A Sustainable and Environmentally Friendly Concrete for Structural Applications

Abstract

The purpose of this study is to utilize waste products—precipitated calcium carbonate (PCC) and upcycled recycled concrete aggregate (upcycled RCA or UCA)—in civil works projects. To do so, tests must be performed to determine the engineering properties of the materials in which PCC and UCA are sequestered. PCC is a fine to coarse grain waste product generated during the production of sugar from sugar beets. UCA is produced from demolished and returned concrete by the extraction of primarily calcium and alkalinity. The study also includes the use of both PCC and UCA in the same concrete mix design. The test results on PCC alone show that the optimum content to achieve a minimum 28 MPa (4000 psi) compressive strength is 25% and 30%. The corresponding compressive strength of mixes in which conventional aggregate was replaced by UCA is about 48 MPa (7000 psi) to 55 MPa (8000 psi) at the same water: cement ratio (0.44) by weight. The compressive strength of concrete with 25% to 30% cement replaced by PCC and varying aggregates replaced with UCA ranges from 19.3 to 40 MPa (2800 to 5800 psi). Other tests on PCC and UCA include tensile strength of 2 to 3 MPa (293 to 423 psi) and flexural strength of 1.3 to 1.9 MPa (183 to 279 psi). Analytical techniques such as X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDX), and scanning electron microscopy (SEM) were used to identify the constituent elements and chemical compounds present in PCC, including calcium carbonate and silica. Based on the test results, the composition of PCC by weight indicates 45.9% calcium, 39.4% oxygen, and 9.2% carbon. Based on the results of this study, we can expect to reduce carbon emissions in the production of cement and aggregates, as well as utilize waste products in the civil engineering field.

1. Introduction

To produce concrete, every year, 20 billion tons of raw materials are consumed [1]. The ingredients needed for the concrete mix are fine aggregate, coarse aggregate, Portland cement, and water. The other materials that can be incorporated into concrete are supplementary cementitious materials (SCMs) and admixtures. Portland cement is the main ingredient in concrete. It has very high carbon dioxide emissions, which is due to the larger quantities of carbon dioxide produced during the process of heating raw materials such as “limestone, shells, and chalk or marl combined with shale, slate, blast furnace slag, silica sand, and iron ore” [2]. The heating temperatures for cement production exceed 2500 degrees Fahrenheit [2]. In addition, concrete produces about 8% of the global carbon emissions [3] and cement alone is responsible for approximately 7% [4]. The Global Alliance for Buildings and Construction opines that building materials and construction are responsible for 11% of global carbon emissions (refer to Figure 1a,b) [5]. Moreover, it explains that cement is responsible for 8% of global carbon emissions. Out of the 8% of global emissions from cement, the carbon emissions from clinker production are approximately 90% and the remaining are from the other stages of cement production [3].

1.1. Research Gaps

The literature review shows both the benefits and drawbacks of replacing cement and aggregates in concrete with different cementitious and non-cementitious materials. Few researchers have identified the advantages of substituting portions of cement with fly ash, silica fume, or slag cement, and the substitution of aggregates with steel fibers, shredded tires, and recycled concrete aggregate (RCA). However, other researchers mentioned that substitutions of cement and aggregates by other cementitious or non-cementitious might not provide a beneficial effect on concrete. Due to contrasting viewpoints from various researchers, it is uncertain whether the substitutions will provide benefits or drawbacks. Lots of new raw materials are still under investigation to reduce the carbon emissions from the concrete but limited materials are available to replace the cement or aggregates in the civil engineering field. Nonetheless, some amounts of cement are required to provide the necessary bond strength to the concrete. Thus, new materials need to be investigated and treatment methods on RCA need to be further investigated to provide beneficial effects to conventional concrete.

The carbon emissions from concrete can be minimized by replacing cement with other supplementary cementitious materials like fly ash, silica fume, and slag cement. Similarly, the aggregate can be replaced with recycled concrete aggregate (RCA), and/or steel fibers, shredded tires/rubber, plastic, brick, recycled glass, etc., in a concrete mix [7,8]. Concrete’s durability and workability can be improved by substituting cement with fly ash [9]. Various research suggests approximately up to 40% of cement can be replaced with fly ash without sacrificing strength but this might require longer setting times. At the same time, concrete might have higher workability and lower fresh density, modulus of rupture, modulus of elasticity, and tensile strength compared to conventional concrete mix [9]. Like fly ash, about 11% of cement can be substituted with silica fume, which results in higher compressive strength in concrete [10]. However, increasing the number of silica fumes above 11% results in lower workability and lower compressive strength compared to conventional concrete. In addition, slag cement can be replaced from 25% to 70% of total cementitious content by weight [11]. Nonetheless, it has lower heat of hydration, which prevents its use in colder climates, can result in lower early strength, and limits the use of concrete in case of emergency [12].

The replacement of 5% of coarse aggregates by steel fibers increases compressive strength, but the replacement of 10% of coarse aggregates by steel fibers decreases the compressive strength [13]. The substitution of coarse aggregates for shredded tires reduces compressive strength [14,15]. Similarly, the replacement of conventional aggregates with plastic (allowance limits to 30%) reduces the compressive strength [16]. Moreover, the substitution of coarse aggregates with bricks results in the reduction in compressive strength by 33% [17]. When the fine aggregates were replaced with recycled glass, there was a 0% to 50% reduction in the compressive strength and a reduction in the modulus of elasticity, split tensile strength, and flexural strength test results [18].

Although various research has been conducted on replacing either cement or aggregates, the research on the substitution of both cement and aggregates with waste materials has not been extensively conducted. So far, no new material has been introduced that can match or exceed the strength provided by Portland cement in concrete. Even ultra-high-performance concrete (UHPC) relies on Portland cement to achieve its maximum strength. It is beneficial to research on utilization of locally available materials in concrete. This paper aims to explain the use of precipitated calcium carbonate (PCC) to replace cement and upcycled recycled concrete aggregates (upcycled RCA or UCA) to replace both fine and coarse aggregates. For this research, Amalgamated Sugar Corporation, located in Twin Falls, Idaho donated the PCC and is denoted as “Producer A”. In addition, UCA was provided by San Francisco Bay Aggregates (SFBA), a Blue Planet Company located in Pittsburg, CA, USA, and is identified as “Producer B”.

1.2. Research Motivation

In the current situation, one of the trending topics for many nations is achieving net zero emissions through various means, especially in the built environment. Net zero is a concept where the volume of greenhouse gas emissions is balanced by their removal from the atmosphere [19]. Concrete, in fact, ranks as the second most widely consumed material in the world after water. It contributes significantly to carbon emissions, primarily due to Portland cement that is used as a binder in concrete. This study aims to investigate novel approaches to reduce cement usage in concrete and substitute fresh (virgin) aggregates with upcycled recycled concrete aggregates. The waste products utilized in this study have the potential to reduce carbon emissions globally and contribute to a sustainable future. To support the goal of achieving net zero carbon emissions from concrete, this research proposes replacing a significant portion of cement with PCC, and similarly replacing fresh aggregates with UCA.

1.3. Research Scope

The scope of this research is to explore the use of waste products from agricultural and demolition industries in concrete. RCA has been well-studied in fresh concrete mixes. However, this research aims to use an upgraded version of RCA called UCA, which results in promoting sustainability and a reduction in the carbon footprint potential while achieving similar strengths as benchmark mixes (control mixes). This research mainly focuses on the utilization of PCC as a portion of cement substitute and the utilization of UCA as an aggregate’s substitutes. Small-scale laboratory testing on concrete containing PCC and UCA was performed. The compressive strength test on a 4 in. diameter by 8 in. height concrete cylinder, the split tensile strength test on a 6 in. diameter by 12 in. height concrete cylinder, and the flexural strength test on a 6 in. width, 6 in. height, and 21 in. length concrete beam were performed. The testing was conducted following the American Society for Testing and Materials (ASTM) procedures.

This study is an initial effort to reduce carbon emissions from concrete to a greater extent by combining both PCC and UCA. The study was conducted at Idaho State University (ISU) Engineering. In this research, durability (freeze and thaw and salt penetration) tests or any other large scale-testing such as beam, columns, or foundation were not performed and are recommended for future tests. Quantitative analysis and analytical approaches were used. The initial phase of the research was the identification of the elements contained in PCC. The second phase involved the use of PCC as a replacement for cement in concrete. In the next study, conventional aggregate was replaced by UCA in different concrete mixes. Finally, the work progressed to using PCC and UCA in the same concrete mix.

1.4. Research Objectives

The objectives of this research are:

• To reduce carbon emissions from the production of cement and aggregates;

• To find beneficial uses of PCC and UCA in structural concrete that not only reduce the carbon footprint of the concrete mix, but also utilizes the waste products from the sugar beet and concrete demolition sectors, along with returned concrete from ready-mix operations;

• To determine the quantities of PCC and UCA that can be incorporated in concrete mixes, while maintaining the same strength.

1.5. Literature Review

1.5.1. Precipitated Calcium Carbonate (PCC)/Carbonation Lime Residue (CLR)/Carbonation Mud/Lime Sludge

Precipitated calcium carbonate (PCC) used in this study is a waste product obtained from the sugar beet industry. It is important to note that the commercially produced precipitated calcium carbonate (PCC) or marble powder from carbonate rocks is different from the PCC used in this project. In 2019/20, sugar consumption was approximately 171.69 million metric tons worldwide and is projected to reach 178.84 million metric tons at the end of 2023 [20]. The production of sugar from beets is roughly 32% worldwide; the remaining percentage is from sugar cane [21]. United States (US), Turkey, Ukraine, China, and Egypt are the top five nations that produce sugar from sugar beets in substantial amounts [22]. In the US alone, the average daily sugar consumption is 126.4 g per individual [23]. The higher consumption of sugar from sugar beets results in substantial amounts of waste annually. Thus, it is important to utilize these waste products to prevent landfills and to reduce carbon emissions.

PCC is also known as carbonation lime residue (CLR) (refer to Figure 2a), carbonation mud, lime sludge, and calcined carbonation lime residue (CCR) when limestone is added to CLR. There are several types of waste created from sugar beets and sugar cane during sugar production. The waste products generated from sugar beets are beet-washing water, diffusion water, lime sludge, and residual products [24]. The lime sludge contains calcium carbonate, which can be used to replace a portion of cement. Nanoplatelets of PCC help to boost the mechanical properties of concrete (refer to Figure 2b) [25]. Nanoplatelets are microscopic particles of PCC with irregular and angular shapes. Sugarcane bagasse ash (SCBA) produced from cane sugar can also be used to replace a portion of cement in concrete. Moreover, the substitution of 10% of cement by SCBA increased the compressive strength of concrete by 15.67% [26].

In the research conducted by Gharieb M. and Rashad A.M., cement was replaced by 5% (CLR 5), 10% (CLR 10), 15% (CLR 15), 20% (CLR 20), and 25% (CLR 25) carbonation lime residue [22]. The sugar beet waste used in the experiments was collected from the Saudi Sugar Company (Jeddah-Saudi Arabia). The authors present the relationship between the CLR content and unconfined compressive strength (refer to Figure 3a). The highest strengths were recorded at CLR contents of 5% (by weight of cement) at 3 days, 7 days, and 28 days. The compressive strength decreased progressively with increasing CLR contents. In this study, it was also found that the higher the CLR (or PCC) content in concrete, the lower the set time and bulk density and the higher the porosity, and fines content. Further, the higher the CLR substitution for cement, the greater the amount of water absorbed in concrete (see Figure 2b). CLR is a lighter material than cement because the specific gravity of CLR is 2.55, which is less than 3.15, the specific gravity of Portland Cement [27].

M. Rashad and Gharieb report the presence of silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), iron oxide (Fe₂O₃), calcium oxide (CaO), magnesium oxide (MgO), sulfur trioxide (SO₃), sodium oxide (Na₂O), potassium oxide (K₂O), titanium oxide (TiO₂), and chloride (Cl⁻) in the CLR from Saudi Arabia [22]. CLR can be used in fertilizer [27], and can also be used as filler for concrete, plastic, rubber, and paper [22,28], and used for the production of acetic acid [29], biogas [30], and bioethanol [27].

1.5.2. Recycled Concrete Aggregate (RCA) and Upcycled Recycled Concrete Aggregate (UCA)

The Environmental Protection Agency (EPA) mentioned that approximately 600 million tons of construction and demolition (C&D) waste were generated in the United States in 2018 [31]. Out of the 600 million tons, 567 million tons of waste were from demolition and the remaining was from construction, as shown in Figure 4 [32]. In addition, the waste from concrete demolition was approximately 381 million tons (the highest of the C&D wastes) and construction-related concrete waste was about 24.2 million tons.

The C&D wastes include a variety of materials like concrete, wood, gypsum drywall, metal, brick, and clay tile, as well as asphalt shingles and pavement [33]. In 2015, the EPA reported approximately 132 million tons of C&D waste disposed of in landfills, while 415 million tons of C&D waste were allocated for future use in the United States (US) (refer to Table 1) [33]. Furthermore, approximately 66,535,034 tons of concrete were disposed of in landfills in the same year. Therefore, the proper utilization of demolished concrete struc-tures from roads, bridges, buildings, etc., unused and returned concrete from ready-mix sources are important for the reduction of carbon emissions and protection of the environment.

Recycled concrete aggregates (RCAs) are mainly produced from demolished concrete and returned concrete from the ready mix. The demolished concrete is the old concrete that has already been used for construction and the returned concrete is the leftover concrete from ready-mix concrete trucks. The returned concrete is deposited at a nearby recycling site, where it hardens before crushing. The demolished and hardened returned concrete is crushed using crushers like jaw-type crushers and impact crushers. Before using RCA in concrete mixes, steel, rebar, wood, plastics, paper, gypsum, asphalt, and unwanted materials are extracted from the RCA [34]. Various countries have different methods to separate undesired materials from the RCA. United States and Russia use magnet separators to separate iron and steel, use crushers to break down the RCA, and, finally, use a double-screening device to refine the remaining materials [34]. China uses manpower to separate the large-scale impurities from RCA and then uses magnetic separation, screening, and washing. The broken pieces of RCA are sieved and analyzed to maintain the quality of aggregates. Finally, the coarse and fine aggregates are separated after crushing and screening. In addition, the coarse aggregates can be further crushed to make fine aggregates. The quality of RCA depends on various factors such as the quality of the parent aggregates used in the concrete mix before, the quantity of mortar present in the RCA, and the type of cement used in the parent concrete.

Shatanut et al. (2022) found RCA concrete has lower durability but higher absorption than companion conventional concrete samples [35]. Moreover, the use of RCA in concrete reduces the compressive strength, tensile strength, elastic modulus, and flexural strength [34,36]. The compressive strength of concrete reduces by 0% to 30% with the replacement of conventional aggregates with RCA [34]. Kwan et al. (2012) report that approximately 70% to 80% of conventional aggregate can be replaced with RCA with no negative impact on the mechanical and durability properties of concrete [37]. However, Limbachiya et al. (2012) mentioned that the optimum amounts of convention aggregates that can be replaced with RCA is about 30% [38]. From various researchers, it is found that RCA con-crete has higher shrinkage, higher absorption, higher porosity, and higher creep than conventional concrete. However, the amounts of cement might need to be increased in some concrete with RCA to achieve the same amounts of workability, slump, and compressive strength as that of conventional concrete mix. As a result, it will result in substantial amounts of carbon emissions from concrete.

RCAs are divided into various classes across different countries. In the USA, RCA is categorized into three types: Class A, Class B, and Class C [34]. The quality of RCA decreases from Class A (the highest quality which can be used for severe exposure) to Class B (which can be used for moderate exposure) to Class C (the lowest quality with negligible exposure). Table 2, from the American Concrete Institute (ACI) Technical Committee, shows that RCA concrete allows certain percentages of impurities by volume (7% lime plaster, 5% soil, 4% wood, 3% hydrated gypsum, 2% asphalt, and 0.2% paint-made vinyl acetate).

Improving the quality of RCA can be achieved by either eliminating or strengthening the adhered mortar (see Figure 5a,b) [32]. The mortar is removed by acids such as hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and acetic acid (CH₃COOH). Alternatively, the mortar is removed by treating RCA to higher temperatures (more than 360 degrees Celsius). The quality of RCA can also be improved by increasing the bonding strength of the adhered mortar. Strengthening the RCA can be carried out by subjecting it to a carbon dioxide (CO₂) chamber with specific techniques for certain periods. As a result, the adhered mortar reacts with CO₂ to produce calcium carbonate (CaCO₃) and silica (SiO₂). This transformation is possible through the reaction of CO₂ with calcium hydroxide (Ca(OH)₂) and calcium silicate hydrate (C-S-H gel).

Like increasing the strength of the adhered mortar in RCA through CO₂ treatment, the upcycled concrete aggregates (UCAs) obtained from “Producer B” are produced by treating RCA with mild acid in the carbon capture and mineralization process (refer to Figure 6a,b) [39,40]. In addition, Producer B produces calcium carbonate aggregate (CaCO₃ Aggregate) for concrete (refer to Figure 6c) [41]. In the production process of CaCO₃ aggregate, Producer B captures and permanently sequesters the carbon dioxide (CO₂) as a solid synthetic mineral, which is known as CaCO₃ aggregate. This type of aggregate is 44% CO₂ by weight.

2. Materials and Methods

2.1. Precipitated Calcium Carbonate (PCC)

Precipitated calcium carbonate (PCC) used in this project was provided by the “Producer B” Sugar Factory in Twin Falls, Idaho. The PCC samples were in fine to granular form with particle sizes up to 1/2 in. in size (see Figure 7). Samples were collected from stockpiles at the factory.

The sieve analysis of PCC was conducted through the guidelines from the American Society of Testing and Materials (ASTM) D6913-04 [42]. The sieve analysis data indicated that 96.09%, 85.22%, 64.57%, 1.09%, 0.11%, and 0.11% of PCC passed through a sieve of 3/8 inches, Number (No.) 4, No. 10, No. 40, No. 100, and No. 200, respectively.

2.2. Upcycled Recycled Concrete Aggregate (Upcycled RCA or UCA)

The third phase of the project was to replace both fine and coarse aggregates with upcycled recycled concrete aggregates (UCA) (see Figure 6a,b) obtained from “Producer B”. Both fine and coarse aggregates in the concrete mix were substituted with fine and coarse UCA at varying percentages of 12.5%, 25%, 37.5%, 50%, 75%, and 100%.

The sieve analysis of UCA fine aggregates and UCA coarse aggregates was performed following the guidelines from ASTM C136-06 [43]. The UCA fine aggregates had passing percentages of 94.76% for No. 4, 63.34% for No. 8, 42.26% for No. 16, 24.37% for No. 30, 9.65% for No. 50, and 1.55% for No. 100. Moreover, the UCA coarse aggregates have passing percentages of 100% for 1 in., 86.51% for ¾ in., 69.69% for 5/8 in., 55.33% for ½ in., and 34.61% for 3/8 in.

2.3. Aggregates

The sieve analysis of fine and coarse aggregates was performed following the guidelines from ASTM C136-06 [43]. Additionally, fine aggregates passing through different sieve sizes were 96.94% for No. 4, 89.06% for No. 8, 71.67% for No. 16, 15.12% for No. 30, 6.01% for No. 50, and 0.24% for No. 100. Similarly, coarse aggregates passing through various sieve sizes were: 100% for 1 in., 99.66% for ¾ in., 92.80% for 5/8 in., 76.12% for ½ in. and 48.70% for 3/8 in., with zero materials passing through the No. 200 sieve. PCC utilized in this study had a water content of approximately 27%, which might be a reason for the limited amount of PCC passing through the No. 40 sieve. From sieve analysis of PCC and Portland cement, it was found that cement was finer than PCC samples. Similarly, smaller amounts of UCA fine aggregates pass through the No. 16 sieve compared to conventional fine aggregates. Moreover, the lesser amounts of UCA coarse aggregates pass through the ½ in. and 3/8 in. sieves in comparison to conventional coarse aggregates. On the other hand, larger amounts of UCA fine aggregates pass through the No. 30 sieve compared to conventional fine aggregates.

2.4. Fly Ash

Since fly ash is commonly used in the concrete industry, the idea was developed to compare the results with concrete containing fly ash and the concrete containing PCC. The replacement of some portions of cement with fly ash in a concrete mix has higher durability and workability compared to conventional concrete [44]. Typically, cement can be replaced with fly ash from 15% to 30%, with a replacement ratio of cement to fly ash of 1:1 to 1:1.5 [45]. A few articles explain that a slightly higher amount of fly ash should be used than the amount of cement replaced by fly ash. There are different classifications of fly ash available: Class N, Class F, and Class C [46]. In this research, class F fly ash was utilized, maintaining the cement-to-fly ash replacement ratio of 1:1.

2.5. Concrete Sample Preparation

In the initial phase of the research, four different control mixes were prepared with varying quantities of aggregate, cement, and water. The mix with the lowest water/cement (w:c) ratio of 0.44 was chosen for the project. The slump of the control concrete mix, measured per ASTM C143/C143M, was recorded to be 1.5 inches (37.5 mm) [47]. The control concrete mix for this research consists of cement, fine aggregate, coarse aggregate, and water. The control mix was considered the benchmark concrete and was used to compare with the results with other trial mixes. The benchmark used in this research was designed using the absolute volume method given by Mamlouk and Zaniewski (2006) [46]. Table 3 displays the amounts of materials used in the concrete mix design per cubic foot volume.

After measuring out the components, the concrete was mixed in a 0.099 m³ (3.5 ft³) mixer. The materials were added in a mixer with one-third quantity increments and mixed after each increment. The total mixing time was roughly five to seven minutes. Slump measurements were made at the end of the mixing period under ASTM C 143 [47]. The concrete was cast in both steel and plastic molds, as well as on wooden boxes (refer to Figure 8a–c and Figure 9, respectively). Two types of molds made from different materials (plastic and steel) were used because only fifteen 4 in. (10.16 cm) diameter by 8 in. (20.32 cm) height steel molds were available in the laboratory and more than 15 cylinders were cast in a day. Furthermore, 6 in. (15.24 cm) diameter by 12 in. (30.48 cm) height black plastic molds were used for the split tensile strength test. ASTMs requires 4 in. diameter by 8 in. height concrete cylinders for compressive strength test and 6 in. diameter by 12 in. height for the split tensile strength test. The concrete cylinders and beams were removed from the molds, 24 h after casting, and then placed in a water bath to cure for 28 days at lab temperature (refer to Figure 8d). Compressive, tensile, and four-point beam tests were performed on the samples at 28 days.

2.6. Compressive Strength Tests

Compressive strength tests were performed following the ASTM C 39 [48]. The unconfined compressive strength tests were carried out with 4 in. (10.16 cm) diameter by 8 in. (20.32 cm) height concrete cylinders (see Figure 8d) using a Gilson Model MC-300M testing machine (see Figure 10a). The concrete cylinders were cast in one-third increments and the concrete was rodded 25 times in each increment. The unconfined compressive strength is determined using Equation (1).

(1)$f_c^{\prime }=\frac{P_{max}}{A}$

where $f_c^{\prime }$ = unconfined compressive strength (psi), P_max = maximum applied load until failure (lbs), and A = actual average cross-sectional area of a concrete cylinder (in²).

2.7. Split Tensile Strength Tests

Split tensile strength tests were carried out using ASTM C 496 [49]. In this research, an indirect method of tensile testing was performed. This test was performed in the standard 6 in. (15.24 cm) diameter by 12 in. (30.48 cm) height concrete cylinders (see Figure 8c). Like concrete cylinders prepared for compressive strength test, the concrete cylinders were cast in a plastic mold with one-third increments and the concrete rodded 25 times in each increment. Equation (2) is used to determine the split tensile strength of the concrete:

(2)$T=\frac{2P}{\pi DL}$

where T = tensile strength (psi), P = maximum load at failure (lbs), D = diameter of a concrete cylinder (in.), and L = length or height of a cylinder (in.).

The split tensile test was conducted using a Gilson Model MC-300M as shown in Figure 10b.

The mixes used in the unconfined compressive strength tests of concrete with PCC were carried out using the same amounts of aggregates and water. The only difference be-tween the control mix and the trials was that Portland cement was replaced by PCC with various percentages: 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, and 100% by weight.

2.8. Flexural Strength Tests

The flexural strength of concrete is the indirect measure of its tensile strength and is an index of concrete quality [50]. Its value is about 10% of its compression strength. From various research, it is found that most concrete beams fail in tension than in compression [48]. Flexural strength specimens were cast in 6 in. × 6 in. × 21 in. (15.24 cm × 15.24 cm × 53.34 cm) wooden forms (see Figure 10b). The specimens were cast in one-third increments and the concrete vibrated in each increment. Two 6 in. × 6 in. × 21 in. (15.24 cm × 15.24 cm × 53.34 cm) ASTM standard beams were cast for each concrete mix. The surface was trowel-finished (see Figure 10a).

The flexural strength tests were carried out following ASTM C 78 (Standard Test Method for Flexural Strength of Concrete [Using Simple Beam with Third-Point Loading]) (see Figure 11) [51]. The tests were performed using Tinius Olsen Machine No. 64867.

The flexural strength of a beam is calculated using Equation (3).

(3)$R=\frac{FL}{b{d}^2}$

where R = modulus of rupture or flexural strength, F = point load or total applied force on a beam, L = length of a beam from one support to another (in.), b = average width of a specimen (in.), and d = average depth of a specimen (in.).

Photos of a failed beam are given in Figure 12.

2.9. Deflection

The determination of deflection is an important serviceability characteristic of concrete [52]. Deflection is mainly affected by the modulus of elasticity, modulus of rupture, creep, and shrinkage. The maximum deflection (Δ) of the beam is determined using Equation (4):

(4)$∆=\frac{Pa\left(3L^2-4a^2\right)}{24EI}$

where $∆$ = deflection (in.), P = maximum load at failure = F/2 (lbs), L = length of beam between supports (in.), a = distance of load (P) from support (in.), E = modulus of elasticity (psi) ≈ $\mathrm{57,000}\sqrt{f_c^{\prime }}$ for normal concrete, E = w_c^1.5 × 33 × $\sqrt{f_C^{\prime }}$ for concrete weighing 90 lb/ft³ to 160 lb/ft³ (14.14 KN/m³ to 25.13 KN/m³), E = [40,000 $\sqrt{f_c^{\prime }}$ + 10⁶] ${\left(\frac{{\omega }_c}{145}\right)}^{1\cdot 5}$ for high strength concrete above 6000 psi to 9000 psi (41.4 MPa to 62.1 MPa), w_c = weight of concrete in pounds per cubic foot [46,53,54], I = moment of Inertia (in.⁴), and $f_c^{\prime }$ = compressive strength in psi. The concrete weight (w_c) utilized in this research is 145.61 lb/ft³ (22.87 KN/m³) (refer to Section 2.3). Since, concrete utilized in this research contained PCC and UCA, the formula for calculating the modulus of elasticity of a concrete beam is shown in Equation (5).

(5)$\mathrm{E}={{\mathrm{w}}_{\mathrm{c}}}^{1.5}\times 33\times \sqrt{f_C^{\prime }}$

3. Results

3.1. Composition of PCC

Samples of PCC from the sugar factory plant were taken to the Idaho National Laboratory (INL) for analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectrometry (EDS). The phase compositional data were obtained using powder X-ray diffraction (XRD) patterns obtained from Bruker D8 Advance (German) equipment with Cu Kα radiation of 1.5060 Angstrom wavelength. The XRD work was performed using LYNXEYE XE-T and Bragg–Brentano geometry in the 20° to 110° 2-theta range to scan with a step size of 0.01° and 0.5 s dwell time. The substrate was rotated at 15 rpm. The XRD analysis was initiated at 20 degrees to ensure the accurate measurement of diffraction peaks while minimizing interference from baseline drift, amorphous content, and X-ray exposure. The radius of the goniometer was 280 mm. SEM and EDX analyses were performed using an FEI Quanta FEG 650 and an EDAX Octane Plus, respectively.

The XRD results show that the PCC has a high calcite (CaCO₃) concentration, followed by quartz (SiO₂) (see Figure 13). The red line in Figure 13 shows the presence of calcite and the blue line shows quartz. The XRD is used for analyzing the crystallographic structure of the materials; however, it does not provide quantitative data on the element concentration. To determine the amounts of calcium, oxygen, and other elements in a sample, other analytical techniques such as energy dispersive X-ray spectroscopy are used.

The display generated from the EDX analysis, referred to as an EDX spectrum, displays the intensity of X-rays emitted at different energy levels. The x-axis on the graph represents the energy or wavelength of the X-rays, whereas the y-axis represents the intensity of the detected X-rays or X-ray counts at each energy level. The higher the peak or count on the y-axis, the more intense the X-ray emission of a particular energy level, indicating a higher concentration of the corresponding element in the sample.

The EDX results are provided in Figure 14. The x-axis represents the energy in kiloelectronvolt (keV), whereas the y-axis represents the intensity count. Further, the inset table in Figure 14 lists the elements detected in the PCC sample along with their respective weight percentages and atomic percentages. Moreover, it provides a quantitative breakdown of the element composition present in the PCC. The EDX analysis reveals that calcium makes up the highest percentage (45.9%), followed by oxygen (39.4%). Other elements include potassium, magnesium, titanium, and iron. It was noted that PCC contains up to 9% carbon by weight, which is justification for sequestering PCC in concrete applications.

The SEM image provides a high-resolution visual image of the surface morphology and microstructure of PCC. Figure 15 reveals an irregular and sub-rounded structure of PCC.

3.2. Compressive and Tensile Strength

To determine the beneficial effect of using PCC in concrete applications and to sequester carbon from it, a series of 28-day unconfined compressive strength and tensile strength tests were performed on samples containing variable amounts of PCC. TThe PCC mixes were compared with conventional concrete in which the same amounts of aggre-gates and water were used. The average compressive strength of the benchmark concrete mix was approximately 48 MPa (7000 psi), and the average tensile strength was about 3.5 MPa (500 psi) ($f_c^{\prime }$/$f_t^{\prime }$ = 13). All the samples were nominally 10.16 cm (10.12 to 10.17 cm) in diameter and 20.32 cm (20.34 to 20.63 cm) in length (L/D ratio of 2). The dimensions of the concrete cylinders were measured using a vernier caliper. Once the measurements were obtained, the cylinders underwent testing, and the maximum load at the point of failure was recorded. The compressive strength of each cylinder was then calculated using its actual dimensions. The average compressive strength for three benchmark concretes was recorded to be 47.92 MPa (6950 psi). Most of the samples failed with side fractures while only a few samples failed with columnar vertical cracks, diagonal cracks, or side fractures at the top or bottom, and the end of the cylinder is pointed (Type 3, 4, 5, and 6, respectively) as shown in Figure 16 and Figure 17.

Figure 17 illustrates examples of compression failures that were observed in the laboratory.

Likewise, vernier calipers were used to measure the dimensions of the conventional concrete cylinders before the split cylinder tests. All the samples were nominally 15.24 cm (15.34 to 15.39 cm) in diameter and 30.48 cm (30.88 to 31.12 cm) in length (L/D ratio of 2). After the measurements were taken and the split cylinder tests were performed, the maximum load at the point of failure was recorded. Then, the tensile strength of each cylinder was calculated using its actual dimensions. The average split tensile test results for three conventional concretes were recorded to be 3.89 MPa (561.28 psi). Figure 18 shows the fracture samples because of the split tensile strength test.

3.2.1. Cement Replaced with PCC

The compressive strength of concrete, in which various portions of cement were substituted by PCC, was determined using the same procedures used to determine the compressive strength of the control mix. Figure 19 displays the compressive strength test results for concrete with varying percentages of PCC. The orange horizontal line in Figure 19, Figure 20, Figure 21, Figure 22, Figure 23, Figure 24 and Figure 25 displays the standard target compressive strength of 28 MPa (4000 psi) and the red vertical line represents the standard deviations of three samples for each concrete mix.

Conventional concrete has the highest compressive strength in contrast to concrete with different percentages of cement replaced by PCC. But concrete containing 0% (benchmark concrete), 5%, 10%, 15%, 25%, and 30% PCC (each) has the target strength of approximately 28 MPa (4000 psi). The concrete with 5% cement replaced by PCC provided a higher compressive strength concrete, than the concrete with 10% to 30% PCC. Nonetheless, for structural performance, concrete containing 35% to 100% cement replaced by PCC should not be used because it has a compressive strength of less than 28 MPa (4000 psi). Concrete with 25% and 30% PCC achieves a compressive strength greater than 28 MPa (4000 psi) and can reduce carbon emissions from concrete in substantial amounts, which is why they were selected for further analysis. The standard deviation for the conventional concrete was 3.33 MPa (483 psi) and that for the concrete containing 25 to 30% PCC was from 0.45 to 1.6 MPa (66 to 230 psi). In structural design, it is important to achieve the required mechanical properties, as well as prepare a sustainable concrete mix design. Thus, the concrete below 28 MPa (4000 psi) is rejected for further research.

The tensile strength of concrete with PCC was found using the same methodology applied to conventional concrete. Figure 20 displays the results of the split tensile strength test of concrete with varying percentages of PCC.

The benchmark concrete has the highest tensile strength, which is followed by 5% PCC. Concrete with 100% cement replaced with PCC has the lowest tensile strength compared to other concrete mixes with PCC. Most of the concrete with 100% PCC were delicate/fragile and broke during demolding and transporting. Concrete containing various percentages of PCC has tensile strengths ranging from 7.5% to 13.5% when compared to its corresponding compressive strength. As shown in Figure 20, the split tensile strength of concrete with various amounts of PCC ranges from 0.02 to 3.87 MPa (3.09 to 561 psi). The standard deviation for conventional concrete, concrete with 25%, and 30% PCC are 0.04 MPa (5 psi), 0.07 MPa (10 psi), and 0.67 MPa (9.8 psi), respectively.

3.2.2. Aggregates Replaced with UCA

The second part of the project focuses on replacing each fine and coarse aggregate with upcycled fine and coarse aggregate (UCA), respectively. For this research, we utilized UCA provided by San Francisco Bay Aggregate (SFBA), a Blue Planet Company located in Pittsburg, CA, USA. The compressive and tensile strength for concrete with aggregate replacements were performed using the same procedures used for conventional concrete.

The percentage of UCA in Figure 21a is the percentage of conventional aggregate replaced by upcycled recycled concrete aggregate. According to the data in Figure 21a, conventional concrete and concrete with 100% UCA have similar compressive strength. Additionally, when the aggregate was replaced with UCA from 12.5% to 50%, the compressive strength of the concrete was increased. However, the compressive strength of the concrete with 75% and 100% UCA decreased as compared to 50% UCA, but still achieves a strength more than that of the control mix of 48.3 MPa (7000 psi). The standard deviation (shown by a vertical red line in Figure 21a) for concrete with various portions of UCA ranges from 0.6 to 3.4 MPa (90 to 394 psi).

The tensile strength of concrete with different percentages of conventional aggregates replaced by UCA was determined using a similar method used to evaluate the tensile strength of conventional concrete. The split tensile strength results for concrete with different percentages of UCA are displayed in Figure 21b. Conventional concrete has the highest compressive strength, followed by a concrete mix containing a 50% replacement of conventional aggregates with UCA. The concrete mixes with UCA had split tensile strength test results of 5.2% to 6.9% according to their corresponding compressive strength. The standard deviation ranges from 5% to 64% (shown by a vertical red line in Figure 21b).

3.2.3. Cement Replaced with PCC and Aggregates Replaced with UCA

To further reduce carbon dioxide emissions, both the cement and aggregates were replaced by PCC and UCA, respectively, in the same concrete mix. The performance of this sustainable and eco-friendly concrete was evaluated through several laboratory tests. The test involves the replacement of cement with 25% and 30% PCC and various percentages of aggregates with UCA by 12.5%, 25%, 37.5%, 50%, 75%, and 100%.

3.2.4. 25% Cement Replacement with PCC and Various Amounts of Aggregates Replaced with UCA

Figure 22 displays the compressive strength of concrete, in which 25% of cement is replaced by PCC, and both fine and coarse aggregates are replaced by UCA ranging from 0% to 100%. The concrete mix containing 25% of cement substituted with PCC and aggregates ranging from 0% to 100% replaced with UCA has achieved the targeted compressive strength of 28 MPa (4000 psi). Moreover, the compressive strength for concrete with UCA ranges from 28.3 MPa to 40 MPa (4100 psi to 5800 psi). In addition, the standard deviation for the mix design was from 0.35 MPa to 4.6 MPa (52 to 666 psi).

Figure 23 shows the split tensile test results for the concrete with 25% of cement replaced with PCC and both fine and coarse aggregates replaced with UCA from 0% to 100%. The concrete with 25% of cement replaced with PCC and 12.5%, 25%, 37.5%, and 50% of aggregates replaced with UCA has higher split tensile test results compared to the concrete with 25% of cement replaced with PCC only. Similarly, the concrete with 75% and 100% of aggregates replaced with UCA has lower split tensile test results than the concrete with 25% of cement replaced with PCC only. The standard deviation of concrete ranges from 0.16 to 1.64 MPa (23 to 238 psi).

3.2.5. 30% Cement Replacement with PCC and Various Amounts of Aggregates Replaced with UCA

Figure 24 displays the compressive strength of concrete for the case where 30% of cement was replaced with PCC, and both fine and coarse aggregates were replaced with UCA from 0% to 100%. The concrete with 30% cement replaced with PCC and 25% and 50% aggregates replaced with UCA has a lower compressive strength compared to the target strength of 4000 psi. But concrete with 30% of cement replaced by PCC and 12.5%, 37.5%, 75%, and 100% of aggregates replaced by UCA has a higher compressive strength above 28 MPa (4000 psi). Furthermore, concrete with 30% of cement replaced by PCC and 12.5%, 37.5%, 75%, and 100% of aggregates replaced by UCA has a higher compressive strength compared to that of concrete with 30% PCC only.

The graph in Figure 25 displays the split tensile test results for concrete where 30% of cement was replaced by PCC, and both fine and coarse aggregates were replaced by UCA. The concrete containing 30% PCC and 25% UCA; and 30% PCC and 37.5% UCA has higher split tensile test results than concrete containing 30% PCC only. However, concrete containing 30% cement replaced with PCC and conventional aggregates replaced with 12.5%, 50%, 75%, and 100% UCA has lower strength. The tensile strength of concrete with 30% of cement replaced by PCC only is about 9.4%, 30% PCC and 12.5% UCA is 6.3%, 30% PCC and 25% UCA is 14.1%, 30% PCC and 37.5% UCA is 7.9%, 30% PCC and 50% UCA is approximately 9.3%, 30% PCC and 75% UCA is approximately 7.8%, and 30% PCC and 100% UCA is about 7.7%. Furthermore, the standard deviation ranges from 0.06 to 0.42 MPa (9 to 61 psi).

3.2.6. Concrete with Various Percentages of Cement

The companion concrete mix with reduced cement and/or water was made to compare the concrete with 25% and 30% of cement replaced by PCC. The companion concrete cylinders consisted of 75% and 70% cement content, out of the total 100% cement (13.32 kg) used in the control mix design, as shown in Table 1 (refer to Section 2.5). Neither PCC nor UCA was utilized in these types of concrete mixes. The water-to-cement ratio was greater than 0.44 for this concrete mix, due to the reduced cement content (see Table 3). However, amounts of aggregates and water were the same for all PCC and non-PCC concrete.

Table 4 presents the compressive strength of concrete with reduced cement content. The standard deviation of 0.69 to 1.31 MPa (100 to 190 psi) of the mix is also shown in the same table. The compressive strength of concrete with 25% and 30% reduced cement is lower than the concrete containing 25% and 30% of cement replaced by PCC (refer to Section 3.2.1). The average compressive strength for concrete with a 25% reduced cement content is 22.2 MPa (3218.20 psi) and the average compressive strength of concrete in which 25% of cement is replaced by PCC is 35.3 MPa (5122.49 psi) (refer to Table 4 and Table 5) Similarly, the average compressive strength for concrete with a 30% reduced cement content is 21 MPa (3047.83 psi) and the average compressive strength of concrete with 30% of cement replaced with PCC is 28.3 MPa (4111.70 psi).

The average split tensile test results for concrete with 25% and 30% reduced cement were compared to those of concrete with 25% and 30% of cement replaced with PCC. Table 6 displays the split tensile results for reduced cement and water content. The split tensile test results with 25% and 30% reduced cement are less than the split tensile test results with 25% and 30% of cement replaced with PCC (see Table 6), respectively (refer to Section 3.2.1). The average split tensile strength test results for 25% reduced cement are 2.3 MPa (330.13 psi) and the average split tensile strength test results for 30% reduced cement is 2.1 MPa (311.30 psi). Additionally, the standard deviations for the split tensile strength test results for the non-PCC and PCC samples are shown in Table 6 and Table 7.

3.2.7. Concrete with Reduced Cement and Reduced Water

To find the cementing capacity of PCC in a concrete mix, the companion mix underwent a reduction of 25% to 30% of cement content but maintained a water-to-cement ratio of 0.44. Table 4 and Table 6 provide the data for the compressive strength and split tensile strength test results, respectively, for concrete with a reduced cement content and a water-to-cement ratio of 0.44. From Table 4 and Table 5, the compressive strength of concrete with 25% reduced cement at a water-to-cement ratio of 0.44 was found to be higher than the concrete with 25% reduced cement only and 25% of cement replaced by PCC only (also refer to Section 3.2.1). On the other hand, the compressive strength of 30% reduced cement at a water-to-cement ratio of 0.44 was found to be lower than concrete with 25% and 30% of cement replaced by PCC only but higher than concrete with 30% reduced cement only.

According to data presented in Table 6, the split tensile strength of concrete with 25% reduced cement at a water-to-cement ratio of 0.44 is found to be higher than the concrete with 25% of cement replaced with PCC only. However, the split tensile strength test results of concrete with 30% reduced cement at a water-to-cement ratio of 0.44 are found to be lower than the concrete with 30% of cement replaced by PCC only (refer to Section 3.2.1). In addition, the standard deviation of the split test results ranges from 0.11 to 0.62 MPa (16 to 90 psi) (refer to Table 5).

3.2.8. Concrete with Cement Replaced with 25% and 30% Fly Ash

Table 5 presents the compressive strength data for concrete mixes where 25% and 30% of cement were replaced with class F fly ash. Additionally, the compressive strength of a concrete mix where 25% and 30% of cement replaced with fly ash higher is found to be higher than the compressive strength of a concrete mix where 25% and 30% of cement was replaced by PCC (refer to Section 3.2.1).

Table 7 presents the split tensile test results for concrete where 25% and 30% of cement was replaced by fly ash. The average split tensile test results of concrete where 25% of cement replaced by fly ash are found to be higher compared to the average tensile strength of concrete with 25% of cement replaced by PCC only (refer to Section 3.2.1). On the other hand, the average split tensile strength of concrete where 30% of cement replaced by fly ash is found to be lower than the average split tensile strength of concrete where 30% of cement was replaced by PCC. To further determine the impact of PCC and UCA, flexural tests on beams were carried out.

3.3. Flexural Test and Deflection

The length, width, and height were measured using vernier calipers for each unreinforced concrete beam. The specimens of approximately 53.34 cm (21 inches) in length, 15.24 cm (6 inches) in width, and 15.24 cm (6 inches) in height were prepared in the lab. The average lengths, widths, and heights were calculated using the three measurements taken for each specimen. Then, the beam was tested per ASTM C 78, the standard test method for the flexural strength of concrete (using a simple beam with third-point loading) [49]. The maximum load was applied on the beam until failure and the maximum load was recorded for the flexural strength calculation. The flexural strength of the beam was calculated using the Equation (3) (refer to Section 2.8).

Concrete beam samples, containing 25% PCC of cement and different amounts of fine and coarse aggregates or UCA fine and coarse aggregates, were cast and subjected to a 28-day curing process. The beam after 28 days was tested using the four-point bending method. Table 8 displays the results for the flexural strength and deflection of a concrete beam. Conventional concrete has higher flexural strength and deflections than other mixes containing 25% PCC and different percentages of UCA. Furthermore, concrete containing 25% PCC and 50% UCA has the minimum flexural strength and deflections compared to other mixes. The average flexural strength and deflections for the two conventional concrete specimens were 5.41 MPa (784.66 psi) and 4.445 × 10⁻⁵ m (0.0018 in.), respectively. However, the average flexural strength and deflections for the two samples containing 25% PCC and 50% UCA samples were 2.28 MPa (330.69 psi), and 2.286 × 10⁻⁵ m (0.0009 in.), respectively.

Like Table 8, Table 9 displays the flexural strength of concrete specimens containing 30% PCC and various amounts of UCA. Conventional concrete has higher flexural strength and deflections compared to mixes containing 30% PCC and varying amounts of UCA. In contrast, the concrete with 30% PCC and 37.5% UCA has the lowest flexural strength and deflections compared to other concrete mixes. The average flexural strength and deflections for the two specimens containing 30% PCC and 37.5% UCA are 2.46 MPa (356.79 psi) and 2.413 × 10⁻³ m (0.00094 in.), respectively.

4. Discussions

4.1. Environmental Considerations of Study and Reduction in Carbon Emissions

Based on the EDX analysis (refer to Section 3.1), the waste product contains 9.2% carbon, 39.4% oxygen, and 45.9% calcium. With the projected increase in global sugar consumption, it is expected that the amount of PCC waste will also rise. One solution is to utilize the waste product in civil works projects, which reduces disposal in landfills, and, at the same time, sequester carbon. Further, carbon emissions in cement production can also be reduced by substituting Portland cement with PCC. A second environmental aspect of this study was to evaluate the use of chemically treated crushed waste concrete (demolished structures [such as buildings, bridges, and roads] or returned from a ready mix) called UCA as a substitute for fine and coarse aggregate in fresh concrete. The intent is to reduce the disposal of waste concrete and decrease the environmental impact associated with the production of fine and coarse aggregate.

According to the findings from this study, 25% to 30% of cement can be substituted with PCC, and 0% to 100% aggregates can be replaced with UCA. After reducing the substantial amounts of cement and aggregates, the production of cement and aggregates can be decreased. These cement and aggregates substitutions have the potential to reduce the carbon dioxide emissions from concrete. Moreover, PCC itself contains calcium carbonate (refer to Section 3.1), which might release carbon dioxide when it breaks down. Additionally, the waste products, namely, PCC, concrete demolition, and returned concrete from a ready mix, which are usually sent to landfills, can be utilized in concrete to permanently trap carbon from PCC and UCA.

4.2. Use of PCC in Concrete

The replacement of cement with PCC from 0% to 30% in a conventional concrete mix achieved a target compressive strength of 28 MPa (4000 psi) in a 28-day curing period. Most conventional concrete used on civil engineering projects has a design compressive strength ranging from 28 MPa (4000 psi) to 41.4 MPa (6000 psi) (as outlined in Section 3.2.1). Split tensile test results of concrete containing 0% to 30% PCC range from 2.66 to 3.87 MPa (385 to 496 psi). These tensile strength values were approximately 7.5% to 13.5% of their corresponding compressive strength. The deflection at failure was 4.572 × 10⁻⁵ m (0.0018 in.) for conventional concrete and 3.81 × 10⁻⁵ to 4.064 × 10⁻⁵ m (0.0015 to 0.0016 in.) for concrete with PCC. Therefore, due to the lower deflection in PCC beams when compared to conventional concrete beams, it can be concluded that PCC beams have a lower modulus of elasticity compared to that of conventional concrete. The average flexural strength for two concrete beams containing 25% PCC is approximately 4.06 MPa (589 psi), which is 11.50% of their average compressive strength. From various literature reviews, the flexural strength of concrete typically ranges around 10% of its compressive strength. In this research, the flexural strength of concrete with PCC nearly falls within an acceptable range. Thus, PCC can be used to replace a portion of the cement in concrete, as well as help to sequester the carbon in the PCC.

4.3. Use of UCA in Concrete

A series of compressive, splitting tensile, and four-point bending strength tests were conducted where conventional aggregates were replaced by UCA at various percentages such as 12.5%, 25%, 37.5%, 50%, 75%, and 100%. The results were mixed at best: the compressive strength of concrete with UCA rises to 57.11 MPa (8283.6 psi) when 50% of conventional aggregates are replaced with UCA, while the strength gradually decreases from 75% to 100%, reaching 48.3 MPa (7000 psi), which is the same amount of strength for a conventional concrete mix (as outlined in Section 3.2.2). It is important to note that the compressive strength of concrete with UCA is higher than the compressive strength of conventional concrete. The enhanced strength of concrete might be due to the chemical treatment employed in the Blue Planet Process, which seems to influence the RCA, thus increasing its performance as an aggregate following the process of upcycling.

The tensile strength of concrete containing UCA is found to be lower compared to the conventional concrete mix. The tensile strength of concrete with an up to 37.5% UCA replacement was lower but increased slightly afterwards. The tensile strength of the concrete containing UCA varies from 5.26% to 8.08% according to its respective compressive strength. However, the split tensile strength test values are within an acceptable range.

4.4. Use of PCC and UCA in Concrete

To further reduce carbon emissions from concrete, both cement and aggregates were replaced with PCC and UCA, respectively. The compressive strength of concrete, with 25% of cement replaced by PCC and different percentages of aggregates replaced with UCA, ranges from 28.3 to 40 MPa (4100 psi to 5800 psi) (as discussed in Section 3.2.4). This value is within the acceptable range for conventional concrete. The tensile strength of concrete, with 25% of cement replaced by PCC and various amounts of aggregates replaced by UCA, ranges from 2.02 MPa (293 psi) to 2.88 MPa (418 psi). These split tensile results are below 10% of its corresponding compressive strength and are within an acceptable range.

The concrete in which 30% of the cement was replaced with PCC and various percentages of aggregates were replaced by UCA has a compressive strength ranging from 19.82 MPa (2874 psi) to 36.74 MPa (5329 psi) (as detailed in Section 3.2.5). Moreover, the compressive strength of concrete containing 30% of cement replaced by PCC and 25% UCA is below the standard target value, which is 28 MPa (4000 psi) for conventional concrete. However, concrete with 30% of cement replaced by PCC and conventional aggregates replaced by 12.5%, 37.5%, 75%, and 100% UCA has a compressive strength above 28 MPa (4000 psi). Most of the samples have an acceptable tensile strength, except for the concrete containing 30% PCC and 25% UCA. The fluctuations in results might be due to the improper concrete mix or failure modes.

Four-point bending tests conducted on concrete containing both PCC and UCA show a decline from 39% to 61% compared to the flexural strength of concrete beams containing 25% of cement replaced by PCC. The flexural strength of concrete with 25% cement replaced by PCC and different amounts of UCA are from 4% to 6.6% of their respective compressive strength. Similarly, the four-point bending tests on a concrete beam containing 30% PCC shows a decrease of 45 to 59% in the flexural strength compared to that of conventional concrete. The flexural strength of concrete with 30% of cement replaced by PCC and various amounts of aggregate replaced by UCA ranges from 3.9% to 8.0% of their respective compressive strength. However, the flexural strength for concrete containing PCC and UCA falls within the acceptable range of 10% of their corresponding compressive strength.

4.5. Comparison between Concrete with PCC and Reduced Cement or Fly Ash in a Concrete

The compressive strength of concrete containing a reduced cement amount of 75% and 70% cement out of 13.32 kg (29.36 lb/ft³), while maintaining the same amount of other materials (see Section 2.5), is lower compared to concrete containing 25% and 30% PCC. Similarly, the split tensile test results for concrete with reduced cement are lower compared to concrete containing 25% and 30% of cement with PCC. When both cement and water quantities were reduced at a water-to-cement ratio of 0.44, the compressive strength of concrete was higher compared to that of concrete containing 25% and 30% PCC. In addition, the tensile strength of concrete with a reduced cement and water content is higher with a 25% reduction in cement but lower than the concrete containing 30% of cement replaced with PCC. Furthermore, the compressive strength and split tensile test results of concrete where 25% and 30% of cement were replaced by fly ash was higher than the compressive strength and split tensile test results of concrete where 25% and 30% of cement were replaced by PCC.

From the various concrete mix designs, 25 to 30% of cement can be replaced with PCC and 0% to 100% of aggregates can be replaced with UCA, which can achieve a compressive strength of 28 MPa (4000 psi). This type of concrete helps to sequester carbon, reduce carbon emissions, and minimize waste disposal in landfills to obtain environmentally friendly concrete (PCC-UCA concrete).

5. Conclusions

In summary, this study has explored the prospects of using precipitated calcium carbonate (PCC) and upcycled recycled concrete aggregate (UCA) in concrete to repurpose waste materials and reduce carbon emissions in the construction industry, while having sufficient strength and stiffness to satisfy the design requirements for concrete structures. Laboratory test results reveal the composition and physical attributes of PCC. Based on the test results, an up to 30% PCC replacement of Portland cement and a 100% replacement of UCA for coarse and fine aggregates meet minimum design values in conventional concrete and have an additional benefit of reducing the carbon footprint. Moreover, the introduction of PCC and UCA as an alternative to Portland cement and conventional aggregates showcased comparable compressive strengths, emphasizing their feasibility as an eco-friendly substitution. While this research represents an initial phase, further investigations encompassing additional assessments, such as freeze–thaw, shrinkage, surface hardness tests, and rebound testing are essential to comprehensively evaluate the suitability of PCC and UCA in concrete applications. Moreover, future studies should explore PCC substitution in ultra-high-performance concrete (UHPC) and its applicability in seismic retrofitting and closure pours in construction, thereby contributing to a sustainable and innovative construction industry.

Author Contributions

Conceptualization, M.M., J.M. and K.P.; methodology, M.M., J.M., K.M. and K.P.; software, K.P. and K.M.; validation, M.M., J.M., K.M. and K.P.; formal analysis, M.M., J.M., K.M., U.S. and K.P.; investigation, M.M., J.M., K.M., U.S. and K.P.; resources, M.M., J.M. and K.P.; data curation, M.M., J.M. and K.P.; writing—original draft preparation, K.P.; writing— M.M., J.M., K.M. and K.P.; visualization, M.M., J.M. and K.P.; supervision, M.M. and J.M.; project administration, M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The corresponding authors can provide the data upon request.

Acknowledgments

The researchers are thankful for the support provided by Jared Cantrell, Laboratory Manager in the Department of Civil and Environmental Engineering at Idaho State University. The authors would also like to express their gratitude to Kathryn Hogarth, Manish Acharya, Saksham Raj Maharjan, Icewal Ghimire, Ashish Deo, and the entire structural team at Idaho State University for their invaluable support. Special thanks go to Kathryn Hogarth for meticulously proofreading the journal article. Recognition is also due to the following entities: Pocatello Ready Mix of Pocatello, Idaho for supplying aggregates, Amalgamated Sugar Corporation in Twin Falls, Idaho for providing precipitated calcium carbonates (PCCs), and Blue Planet and SFBA for their contribution of upcycled recycled concrete aggregates (UCAs). The authors extend their appreciation to Bruce Savage for facilitating the acquisition of PCC from Amalgamated Sugar Cooperation, and heartfelt thanks are given to Jacob Schneider and Mahesh Mahat for arranging the acquisition of UCA from SFBA. The comprehensive presentations on UCA and calcium carbonate (CaCO₃) aggregates were instrumental in enhancing the project’s understanding. Kunal Mondal (co-author) would also like to thank Oak Ridge National Laboratory, USA, for their support.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Table 1

Disposition of construction and demolition debris in tons in the United States (2015) [33].

Material Type in C&D Debris	Landfill	Next Use					Total Next Use
		Compost and Mulch	Manufactured Products	Aggregate, Other	Fuel	Soil Amendment
Concrete	66,535,034	0	30,962,635	284,260,331	0	0	315,222,966
Wood	27,053,922	2,611,131	1,296,159	0	7,988,787	0	11,896,077
Gypsum drywall	10,803,717	0	234,675	0	0	2,003,608	2,238,283
Metal	670,495	0	3,784,505	0	0	0	3,784,505
Brick and clay tile	10,587,745	0	0	1,559,255	0	0	1,559,255
Asphalt shingles	11,491,724	0	1,931,000	80,045	22,231	0	2,033,276
Asphalt pavement	5,042,361	0	70,347,585	7,769,079	0	0	78,116,664
TOTAL	132,184,998	2,611,131	108,556,559	293,668,710	8,011,018	2,003,608	414,851,026

Table 2

Impurities allowed in RCA [34].

Impurities	Lime Plaster	Soil	Wood	Hydrated Gypsum	Asphalt	Paint-Made Vinyl Acetated
Percentage of aggregate by volume	7%	5%	4%	3%	2%	0.20%

Table 3

Control concrete mix design per cubic foot.

Materials	Weight (kg)
Water	5.82 (12.82 lb)
Cement	13.32 (29.36 lb)
Fine aggregate	15.53 (34.24 lb)
Coarse aggregate	31.38 (69.19 lb)
Total weight	66.05 (145.61 lb)

Table 4

Average 28-day compressive strength of concrete with reduced cement content or reduced content of both cement and water.

Percentage of Cement in a Concrete Mix	Water-to-Cement Ratio (w:c)	Compressive Strength (MPa)			Average Compressive Strength (MPa)	Standard Deviation (MPa)
		Sample 1	Sample 2	Sample 3
75	0.58	21.96	21.48	23.13	22.19	0.69
70	0.62	19.56	20.75	22.74	21.01	1.31
75	0.44	52.81	51.87	45.70	50.13	3.15
70	0.44	25.73	24.84	27.88	26.15	1.28

Table 5

Average 28-days compressive strength of a concrete mix with various percentages of PCC or fly ash.

Percentage of PCC	Percentage of Fly Ash	Compressive Strength (MPa)			Average Compressive Strength (MPa)	Standard Deviation (MPa)
		Sample 1	Sample 2	Sample 3
25	0	35.71	34.81	35.43	35.32	0.46
30	0	26.83	29.99	28.22	28.35	1.58
0	25	43.71	44.74	33.41	40.62	6.27
0	30	38.69	40.36	40.54	39.86	1.02

Table 6

Average 28-day tensile strength of concrete with reduced cement content or reduced content of both cement and water.

Percentage of Cement in a Concrete Mix	Water-to-Cement Ratio (w:c)	Split Tensile Test Results (MPa)			Average Split Tensile Test Results (MPa)	Standard Deviation (MPa)
		Sample 1	Sample 2	Sample 3
75	0.58	2.08	2.49	2.26	2.28	0.21
70	0.62	1.86	2.63	1.95	2.15	0.42
75	0.44	2.85	2.63	2.81	2.76	0.12
70	0.44	2.75	1.72	2.81	2.43	0.62

Table 7

Average 28-day split tensile test results of a concrete mix with various percentages of PCC or fly ash.

Percentage of Cement Replaced by PCC	Percentage of Fly Ash	Split Tensile Test Results (MPa)			Average Split Tensile Test Results (MPa)	Standard Deviation (MPa)
		Sample 1	Sample 2	Sample 3
25	0	2.59	2.73	2.67	2.66	0.07
30	0	2.59	2.73	2.65	2.66	0.07
0	25	3.18	2.51	2.41	2.70	0.42
0	30	2.45	2.84	2.78	2.69	0.21

Table 8

Average 28-day flexural strength and deflection of a concrete beam with 25% PCC and various percentages of UCA.

PCC (%)	UCA (%)	Specimens Number	Average Length (m)	Average Width (m)	Average Height (m)	Moment of Inertia (m4)	Maximum Point Load before Failure (F)	Flexural Strength (MPa)	Modulus of Elasticity (MPa)	Deflection (m)
						(×10−5)				(×10−5)
0	0	1	0.54	0.16	0.16	5.24	10,100.00	5.21	33,328.66	4.32
		2	0.53	0.16	0.16	5.27	10,900.00	5.60	33,328.66	4.57
25	0	1	0.53	0.16	0.16	5.41	8200.00	4.09	28,612.81	4.06
		2	0.51	0.16	0.16	5.45	8200.00	4.10	28,612.81	4.06
25	12.5	1	0.53	0.16	0.16	5.34	5620.00	2.86	30,604.64	2.54
		2	0.53	0.16	0.16	5.38	5220.00	2.65	30,604.64	2.29
25	25	1	0.54	0.16	0.16	5.03	7230.00	3.83	26,877.78	4.06
		2	0.54	0.16	0.16	4.95	5040.00	2.69	26,877.78	2.79
25	37.5	1	0.53	0.16	0.16	5.28	4840.00	2.48	26,094.57	2.54
		2	0.54	0.16	0.16	5.16	5980.00	3.11	26,094.57	3.30
25	50	1	0.53	0.16	0.16	5.24	3640.00	1.87	25,707.24	2.03
		2	0.54	0.16	0.17	5.95	5150.00	2.44	25,707.24	2.54
25	75	1	0.54	0.16	0.16	5.26	5030.00	2.59	25,707.24	2.79
		2	0.53	0.16	0.16	5.27	5700.00	2.93	25,707.24	3.30
25	100	1	0.53	0.16	0.16	5.33	6920.00	3.52	29,190.38	3.30
		2	0.54	0.16	0.16	5.51	5790.00	2.87	29,190.38	2.79

Table 9

Average 28-day flexural strength and deflection of a concrete beam with 30% PCC and various percentages of UCA.

PCC (%)	UCA (%)	Specimens Number	Average Length (m)	Average Width (m)	Average Height (m)	Moment of Inertia (m4)	Maximum Point Load before Failure (F)	Flexural Strength (MPa)	Modulus of Elasticity (MPa)	Deflection (m)
						(×10−5)				(×10−5)
0	0	1	0.54	0.16	0.16	5.24	10,100.00	5.21	33,328.66	4.32
		2	0.53	0.16	0.16	5.27	10,900.00	5.60	33,328.66	4.57
30	0	1	0.54	0.16	0.16	5.52	7260.00	3.60	25,634.84	3.81
		2	0.54	0.16	0.16	5.67	7960.00	3.87	25,634.84	4.06
30	12.5	1	0.53	0.16	0.16	5.39	5530.00	2.79	28,781.11	2.79
		2	0.54	0.16	0.16	5.46	6730.00	3.35	28,781.11	3.30
30	25	1	0.53	0.16	0.16	5.38	5340.00	2.71	21,433.15	3.56
		2	0.53	0.16	0.16	5.14	4810.00	2.51	21,433.15	3.30
30	37.5	1	0.54	0.15	0.16	5.15	4820.00	2.52	29,184.82	2.54
		2	0.53	0.16	0.16	5.08	4580.00	2.41	29,184.82	2.29
30	50	1	0.53	0.15	0.16	5.63	6330.00	3.12	23,337.40	3.56
		2	0.54	0.15	0.16	5.26	6330.00	3.27	23,337.40	3.81
30	75	1	0.53	0.15	0.16	4.98	5750.00	3.07	27,885.64	3.05
		2	0.54	0.16	0.16	5.13	5220.00	2.73	27,885.64	2.79
30	100	1	0.53	0.15	0.16	5.08	6170.00	3.26	26,772.39	3.30
		2	0.53	0.16	0.16	5.23	5650.00	2.91	26,772.39	3.05

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