Influence of Super Absorbent Polymer (SAP) on Mechanical, Durability Properties, and Internal Curing Mechanism of Cement Stabilized Macadam in Desert Environment

Abstract

Semi-rigid base materials in desert areas are susceptible to fast evaporation of water due to high-temperature and dry environment during construction and curing, leading to insufficient cement hydration and shrinkage cracking, thus affecting the durability of the pavement. In this paper, through indoor simulation of desert curing conditions, based on the study of water absorption and water retention characteristics of super absorbent polymer (SAP), the mix proportion of SAP cement stabilized macadam (CSM) was designed, and the influence of SAP on compaction characteristics was analyzed. The variation of mechanical properties and durability of SAP internal curing CSM was studied. The x-ray diffraction (XRD) and scanning electron microscope (SEM) tests were used to clarify the internal curing effect and microscopic mechanism of SAP on CSM in desert environment. The results show that the mechanical properties of SAP CSM under different curing methods were improved. When the content was 1.0%, the internal curing effect of SAP-B with small particle size was the best in high-temperature curing environment. The dry shrinkage with 1.0% SAP-B was 31.6% lower than that without SAP. SAP can reduce the effect of temperature and humidity on the migration of water inside the specimen. In addition, the incorporation of SAP under high-temperature and drought curing improved the hydration degree of cement, and the microstructure was more compact.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Northwest China has a harsh climatic environment, especially in the desert area where high-temperature and drought persist for years [1, 2]. Under this high evaporation environment, the semi-rigid base is prone to the phenomenon of insufficient cement hydration due to rapid water evaporation, which not only reduces the overall stability of the semi-rigid base, but also further exacerbates the shrinkage and cracking, and prevails the phenomenon of insufficient durability during the service of the pavement [3, 4]. Semi-rigid base is usually cured externally to reduce early internal water evaporation. However, due to the dense structure of semi-rigid base, the moisture of external maintenance only stays on the surface, and it is difficult to enter into the interior, and it is difficult to essentially solve the problems of insufficient hydration and drying shrinkage due to the lack of internal humidity [5]. Therefore, it is of great significance to carry out research on the maintenance problems of semi-rigid base materials in desert areas to improve the service life.

The later strength development of cement mixtures mainly depends on the hydration degree of cement [6]. With the continuous hydration time, the strength of cement increases continuously. Although continuous high-temperature can accelerate the development of cement mixtures strength, resulting in insufficient hydration of cement due to insufficient water [7]. The rapid growth of early strength and the shrinkage of cement will result in an increased risk of cracking at the base, which in turn affects the durability of the pavement [8]. The strength and shrinkage resistance of cement stabilized macadam (CSM) are affected by internal humidity, which means that the curing process plays an important role in the performance of CSM [9]. The curing of CSM mainly adopts external maintenance methods, such as watering and covering wet cloth [10]. For the high-temperature and arid environment in the desert, it is difficult to fundamentally solve the problem of insufficient hydration and shrinkage cracking due to the lack of internal humidity of CSM [4]. Due to the sensitivity of CSM to temperature and humidity, the external curing method is easy to make the base in a dry and wet cycle state, thus aggravating the probability of microcracks and dry shrinkage cracking in the base [11–14]. Compared with the external maintenance, the internal maintenance does not need to bear the cost of long-distance water transportation, and the construction is convenient.

Internal curing refers to the formation of a micro “reservoir” in cement-based materials through water-absorbing materials, and can release water according to the needs of cement hydration, or supplement the water lost due to evaporation or self-drying, thereby increasing the degree of hydration of cement and alleviating shrinkage deformation [5, 15]. Super absorbent polymer (SAP) is a cross-linked polymer containing hydrophilic groups, which can usually absorb and store a large amount of water or aqueous solution [16]. Therefore, SAP is currently the most concerned internal curing material for concrete [17]. Zhu et al. [18] found that SAP can improve the compressive strength of cement mixture. Esteves et al. [19] reported that the compressive strength of ordinary cement mortar decreased under the condition of dry curing environment, while the compressive strength of SAP was almost unchanged, indicating that SAP had better curing effect in dry environment. Dudziak and Mechtcherine [20] and Kong et al. [21] suggested that SAP can delay the decrease of capillary liquid level and reduce the shrinkage of cement mixture. Kang et al. [22] proposed that after the concrete with SAP was hardened, the internal humidity increased significantly and the risk of shrinkage cracking was reduced. Guan et al. [5] showed that SAP can reduce the early water evaporation of CSM, and significantly improve the internal humidity and early and late strength of the base. Zhao and Li [23] demonstrated significantly lower drying shrinkage in SAP-modified bases compared to conventional ones, particularly at 14 days (50% reduction). After 3 months, drying shrinkage remained stable at 0.02% for SAP-modified bases versus 0.04% for conventional ones, confirming SAP’s enhanced cracking resistance. Qiao et al. [24] found that the incorporation of SAP reduced the cumulative thermal shrinkage strain and average thermal shrinkage coefficient of CSM base layers by 14.1% and 15.0%, respectively. Wang [25] found that when using composite-type SAP with a content of 0.05% and an additional water entrainment ratio of 20 times, the average drying shrinkage coefficient of the cement-fly ash-stabilized eolian sand-crushed stone semi-rigid base course was reduced by approximately 60% compared to the reference group. Research has demonstrated that SAP exhibit significant potential in internally cured semi-rigid cementitious base layers, particularly in optimizing mechanical performance and enhancing environmental adaptability. However, desert environments exhibit unique challenges, such as extreme high temperatures and arid conditions, posing severe demands on the crack resistance and durability of semi-rigid base materials. Although SAP based internal curing technology has demonstrated excellent anti-shrinkage performance in road engineering under conventional climatic conditions, its practical application in desert regions remains in the exploratory phase.

In summary, aiming at the problem of insufficient hydration and shrinkage cracking of semi-rigid base in desert environment, the effect of SAP on the mechanical properties and durability of CSM in desert environment was studied. The variation law of internal humidity of CSM under different SAP content and particle size was analyzed, and the internal curing mechanism of SAP was explored by means of x-ray diffraction (XRD) and scanning electron microscope (SEM) tests.

2. Materials and Methods

2.1. Materials

Ordinary P.O 42.5 cement was used as cementitious material, the properties of cement and the strength of cement mortars are shown in Tables 1 and 2. As shown in Table 3, the SAP (sodium polyacrylate polymer with low cross-linking density) with two particle sizes were used, the water absorption of SAP is shown in reference [26], and the water retention characteristics of SAP in pure water is shown in Figure 1. It can be observed that the water retention rate of SAP gradually decreases over time, with the larger-particle-sized SAP-A exhibiting superior water retention characteristics compared to SAP-B. Within 4 h, both particle sizes of SAP maintained a water retention rate of ~80%, demonstrating their effective water retention properties. At 12 h, the water retention rates of SAP-A and SAP-B were 44.1% and 37.0%, respectively. This difference arises because the smaller-particle-sized SAP has a larger specific surface area, leading to a faster water evaporation rate [26]. The aggregate used in this paper was limestone in different particle size ranges after crushing.

[figure(s) omitted; refer to PDF]

Table 1

Physical properties of cement.

Items	Technical indexes	Results	Standard
Fineness (%)	≤10	5.4	GB175-2007
Standard consistency water consumption (%)	—	28.5	GB175-2007
Stability (mm)	≤5	1.0	JTGE30-2005
Setting time
Initial (min)	≥45	182	JTGE30-2005
Final (min)	≤600	420	JTGE30-2005

Table 2

Strength of cement mortars.

Items	Compressive strength (MPa)		Flexural strength (MPa)
Age	3 days	28 days	3 days	28 days
Standard	≥21	≥42.5	≥4.0	≥6.5
Results	28.3	50.9	5.3	8.2

Table 3

Basic indicators of SAP.

Particle-size (mesh)	Abbreviation	Bulk density (g·cm−3)	pH	Volatile component (%)
30–80	SAP-A	0.68	6.7	5.2
60–100	SAP-B

2.2. Desert Environment Simulation

The curing environment for indoor experiments of CSM is generally constant temperature and humidity, while for the construction site curing temperature and humidity will change with the local climate. To better combine with the construction site and determine the change law of the performance of CSM in the desert environment, it is necessary to simulate the desert environment to curing the specimens. In this paper, the desert environment was simulated by high-temperature and drought environmental conditions, and compared with the performance of the specimens after standard curing (20 ± 2°C). The specific parameter settings are shown in Table 4. The high-temperature curing method is as follows: According to the analysis of the desert climate environment, the temperature of the road surface is about 60°C [27]. The specimens were wrapped with cling film after demoulding, and then placed the specimens in the oven at 60°C for curing.

Table 4

Curing condition.

Curing method	Temperature (°C)	Humidity (%)	Time (days)
Standard curing	20 ± 2	≥95	3, 7, and 28
High-temperature curing	60 ± 2	1020	3, 7, and 28

2.3. SAP-CSM Mix Proportion Design

According to China test specification JTG/T-F20-2015, the aggregate gradation design is shown in Table 5. The skeleton compactness gradation was adopted, and the specific gradation is shown in Figure 2. In this paper, 5% cement content was used for mixture ratio design, and SAP content was 0.5%, 1.0%, and 1.5% of cement weight. The optimum water content and maximum dry density for different mix proportions are shown in Table 6.

[figure(s) omitted; refer to PDF]

Table 5

The gradation of aggregate.

Size of screen mesh (mm)	31.5–19 (mm)	9.5–19 (mm)	4.75–10 (mm)	4.75–2.36 (mm)	2.36–0.075 (mm)	Design gradation	Up limit	Lower limit
31.5	100	100	100	100	100	100	100	100
26.5	72.96	100	100	100	100	95.13	100	90
19	7.35	84.1	100	100	100	79.51	87	73
16	0.92	58	100	100	100	72.09	82	65
13.2	0.73	32.9	100	100	100	66.03	75	58
9.5	0.66	5.2	86.7	98.4	100	56.77	66	47
4.75	0.65	0.7	9.7	92.5	100	40.82	50	30
2.36	0.64	0.67	1.1	11.90	85.8	30.99	36	19
1.18	0.63	0.65	1.02	5.70	57.52	20.82	26	12
0.6	0.62	0.64	0.98	3.90	40.21	14.68	19	8
0.3	0.61	0.59	0.92	2.80	25.49	9.45	14	5
0.15	0.41	0.45	0.72	2.10	13.33	5.07	10	3
0.075	0.20	0.20	0.64	1.60	6.51	2.55	7	2

Table 6

The optimum water content and maximum dry density for different mix proportions.

Mix proportion	SAP content (%)	Optimum water content (%)	Maximum dry density (g·cm−3)
REF	0	5.419	2.4148
CSM-A-0.5	0.5	5.476	2.4172
CSM-A-1.0	1.0	5.797	2.4184
CSM-A-1.5	1.5	6.274	2.4039
CSM-B-0.5	0.5	5.512	2.4250
CSM-B-1.0	1.0	5.558	2.4118
CSM-B-1.5	1.5	5.604	2.4086

Note: REF represents cement stabilized macadam without SAP; CSM-A-0.5 represents cement stabilized macadam with SAP-A content of 0.5%.

2.4. Test Methods

2.4.1. Unconfined Compressive Strength

According to China test specification JTGE51-2009, the TYE-300B pressure testing machine was used to pressurize the specimens at a speed of 1 mm/min. The calculation of the compressive strength (R_c) is shown in Equation (1).$$\begin{array}{}\text{(1)}& R_C=\frac{P}{A},\end{array}$$where P is the maximum pressure at specimen failure, N; A is the sectional area of the specimen, mm².

2.4.2. Splitting Strength

The specimen was the cylindrical specimen of φ 150 mm × 150 mm. The test method of hydraulic pressure tester with splitting pressure bar was adopted, and the pressure bar with an arc diameter of 150 mm was selected. The CSM specimens were loaded at a loading rate of 1 mm/min. The calculation of the splitting strength (R_i) is shown in Equation (2).$$\begin{array}{}\text{(2)}& R_i=0.004178\frac{P}{h},\end{array}$$where P is the maximum pressure at specimen failure, N; h is the height of the specimen after immersion in water, mm.

2.4.3. Flexural-Tensile Strength

The electronic universal testing machine (0–100 kN) CMT5105 was used to test the bending resistance of the specimens. The specimens were pressurized at a loading rate of 50 mm/min. The calculation of the flexural-tensile strength (R_s) is shown in Equation (3).$$\begin{array}{}\text{(3)}& R_s=\frac{PL}{b^{2}h},\end{array}$$where P is the failure limit load, N; L is the distance between two fulcrums, mm; b is the width of specimen, mm; h is the height of the specimen, mm.

2.4.4. Dry Shrinkage Test

The dry shrinkage test used the 100 mm × 100 mm × 400 mm middle beam specimen. The molding method was static pressure molding. After the specimen was molded, it was placed at room temperature for 1 day and then demoulded to prevent the specimen from being insufficient in strength and scattered. To simulate the natural air drying of pavement in desert environment, the dry shrinkage test was carried out in high-temperature curing environment. After 7 days of standard curing, the dry shrinkage test of the specimens was carried out in the oven at 60°C. According to the Equations (4)–(8), the dry shrinkage parameters of CSM were calculated.$$\begin{array}{}\text{(4)}& {\omega }_i=\frac{m_i-m_i+1}{m_p},\end{array}$$$$\begin{array}{}\text{(5)}& {\delta }_i=\frac{\sum _{j=1}^4{X}_{i,j}-\sum _{j=1}^4{X}_{i+1,j}}{2},\end{array}$$$$\begin{array}{}\text{(6)}& {\epsilon }_i=\frac{{\delta }_i}{l},\end{array}$$$$\begin{array}{}\text{(7)}& {\alpha }_{di}=\frac{{\epsilon }_i}{{\omega }_i},\end{array}$$$$\begin{array}{}\text{(8)}& {\alpha }_d=\frac{\sum {\epsilon }_i}{\sum {\omega }_i},\end{array}$$where ${\omega }_i$ is the rate of water loss at time i, %; ${\delta }_i$ is dry shrinkage at time i, %; ${\epsilon }_i$ is the dry shrinkage strain at time i, %; ${\alpha }_{di}$ is the dry shrinkage coefficient at time i, %; $m_i$ is the weight of standard specimen at time i, g; $X_{i,j}$ is the the reading of the j^th micrometer at the i^th test, mm; l is the length of standard specimen, mm; $m_p$ is the constant of standard specimen after drying, g.

2.4.5. Internal Relative Humidity Test

The cement specimens with different mix proportions were cured in the standard curing room for 2 days, and then the drilling treatment was carried out to place the sensor, in which the depth of the hole was 7 cm. After the sensor is placed in a reserved hole, it was sealed with sealant. Then the specimen was placed in an oven at 60°C for curing. When the temperature and humidity in the hole reached equilibrium, the change of humidity inside the CSM was measured by a digital temperature-humidity meter. The humidity measurement range was 0%–100%, as shown in Figure 3.

[figure(s) omitted; refer to PDF]

2.4.6. SEM

SEM can observe and analyze the morphology and structure of the sample surface by detecting the signals generated by electrons acting on the sample [28]. The micromorphology of CSM was tested by SEM (Quattro), and the acceleration voltage was 3 kV. The specimens that reached the specified age were broken and soaked in anhydrous ethanol. The fragments with a diameter of 2–5 mm were taken for SEM analysis, and the samples were sprayed with metal before the test. To ensure complete termination of the hydration process, anhydrous ethanol was employed for solvent exchange treatment to effectively arrest the cement hydration reaction.

2.4.7. XRD

The hydration products of cement can be tested by XRD [29]. To further reveal the curing mechanism of SAP in CSM. This paper used cement paste with water-cement ratio of 0.4 for XRD (Bruker D8 ADVANCE) test, in which the additional water diversion rate of SAP was 10 times the mass of cement. The 20 mm × 20 mm × 20 mm six-in-one test mold was used to mold the cement paste. After 24 h, the mold was demoulded for standard curing and high-temperature curing (60°C), as shown in Figure 4. The specimens that reached the specified age were broken and soaked in anhydrous ethanol, and then dried in an oven at 50°C. The broken sample was ground into fine powder for testing. The test was performed using a Cu-targeted Kα line with a voltage of 40 kV. And the scans were performed from 10° to 70° at a rate of 10°/min.

[figure(s) omitted; refer to PDF]

2.4.8. Thermogravimetry-Differential Scanning Calorimetry Analysis (TG-DSC)

Thermogravimetric analysis (TGA) is a technique that measures the relationship between mass and temperature. When cement is heated, processes, such as moisture evaporation, loss of bound water, and decomposition of CaCO₃ occur, resulting in mass changes. Among these, Ca(OH)₂—a primary hydration product of cement—serves as a key indicator of hydration degree, as variations in its content can characterize the extent of cement hydration. The variations in chemically bound water and Ca(OH)₂ content in cement were analyzed using a NETZSCH STA 449 F3 synchronous thermal analyzer. The temperature was increased from 30 to 800°C at a heating rate of 10°C/min. Nitrogen was employed as the protective atmosphere during the heating process to prevent carbonation of the cement.

3. Results and Discussions

3.1. Unconfined Compressive Strength

As shown in Figure 5a, SAP had little effect on the compressive strength under standard curing. Under standard curing, the water loss in specimen was very small, so the internal curing effect of SAP was relatively small. Figure 5b shows that when the SAP content reached 1.0%, the specimen corresponded to the highest compressive strength. In the environment of high-temperature and drought, the hydration of cement and the evaporation of water were faster. The addition of SAP can store part of the water, thus reducing the volatilization of water. Under the action of internal humidity difference, the water was gradually released to promote the hydration of cement, and the distribution of water inside the specimen tended to be uniform [30]. But the high content of SAP may reduce the contact area between aggregate and cement due to the increase of pores, resulting in weak interface transition zone and easy damage under the action of external force. And the overuse of SAP can absorb excessive water, potentially hindering the complete hydration of cement. This may weaken the formation of hydration products (e.g., C─S─H gel), lowering the compressive strength of the stabilized CSM [31]. In addition, SAP-B with relatively small particle size had better curing effect. When the content of SAP was 1.0%, the 7 days compressive strength of specimen with SAP-B was increased by 1.03% and 0.24% respectively compared with that of SAP-A under standard curing and high-temperature curing environment.

[figure(s) omitted; refer to PDF]

3.2. Splitting Strength

Figure 6 is the splitting strength results. Under standard curing, the splitting strength increased with the increase of age, and SAP has little effect on the splitting strength. As shown in Figure 6a, under natural curing conditions, varying content of SAP improved the splitting tensile strength of the specimens to different degrees. The specimen with a 1.0% SAP content demonstrated the best internal curing effect, and it was observed that the increases in splitting tensile strength at 7 and 28 days were more pronounced. Figure 6b shows that the addition of SAP in high-temperature curing environment improved the splitting strength. The splitting strength of 3 days can reach more than 0.7 MPa, but the growth rate of splitting strength became slower after 3 days, especially after 7 days. The temperature in the early stage of high-temperature curing played a decisive role in the strength of the specimen, which promoted the rapid growth of the splitting strength. And the high-temperature seriously reduced the humidity inside the specimen, which became the main factor restricting the continuous growth of the strength. However, the strength growth of the specimens in later stage cured at high-temperature was limited due to the decrease of internal humidity. Further observation revealed that with increasing SAP content, the strength of the specimens decreased under both curing conditions. Excessive SAP increases porosity and causes competitive water absorption, reducing the compactness and hydration degree of the cement matrix, thereby diminishing splitting strength [32].

[figure(s) omitted; refer to PDF]

The growth rate of splitting strength under different curing conditions is shown in Table 7. When the curing age was less than 28 days, the splitting strength of specimen under standard curing had maintained a high growth rate. For the specimens cured at high-temperature, the temperature in the early stage of curing was the decisive factor of cement hydration, and the high-temperature accelerated the hydration process of cement. After 3 days, the strength growth became extremely slow due to the decrease of internal humidity. In addition, compared with REF, the addition of SAP increased the growth rate of the splitting strength, and the increase of the growth rate was more significant in a relatively dry environment, indicating that SAP had a good internal curing effect in desert areas.

Table 7

The growth rate of splitting strength.

Mix proportion	Age (days)	CSM–REF (%)	CSM-B −0.5 (%)	CSM-B −1.0 (%)	CSM-B −1.5 (%)	CSM-A −0.5 (%)	CSM-A −1.0 (%)	CSM-A −1.5 (%)
Standard curing	3–7	14.8	14.4	14.1	15.9	14.3	16.1	17.8
	7–28	15.8	14.6	14.3	14.7	13.5	13	11.3
High-temperature curing	3–7	6.9	7.5	6.3	8.6	8.3	7.1	5.5
	7–28	0.9	1.7	0.9	1.8	1.7	1.7	0.9

3.3. Flexural-Tensile Strength

As shown in Figure 7, SAP improved the flexural-tensile strength of the specimens in different curing environments, especially in the high-temperature and drought curing environment. SAP provided continuous water for cement hydration by delaying the decrease of internal humidity. However, compared with standard curing, high-temperature curing reduced the flexural strength of the specimens, mainly because the water loss under standard curing was slower and the degree of cement hydration was higher. The early cement hydration of the specimens in high-temperature environment was faster, but the lack of internal humidity in the later period led to insufficient cement hydration. In addition, the effects of SAP particle size and content on flexural strength exhibit similar patterns to those on compressive strength, the effect of SAP-B internal curing with 1.0% content was the best. Compared with REF, the flexural tensile strength of standard curing and high-temperature curing specimens with 1.0% SAP-B increased by 11.1% and 12.6%, respectively. Therefore, adding SAP can play a good internal curing effect in desert areas.

[figure(s) omitted; refer to PDF]

3.4. Dry Shrinkage Test

Figure 8 is the water loss rate of CSM under high-temperature curing. The cumulative water loss rate progressively elevated with varying mix proportions as the curing time increased. And SAP can significantly reduce the water loss of the CSM. However, when the SAP content exceeded 1.0%, the reduction of water loss rate decreased, especially the final cumulative water loss rate was almost the same. The final cumulative water loss rate of REF specimen was 4.14%, which was 1.13 times of the specimen with 1.0% SAP-B. In the high-temperature curing environment, the water in the CSM had almost completely evaporated in the first 7 days, and the water evaporation was mainly concentrated in the first 3 days. The water loss rate of different mix proportion specimens for 3 days was 93.9%, 92.2%, 92.1%, 90.12%, 86.92%, 90.21%, and 90.04% of the final cumulative water loss rate, respectively. Due to the high-temperature expediting the cement hydration process and the relative humidity within the specimen, the evaporation rate of water was also accelerated. It can be found from the slope that SAP can delay the loss rate of water, because the water stored in SAP was mainly combined in the form of hydrogen bonds, which had good water retention, gradually released water under the humidity gradient, and inhibited the reduction of internal humidity of the specimen [33].

[figure(s) omitted; refer to PDF]

As shown in Figure 9, the dry shrinkage strain of different mix proportion specimens increased with the increase of curing age, and the dry shrinkage strain of specimen with SAP was significantly lower than REF. With the increase of SAP content, the drying shrinkage strain has shown a gradual reduction, mainly because more SAP can store the internal water in the SAP crosslinking network, which reduced the evaporation rate of water. The dry shrinkage strain of REF was 476.5 × 10⁻⁶, while the specimen with 0.5%, 1.0%, and 1.5% SAP-B were 364.7 × 10⁻⁶, 290.1 × 10⁻⁶ and 275.7 × 10⁻⁶, respectively, indicating that SAP effectively reduced the dry shrinkage of CSM. With the increase of SAP, the growth rate of the dry shrinkage strain gradually decreased. The high content of SAP formed multiple “water release area” inside the specimen, and the internal humidity was more uniform, thereby reducing the shrinkage of the specimen. The maximum dry shrinkage strain age of REF specimen was 5 days, and the maximum dry shrinkage strain age with SAP content of 0.5%, 1.0%, and 1.5% was 6 days, 7 days, and 8 days, respectively. Therefore, SAP can slow down the early shrinkage rate. The dry shrinkage strain of REF tended to stabilize in the later stage, whereas the specimen with SAP exhibited a continued gradual increase in dry shrinkage strain, which was mainly due to the water released by SAP, delaying the decrease of internal humidity of the specimen, resulting in the slow growth of dry shrinkage strain. And SAP-B with smaller particle size had better shrinkage reduction effect due to the larger specific surface area [18]. After water release, water migration can make cement in different regions get water supplement, and the humidity distribution inside the specimen was more uniform [15]. Although water release from SAP leaves pores and induces material performance degradation, the results of this study demonstrate that the hydration enhancement effect provided by SAP in arid desert environments significantly outweighs the detrimental impacts caused by pore formation [34–36].

[figure(s) omitted; refer to PDF]

As shown in Figure 10, the early dry shrinkage coefficient of REF increased rapidly and then tended to be gentle. The phenomenon was primarily a result of the synergistic impact of the swift evaporation of water and the process of cement hydration. The dry shrinkage coefficient of specimens with SAP increased slowly, indicating that SAP reduced the humidity inside the specimen by releasing water. The dry shrinkage coefficient of SAP-B specimens with 0.5%, 1.0%, and 1.5% SAP-B was 17.9%, 31.6%, and 34.9% lower than that of REF, respectively. The curing effect of SAP with smaller particle size was better. The large pores left after SAP release also affected the strength. SAP with relatively small size can be uniformly dispersed, and the range of water migration was wider, so that cement particles can be supplemented to water [33]. Under the high-temperature environment curing, the primary occurrence of drying shrinkage in specimens was observed during the initial 7 days. When the construction is carried out in the desert area, 1.0% SAP-B can be added for internal curing.

[figure(s) omitted; refer to PDF]

3.5. Internal Relative Humidity Test

As shown in Figure 11, with the increase of curing time, the humidity gradient formed inside the specimen gradually increased, so that the humidity of the specimen began to decrease. Compared with REF, SAP delayed the decrease of internal humidity and increased the internal humidity of a certain age. Under high-temperature and drought curing, the internal humidity of specimens decreased rapidly, especially REF. The rapid decrease in humidity can be attributed to the cement’s hydration process and the subsequent outward migration of water. High-temperature not only accelerated the hydration rate of cement and the occurrence of self-drying, but also accelerated the evaporation of water, resulting in the decrease of humidity and the stress of capillary pores, and the increase of humidity gradient also aggravated the evaporation of water. Therefore, increasing the internal humidity by adding SAP can weaken the migration effect of temperature on the internal humidity of the specimen, thus delaying the decrease of humidity and promoting the further hydration of cement, and improving the mechanical properties [37–39]. With the increase of SAP, the internal humidity of the specimen was higher. The total water absorption capacity of SAP with high content increased, and it was evenly distributed inside the CSM to form multiple micro “reservoirs,” which changed the distribution form of humidity inside the specimen and made the internal humidity distribution more uniform, which can effectively reduce the moisture loss caused by the humidity gradient generated by the temperature effect [15].

[figure(s) omitted; refer to PDF]

3.6. SEM

Figure 12 shows the SEM results of the interfacial transition area of CSM at different ages under high-temperature curing. Compared with REF, the interfacial transition area of CSM with SAP produced more hydration products, mainly because the hydrogel state formed by SAP after water absorption was located between cement slurry and aggregate. Under the action of humidity gradient, SAP mitigated the humidity disparity by introducing water, fostering enhanced cement hydration, thereby fortifying the bond within the interfacial transition zone. Figure 12a shows that more hydration products were generated at the aggregate interface of REF at 3 days age, but the structure was relatively loose, with more pores, and some microcracks can be seen. High temperatures enhanced the early cement hydration rate, leading to improved initial strength. However, this also accelerated water evaporation, causing microcracks due to material shrinkage. Figure 12b,c shows that there were more needle-like ettringite, hexagonal flaky calcium hydroxide, and flocculent C─S─H gel. With the increase of SAP content, the hydration products were denser, especially around the pores of SAP, and the humidity around SAP was higher, which provided sufficient water for the growth of ettringite and C─S─H gel and filled some pores [40–42]. SAP particles absorb water and swell, occupying a certain volume. After releasing water, they form microscopic pores, directly increasing the material’s porosity. If the pore size exceeds a specific threshold, it may significantly weaken the matrix densification, creating mechanical weak zones [34]. Additionally, the edges of these pores are prone to localized stress concentration, acting as initiation points for microcracks [35]. These microcracks may interconnect to form propagation pathways, accelerating material failure and reducing toughness and durability [36]. As the SAP content increased, the pores left after water release also increased, resulting in a reduction in the mechanical strength of the specimens. Figure 12d,e illustrates that as the cement hydration age progressed, there was an increased formation of hydration products at the aggregate interface, leading to a denser overall structure. Adding SAP can promote the hydration degree of cement under high-temperature curing, especially the hydration degree of cement in the interface transition area between cement and aggregate. In addition, SAP released water to delay the reduction of humidity and reduce the probability of internal microcracks, thereby improving the strength of the specimen [43, 44].

[figure(s) omitted; refer to PDF]

3.7. XRD

Figure 13 is the XRD of cement paste at different ages under high-temperature curing. The hydration crystallization products of cement with SAP were mainly Ca(OH)₂, CaCO₃ and unhydrated cement clinker (C₃S and C₂S), indicating that SAP did not alter the composition of hydration products, but rather influenced the diffraction peak intensity of these products [45, 46]. When the curing duration is 3 days, both CS-REF and SAP cement pastes exhibit a significant generation of Ca(OH)₂, suggesting that elevated temperature expedited the cement hydration process. Compared with CS-REF, the presence of SAP in cement paste led to a gradual increase in Ca(OH)₂, while C₃S and C₂S exhibited a gradual decrease. This suggests that SAP has the potential to enhance the hydration process of cement, particularly in high-temperature and drought conditions [47]. Cement hydration reactions were expedited, leading to a more rapid loss of water in desert regions. The water released by SAP could promptly supplement the water needed for cement hydration, thereby enhancing the degree of hydration. This effect remained consistent with the extension of time. The peak intensities of C₃S and C₂S in the XRD of CS-REF, CS-A-1.0, and CS-B-1.0 cement pastes at 7 days age were further reduced, and the promotion of SAP on cement hydration process was more obvious.

[figure(s) omitted; refer to PDF]

Figure 14 depicts the XRD analysis of cement paste at 28 days under various curing conditions. The impact of curing temperature and humidity on the composition of hydration products in cement slurry cured with SAP was observed to be minimal. The peak intensity of cement hydration products under standard curing exceeded that observed under high-temperature curing. The primary factor was that standard curing provides the necessary water for cement hydration, leading to a substantial enhancement in the degree of hydration. Under high-temperature and drought curing, the cement slurry was evaporated due to water, which limited the further hydration of cement in later stage. The impact of temperature on the degree of cement hydration was primarily observed during the early stages of hydration and had minimal influence on later stages. The internal humidity was the key factor affecting the cement hydration in the later stage. With the elevation of SAP content, the Ca(OH)₂ content exhibited a gradual increase. This suggests that a higher quantity of water preabsorbed by SAP resulted in a more gradual release of water. Consequently, this phenomenon enhanced the hydration degree.

[figure(s) omitted; refer to PDF]

3.8. TG-DSC

The TG-DSC test results of SAP-internally cured cement paste are shown in Figure 15, while the hydration product contents are listed in Table 8. Under elevated-temperature curing, the incorporation of SAP and additional water influenced the TG-DSC curves of the cement paste, significantly increasing the Ca(OH)₂ and chemically bound water content at different curing ages. This indicates that SAP enhanced the hydration degree of the cement paste. As shown in Table 8, the Ca(OH)₂ content of the reference sample (REF) at 3 days was 9.85%, whereas the SAP-incorporated samples CS-A10-1.0 and CS-B10-1.0 exhibited Ca(OH)₂ contents of 10.63% and 11.70% at 3 days, representing increases of 7.92% and 18.78%, respectively. At 7 days, the improvements in Ca(OH)₂ content for CS-A10-1.0 and CS-B10-1.0 were 2.75% and 22.23%, respectively. Additionally, the chemically bound water content in these samples was higher than that of CS-REF, demonstrating that SAP-B more significantly accelerated the hydration process. The water released by SAP during hydration provided moisture for further hydration, which macroscopically manifested as enhanced mechanical performance [35].

[figure(s) omitted; refer to PDF]

Table 8

The content of Ca(OH)₂ and chemically bound water in cement slurry at different ages.

Curing environment	Specimen	Mass fraction (%)
		Ca(OH)2	Chemically combined water
High-temperature	CS-REF (3 days)	9.85	5.96
High-temperature	CS-A10-1.0 (3 days)	10.63	6.32
High-temperature	CS-B10-1.0 (3 days)	11.70	6.76
High-temperature	CS-REF (7 days)	11.29	6.88
High-temperature	CS-A10-1.0 (7 days)	11.60	6.73
High-temperature	CS-B10-1.0 (7 days)	13.80	7.98
High-temperature	CS-B10-0.5 (28 days)	13.56	7.45
High-temperature	CS-B10-1.0 (28 days)	14.61	8.32
High-temperature	CS-B10-1.5 (28 days)	12.76	6.90
Standard temperature	CS-B10-1.0 (28 days)	19.21	12.53

From Figure 15c, it is evident that as the SAP dosage increased, the Ca(OH)₂ and chemically bound water content initially rose and then declined. A SAP content of 1.0% optimally promoted the hydration process, consistent with the trends observed in mechanical properties. Furthermore, the hydration product content in cement paste under standard curing was substantially higher than that under elevated-temperature curing. Under high-temperature conditions, the steep humidity gradient between the interior and exterior of the paste accelerated outward moisture migration due to combined thermal and humidity effects. Meanwhile, the release of water from SAP was insufficient to compensate for the moisture loss caused by the humidity gradient, leading to reduced hydration degrees. This also highlights that the long-term hydration degree of cement paste primarily depends on the availability of sufficient moisture within the matrix.

3.9. Cost and Environmental Impact Assessment of SAP

The current market price of SAP is approximately 16 RMB per kilogram, which is slightly higher than conventional curing agents. However, its internal curing efficiency can reduce long-term maintenance frequency. Furthermore, SAP costs are projected to decrease by 30%–50% with large-scale industrial adoption. Life cycle assessment (LCA) models demonstrate the superior economic performance of SAP-containing concrete, with total costs reduced by up to 11% [48]. Additionally, SAP’s compatibility with standard construction practices and its additional environmental benefits, such as reduced water consumption, further enhance its economic viability in sustainable pavement engineering. SAP exhibits good biodegradability and can be decomposed by microorganisms in natural environments. The degradation products primarily consist of small molecular compounds, such as acrylic acid and acrylamide, which are relatively stable in the environment and do not cause significant environmental pollution. Studies have shown that SAP-modified concrete can reduce environmental burdens by up to 55%, and its application value increases significantly with longer service life [48]. In conclusion, SAP holds significant potential for application in the maintenance of CSM in desert regions.

4. Conclusions

To provide technical guidance for the curing of semi-rigid base in desert area, the influence of SAP on the properties of CSM in desert environment was conducted. And the curing mechanism of SAP was explored by means of microscopic tests. The conclusions are as follows.

1. The inclusion of SAP enhanced the mechanical properties of CSM. Optimal curing effects were observed when the SAP content reached 1.0%. Compared with standard curing, SAP had better effect in high-temperature curing environment.

2. SAP had a notable capacity to effectively mitigate the dry shrinkage of CSM. As the SAP content increases, both the dry shrinkage strain and dry shrinkage coefficient exhibit a gradual decrease, consequently decelerating the early shrinkage rate.

3. High-temperature seriously reduced the later hydration degree. SAP can mitigate the impact of temperature and humidity on the internal water migration, exhibiting a notable enhancement in the hydration degree of cement during high-temperature curing. SAP released water to generate additional hydration products in the interfacial transition area, thereby mitigating the formation of microcracks.

It is recommended to systematically investigate the long-term durability under coupled multifactor conditions (freeze–thaw cycles, sulfate attack, and water exposure), evaluate the impact of degradation byproducts on matrix performance and the environment, and optimize SAP content, particle size, and degradation rate to balance mechanical properties, porosity, and long-term stability.

Author Contributions

Jiachen Ma: investigation, writing – original draft, methodology. Wenhui Zhao: writing – original draft, methodology. Zewen He: investigation, supervision. Leilei He: supervision, methodology. Fengyin Liu: visualization, methodology, funding acquisition. Yana Shi: supervision, funding acquisition.

References

[1] H.-H. Meng, X.-Y. Gao, J.-F. Huang, M.-L. Zhang, "Plant Phylogeography in Arid Northwest China: Retrospectives and Perspectives," Journal of Systematics and Evolution, vol. 53 no. 1, pp. 33-46, DOI: 10.1111/jse.12088, 2015.

[2] Z. Zhang, D. Huisingh, "Combating Desertification in China: Monitoring, Control, Management and Revegetation," Journal of Cleaner Production, vol. 182, pp. 765-775, DOI: 10.1016/j.jclepro.2018.01.233, 2018.

[3] H. Zhang, R. Zhang, R. Wu, "Numerical Simulation of Expansion Joint Setting of Semi-Rigid Base Layer," International Journal of Pavement Research and Technology, vol. 16 no. 1, pp. 158-166, DOI: 10.1007/s42947-021-00122-1, 2023.

[4] X. H. Sun, Study on Preparation and Performance of Cement Stabilized Macadam External Curing Agent in Desert Environment, 2021.

[5] Y. S. Guan, Z. X. Zhang, X. R. Zhang, "Effect of Superabsorbent Polymer on Mechanical Properties of Cement Stabilized Base and Its Mechanism," Transportation Safety and Environment, vol. 2 no. 1, pp. 58-68, DOI: 10.1093/tse/tdaa001, 2020.

[6] M. M. H. W. Bandara, W. K. Mampearachchi, "Mitigate the Curing Problems of Concrete Pavements by Using Heated Non-Expansive Clay Particles as Internal Curing Agent," Road Materials and Pavement Design, vol. 22 no. 4, pp. 969-982, DOI: 10.1080/14680629.2019.1700824, 2021.

[7] J. Liu, C. Shi, X. Ma, K. H. Khayat, J. Zhang, D. Wang, "An Overview on the Effect of Internal Curing on Shrinkage of High Performance Cement-Based Materials," Construction and Building Materials, vol. 146, pp. 702-712, DOI: 10.1016/j.conbuildmat.2017.04.154, 2017.

[8] M. Wyrzykowski, S.-I. Igarashi, P. Lura, V. Mechtcherine, "Recommendation of RILEM TC 260-RSC: Using Superabsorbent Polymers (SAP) to Mitigate Autogenous Shrinkage," Materials and Structures, vol. 51 no. 5,DOI: 10.1617/s11527-018-1241-9, 2018.

[9] S. T. Lv, C. D. Xia, H. F. Liu, "Strength and Fatigue Performance for Cement-Treated Aggregate Base Materials," International Journal of Pavement Engineering, vol. 22 no. 6, pp. 690-699, DOI: 10.1080/10298436.2019.1634808, 2021.

[10] D. B. Liu, Study on Maintenance Program Selection of the Semi-Rigid Base Asphalt Pavement, 2021.

[11] A. Dixit, S. Gupta, S. D. Pang, H. W. Kua, "Waste Valorisation Using Biochar for Cement Replacement and Internal Curing in Ultra-High Performance Concrete," Journal of Cleaner Production, vol. 238,DOI: 10.1016/j.jclepro.2019.117876, 2019.

[12] P. Zhong, Z. Hu, M. Griffa, M. Wyrzykowski, J. Liu, P. Lura, "Mechanisms of Internal Curing Water Release From Retentive and Non-Retentive Superabsorbent Polymers in Cement Paste," Cement and Concrete Research, vol. 147,DOI: 10.1016/j.cemconres.2021.106494, 2021.

[13] Y. Tan, H. Chen, R. He, "Water Distribution and Transport-Kinetics Model in Fresh Cement-Based Mixtures Containing Superabsorbent Polymers Based on 1H Low-Field NMR," Cement and Concrete Composites, vol. 127,DOI: 10.1016/j.cemconcomp.2021.104396, 2022.

[14] Y. W. Tan, H. X. Chen, Z. D. Wang, C. Xue, R. He, "Performances of Cement Mortar Incorporating Superabsorbent Polymer (SAP) Using Different Dosing Methods," Materials, vol. 12 no. 10,DOI: 10.3390/ma12101619, 2019.

[15] A. Assmann, H. W. Reinhardt, "Tensile Creep and Shrinkage of SAP Modified Concrete," Cement and Concrete Research, vol. 58, pp. 179-185, DOI: 10.1016/j.cemconres.2014.01.014, 2014.

[16] J. Yang, J. Huang, X. He, Y. Su, S.-K. Oh, "Shrinkage Properties and Microstructure of High Volume Ultrafine Phosphorous Slag Blended Cement Mortars With Superabsorbent Polymer," Journal of Building Engineering, vol. 29, pp. 101121-101121, DOI: 10.1016/j.jobe.2019.101121, 2020.

[17] O. M. Jensen, P. F. Hansen, "Water-Entrained Cement-Based Materials: II. Experimental Observations," Cement and Concrete Research, vol. 32 no. 6, pp. 973-978, DOI: 10.1016/S0008-8846(02)00737-8, 2002.

[18] H. Zhu, Z. Wang, J. Xu, Q. Han, "Microporous Structures and Compressive Strength of High-Performance Rubber Concrete With Internal Curing Agent," Construction and Building Materials, vol. 215, pp. 128-134, DOI: 10.1016/j.conbuildmat.2019.04.184, 2019.

[19] L. P. Esteves, P. Cachim, V. M. Ferreira, C. U. Grosse, "Mechanical Properties of Cement Mortars With Superabsorbent Polymers," Advances in Construction Materials, pp. 451-462, DOI: 10.1007/978-3-540-72448-3_45, 2007.

[20] L. Dudziak, V. Mechtcherine, "Enhancing Early-Age Resistance to Cracking in High-Strength Cement Based Materials by Means of Internal Curing Using Super Absorbent Polymers," International RILEM Conference on Material Science, pp. 129-139, 2010.

[21] X.-M. Kong, Z.-L. Zhang, Z.-C. Lu, "Effect of Pre-Soaked Superabsorbent Polymer on Shrinkage of High-Strength Concrete," Materials and Structures, vol. 48 no. 9, pp. 2741-2758, DOI: 10.1617/s11527-014-0351-2, 2015.

[22] S.-H. Kang, S.-G. Hong, J. Moon, "Shrinkage Characteristics of Heat-Treated Ultra-High Performance Concrete and Its Mitigation Using Superabsorbent Polymer Based Internal Curing Method," Cement and Concrete Composites, vol. 89, pp. 130-138, DOI: 10.1016/j.cemconcomp.2018.03.003, 2018.

[23] Y. Zhao, Z. N. Li, "Application of Superabsorbent Resin in Cement Stabilized Gravel Base," Hunan Communication Science and Technology, vol. 46 no. 3, pp. 54-59, .

[24] J. G. Qiao, R. Peng, P. Z. Yin, Y. Wang, "Shrinkage Characteristics of Self-Curing Cement Stabilized Macadam Base With Super Absorbent Polymer," Thermosetting Resin, vol. 37 no. 1, pp. 33-37 43, 2022.

[25] K. Wang, Study on Road Performance of Aeolian Sand Semi-Rigid Base Based on Self-Curing Technology, 2023.

[26] J. Geng, L. He, M. Chen, Y. Niu, "Applicability of Rheological Properties of Superabsorbent Polymer Cement-Based Materials Using Apparent Viscosity," Materials and Structures, vol. 55 no. 2,DOI: 10.1617/s11527-022-01924-w, 2022.

[27] P. J. Shi, S. Sun, M. Wang, "Climate Change Regionalization in China (1961–2010)," Science China Earth Sciences, vol. 57 no. 11, pp. 2676-2689, DOI: 10.1007/s11430-014-4889-1, 2014.

[28] B. Gaël, T. Christelle, E. Gilles, G. Sandrine, S.-F. Tristan, "Determination of the Proportion of Anhydrous Cement Using SEM Image Analysis," Construction and Building Materials, vol. 126, pp. 157-164, DOI: 10.1016/j.conbuildmat.2016.09.037, 2016.

[29] C. M. Gomes, A. D. S. de Oliveira, "Chemical Phases and Microstructural Analysis of Pastes Based on Magnesia Cement," Construction and Building Materials, vol. 188, pp. 615-620, DOI: 10.1016/j.conbuildmat.2018.08.083, 2018.

[30] D. Shen, X. Wang, D. Cheng, J. Zhang, G. Jiang, "Effect of Internal Curing With Super Absorbent Polymers on Autogenous Shrinkage of Concrete at Early Age," Construction and Building Materials, vol. 106, pp. 512-522, DOI: 10.1016/j.conbuildmat.2015.12.115, 2016.

[31] J. Liu, N. Farzadnia, K. H. Khayat, C. Shi, "Effects of SAP Characteristics on Internal Curing of UHPC Matrix," Construction and Building Materials, vol. 280,DOI: 10.1016/j.conbuildmat.2021.122530, 2021.

[32] Y. Li, C. Zhang, T. Li, G. Sun, "Exploring the Role of SAP Chemical Composition in the Internal Curing of High-Performance Cementitious Materials," Construction and Building Materials, vol. 451,DOI: 10.1016/j.conbuildmat.2024.138784, 2024.

[33] Y. Tan, X. Lu, R. He, H. Chen, Z. Wang, "Influence of Superabsorbent Polymers (SAPs) Type and Particle Size on the Performance of Surrounding Cement-Based Materials," Construction and Building Materials, vol. 270,DOI: 10.1016/j.conbuildmat.2020.121442, 2021.

[34] R. He, N. Lu, "Air Void System and Freezing-Thawing Resistance of Concrete Composite With the Incorporation of Thermo-Expansive Polymeric Microspheres," Construction and Building Materials, vol. 419,DOI: 10.1016/j.conbuildmat.2024.135535, 2024.

[35] R. He, T. Nantung, N. Lu, "Unraveling Microstructural Evolution in Air-Entrained Mortar and Paste: Insights From MIP and micro-CT Tomography Amid Cyclic Freezing-Thawing Damage," Journal of Building Engineering, vol. 94,DOI: 10.1016/j.jobe.2024.109922, 2024.

[36] R. He, N. Lu, "Hydration, Fresh, Mechanical, and Freeze-Thaw Properties of Cement Mortar Incorporated With Polymeric Microspheres," Advanced Composites and Hybrid Materials, vol. 7 no. 3,DOI: 10.1007/s42114-024-00899-2, 2024.

[37] S. G. Hu, Y. F. Zhou, F. Z. Wang, B. Peng, "Effect of Super Absorbent Polymer (SAP) Particles on Autogenous Shrinkage and Strength of Concrete," Journal of Huazhong University of Science and Technology (Urban Science Edition), vol. 25 no. 1, 2008.

[38] B. Craeye, M. Geirnaert, G. D. Schutter, "Super Absorbing Polymers as an Internal Curing Agent for Mitigation of Early-Age Cracking of High-Performance Concrete Bridge Decks," Construction and Building Materials, vol. 25 no. 1,DOI: 10.1016/j.conbuildmat.2010.06.063, 2011.

[39] X. Ma, Y. Zhang, S. Wu, "Effect of Super Absorbent Polymer on the Properties of Cement Mortar With High Water-Cement Ratio," Concrete, vol. 7, pp. 124-127, 2015.

[40] R. Rostami, A. J. Klemm, F. C. R. Almeida, "Combined Effect of Polymeric Fibres and SAP on the Performance of Concrete and Repair Mortars: A Review," Innovation in Construction Materials, 2018.

[41] M. S. Amin, S. M. A. El-Gamal, F. S. Hashem, "Effect of Addition of Nano-Magnetite on the Hydration Characteristics of Hardened Portland Cement and High Slag Cement Pastes," Journal of Thermal Analysis and Calorimetry, vol. 112 no. 3, pp. 1253-1259, DOI: 10.1007/s10973-012-2663-1, 2013.

[42] L. P. Esteves, I. Lukošiūtė, J. Čėsnienė, "Hydration of Cement With Superabsorbent Polymers," Journal of Thermal Analysis and Calorimetry, vol. 118 no. 2, pp. 1385-1393, DOI: 10.1007/s10973-014-4133-4, 2014.

[43] Z. H. Lyu, A. Q. Shen, Z. M. He, "Absorption Characteristics and Shrinkage Mitigation of Superabsorbent Polymers in Pavement Concrete," International Journal of Pavement Engineering, vol. 23 no. 2, pp. 270-284, DOI: 10.1080/10298436.2020.1742334, 2022.

[44] X. Qin, A. Shen, Z. Lyu, L. Shi, J. Yang, H. Liu, "Research on Water Transport Behaviors and Hydration Characteristics of Internal Curing Pavement Concrete," Construction and Building Materials, vol. 248,DOI: 10.1016/j.conbuildmat.2020.118714, 2020.

[45] V. S. Harutyunyan, "Adsorption Energy of Stoichiometric Molecules and Surface Energy at Morphologically Important Facets of a Ca(OH) 2 Crystal," Materials Chemistry and Physics, vol. 134 no. 1, pp. 200-213, DOI: 10.1016/j.matchemphys.2012.02.052, 2012.

[46] V. S. Sujitha, B. Ramesh, J. R. Xavier, "Influence of Nano Alumina Reinforced Superabsorbent Polymer on Mechanical, Durability, Microstructural and Rheological Properties of Cementitious Materials," Journal of Building Engineering, vol. 79,DOI: 10.1016/j.jobe.2023.107780, 2023.

[47] K. Farzanian, A. Ghahremaninezhad, "Desorption of Superabsorbent Hydrogels With Varied Chemical Compositions in Cementitious Materials," Materials and Structures, vol. 51 no. 1,DOI: 10.1617/s11527-017-1128-1, 2018.

[48] D. di Summa, D. Snoeck, J. R. Tenorio, "Sustainability and Economic Viability of Self-Healing Concrete Containing Super Absorbent Polymers," Proceedings of the 75th Rilem Annual Week, vol. 40, pp. 335-343, DOI: 10.1007/978-3-031-21735-7, 2023.