of materials.

	Bulk Density (kg/m3)	Apparent Density(kg/m3)	Specific Surface Area (m2/kg)	Porosity(%)
GBFS	843.13	2832.00	623.45	70.23
FA	863.44	2381.60	551.50	63.75
OPC	1163.34	3090.12	376.40	62.35
DS	1632.13	2616.47	10.48	37.62
DS-G20	1076.13	2679.24	314.41	59.83
DS-G30	942.5	2790.12	448.80	66.93

Table 3

Mix proportions.

	OPC(%)	GBFS(%)	FA(%)	DS(%)	DS-T(%)	DS-G20(%)	DS-G30(%)	Na2SiO3·9H2O(%)	Water(%)
OPC	100	0	0	0				0	40
DS-0		80	20	0				13.75	32.16
DS-5		75	20	5
DS-10		70	20	10
DS-15		65	20	15
DS-20		60	20	20
DS-T-5		75	20		5
DS-T-10		70	20		10
DS-T-15		65	20		15
DS-T-20		60	20		20
DS-G20-5		75	20			5
DS-G20-10		70	20			10
DS-G20-15		65	20			15
DS-G20-20		60	20			20
DS-G30-5		75	20				5
DS-G30-10		70	20				10
DS-G30-15		65	20				15
DS-G30-20		60	20				20

References

1. Barcelo, L.; Kline, J.; Walenta, G.; Gartner, E. Cement and carbon emissions. Mater. Struct.; 2014; 47, pp. 1055-1065. [DOI: https://dx.doi.org/10.1617/s11527-013-0114-5]

2. Miller, S.A.; Horvath, A.; Monteiro, P.J. Readily implementable techniques can cut annual CO₂ emissions from the production of concrete by over 20%. Environ. Res. Lett.; 2016; 11, 074029. [DOI: https://dx.doi.org/10.1088/1748-9326/11/7/074029]

3. Dunant, C.F.; Joseph, S.; Prajapati, R.; Allwood, J.M. Electric recycling of Portland cement at scale. Nature; 2024; 629, pp. 1055-1061. [DOI: https://dx.doi.org/10.1038/s41586-024-07338-8] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38778099]

4. Zunino, F.; Dhandapani, Y.; Ben Haha, M.; Skibsted, J.; Joseph, S.; Krishnan, S.; Parashar, A.; Juenger, M.C.; Hanein, T.; Bernal, S.A. Hydration and mixture design of calcined clay blended cements: Review by the RILEM TC 282-CCL. Mater. Struct.; 2022; 55, 234. [DOI: https://dx.doi.org/10.1617/s11527-022-02060-1]

5. Al-Noaimat, Y.A.; Chougan, M.; Al-kheetan, M.J.; Al-Mandhari, O.; Al-Saidi, W.; Al-Maqbali, M.; Al-Hosni, H.; Ghaffar, S.H. 3D printing of limestone-calcined clay cement: A review of its potential implementation in the construction industry. Results Eng.; 2023; 18, 101115. [DOI: https://dx.doi.org/10.1016/j.rineng.2023.101115]

6. Panda, B.; Tan, M.J. Rheological behavior of high volume fly ash mixtures containing micro silica for digital construction application. Mater. Lett.; 2019; 237, pp. 348-351. [DOI: https://dx.doi.org/10.1016/j.matlet.2018.11.131]

7. Wang, J.; Che, Z.; Zhang, K.; Fan, Y.; Niu, D.; Guan, X. Performance of recycled aggregate concrete with supplementary cementitious materials (fly ash, GBFS, silica fume, and metakaolin): Mechanical properties, pore structure, and water absorption. Constr. Build. Mater.; 2023; 368, 130455. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.130455]

8. Lizarazo-Marriaga, J.; Claisse, P.; Ganjian, E. Effect of steel slag and portland cement in the rate of hydration and strength of blast furnace slag pastes. J. Mater. Civ. Eng.; 2011; 23, pp. 153-160. [DOI: https://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0000149]

9. Gökçe, H.; Tuyan, M.; Nehdi, M. Alkali-activated and geopolymer materials developed using innovative manufacturing techniques: A critical review. Constr. Build. Mater.; 2021; 303, 124483. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.124483]

10. Sun, B.; Ye, G.; De Schutter, G. A review: Reaction mechanism and strength of slag and fly ash-based alkali-activated materials. Constr. Build. Mater.; 2022; 326, 126843. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.126843]

11. Liu, H.; Song, J. Research on the Mechanical Properties of Desert Sand Concrete and Its Resistance to Carbonation and Chloride Ion Permeation; Yang Guang Chu Ban She: Beijing, China, 2019.

12. Liu, X.; Liu, R.; Lyu, K.; Gu, Y. A Quantitative Evaluation of Size and Shape Characteristics for Desert Sand Particles. Minerals; 2022; 12, 581. [DOI: https://dx.doi.org/10.3390/min12050581]

13. Liu, Y.; Li, Y.; Jiang, G. Orthogonal experiment on performance of mortar made with dune sand. Constr. Build. Mater.; 2020; 264, 120254. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.120254]

14. Zhang, Q.; Liu, H.; Liu, Q.; Che, J.; Yang, W.; Ma, J.; Doh, S.I.; Lim, K.S. Study the fire resistance of desert sand concrete (DSC) with interface phase through uniaxial compression tests and analyses. Adv. Civ. Eng.; 2021; 2021, 8863136. [DOI: https://dx.doi.org/10.1155/2021/8863136]

15. Yan, W.; Wu, G.; Dong, Z. Optimization of the mix proportion for desert sand concrete based on a statistical model. Constr. Build. Mater.; 2019; 226, pp. 469-482. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2019.07.287]

16. Jiang, J.; Feng, T.; Chu, H.; Wu, Y.; Wang, F.; Zhou, W.; Wang, Z. Quasi-static and dynamic mechanical properties of eco-friendly ultra-high-performance concrete containing aeolian sand. Cem. Concr. Compos.; 2019; 97, pp. 369-378. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2019.01.011]

17. Zhang, G.; Song, J.; Yang, J.; Liu, X. Performance of mortar and concrete made with a fine aggregate of desert sand. Build. Environ.; 2006; 41, pp. 1478-1481. [DOI: https://dx.doi.org/10.1016/j.buildenv.2005.05.033]

18. Alhozaimy, A.; Al-Negheimish, A.; Alawad, O.; Jaafar, M.; Noorzaei, J. Binary and ternary effects of ground dune sand and blast furnace slag on the compressive strength of mortar. Cem. Concr. Compos.; 2012; 34, pp. 734-738. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2012.03.002]

19. Naas, A.; Taha-Hocine, D.; Salim, G.; Michèle, Q. Combined effect of powdered dune sand and steam-curing using solar energy on concrete characteristics. Constr. Build. Mater.; 2022; 322, 126474. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.126474]

20. Luo, F.J.; He, L.; Pan, Z.; Duan, W.H.; Zhao, X.L.; Collins, F. Effect of very fine particles on workability and strength of concrete made with dune sand. Constr. Build. Mater.; 2013; 47, pp. 131-137. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2013.05.005]

21. Benabed, B.; Azzouz, L.; Kadri, E.-H.; Kenai, S.; Belaidi, A.S.E. Effect of fine aggregate replacement with desert dune sand on fresh properties and strength of self-compacting mortars. J. Adhes. Sci. Technol.; 2014; 28, pp. 2182-2195. [DOI: https://dx.doi.org/10.1080/01694243.2014.950625]

22. Al-Harthy, A.S.; Halim, M.A.; Taha, R.; Al-Jabri, K. The properties of concrete made with fine dune sand. Constr. Build. Mater.; 2007; 21, pp. 1803-1808. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2006.05.053]

23. Li, W.; Huang, Z.; Zu, T.; Shi, C.; Duan, W.H.; Shah, S.P. Influence of nanolimestone on the hydration, mechanical strength, and autogenous shrinkage of ultrahigh-performance concrete. J. Mater. Civ. Eng.; 2016; 28, 04015068. [DOI: https://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0001327]

24. Craeye, B.; De Schutter, G.; Desmet, B.; Vantomme, J.; Heirman, G.; Vandewalle, L.; Cizer, Ö.; Aggoun, S.; Kadri, E. Effect of mineral filler type on autogenous shrinkage of self-compacting concrete. Cem. Concr. Res.; 2010; 40, pp. 908-913. [DOI: https://dx.doi.org/10.1016/j.cemconres.2010.01.014]

25. Deschner, F.; Winnefeld, F.; Lothenbach, B.; Seufert, S.; Schwesig, P.; Dittrich, S.; Goetz-Neunhoeffer, F.; Neubauer, J. Hydration of Portland cement with high replacement by siliceous fly ash. Cem. Concr. Res.; 2012; 42, pp. 1389-1400. [DOI: https://dx.doi.org/10.1016/j.cemconres.2012.06.009]

26. Vance, K.; Aguayo, M.; Oey, T.; Sant, G.; Neithalath, N. Hydration and strength development in ternary portland cement blends containing limestone and fly ash or metakaolin. Cem. Concr. Compos.; 2013; 39, pp. 93-103. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2013.03.028]

27. Krauss, H.; Budelmann, H. Hydration kinetics of cement paste with very fine inert mineral additives. Proceedings of the International RILEM Conference on Advances in Construction Materials Through Science and Engineering; Hong Kong, China, 5–7 September 2011; pp. 93-99.

28. Wu, P.; Lyu, X.; Wang, J.; Hu, S. Effect of mechanical grinding on the hydration activity of quartz in the presence of calcium hydroxide. Adv. Cem. Res.; 2017; 29, pp. 269-277. [DOI: https://dx.doi.org/10.1680/jadcr.16.00159]

29. Wei, B.; Zhang, Y.; Bao, S. Preparation of geopolymers from vanadium tailings by mechanical activation. Constr. Build. Mater.; 2017; 145, pp. 236-242. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2017.03.234]

30. Kumar, S.; Mucsi, G.; Kristály, F.; Pekker, P. Mechanical activation of fly ash and its influence on micro and nano-structural behaviour of resulting geopolymers. Adv. Powder Technol.; 2017; 28, pp. 805-813. [DOI: https://dx.doi.org/10.1016/j.apt.2016.11.027]

31. Kai, L.; Liu, X.; Xie, X.; Liu, R.; Li, T.; Shah, S.P. The feasibility of utilizing sifted desert sand (<75 μm) as sustainable supplementary cementitious materials (SCM). Constr. Build. Mater.; 2023; 406, 133375.

32. Guettala, S.; Mezghiche, B. Compressive strength and hydration with age of cement pastes containing dune sand powder. Constr. Build. Mater.; 2011; 25, pp. 1263-1269. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2010.09.026]

33. Liu, M.; Liu, E.; Hao, J.L.; Di Sarno, L.; Xia, J. Hydration and material properties of blended cement with ground desert sand. Constr. Build. Mater.; 2023; 389, 131624. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.131624]

34. Li, G.; Shen, X.; Yuan, Q.; Zou, Y.; Xiong, L. Study on the activity of aeolian sand powder and alkali excitation modification. JOM; 2019; 71, pp. 984-994. [DOI: https://dx.doi.org/10.1007/s11837-018-3163-y]

35. Van Tran, M.; Ly, D.-K.; Nguyen, T.; Tran, N. Robust prediction of workability properties for 3D printing with steel slag aggregate using bayesian regularization and evolution algorithm. Constr. Build. Mater.; 2024; 431, 136470. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2024.136470]

36. Tsoar, H. The ecological background, deterioration and reclamation of desert dune sand. Agric. Ecosyst. Environ.; 1990; 33, pp. 147-170. [DOI: https://dx.doi.org/10.1016/0167-8809(90)90239-A]

37. Wang, M.; Xu, J.; Zhang, X.; Tan, L.; Mei, Y. Mechanical Performance Optimization and Microstructural Mechanism Study of Alkali-Activated Steel Slag–Slag Cementitious Materials. Buildings; 2024; 14, 1204. [DOI: https://dx.doi.org/10.3390/buildings14051204]

38. Salas Montoya, A.; Rodríguez-Barboza, L.I.; Colmenero Fonseca, F.; Cárcel-Carrasco, J.; Gómez-Zamorano, L.Y. Composite Cements Using Ground Granulated Blast Furnace Slag, Fly Ash, and Geothermal Silica with Alkali Activation. Buildings; 2023; 13, 1854. [DOI: https://dx.doi.org/10.3390/buildings13071854]

39. Zhao, Y.; Yang, C.; Li, K.; Yang, J.; Wu, Z.; Yan, C. Mechanical Performances and Frost Resistance of Alkali-Activated Coal Gangue Cementitious Materials. Buildings; 2022; 12, 2243. [DOI: https://dx.doi.org/10.3390/buildings12122243]

40. Ma, H.; Li, X.; Zheng, X.; Niu, X.; Fang, Y. Effect of active MgO on the hydration kinetics characteristics and microstructures of alkali-activated fly ash-slag materials. Constr. Build. Mater.; 2022; 361, 129677. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.129677]

41. Huang, G.; Zhang, X.; Liu, M.; Fang, B.; Wang, C.; Mi, H. Compatibility of sodium hydroxide, sodium silicate and calcium-enriched additives in alkali-activated materials: From the perspectives of flowability, strength and microstructure. Constr. Build. Mater.; 2023; 403, 133102. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.133102]

42. Srinivasamurthy, L.; Chevali, V.S.; Zhang, Z.; Longhi, M.A.; Loh, T.W.; Wang, H. Mechanical property and microstructure development in alkali activated fly ash slag blends due to efflorescence. Constr. Build. Mater.; 2022; 332, 127273. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.127273]

43. Kamath, M.; Prashant, S.; Ralegaonkar, R. Microstructure Properties of Popular Alkali-Activated Pastes Cured in Ambient Temperature. Buildings; 2023; 13, 858. [DOI: https://dx.doi.org/10.3390/buildings13040858]

44. Nedeljković, M.; Ghiassi, B.; van der Laan, S.; Li, Z.; Ye, G. Effect of curing conditions on the pore solution and carbonation resistance of alkali-activated fly ash and slag pastes. Cem. Concr. Res.; 2019; 116, pp. 146-158. [DOI: https://dx.doi.org/10.1016/j.cemconres.2018.11.011]

45. Song, S.; Jennings, H.M. Pore solution chemistry of alkali-activated ground granulated blast-furnace slag. Cem. Concr. Res.; 1999; 29, pp. 159-170. [DOI: https://dx.doi.org/10.1016/S0008-8846(98)00212-9]

46. Ahmad, M.R.; Khan, M.; Wang, A.; Zhang, Z.; Dai, J.-G. Alkali-activated materials partially activated using flue gas residues: An insight into reaction products. Constr. Build. Mater.; 2023; 371, 130760. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.130760]

47. Ahmad, M.R.; Qian, L.-P.; Fang, Y.; Wang, A.; Dai, J.-G. A multiscale study on gel composition of hybrid alkali-activated materials partially utilizing air pollution control residue as an activator. Cem. Concr. Compos.; 2023; 136, 104856. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2022.104856]

48. Wei, X.; Li, D.; Ming, F.; Yang, C.; Chen, L.; Liu, Y. Influence of low-temperature curing on the mechanical strength, hydration process, and microstructure of alkali-activated fly ash and ground granulated blast furnace slag mortar. Constr. Build. Mater.; 2021; 269, 121811. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.121811]

49. GB/T 17671-2021 Test Method of Cement Mortar Strength (ISO Method); Standardization Administration of China: Beijing, China, 2021.

50. Aygörmez, Y. Evaluation of the red mud and quartz sand on reinforced metazeolite-based geopolymer composites. J. Build. Eng.; 2021; 43, 102528. [DOI: https://dx.doi.org/10.1016/j.jobe.2021.102528]

51. Nedunuri, S.S.S.A.; Sertse, S.G.; Muhammad, S. Microstructural study of Portland cement partially replaced with fly ash, ground granulated blast furnace slag and silica fume as determined by pozzolanic activity. Constr. Build. Mater.; 2020; 238, 117561. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2019.117561]

52. Chithiraputhiran, S.; Neithalath, N. Isothermal reaction kinetics and temperature dependence of alkali activation of slag, fly ash and their blends. Constr. Build. Mater.; 2013; 45, pp. 233-242. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2013.03.061]

53. Salim, M.U.; Moro, C. Towards sustainable construction: Performance evaluation of slag-cenosphere geopolymers under different NaOH concentrations. J. Build. Eng.; 2024; 91, 109605. [DOI: https://dx.doi.org/10.1016/j.jobe.2024.109605]

54. Liu, S.; Xie, G.; Wang, S. Effect of curing temperature on hydration properties of waste glass powder in cement-based materials. J. Therm. Anal. Calorim.; 2015; 119, pp. 47-55. [DOI: https://dx.doi.org/10.1007/s10973-014-4095-6]

55. Real, S.; Carriço, A.; Bogas, J.A.; Guedes, M. Influence of the treatment temperature on the microstructure and hydration behavior of thermoactivated recycled cement. Materials; 2020; 13, 3937. [DOI: https://dx.doi.org/10.3390/ma13183937] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32899578]

56. Xia, D.; Chen, R.; Cheng, J.; Tang, Y.; Xiao, C.; Li, Z. Desert sand-high calcium fly ash-based alkali-activated mortar: Flowability, mechanical properties, and microscopic analysis. Constr. Build. Mater.; 2023; 398, 131729. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.131729]

57. Ates, F.; Park, K.T.; Kim, K.W.; Woo, B.-H.; Kim, H.G. Effects of treated biomass wood fly ash as a partial substitute for fly ash in a geopolymer mortar system. Constr. Build. Mater.; 2023; 376, 131063. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.131063]

58. Scrivener, K.; Snellings, R.; Lothenbach, B.; Press, C. A Practical Guide to Microstructural Analysis of Cementitious Materials; CRC Press: Boca Raton, FL, USA, 2016; Volume 540.

59. Silva, D.; John, V.; Ribeiro, J.; Roman, H. Pore size distribution of hydrated cement pastes modified with polymers. Cem. Concr. Res.; 2001; 31, pp. 1177-1184. [DOI: https://dx.doi.org/10.1016/S0008-8846(01)00549-X]

60. Wang, S.; Zhang, G.; Wang, Z.; Huang, T.; Wang, P. Evolutions in the properties and microstructure of cement mortars containing hydroxyethyl methyl cellulose after controlling the air content. Cem. Concr. Compos.; 2022; 129, 104487. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2022.104487]

61. Fernández-Jiménez, A.; Palomo, A.; Criado, M. Microstructure development of alkali-activated fly ash cement: A descriptive model. Cem. Concr. Res.; 2005; 35, pp. 1204-1209. [DOI: https://dx.doi.org/10.1016/j.cemconres.2004.08.021]

62. Sultan, M.E.; Abo-El-Enein, S.A.; Sayed, A.Z.; EL-Sokkary, T.M.; Hammad, H.A. Incorporation of cement bypass flue dust in fly ash and blast furnace slag-based geopolymer. Case Stud. Constr. Mater.; 2018; 8, pp. 315-322. [DOI: https://dx.doi.org/10.1016/j.cscm.2018.02.009]

63. Ju, C.; Liu, Y.; Jia, M.; Yu, K.; Yu, Z.; Yang, Y. Effect of calcium oxide on mechanical properties and microstructure of alkali-activated slag composites at sub-zero temperature. J. Build. Eng.; 2020; 32, 101561. [DOI: https://dx.doi.org/10.1016/j.jobe.2020.101561]

64. Luan, Y.; Wang, J.; Ma, T.; Wang, S.; Li, C. Modification mechanism of flue gas desulfurization gypsum on fly ash and ground granulated blast-furnace slag alkali-activated materials: Promoting green cementitious material. Constr. Build. Mater.; 2023; 396, 132400. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2023.132400]

65. Zhang, J.; Li, B.; Zhang, M.; Shan, Z.; Jin, Q. Mechanical behavior and microstructure evolution of slag-fly ash based geopolymer stabilized sand. Case Stud. Constr. Mater.; 2024; 20, e03279. [DOI: https://dx.doi.org/10.1016/j.cscm.2024.e03279]

66. Song, W.; Zhu, Z.; Pu, S.; Wan, Y.; Huo, W.; Song, S.; Zhang, J.; Yao, K.; Hu, L. Efficient use of steel slag in alkali-activated fly ash-steel slag-ground granulated blast furnace slag ternary blends. Constr. Build. Mater.; 2020; 259, 119814. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.119814]

67. Kovtun, M.; Kearsley, E.P.; Shekhovtsova, J. Chemical acceleration of a neutral granulated blast-furnace slag activated by sodium carbonate. Cem. Concr. Res.; 2015; 72, pp. 1-9. [DOI: https://dx.doi.org/10.1016/j.cemconres.2015.02.014]

68. Puertas, F.; Torres-Carrasco, M. Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterisation. Cem. Concr. Res.; 2014; 57, pp. 95-104. [DOI: https://dx.doi.org/10.1016/j.cemconres.2013.12.005]

69. Revathi, T.; Jeyalakshmi, R. Fly ash–GGBS geopolymer in boron environment: A study on rheology and microstructure by ATR FT-IR and MAS NMR. Constr. Build. Mater.; 2021; 267, 120965. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.120965]

70. Lodeiro, I.G.; MacPhee, D.E.; Palomo, A.; Fernández-Jiménez, A. Effect of alkalis on fresh C–S–H gels. FTIR analysis. Cem. Concr. Res.; 2009; 39, pp. 147-153. [DOI: https://dx.doi.org/10.1016/j.cemconres.2009.01.003]

71. Rafeet, A.; Vinai, R.; Soutsos, M.; Sha, W. Effects of slag substitution on physical and mechanical properties of fly ash-based alkali activated binders (AABs). Cem. Concr. Res.; 2019; 122, pp. 118-135. [DOI: https://dx.doi.org/10.1016/j.cemconres.2019.05.003]

72. García-Lodeiro, I.; Palomo, A.; Fernández-Jiménez, A.; Macphee, D. Compatibility studies between NASH and CASH gels. Study in the ternary diagram Na₂O–CaO–Al₂O₃–SiO₂–H₂O. Cem. Concr. Res.; 2011; 41, pp. 923-931. [DOI: https://dx.doi.org/10.1016/j.cemconres.2011.05.006]

73. Shi, C.; Roy, D.; Krivenko, P. Alkali-Activated Cements and Concretes; CRC Press: Boca Raton, FL, USA, 2003.

74. Li, G.; Tan, H.; Zhang, J.; Deng, X.; Liu, X.; Luo, Z. Ground granulated blast-furnace slag/fly ash blends activated by sodium carbonate at ambient temperature. Constr. Build. Mater.; 2021; 291, 123378. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.123378]

75. Luo, X.; Huang, L.; Wei, L.; Chen, M.; Zhou, Z.; Zhang, T. A technique for preparing one-part geopolymers by activating alkali-fused lithium slag with solid sodium silicate. Constr. Build. Mater.; 2024; 435, 136817. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2024.136817]

76. Valencia, N.C.; Izadifar, M.; Ukrainczyk, N.; Koenders, E. Coarse-Grained Monte Carlo Simulations with Octree Cells for Geopolymer Nucleation at Different pH Values. Materials; 2023; 17, 95. [DOI: https://dx.doi.org/10.3390/ma17010095] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38203949]

77. Izadifar, M.; Valencia, N.C.; Xiao, P.; Ukrainczyk, N.; Koenders, E. 3D off-lattice coarse-grained Monte Carlo simulations for nucleation of alkaline aluminosilicate gels. Materials; 2023; 16, 1863. [DOI: https://dx.doi.org/10.3390/ma16051863] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36902975]

78. Duxson, P.; Fernández-Jiménez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; Van Deventer, J.S.J. Geopolymer technology: The current state of the art. J. Mater. Sci.; 2007; 42, pp. 2917-2933. [DOI: https://dx.doi.org/10.1007/s10853-006-0637-z]