Pore Structure, Hardened Performance and Sandwich Wallboard Application of Construction and Demolition Waste Residue Soil Recycled Foamed Concrete

As shown in Figure 5a, the dry density of the unmodified foamed concrete containing CDWRS was 580 kg/m³, indicating that the target design density of 600 kg/m³ was achieved. However, as shown in Figure 5b, the compressive strengths at various ages were extremely low, due to the introduction of a high content of CDWRS (1126 kg/m³, as shown in Table 3) for the sustainability of city development; therefore, modification based on the characteristics of CDWRS was necessary. The water glass was firstly chosen to enhance the strength, because it often used to destroy and reconstruct the main mineral phases in CDWRS [58,59]. As shown in Figure 5, the dry density of the foamed concrete changed slightly, and the thermal conductivity also rose slightly with the increasing dosage of water glass; the compressive strength increased firstly and subsequently decreased. When the content of the water glass rose from 0 to 68 kg/m³, the dry density of the sample slightly varied from 528 kg/m³ to 596 kg/m³, and the value of thermal conductivity changed from 0.08 W/(m·K) to 0.11 W/(m·K). The compressive strength was the largest when the dosage of the water glass was 51 kg/m³. Compared with blank foamed concrete (water glass dosage of 0 kg/m³), the compressive strength of the foamed concrete (51 kg/m³) at 7 days, 28 days, and 56 days increased by 460.0%, 144.4%, and 118.2%, respectively. The improvement in early strength was obvious. This enhancement of strength at all ages can be attributed to the destruction of the mineral phases (such as the clay-based minerals in CDWRS) and hydration product generation in the curing process because water glass was widely used in the modification of clay-based materials for destroying structure and reconstructing networks of hydration products [60,61,62]. However, the water in the water glass would increase the content of free water in the system when the content of the water glass further rose, thus weakening the mechanical performance of the foamed concrete [63].

As mentioned above, 51 kg/m³ of water glass in the CDWRS recycled foamed concrete system can improve strength, but it was still low; however, gypsum was used as an activator to further modify the foamed concrete. As shown in Figure 6, with gypsum varying from 0 kg/m³ to 12.5 kg/m³, the dry density and thermal conductivity of the foamed concrete slightly changed, and the relationship between dry density and thermal conductivity maintained a good corresponding correlation. When the dosage of gypsum rose from 0 kg/m³ to 12.5 kg/m³, the dry density of the sample varied between 544 kg/m³ and 626 kg/m³, and the thermal conductivity slightly changed from 0.09 W/(m·K) to 0.11 W/(m·K). For compressive strength, when the content of gypsum increased from 0 kg/m³ to 7.5 kg/m³, the strength of the sample decreased firstly and then increased, but a further increase in gypsum (12.5 kg/m³) reduced the strength of the foamed concrete. The largest value of compressive strength was achieved when the dosage of gypsum was 7.5 kg/m³. Compared with the sample with a gypsum dosage of 0 kg/m³, the compressive strength at 7 days, 28 days, and 56 days was 0.44 MPa, 0.55 MPa, and 0.60 MPa, and increased by 57.1%, 25.0%, and 25.0%, respectively. The primary reason for this improvement was the activity activation of slag through introducing gypsum. Lots of ettringite and hydrated calcium silicate were generated, which guaranteed the strength improvement. However, a low content or high content of gypsum would cause the strength to decrease because a low content of gypsum would lead to weak generation of hydration products. Phase transition or dissolution due to excess gypsum could also cause strength reduction.

Based on the optimal dosages of water glass and gypsum obtained above, modified CDWRS recycled foamed concretes with various densities (density grade, 600 kg/m³ to 900 kg/m³) were designed and prepared with the same volume replacement percentage of foam. The content of the original CDWRS was extremely high and changed from 988 kg/m³ to 1126 kg/m³, thus contributing to a large consumption of waste for sustainable development. In this situation, the dry density of the modified sample changed from 948 kg/m³ to 626 kg/m³, and the density of the original sample decreased from 928 kg/m³ to 580 kg/m³ (Figure 7). As shown in Figure 7 and Figure 8, both the thermal conductivity and the compressive strength of the foamed concrete rose with the increase in the dry density of foamed concrete. Specifically, as the dry density of the modified foamed concrete increased from 626 kg/m³ to 948 kg/m³, the thermal conductivity slightly increased from 0.11 W/(m·K) to 0.14 W/(m·K), and the compressive strength at 7 days, 28 days, and 56 days rose from 0.44 MPa to 4.96 MPa, 0.55 MPa to 7.76 MPa, and 0.60 MPa to 8.01 MPa, respectively. For the recycled foamed concrete at the same density grade, the thermal conductivity and compressive strength increased, after the modification of activators. For instance, when the design density grade was 700 kg/m³, the actual dry density of the original and modified samples was 682 kg/m³ and 688 kg/m³, the thermal conductivity was 0.10 W/(m·K) and 0.11 W/(m·K), and the 28-day strength was 0.78 MPa and 1.3 MPa, respectively. More importantly, the thermal conductivity of the modified sample was much lower than the upper limit of the thermal conductivity of the foamed concrete in Chinese standard, JG/T 266-2011 [53], showing great potential in the application of building thermal insulation. This also indicated that the addition of water glass and gypsum not only enhanced the compressive strength of foamed concrete but also ensured the necessary thermal insulation performance.

Figure 9 presents the appearance of foamed concretes. There is no significant difference between these samples, except that the sample at low density was light red because of the high content of CDWRS. To detect the difference in microstructure, the air-void image of the foamed concrete was obtained using an optical microscope, as shown in Figure 9. A modifier, such as water glass, will change the liquid environment and generate an adverse effect on the stability of the air bubble; therefore, many bubbles in the foamed concrete were broken to form big bubbles, which finally led to the formation of bigger voids in the modified foamed concrete. However, when the design density of the foamed concrete increased, the content of the binder (Table 3) rose and the hydration accelerating effect of the modifier (water glass) played the main role. The change in the air bubbles in the fresh unmodified foamed concrete due to thermodynamic instability was highly limited, forming smaller voids in the high-density modified CDWRS recycled foamed concrete (such as density ≥700 kg/m³). To quantitatively evaluate the change in the macroscopic air-void structure, cumulative air-void size distribution was obtained using the image analysis method. As shown in Figure 10a, the air-void content of ³ promoted the air bubbles to enlarge, this also reflected that the adverse effect (as mentioned above) of the modifier on the stability of the air bubble was fully shown in low-density foamed concrete (600 kg/m³), and this disproportionation phenomenon can simultaneously increase the extremely small void content [64,65]. It is mainly caused because the air bubbles were all in a thermodynamically unstable state; therefore, small bubbles were merged into big bubbles, and this state was stopped by the setting and hardening of the cement matrix. When the density was low and the modifier was added, the low content of the binder meant that there was a prolonged cement setting time. This adverse effect accelerated the merging of the air bubbles (disproportionation phenomenon), forming many extremely big voids and extremely small voids in the modified foamed concrete [66,67]. However, under a high density, the acceleration effect of hydration, due to the high content of the binder (Table 3), dominated the whole hydration process. The setting and hardening of the cement matrix were shortened, and the age of the air bubble merging was reduced; therefore, the merging phenomenon was weakened, finally forming more small voids in the modified foamed concrete, as shown in Figure 10b–d.

Figure 11 presents the results of the mercury intrusion porosimetry, which reflects the microscopic pore size distribution and the cumulative pore content of the recycled foamed concrete in different dry densities. When the foamed concrete was modified by the water glass and gypsum, ettringite and newly-formed hydration products generated and occupied part of the space of the pores and voids, causing the slight reduction in cumulative pore volume, as shown in Figure 11. Specifically, the cumulative pore volume decreased from 1.50 mL/g to 1.03 mL/g, when the design density of the foamed concrete was 600 kg/m³ and modifiers were added. A slight decrease in the cumulative pore volume in the modified foamed concrete also occurred at a design density of 700 kg/m³, 800 kg/m³, and 900 kg/m³, which changed from 0.76 mL/g to 0.62 mL/g, 0.52 mL/g to 0.49 mL, and 0.43 mL/g to 0.39 mL/g, respectively. Due to the same fact, the microscopic pore was refined, and the percentage of the pore dropped at a large size range of the pore. For example, when the design density was 600 kg/m³, the peaks at 10~50 μm and 2 μm on the pore size distribution curve became lower, compared with the blank foamed concrete (unmodified foamed concrete in Figure 11a), the small pores, such as the pores from 10 nm to 400 nm, increased significantly. According to references [64,65,66,67], these microscopic pores also can be classified into small capillary pores (100 nm), as shown in Figure 12. Remarkedly, the large pores reduced under modification, for example, when the density was 600 kg/m³. This content of pore decreased from 1.36 mL/g to 0.77 mL/g under modification. These big pores transferred into smaller pores, such as capillary pores, because the contents of the small, medium, and large capillary pores all rose at a density of 600 kg/m³, and the contents were 0.12 mL/g, 0.14 mL/g, and 0.11 mL/g, respectively. For a higher density of sample (700 kg/m³), the content of the large capillary pore increased by 71.4% after being modified. At a density of 800 kg/m³, the medium and large capillary pore were also refined into a small capillary pore, and the content of this small pore increased to 0.06 mL/g. For the modified foamed concrete with a density of 900 kg/m³, all the capillary pores increased, as shown in Figure 12. These facts all attributed to the refinement of newly-formed hydration products, because they occupied part of the space of the pores.

The Fourier transform infrared spectrum (FIIR) of foamed concrete was used to detect the composition, as shown in Figure 13. The band at 3429 cm⁻¹ was associated with the Al-OH stretching vibration of ettringite, indicating that ettringite existed in the foamed concrete [64]. The peak at 1633 cm⁻¹ represented the bending vibration of the hydroxyl groups in crystalline water. The bands near 1420 cm⁻¹ and 873 cm⁻¹ arose from O-C-O stretching vibrations, which were associated with the presence of carbonates [64]. The peak near 466 cm⁻¹ represented the bending vibrations of Si-O in the raw material, and the peak for the Si-O-T (T for Al or Si) vibrations in the sample was found at 1016 cm⁻¹ [65]. The bands near 782 cm⁻¹ and 527 cm⁻¹ corresponded to Si-O-Si and Si-OH [64,65]. To detect the detailed information of the hydration products, the XPS results were obtained, as shown in Figure 14 and Figure 15. Na existed in the cement. For the low-density sample, the Na in the foamed concrete could not be easily found, but the Na from the modifier was brought into the foamed concrete system after being modified. Due to high content of cement, the Na also could be found in the foamed concrete at a high density [68,69]. For the results of Figure 15a–c, the binding energy showed a slight change after being modified. For example, the peak in the Si 2p spectra shifted from a high binding energy to a low binding energy after being modified, which indicated that the silica chain length reduced. This suggests a depolymerized structure, which is attributed to the fact that the modifiers destroyed the polymerized silicate structure and gradually formed newly-formed hydration products with a low polymerization degree; the same trend was also observed in the spectra of Al 2p [70]. The Ca from various hydration products (such as CaCO₃ or Ca(OH)₂) or raw materials (CaSO₄·2H₂O) showed close peak in Ca 2p of spectra; however, this result could reflect that these products tended to decompose and contributed to the formation of low-polymerized gel, which has also been confirmed in the spectra of Si 2p and Al 2p in Figure 15 [71].

Modified CDWRS recycled foamed concrete showed an excellent thermal insulation performance, and the strength characteristics indicated that it can be used as an insulation material in the sandwich wallboard, which may contribute to energy savings of buildings for promoting the sustainable development of society. Importantly, sandwich structure balanced the safety and insulation performance, giving enough possibility to consume CDWRS, which was also important for the sustainability of city development. Meanwhile, similarly to references [72,73], this structure can prevent water from entering the foamed concrete. Figure 4b presents the structural style of the sandwich wallboard, and the performance of this wallboard is shown in Table 5. When CDWRS recycled foamed concretes with various dry densities were applied in this sandwich wall, the surface density, compressive strength, and heat transfer coefficient of the sample rose as the density of the foamed concrete increased. The compressive strength and heat transfer coefficient of the wallboard were closely related to the strength and thermal conductivity of the foamed concrete in the sandwich wallboard. When the thermal conductivity and strength of the foamed concrete increased, the related strength and heat transfer coefficient of the wallboard rose. Specifically, as the dry density of the foamed concrete increased from 626 kg/m³ to 948 kg/m³, the surface density of the wallboard increased from 286 kg/m² to 362 kg/m², and the compressive strength of the wallboard increased from 16.5 MPa to 24.6 MPa. When the surface density of the sandwich wallboard increased from 286 kg/m² to 362 kg/m², the wet heat transfer coefficient increased from 0.86 W/m²·K to 1.16 W/m²·K, and the dry heat transfer coefficient increased from 0.75 W/m²·K to 1.01 W/m²·K.

When this sandwich wallboard was applied in the building, as shown in Figure 16, the energy consumption and change in energy saving efficiency of the building in typical hot-summer/cold-winter areas was simulated using Energy Plus software (Cond FD). The density of the recycled concrete was 2110 kg/m³, the thermal conductivity and specific heat were 0.99 W/(m·K) and 1120 J/(kg·K), respectively. When the surface density of the wallboard increased from 286 kg/m² to 362 kg/m², the energy consumption of the building in Chongqing changed from 723.1 MJ/m² to 732.7 MJ/m², the energy consumption of the blank building (concrete wall) was 809.8 MJ/m², and the energy saving efficiency improved by 9.5~10.7%. When the building was in Wuhan (a typical city of the Hubei province), the energy consumption of the building changed from 850.3 MJ/m² to 862.2 MJ/m², the energy consumption of the blank group (concrete wall) was 956.5 MJ/m², and the energy saving efficiency rose by 9.8~11.1%. Nanjing is a typical city of Jiangsu province, and the building in this area generated 849.6~860.6 MJ/m² of energy consumption. The energy-saving efficiency rose by 10.7~11.8% when the density of the core-foamed concrete increased from 626 kg/m³ to 948 kg/m³. The energy consumption of the building in Changsha (Hunan Province) changed from 797.9 MJ/m² to 804.3 MJ/m², and the energy saving efficiency increased by 12.9~13.6%. The energy consumption of the building in Hangzhou (Zhejiang province) changed from 792.0 MJ/m² to 802.3 MJ/m², and an improving rate of energy-saving efficiency of approximately 9.98~11.13% was achieved when the wallboard was applied in the building. The energy consumption of the sandwich wallboard in hot-summer/cold-winter areas rose as the surface density of the wallboard increased (286 kg/m² to 362 kg/m²); however, compared with the concrete wall in the building, the energy-saving efficiency of the building can significantly improve.

CO₂ emissions was obtained by transforming the energy consumption of buildings into the content of standard coal, and 1 kg of this coal was equal to a CO₂ emission coefficient of 2.46 [57]. After acquiring the content of the CO₂ emissions, the CO₂ emissions reduction of the building using the sandwich wallboard in the hot-summer/cold-winter areas is shown in Table 6. With the increase in the surface density of the wallboard, the CO₂ emissions reduction gradually decreased, and the content of the CO₂ emissions increased, but this emission content was significantly smaller than that of the blank building (concrete wall, control). When the surface density of the wallboard changed from 286 kg/m² to 362 kg/m², the CO₂ emission reduction of the building in Chongqing was 6.49~7.29 kg/(m²·year), compared with control group, this reduced value of the buildings in Wuhan, Nanjing, Changsha, and Hangzhou was 7.91~8.91 kg/(m²·year), 8.63~9.56 kg/(m²·year), 10.06~10.59 kg/(m²·year), and 7.48 ~8.35 kg/(m²·year), respectively. Thes data indicate that the sandwich wallboard could reduce the CO₂ emissions of the building in hot-summer/cold-winter areas. Due to a comparison of the CO₂ emission result of the building in various cities, the reduction phenomenon of CO₂ emissions while using this wallboard was universal. The reduced CO₂ emissions can be attributed to the energy-saving ability of the foamed concrete due to the low thermal conductivity. Simultaneously, the increase in density caused relatively large thermal conductivity, weakening the ability of CO₂ emission reduction. However, the recycled foamed concrete maintained an excellent energy-saving ability, giving strong confidence for CDWRS consumption in sandwich wallboards, and also contributing to the sustainable development of cities and buildings.