Utilization of Construction and Demolition Waste in Concrete as Cement and Aggregate Substitute: A Comprehensive Study on Microstructure, Performance, and Sustainability

Abstract

Construction and demolition waste (CDW) was successfully utilized as an aggregate with 100% replacement of natural aggregates and mineral admixtures, with up to 60% replacement of ordinary Portland cement (OPC) in the production of recycled concrete. The effects of ratios of concrete-based CDW (concrete-CDW)/brick-based CDW (brick-CDW) in both aggregates and CDW mineral admixture contents in the binder on recycled concrete were investigated in terms of their workability and compressive strength, microstructure, and sustainability. The results showed that with an increase in the ratios of brick-CDW/concrete-CDW aggregates, the concrete workability continuously deteriorated, while the compressive strength firstly increased and then decreased. Compared to the 100% dosage of concrete-CDW aggregates, the 28-day compressive strength of the recycled concrete was 37.4 MPa; the optimized relative proportions of brick-CDW and concrete-CDW aggregates were 20% and 80%, respectively; and the 28-day compressive strength was the highest, reaching to 46.7 MPa, and increasing by 24.9%. In a binder study, the microstructure of the paste was found to be improved, with the dosage of brick-CDW and concrete-CDW admixtures at up to 20%. In this range, the workability changed slightly when the relative proportion of brick-CDW admixture increased, the 28-day compressive strength of the recycled concrete increased, and the pore structure was refined. Furthermore, the utilization of a large amount of CDW as a mineral admixture and aggregate in concrete significantly reduced costs and CO₂ emissions in different regions.

1. Introduction

According to research and statistical analyses in the literature, urbanization has resulted in the annual generation of approximately 1.3–1.7 billion tons of construction and demolition waste (CDW) in China per year, leading to the significant accumulation of this waste [1,2,3]. Seeking reasonable approaches to achieve large-scale consumption of CDW has become urgent. To address this issue, some studies from different regions have employed various recycling methods and management strategies, and methods for the CDW recycling industry have been explored in 12 pilot cities of China [4]. For instance, CDW has been commonly utilized as filler in road bases and sub-bases [5]. Concrete is one of the most widely used building materials in the world [6], and its production consumes large amounts of raw materials, such as sand, stone, and cement, causing significant environmental damage and adverse impacts. CDW as a substitute for raw materials can mitigate these environmental impacts.

Many studies have tried to use solid waste for the replacement of these raw materials, such as using circulating fluidized bed combustion ash (CFBA) and phosphorus slag as mineral admixtures for the replacement of cement. With the help of potential pozzolanic activity, Gao et al. [7] and Zhang et al. [8] designed concretes containing CFBA and phosphorus slag, respectively. Additionally, other industrial solid wastes, such as titanium-bearing blast furnace slag and steel slag, have also been used for the substitution of natural aggregate to mitigate environmental impact. Jiang et al. [9] and Shen et al. [10] prepared mortar with expanded titanium-bearing blast furnace slag and asphalt concrete with porous steel slag, respectively. As one of the most widely generated solid wastes, CDW has also shown enormous potential to replace natural aggregate or cement. For example, adding CDW as an aggregate and mineral admixture to concrete not only prevents the accumulation of waste but also reduces the consumption of natural resources and cement in concrete manufacturing. CDW consists of waste concrete, brick, glass, tile, and plastic, which are different from other solid wastes. Usually, each component shows special characteristics, thus presenting different effects on material performance. For instance, with the full utilization of high-strength waste tiles, Zhang et al. [11] fabricated ultra-high-performance concrete, achieving 5.5% and 26.5% improvements in compressive and flexural strength, respectively. Owing to its inherent chemical stability, waste plastic can be upcycled into high-value chemicals through novel chemical processes [12], and waste glass has been recycled to fabricate new glass products through melting, including flat glass panels and containers [13]. These types of waste have been fully consumed in special fields. However, waste brick and concrete, as the main components of CDW [14,15,16], cannot be fully utilized and consumed. Thus, to fully consume concrete and brick, their composition and characteristics were analyzed. Specifically, concrete-CDW contains natural aggregates and attached mortar [17], indicating that CDW shows great potential in the replacement of aggregate and cement, and brick-CDW contains 11.2–17.0% amorphous SiO₂ and Al₂O₃, 26.8–39.3% of quartz and calcite, and other crystalline phases [18]. The amorphous phase offers pozzolanic activity, for use as an admixture. The crystalline phases provide a robust skeleton, for use as a recycled aggregate. Therefore, Soares et al. [19] utilized brick-CDW recycled aggregates to replace 20% of conventional aggregate, and the drying shrinkage of the recycled concrete was not affected significantly. Similarly, Ozcelikci et al. [20] developed low-quality concrete using concrete-CDW recycled aggregates at water/cement ratios of 0.60 and 0.74, and the 7-day and 28-day compressive and splitting-tensile strength increased. For the substitute for cement, Senol et al. [21] used 20% of brick recycled powder to successfully produce high-strength self-compacting concrete. Tang et al. [22] designed a kind of limestone calcined clay cement containing 30% recycled powder, and the dry shrinkage of mortar containing this cement decreased by 18.75% at 90 days.

Although introducing concrete-CDW and brick-CDW in concrete and mortar improves their performance, the dosage of introduction is relatively low. To address the accumulation of these two types of CDW, large-scale recycling and reuse of these wastes in concrete production are urgent. Yang et al. [23] tried to use a large content of CDW to prepare lightweight concrete, and 60% brick-CDW aggregates and 20% brick-CDW powder were finally successful to prepare recycled lightweight aggregate concrete with a dry density of 1935 kg/m³, a 28-day compressive strength of 39.4 MPa, and a thermal conductivity of 0.63 W/(m·K). Ma et al. [24] utilized 100% concrete-CDW recycled aggregate and 30% concrete-CDW recycled powder to fabricate recycled mortar, and this mortar exhibited better chloride resistance than a reference mortar. However, some researchers found that the large-scale incorporation of concrete-CDW and brick-CDW into concrete and mortar weakened their properties and durability significantly. Ma et al. [25] developed a kind of low-carbon ultra-high-performance concrete using carbonated recycled fine aggregate, and the 28-day compressive strength decreased by 14.5%. Abbas et al. [26] reported the integrated utilization of brick-CDW in concrete, indicating a 71.43% workability decline in mixtures containing 100% recycled brick aggregate and a substantial 47.20% strength reduction. Yang [27] reported a kind of recycled concrete containing 50% recycled aggregate, where the 28-day compressive strength decreased by 18.12%, the sulfate resistance and frost resistance performance of recycled concrete decreased, and the dry shrinkage increased significantly. Thus, the large-scale incorporation of CDW entailed a significant reduction in mechanical performance and durability of recycled concrete. There is a contradiction between large-scale utilization of these wastes and performance retention of recycled concrete: when concrete-CDW and brick-CDW were the main components of CDW, under the condition of the large-scale incorporation of brick-CDW and concrete-CDW into concrete, the relative proportion of concrete-CDW and brick-CDW as aggregate and admixture could be optimized, thereby showing potential for maintaining the performance of the recycled concrete.

In this paper, concrete-CDW and brick-CDW were simultaneously used as aggregates with 100% replacement of natural aggregates and their ground powders used as mineral admixtures with up to 60% replacement of cement to prepare recycled concrete. Its workability, compressive strength, and microstructure were investigated and its sustainability issues discussed. This work provides a comprehensive assessment of the utilization of both concrete-CDW and brick-CDW in cementitious materials and aggregates, considering performance, environmental, and economic impacts. The findings offer valuable guidance for the development of low-carbon, cost-effective concrete incorporating both concrete-CDW and brick-CDW.

2. Materials and Methods

2.1. Raw Materials

In this study, ordinary Portland cement (OPC, P·O 42.5R) from the Jiangyou Lafarge Cement Plant (Mianyang, China) was used as a binder. The particle size and chemical composition of OPC are shown in Figure 1 and

Table 1. Chemical components of raw materials (%).

Binder	SiO2	CaO	Al2O3	Fe2O3	SO3	MgO	K2O	Other
Cement	19.51	65.47	4.33	2.88	2.45	1.42	0.65	3.26
Concrete-CDW	55.12	28.83	6.79	4.02	1.81	1.14	0.77	1.52
Brick-CDW	63.71	14.00	11.74	5.51	0.71	0.93	1.53	1.87

, respectively. The recycled concrete was prepared by two types of CDW (waste concrete and waste brick), which were sourced from Ningbo city, and CDW was collected by Ningbo Construction Engineering Group Co., Ltd. (Ningbo, China). These materials were used to prepare concrete-based CDW (concrete-CDW) and brick-based CDW (brick-CDW) aggregates and mineral admixtures. Specifically, for these two kinds of CDW aggregates, these CDWs (particle size > 63.0 mm) were crushed using a jaw crusher (PC-200×300, Henan Zhaochen Machinery Equipment Co., Ltd., Zhengzhou, China). Then, these CDW aggregates were then sieved through the 4.75 mm square mesh sieve, particles ranging from 37.5 mm to 4.75 mm were used as coarse aggregates, while those between 4.75 mm and 0.15 mm were used as fine aggregates. The properties and particle size distribution of CDW aggregates are presented in

Table 2. Characteristics of CDW fine aggregates.

ID	Apparent Density (kg/m3)	Bulk Density (kg/m3)	Crushing Value (%)	Water Absorption (%)
Brick-CDW	2260	1210	34.0	17.5
Concrete-CDW	2570	1260	24.0	11.9

,

Table 3. Characteristics of CDW coarse aggregates.

ID	Apparent Density (kg/m3)	Bulk Density (kg/m3)	Crushing Value (%)	Water Absorption (%)
Brick-CDW	2200	1150	35.0	9.3
Concrete-CDW	2580	1700	25.0	4.9

,

Table 4. Particle size distribution of CDW fine aggregate (%).

ID	0.15 mm	0.315 mm	0.630 mm	1.18 mm	2.36 mm	4.75 mm
Brick-CDW	10.0	25.6	57.0	87.0	100.0	100.0
Concrete-CDW	13.0	30.0	54.0	75.0	98.0	100.0

and

Table 5. Particle size distribution of CDW coarse aggregate (%).

ID	4.75 mm	9.50 mm	16.0 mm	19.0 mm	26.5 mm	31.5 mm	37.5 mm
Brick-CDW	28.3	54.9	71.6	89.4	95.5	98.6	100
Concrete-CDW	0.0	11.0	64.0	73.0	84.0	93.0	100

. The CDW admixtures were prepared by dry-grinding CDW aggregates (particle size < 4.75 mm) for 30 min in a ball mill (SM500, Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China). The particle size distribution and chemical composition of the CDW admixtures are shown in Figure 1 and Table 1, respectively. The true density of concrete-CDW and brick-CDW admixtures were 2680 kg/m³ and 2670 kg/m³. A polycarboxylate superplasticizer (SP) with 18% solid content was sourced from the Henan Xinxiang Jiesheng Co., Ltd. (Xinxiang, China).

The mineralogical compositions of the brick-CDW admixture and concrete-CDW admixture are shown in Figure 2. The main components of concrete-CDW and brick-CDW admixtures are quartz, calcite, gismondine, and clinochlore. The micromorphology images of CDW admixtures are presented in Figure 3.

2.2. Mix Proportions and Experimental Procedure

To investigate the effect of concrete-CDW and brick-CDW on recycled concrete, these two types of CDW aggregates and mineral admixtures were used to replace natural aggregates and cement. Under the condition of a constant water and superplasticizer content, in a series of utilizing CDW as aggregates, the relative proportion of concrete-CDW aggregates was 100%, used to replace both fine and coarse natural aggregates with brick-CDW/concrete-CDW ratios varying from 0/100% to 60%/40%, and pure OPC was used; in the series utilizing CDW as mineral admixtures, the brick-CDW/concrete-CDW ratio for aggregate was fixed at 20%/80%, as this proportion demonstrated the optimal workability and compressive strength. Based on this optimal aggregate proportion, a ternary binder system was formulated with the following constraints for cement (x₁), brick-CDW admixture (x₂), and concrete-CDW admixture (x₃) (0.4 ≤ x₁ ≤ 1, 0 ≤ x₂ ≤ 0.6, 0 ≤ x₃ ≤ 0.6, and x₁ + x₂ + x₃ = 1). The composition design of the ternary system is shown in Figure 4. The mix proportions were determined using the simplex-centroid experimental design method [28]. Accordingly, seven distinct concrete mixtures incorporating brick-CDW and concrete-CDW were prepared, corresponding to the three vertices, three edge midpoints, and the centroid of the ternary diagram, as illustrated in Figure 4. The performances of these mixtures were measured many times. This data was subsequently fitted using Design-Expert software (v.13) to develop a predictive model for estimating properties at other mixture proportions (Equation (1)).

$$\mathrm{y}={\mathsf{\beta }}_1{\mathrm{x}}_1+{\mathsf{\beta }}_2{\mathrm{x}}_2+{\mathsf{\beta }}_3{\mathrm{x}}_3+{\mathsf{\beta }}_{12}{\mathrm{x}}_1{\mathrm{x}}_2+{\mathsf{\beta }}_{13}{\mathrm{x}}_1{\mathrm{x}}_3+{\mathsf{\beta }}_{23}{\mathrm{x}}_2{\mathrm{x}}_3+{\mathsf{\beta }}_{123}{\mathrm{x}}_1{\mathrm{x}}_2{\mathrm{x}}_3$$

(1)

where ${\mathsf{\beta }}_i$ represents the value of the seven groups.

The mixture proportions are presented in

Table 6. Mixture proportion of concrete (kg/m³).

Mix-ID	Fine Aggregate		Coarse Aggregate		Binder
	Brick-CDW	Concrete-CDW	Brick-CDW	Concrete-CDW	OPC	Brick Admixture	Concrete Admixture	Water	SP
Series of use CDW as aggregates (Brick-CDW/Concrete-CDW)
A1 (0/100)	0	807	0	873	450	0	0	153	20.3
A2 (20/80)	147	647	160	700	450	0	0	153	20.3
A3 (40/60)	293	487	320	527	450	0	0	153	20.3
A4 (60/40)	440	320	480	347	450	0	0	153	20.3
Series of use CDW powders as mineral admixtures (OPC/Brick-CDW/Concrete-CDW)
B1 (100/0/0)	147	647	160	700	450	0	0	153	20.3
B2 (40/60/0)	147	647	160	700	180	270	0	153	20.3
B3 (40/30/30)	147	647	160	700	180	135	135	153	20.3
B4 (70/30/0)	147	647	160	700	315	135	0	153	20.3
B5 (40/0/60)	147	647	160	700	180	0	270	153	20.3
B6 (70/0/30)	147	647	160	700	315	0	135	153	20.3
B7 (60/20/20)	147	647	160	700	270	90	90	153	20.3

, with the reference (A1) representing a commonly used mixture with 100% concrete-CDW replacement in local concrete factories. This reference proportion was also pre-tested by adjusting workability with the target of obtaining a slump of 220 mm and a slump flow of 550 mm. For these proportions, all raw materials after homogeneous sampling were pre-mixed in a concrete mixer for 2 min. Then, water and admixtures were added and mixed for 3 min. After assessing the workability of the fresh mixture, the fresh mixture was cast into molds and cured for 24 h (at 20 ± 1 °C and 60% RH). After demolding, the samples were placed in a standard curing room (20 ± 1 °C, ≥95% RH) for curing.

2.3. Test Method

To evaluate the workability of the recycled concrete, the fresh concrete mixture was placed in a slump cone and tested three times to determine the slump and slump flow values by following the Chinese Standard GB/T 50080-2016 [29].

To obtain the compressive strength, three specimens (100 mm × 100 mm × 100 mm) were tested using a 3000 kN electro-hydraulic servo pressure testing machine (YL64.306, Shenzhen Labsans Machine Co., Ltd., Shenzhen, China) in accordance with the Chinese Standard GB/T 50081-2019 [30], with a loading rate of 0.4 MPa/s.

After 28-day curing, the binder paste was dried at 50 °C until the mass remained constant and then ground. This powder was used for thermogravimetric analysis (50–1000 °C, 20 °C/min, N₂) using a simultaneous thermal analyzer (STA 449F5, NETZSCH Instrument Manufacturing GmbH, Selb, Germany).

The pore structure characteristics of the concrete matrix were analyzed using a fully automated pore tester (poreMaster 60, Anton Paar, Ashland, Virginia (VA), USA), with pressure ranging from 0.1 to 214 MPa. Relevant pore structure parameters were then obtained.

To examine the micromorphology and hydration products, broken specimens were observed using a scanning electron microscope and energy dispersive spectroscopy (SEM/EDS ULTRA55, Zeiss Instruments, Ostfildern, Germany) after gold coating.

3. Results

3.1. Series One: Use CDW as Aggregates

The slump and slump flow of the recycled concretes are shown in Figure 5. As the proportion of brick-CDW aggregates increased, both the slump and slump flow of the recycled concrete decreased. As the relative proportions of brick-CDW and concrete-CDW reached to 20% and 80%, respectively, the slump and slump flow of recycled concrete were 260 mm and 530 mm, respectively. Compared to recycled concrete with a ratio of 0%/100% (brick-CDW/concrete-CDW aggregates), the workability of concrete changed slightly. When the ratio of brick-CDW to concrete-CDW aggregates reached to 60%/40%, the slump and slump flow were 5 mm and 200 mm, respectively, and the slump and slump flow were reduced by 80% and 64%, respectively. This decrease can be attributed to the higher water absorption of the brick-CDW aggregates compared to the concrete-CDW aggregates. As the proportion of brick-CDW aggregates increased, the water demand of the mixture also increased, leading to a reduction in the slump and slump flow of the recycled concrete.

The compressive strength of the recycled concrete is shown in Figure 6. As the proportion of brick-CDW aggregates in the recycled concrete increased, the compressive strength firstly increased and then decreased. When the volume ratio of brick-CDW to concrete-CDW reached 20%/80%, the maximum compressive strengths were observed as 33.3 MPa, 38.9 MPa, and 46.7 MPa for 3 days, 7 days, and 28 days, respectively. Compared to the mixture with concrete-CDW aggregates, the compressive strengths at 3 days, 7 days, and 28 days increased by 42.4%, 26.7%, and 24.9%, respectively.

Since the water absorption of brick-CDW aggregates is higher than that of concrete-CDW aggregates (Table 2 and Table 3), the actual water-to-binder ratio of the paste decreased when the proportion of brick-CDW aggregates increased. As the recycled concrete hardened, the paste developed a more compact microstructure while reducing the pore content. The densified microstructure contributed to improving the mechanical properties of the concrete [31,32], which led to the increase in compressive strength. In addition, the enhancement of strength was also attributed to the chemical activity of the brick-CDW aggregates. The waste bricks, containing rich reactive aluminosilicate compounds, showed pozzolanic reactivity [33]. These components reacted with the Ca(OH)₂ liberated from cement hydration in the slurry, generating secondary reaction products (C-S-(A)-H gel), which could contribute to compressive strength development. The help of the lower actual water-to-binder ratio and the chemical effect of the pozzolanic reaction synergistically led to the increase in the 3-day, 7-day, and 28-day compressive strength.

However, with a further increase in the proportion of brick-CDW aggregates, the effect of apparent and bulk density became more significant. The apparent and bulk density of the brick-CDW aggregates was lower than that of concrete-CDW aggregates (Table 2), indicating that the brick-based aggregates showed a lower strength when compared with concrete-based aggregates. As the content of brick-CDW aggregates further increased, the 3-day, 7-day, and 28-day compressive strengths of the recycled concrete gradually decreased.

3.2. Series Two: Use of CDW Powders as Mineral Admixtures

To investigate the impact of the matrix containing brick-CDW and concrete-CDW admixtures on the workability of recycled concrete, the effects of concrete-CDW admixture, brick-CDW admixture, and cement on slump and slump flow are shown in Figure 7a and Figure 7b, respectively. When the cement content was maintained at 40–70%, the dosage of brick-CDW admixture increased, and the concrete-CDW admixture decreased, while the slump and slump flow firstly increased and then decreased. For example, when the cement content was 70%, we increased the brick-CDW admixture from 0% to 40% and decreased the concrete-CDW admixture from 40% to 0%, and the slump increased from 55 mm to 230 mm and then decreased from 230 mm to 160 mm, while the slump flow increased from 240 mm to 455 mm and then reduced from 455 mm to 295 mm. Specifically, the dosage of brick-CDW and concrete-CDW admixture reached to 20%, respectively, the content of cement reached to 60%, and the highest slump and slump flow were 230 mm and 455 mm, respectively. This can be attributed to the particle packing effect of brick-CDW and concrete-CDW admixtures with the help of superplasticizer, which filled the spaces among particles, thereby reducing the water demand of the binders; consequently, the workability of recycled concrete improved under a constant water content. When the cement content was maintained at 70–100%, increasing the brick-CDW admixture and decreasing the concrete-CDW admixture led to an increase in slump and slump flow. For instance, when the cement content was 80%, the brick-CDW admixture increased from 0% to 20% and the concrete-CDW admixture decreased from 20% to 0%, the slump increased from 80 mm to 225 mm, and the slump flow increased from 280 mm to 395 mm. Specifically, when the content of cement was maintained at 70–80%, with the concrete-CDW admixture at 0–5% and brick-CDW admixture at 20–25%, the slump changed slightly.

The particle size distribution of the brick-CDW and concrete-CDW admixtures, shown in Figure 1, mostly ranged from 1 to 100 μm and was broader than that of cement. Some voids among cement particles were filled with particles from CDW admixtures, reducing the water demand for cementitious materials. Thus, when the cement content is maintained at 70–80%, with the concrete-CDW admixture of 0–5% and the brick-CDW admixture of 20–25%, the slump and slump flow might show positive change. However, the micromorphology of these admixtures, shown in Figure 3a,b, revealed that they contain some pores, and compared with cement, the water absorption rate of these two admixtures was higher. As the dosage of both CDW admixtures increased, the water absorption rate of the cementitious material became higher. Therefore, when the cement content was maintained at 40–70%, and the dosage of the two admixtures increased from 30% to 60%, the slump and slump flow of the recycled concrete gradually decreased.

The 3-day, 7-day, and 28-day compressive strengths of the recycled concrete containing brick-CDW and concrete-CDW admixtures are presented in Figure 8a, Figure 8b, and Figure 8c, respectively. As the dosage of brick-CDW and concrete-CDW admixtures increased, the compressive strength at all ages (3-day, 7-day, and 28-day) decreased. For example, when the dosage of concrete-CDW admixture increased from 0% to 30%, the dosage of brick-CDW admixture increased from 0% to 30%, and the cement content decreased from 100% to 40%, while the 3-day, 7-day, and 28-day compressive strengths decreased by 47.5%, 40.5%, and 49.2%, respectively. Notably, the dosage of CDW admixtures reached to 60%, and the relative proportion of brick-CDW admixture (relative to concrete-CDW) increased from 0% to 60%, while the 28-day compressive strength increased from 19.8 MPa to 29.0 MPa, by 46.5%; when the same dosage of CDW admixtures was incorporated into the paste, the relative proportion of brick-CDW admixture increased, and the 28-day compressive strength of the recycled concrete was higher, so brick-CDW demonstrated greater suitability for application as a mineral admixture in recycled concrete.

3.3. Series Three: Microstructure of Recycled Concrete

The microstructure between brick-CDW/concrete-CDW aggregate and the matrix is shown in Figure 9. When the brick-CDW aggregates were tightly bonded to the matrix, a significant amount of fibrous and flocculent hydration products appeared in the paste. These elemental compositions of the matrix are shown in Figure 9b and Figure 9d, respectively, which revealed that the matrix contained numerous hydration products. The interface between concrete-CDW aggregate and the paste exhibited a gap of approximately 5 μm, indicating a weaker bond between concrete-CDW aggregate and the paste. The water absorption rates of the brick-CDW fine and coarse aggregates were 34.0% and 35.0% (Table 2 and Table 3), respectively. Compared with the concrete-CDW aggregate, the water absorption rate of the brick-CDW aggregate was higher. Therefore, when the brick-CDW aggregate-to-concrete-CDW aggregate volume ratio reached 20%/80%, the porous brick aggregates absorbed the surrounding free water, effectively reducing the actual water-to-binder ratio. Concurrently, the abundant fine particles in the brick-CDW (smaller than 0.15 mm) generated a pozzolanic reaction. The amorphous siliceous and aluminous components from these fines reacted with calcium hydroxide (Ca(OH)₂) from cement hydration, generating additional products (such as calcium silicate hydrate (C-S-H) gels). These reaction products filled the micropores and created a robust bond between the brick-CDW aggregate and the cement paste, thereby densifying the microstructure of the interfacial transition zone [34]. Consequently, these led to an increase in the 28-day compressive strength of the recycled concrete.

The phase compositions of the pastes are shown in Figure 10a. When 60% of concrete-CDW admixture, 60% of brick-CDW admixture, and 30% of concrete-CDW and brick-CDW admixture were incorporated into the system, the diffraction peak of quartz and calcite enhanced significantly. As shown in Figure 2, brick-CDW and concrete-CDW admixture contained quartz and calcite, and the cement was replaced by brick-CDW and concrete-CDW admixture, so their crystallinity enhanced in the paste. The peak of Ca(OH)₂ decreased for paste containing CDW, and one reason of this result was that the cement was replaced by concrete-CDW and brick-CDW admixture, while the content of hydration products could reduce, which was unfavorable for the generation and crystallization of Ca(OH)₂, while the Ca(OH)₂ peak of the paste containing two types of CDW admixtures reduced. The other reason may be that CDW admixture contained an amorphous phase, which could react with Ca(OH)₂ to reduce the crystallinity of Ca(OH)₂ [18]. Furthermore, the main contents of concrete-CDW were natural aggregate and attached mortar, while the main components of brick-CDW were the bricks; the quartz and calcite components of concrete-CDW admixture could show better crystallization than those of the brick-CDW admixture, so the quartz and calcite peak of the paste containing 60% of concrete-CDW admixture was higher than that of paste containing 60% of brick-CDW admixture, and the quartz and calcite peak of the paste containing 30% of concrete-CDW admixture and 30% of brick-CDW admixture was higher than that of the paste containing 60% of brick-CDW admixture, but was lower than that of the paste containing 60% of concrete-CDW admixture. Therefore, when concrete-CDW and brick-CDW admixtures were incorporated into the paste, the hydration reaction of the paste weakened, the hydration product content decreased, the bonding capacity of the paste reduced, the unhydrated residual water increased, and the porosity of the paste increased after the residual water loss. These last two outcomes resulted in the decrease in 28-day compressive strength of concrete. When the same dosages of CDW admixtures were incorporated into the paste and the relative proportion of concrete-CDW and brick-CDW admixture changed, the Ca(OH)₂ crystallinity of the paste containing 60% of concrete-CDW admixture, 60% of brick-CDW admixture, and 30% of concrete-CDW and brick-CDW admixture changed slightly.

To further quantitatively analyze the content of hydration products of the paste containing brick-CDW and concrete-CDW admixture, the thermogravimetric curve and quantitative result were obtained, as shown in Figure 10. When the temperature ranged from 50 °C to 395 °C, the main peaks of the curves corresponded to ettringite (AFt), hydrated calcium silicate (C-S-H), or hydrated calcium aluminate silicate (C-(A)-S-H), and the dehydration percentages were as follows: control (8.98%) > concrete-CDW admixture 60% (8.05%) > brick-CDW admixture 60% (7.85%) > brick-CDW admixture 30% and concrete-CDW admixture 30% (7.58%). When 60% of the cement was substituted with concrete-CDW admixture, brick-CDW admixture, or both brick-CDW and concrete-CDW admixture, respectively, the degree of hydration reaction reduced, so the AFt, C-(A)-S-H, and C-S-H contents of the matrix containing these two types of CDW admixtures decreased. Additionally, concrete-CDW admixture contained C-S-H and C-(A)-S-H particles, which provided nucleation sites, contributing to the hydration reaction of the paste and promoting the growth of hydration products [35]. Consequently, when the same dosages of CDW admixtures were incorporated into the paste, the relative proportion of concrete-CDW (relative to brick-CDW) increased, and the hydration product content of the matrix containing 60% of concrete-CDW admixture was higher than that of the paste with 60% of brick-CDW admixture. Meanwhile, the hydration products of the matrix containing 30% of brick-CDW admixture and 30% of concrete-CDW admixture had undergone severe carbonization, so the hydration products of the matrix containing 30% of brick-CDW admixture and 30% of concrete-CDW admixture were lowest.

When the temperature was at 395–580 °C, the main peaks of the curves mainly represented the decomposition of Ca(OH)₂, with the following percentages: control (3.91%) > concrete-CDW admixture 60% (2.01%) > concrete-CDW admixture 30% and brick-CDW admixture 30% (1.90%) > brick-CDW admixture 60% (1.61%). Similarly, when these CDW admixtures were incorporated into the paste, the degree of hydration reduced, and the Ca(OH)₂ content of the paste containing two types of CDW admixture also decreased. Notably, when the same dosages of CDW admixtures were incorporated into the paste, and the relative proportion of concrete-CDW (relative to brick-CDW) increased, the hydration degree of the paste containing concrete-CDW admixture increased, so the Ca(OH)₂ content of the paste containing 60% of concrete-CDW admixture was higher than that of the paste containing 60% of brick-CDW admixture, and the Ca(OH)₂ content of the paste containing 30% of concrete-CDW admixture and 30% of brick-CDW admixture was lower than that of the paste containing 60% of concrete-CDW admixture, but higher than that of the paste containing 60% of brick-CDW admixture.

From 580 °C to 820 °C, the main decomposition was calcite, with the following percentages: control (4.76%) < brick-CDW admixture 60% (6.29%) < concrete-CDW admixture 30% and brick-CDW admixture 30% (7.32%) < concrete-CDW admixture 60% (11.18%). Because brick-CDW and concrete-CDW admixture contained calcite, when 60% of concrete-CDW admixture, 60% of brick-CDW admixture, and 30% of brick-CDW and concrete-CDW admixture were incorporated into the matrix, the content of calcite increased. Moreover, the major content of concrete-CDW was a natural aggregate and attached mortar, while the main component of brick-CDW was the sintered bricks, and the calcite components of concrete-CDW admixture could be greater than those of brick-CDW admixture [36]. Consequently, under the same dosage of CDW admixtures, when the relative proportion of concrete-CDW (relative to brick-CDW) increased, the content of CaCO₃ increased in the paste. The calcite content of the matrix containing 60% of concrete-CDW admixture was higher than that of paste with 60% of brick-CDW admixture, and the calcite content of the paste containing 30% of concrete-CDW admixture and 30% of brick-CDW admixture was lower than that of the paste containing 60% of concrete-CDW admixture, but higher than that of the paste containing 60% of brick-CDW admixture.

The decreased content of hydration products (such as C-S-H gel and AFt) coupled with an increased calcite content, leading to a decrease in the 28-day compressive strength of recycled concrete containing CDW admixtures. Moreover, the same dosages of CDW admixtures were incorporated into the paste, and the relative proportion of concrete-CDW (relative to brick-CDW) increased, benefiting the formation of hydration products. However, as demonstrated in Section 3.2, when the same dosages of CDW admixtures were incorporated into the paste, the relative proportion of brick-CDW admixture (relative to concrete-CDW admixture) increased, and the 28-day compressive strength of the recycled concrete increased. To further identify the reasons for the change in compressive strength of recycled concrete containing CDW, the pore structure was characterized by mercury intrusion porosimetry (MIP).

The pore structure parameters of the paste containing 60% concrete-CDW admixture, 60% brick-CDW admixture, and 30% of brick-CDW and concrete-CDW admixture are shown in Figure 11. The porosities of the paste containing 60% concrete-CDW admixture, 60% brick-CDW admixture, and 30% of brick-CDW and concrete-CDW admixture were 29.6%, 24.7%, and 28.2%, respectively. Compared to the paste containing 60% concrete-CDW admixture, the porosity of the paste containing 60% brick-CDW admixture and that with 30% of concrete-CDW and brick-CDW admixture decreased by 4.9% and 1.4%, respectively. The pore size distribution shifted towards the direction of less harmful and harmless pores, and the less harmful and harmless pores increased by 4.4% and 2.0%, respectively. As shown in Figure 1, compared with the particle size distribution of concrete-CDW admixture, brick-CDW admixture was wider, and the particles of brick-CDW admixture filled the voids of cement particles, improving the compactness of the paste. Consequently, when the same dosages of CDW admixtures were incorporated into the paste, the relative proportion of brick-CDW (relative to concrete-CDW) increased, and the pore structure of the paste improved, so the porosity of the matrix containing 60% of concrete-CDW admixture was higher than that of paste with 60% of brick-CDW admixture, while the porosity of the paste containing 30% of concrete-CDW admixture and 30% of brick-CDW admixture was lower than that of the paste containing 60% of concrete-CDW admixture, but higher than that of the paste containing 60% of brick-CDW admixture. Therefore, when the same dosages of CDW admixtures were incorporated into the paste, the 28-day compressive strength of the recycled concrete containing brick-CDW admixture increased, and concrete containing another CDW admixture decreased.

3.4. Series Four: Cost and CO₂ Emission Analysis

To validate the robustness and credibility of the cost and CO₂ emission assessments for recycled concrete incorporating CDW, costs and CO₂ emission factors of the raw material from four major Chinese regions were employed in the calculations. The results for these input parameters are presented in

Table 7. Cost of raw materials reported in different regions (CNY/t) [37,38,39,40,41].

Raw Materials	Chengdu	Beijing	Shanghai	Guangzhou
OPC	365	475	445	455
CDW aggregates	30	35	55	40
CDW admixtures	65	74	93	78

and

Table 8. CO₂ emission factors of each raw material in different regions (kg CO₂-eq/t) [42,43,44,45,46].

Type	Chengdu	Beijing	Shanghai	Guangzhou
OPC	735.0	745.0	745.0	735.0
CDW aggregates	19.0	6.5	7.3	7.8
CDW admixtures	147.0	135.0	135.0	136.0

.

The cost of the prepared recycled concrete with two types of CDW admixtures and aggregates was calculated using Equation (2):

$$\mathrm{Cos}t={\displaystyle \sum (M_i}\times C_i)$$

(2)

where $C_i$ and $M_i$ represent the price (CNY/t) and the proportion (t/m³) of each raw material used for preparing the recycled concrete, respectively.

Based on the Chinese standard [47], the CO₂ emissions for the prepared recycled concrete with CDW admixtures and aggregates was calculated using Equation (3):

$$C{O}_2 \mathrm{e}mission = {\displaystyle \sum (M_i}\times F_i)$$

(3)

where $F_i$ and $M_i$ represent the CO₂ emission content for each material (kg CO₂-eq/t) and each proportion of raw materials in recycled concrete (t/m³), respectively.

As shown in Figure 12 and Figure 13, when the dosage of brick-CDW and concrete-CDW admixture increased, the cost and CO₂ emissions decreased in four regions significantly. As the dosage of brick-CDW and concrete-CDW admixture reached to 60%, respectively, the cost and CO₂ emissions of recycled concrete were lowest, the result of recycled concrete containing brick-CDW and concrete-CDW admixture in four regions, as shown in Figure 14. As the dosage of brick-CDW and concrete-CDW admixture reached 60%, respectively, the lowest cost of the recycled concrete reached 175 CNY/m³ in Chengdu, 162 CNY/m³ in Beijing, 196 CNY/m³ in Shanghai, and 169 CNY/m³ in Guangzhou, and compared to the concrete (no CDW admixture), the cost decreased by 32.3% in Chengdu, 39.8% in Beijing, 32.6% in Shanghai, and 37.6% in Guangzhou; as the dosage of brick-CDW and concrete-CDW admixture reached to 60%, the lowest CO₂ emissions of the recycled concrete reached 210.9 kgCO₂-eq/m³ in Chengdu, 181.3 kgCO₂-eq/m³ in Beijing, 182.6 kgCO₂-eq/m³ in Shanghai, and 181.9 kgCO₂-eq/m³ in Guangzhou, and compared to the concrete (no CDW admixture), the CO₂ emissions decreased by 41.5% in Chengdu, 47.6% in Beijing, 47.4% in Shanghai, and 47.1% in Guangzhou. The sensitivity of the lowest cost and CO₂ emissions was analyzed in these four regions, as shown in Figure 14; the value of $\left|K_1\right|$ was close to 1, yielding the concrete cost variation rate of 1.0 CNY/m³ across different regions, and the regional differences had a negligible impact on the cost of the recycled concrete; the value of $\left|K_2\right|$ was approximately 10, resulting in a carbon emission variation rate of 10 kgCO₂-eq/m³. This result implied the regional influence on the carbon footprint of concrete, primarily attributable to differences in the energy consumption and transportation involved in CDW management, which led to variations in CO₂ emissions of the recycled concrete. Furthermore, the sensitivity analysis of recycled concrete containing CDW admixture regarding the benefit of CDW utilization remained widely applicable.

4. Conclusions

Concrete-CDW and brick-CDW were successfully used as aggregates with 100% replacement of natural aggregates and mineral admixtures with up to 60% replacement of OPC to prepare recycled concrete, and the effects of using these CDWs on workability, hardened performance, microstructure, cost, and CO₂ emissions of the concretes were investigated. The main conclusions are as follows:

(1)

The increase in portion of brick-CDW aggregates weakened the workability of the recycled concrete.

(2)

When the volume ratio of brick-CDW aggregate to concrete-CDW aggregate was 20%/80%, the optimal strength was obtained, and the 28-day compressive strength increased by 24.9%.

(3)

With the dosage of both types of CDW admixtures maintained at 0–20%, respectively, the slump and slump flow changed slightly.

(4)

The 28-day compressive strength increased with an increase in relative proportion of brick-CDW admixture.

(5)

The increase in the 28-day compressive strength can be attributed to the improved microstructure of the interfacial transition zone between brick-CDW aggregate and paste, as well as the refined pore structure caused by the brick-CDW admixture.

(6)

After concrete-CDW and brick-CDW admixture were incorporated into the recycled concrete, the cost and CO₂ emissions could decrease by 40.1% and 47.6%, respectively.

When CDWs were employed as aggregates, if the volume ratio of brick-CDW content exceeded 20%, the workability and compressive strength of recycled concrete declined. The 20%/80% volume ratio of brick-CDW aggregate to concrete-CDW aggregate yielded the highest compressive strength. When CDW was used as an admixture, the workability of recycled concrete reduced at high incorporation levels. As the content of CDW admixture increased, the 28-day compressive strength generally decreased. Compared to concrete-CDW, brick-CDW admixture contributed to a high compressive strength.

Further pilot tests with our industry partners to verify the optimal concrete mix proportions for repeatability and reproducibility will be carried out in our following study, with long-term performance investigations to promote this green concrete in sustainable construction.