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Article

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

by
Fengyuan Yang
1,
Chenxi Yang
1,
Chao Jin
2,
Tie Liu
2,
Renshuang Li
1,
Jun Jiang
1,*,
Yanping Wu
2,
Zhongyuan Lu
1,* and
Jun Li
2
1
State Key Laboratory of Environment-Friendly Energy Materials, School of Materials and Chemistry, Southwest University of Science and Technology, Mianyang 621010, China
2
Ningbo Construction Engineering Group Co., Ltd., Ningbo 345040, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(6), 2308; https://doi.org/10.3390/su16062308
Submission received: 5 January 2024 / Revised: 6 February 2024 / Accepted: 29 February 2024 / Published: 11 March 2024
(This article belongs to the Special Issue Porous Materials for Sustainable Futures)
Browse Figures
Versions Notes

Abstract

:
Construction and demolition waste residue soil (CDWRS) recycled foamed concretes were prepared by introducing the original CDWRS into modified binders. Pore structure, hardened performance, and sandwich wallboard application were also investigated. The results indicated that 51 kg/m3 of water glass and 7.5 kg/m3 of gypsum could significantly increase the strength and generate a slight influence on the thermal insulation performance of CDWRS recycled foamed concrete. The largest enhancing rate of 28-day compressive strength at a density of 600 kg/m3 could reach 205.5%. Foamed concrete with 1126 kg/m3 of CDWRS, modified with water glass and gypsum, showed a low thermal conductivity of 0.11 W/(m·K) and a dry density of 626 kg/m3. In total, 988 kg/m3 of CDWRS in foamed concrete led to a compressive strength of 7.76 MPa, a thermal conductivity of 0.14 W/(m·K), and a dry density of 948 kg/m3. Utilization of the foamed concrete in the sandwich structure could fabricate energy-saving wallboards with a minimum heat transfer coefficient of 0.75 W/(m2·K) and a relatively high compressive strength of 16.5 MPa, providing great confidence of CDWRS consumption in the building energy-saving field.

1. Introduction

Energy is the basis of human survival, but energy consumption is increasingly large in nations across the world with developing societies. Approximately 35% of the energy consumption is in the field of building, which has been one of the most energy-intensive fields, severely threatening the sustainability of human society [1,2]. Applying porous insulation materials in building envelopes was one of the most important approaches to save energy and reduce CO2 emissions, thus contributing to the sustainable development of society [3,4]. Due to the low cost, fire safety, and good durability, porous cement-based materials have been rapidly developed and widely applied, and the main methods to form pores for lightweight and porosification include: (a) adding pre-fabricated foam into cement matrices or mortar; this cement-based material is referred to as foamed concrete [5]; (b) introducing H2O2 or aluminum powder into cementitious system; this kind of material is often called aerated concrete or cement [6]; (3) substitution of lightweight aggregate for cement matrix; these materials are regarded as lightweight aggregate concretes [7]. All of them, due to rich pores, are used in the insulation field. Foamed concrete, as one of the typical porous cement-based materials, can be produced at construction sites or in factories, due to excellent workability, which has been developing as one of the most important insulation materials [8,9,10]. However, compared with organic insulation materials, its insulation performance was relatively poorer [11,12]; therefore, the ability for promoting the sustainable development of society was relatively insufficient. Seeking suitable approaches and technologies to improve the insulation performance of foamed concrete is becoming urgent, attracting plenty of attention.
To solve this crucial problem, air-void structure optimizations, such as the refinement of air voids and the improvement of porosity, have been studied. These results have indicated that the approaches could be used to improve functional performance [13,14]. For example, the formation of hexagon-shape voids instead of spherical pores could reduce the thermal conductivity of foamed concrete, and high porosity could also result in a significant reduction in the thermal conductivity. Thermal conductivities of 0.04~0.06 W/(m·K) could be obtained at 88.5~95.4% porosity, but high porosity means a low content of the solid phase in foamed concrete and a low heat capacity, thus weakening the thermal comfort of indoor building and the mechanical strength [15]. To improve the mechanical strength and reduce the thermal conductivity of foamed concrete, many approaches have been conducted and investigated, as shown in Table 1 [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]; however, these approaches could not significantly enhance insulation performance under suitable mechanical strength [35,36]. Other approaches were also developed under the suitable content of the cement matrix (suitable dry density). For instance, decreasing the thermal conductivity of the cement matrix in foamed concrete by prolonging the curing time was also effective, but the reduction value was relatively small and could not dramatically promote the sustainable development of building. For example, Batool et al. [37,38] showed that a maximum of 0.04 W/(m·K) could be realized by extending the curing time from 60 days to 300 days. To dramatically improve thermal insulation performance at the same density or porosity, Jiang et al. [39,40] proposed and successfully achieved the formation of nanopores in a cement matrix, which led to a significant reduction in the thermal conductivity of foamed concrete under the Knudsen effect. The related approaches were regarded as promising ways to narrow the insulation performance gap between organic materials and foamed concretes, which may be significative in the sustainable development of building in the future. However, aerogel or high-purity montmorillonite were used as nanopore-forming agents to prepare these foamed concretes with rich nanopores, and the cost of these nanopore-forming agents was high; therefore, seeking low-cost pore-forming agents is urgent.
Approximately 2.0 billion tons of construction and demolition waste (CDW) per year was produced in China, and the United States produced 600 million tons of CDW per year, while Europe produced 450–500 million tons of CDW per year [41,42]. Currently, CDW has resulted in a worldwide crisis affecting human survival and sustainable development, because many severe environmental problems, such as the “garbage siege” phenomenon [43,44], have happened frequently. Therefore, the full utilization of CDW was widely conducted. Usually, CDW consists of rebar, waste glass, plastic, concrete, brick, mortar, and residue soil. For waste rebar, waste glass, and plastic, these were picked out for reproduction, which has solved their sustainability problem. Waste concrete, brick, and mortar were used to produce mineral admixtures and aggregates, which can also be fully consumed in building materials. For instance, Atyia et al. [45] utilized waste bricks as lightweight aggregates to successfully fabricate structural lightweight concrete, and Abuellella et al. [46] showed that natural aggregates could be fully replaced by waste bricks and concretes to produce concretes with various strength grades. However, construction and demolition waste residue soil (CDWRS) was the largest waste in construction and demolition waste. Approximately 0.3 billion steres of CDWRS per year was generated in China, most of them accumulated and largely occupied land, resulting in many environmental problems and threatening the sustainable development of society, such as the leaching of harmful substances. More importantly, the collapse of CDWRS dams highly threatened the life and property safety of human beings [47,48,49]; therefore, it was urgent to deal with CDWRS. For CDWRS, aluminosilicate minerals, such as montmorillonite, were the main mineralogical phases, showing great potential for use as nanopore-forming agents, to strengthen the thermal insulation performance of foamed concrete [39,50,51,52], and contribute to the energy saving of building and the reduction of CO2 emissions, which may be very vital in the sustainable development of society. Meanwhile, the successful utilization might also provide a promising approach to consume CDWRS, which could relieve environmental stress caused by waste accumulation, also contributing to sustainability of city development.
Motivated by the large consumption of CDWRS, a high content of original construction and demolition waste residue soil (CDWRS) was used to fabricate recycled foamed concretes; pore structure, hardened performance, and sandwich wallboard application were also investigated in this work. The results of this work may provide an effective approach to prepare low-cost insulation materials for improving building energy-saving efficiency and a promising way to largely consume CDWRS for the reduction of environmental stress, finally contributing to the sustainable development of cities and buildings.

2. Materials and Methods

2.1. Raw Materials

Ordinary Portland cement (OPC, P·O 42.5R, from local cement plant), fly ash (Grade Ⅱ, from Ningbo Ailipu Building Material Co., Ltd., Ningbo, China), and slag (Grade S95, from Hongyuan Building Materials Co., Ltd., Emei, China) were used as binders of concrete in this study. The modifiers were water glass (1.5 of modulus) and gypsum, which were respectively purchased from Xinjie Chemical Co., Ltd. (Mianyang, China) and Tongqing Nanfeng Co., Ltd. (Meishan, China). A protein-based foaming agent (Ketai Building Materials Co., Ltd., Linyi, China, dilution ratio of 1:20 by weight) was used to produce pre-fabricated foam (wet density of 30 kg/m3). The water reducer was a high-efficiency polycarboxylate superplasticizer with a solid content of 36.0%, which was from Xinxiang Jiesheng Admixture Co., Ltd., Xinxiang, China. CDWRS was from the construction site of Ningbo City, it had a density of 1300 kg/m3 and a solid content percentage of 42.5%. The appearance of CDWRS is shown in Figure 1. The chemical components and mineral phases of these raw materials are shown in Table 2 and Figure 2, respectively. The main phases in CDWRS were montmorillonite, quartz, muscovite, and calcite.

2.2. Sample Preparation

Similarly to the montmorillonite in reference [39], CDWRS was also used as pore-forming agent, and the pores generated by CDWRS were used to replace air voids in the pre-fabricated foam. To largely consume CDWRS and enhance research significance in sustainability, the volume replacement rate of air bubbles by CDWRS was set as 90.0% to fabricate all foamed concretes. However, due to the introduction of CDWRS in the foamed concrete, the solid content would increase due to the solid content in CDWRS; therefore, the binder content of foamed concrete containing CDWRS was reduced, and the reducing content was equal to the solid content of the introduced CDWRS. Based on these considerations, the mix proportion of the foamed concrete was calculated, as shown in Table 3.
As shown in Figure 3, pre-fabricated foam with a density of 30 kg/m3 was prepared using an air compressor, before the preparation of foamed concrete. According to the mix proportion (Table 3), CDWRS binder (cement:slag = 1:1), CDWRS slurry, and the modifier were stirred using a mortar mixer to prepare cement paste. The pre-fabricated foam was then added into the paste until uniform foamed concrete paste was obtained. Subsequently, the paste was immediately placed into molds and covered with plastic film. After a 1-day curing, the molds were removed and placed in a standard curing environment (temperature 20 ± 2 °C, relative humidity ≥ 95% RH) for curing. For the fabrication of sandwich wallboard (Figure 3), fresh CDW recycled concrete of C40 grade was firstly prepared (the mix proportion is shown in Table 4) and put into a mold. The unmolded foamed concrete was then placed at the surface of the fresh concrete. Subsequently, residual fresh C40 concrete was filled into the whole mold and covered by a plastic film. After a 1-day curing, the sample was unmolded and placed in a standard curing environment (temperature 20 ± 2 °C, relative humidity ≥ 95%RH) for curing.

2.3. Test Methods

Based on the Chinese standard, JG/T 266-2011 [53], three samples at 28 days for each mixture were placed into a drying oven and dried to a constant weight at 60 ± 5 °C. Subsequently, the sample sizes were measured to obtain the volume, and the dry density was calculated using the mass and volume.
Sample pretreatment for the density test was also used for the compressive strength test, and the loading rate for all foamed concretes was 500 N/s, according to JG/T 266-2011 [53]. The equipment was a micro-controlled electronic universal testing machine (SANS, CMT5105, Shenzhen, China). For the strength of the wallboard, the loading rate was 0.1 Mpa/s, according to the Chinese standard, GB/T 30100-2013 [54], and the related equipment was a similar universal testing machine (SANS, YL64, Shenzhen, China).
According to the Chinese standard, GB/T 32064-2015 [55], the dried sample for dry density measurement was also used for the thermal conductivity test. Before this measurement, the sample was processed to obtain a flat surface, and the surface was coated with a silicone thermal grease. Subsequently, the sensor was clamped by these sample surfaces, and the transient plane source method was used to measure the thermal conductivity of the sample, using the equipment of DRE 2C (Xiangtan Xiangyi Instrument Co., Ltd., Xiangtan, China). For the heat transfer coefficient test, the wallboard (1000 mm × 1000 mm × 200 mm) was placed into a temperature-controlled box, and the equipment of JTRG-I (Beijing Jantytech Co., Ltd., Beijing, China) was used to obtain the result, based on the Chinese standard, GB/T 34342-2017 [56].
For the composition characterization of foamed concrete, the sample was firstly dried at a temperature of 60 ℃. The dried sample was then grinded until all particles were lower than 80 μm. Subsequently, these particles were used to obtain Fourier transform infrared spectroscopy curves using a SPECTRUM ONE (PerkinElmer, Waltham, MA, USA) in range 4000–400 cm−1. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA; X-ray source operating at 12 kV and 6 mA) with a monochromatic Al Kα at 1486.6 eV was also used to detect the characteristics of the foamed concrete.
To obtain an air-void structure, the dried sample for the dry density test was cut open, and the center of this open sample surface was imaged using an optical microscope (Dongguan Bosheng BC200-3 electron microscope, China). This image was then processed based on the same operations (dilation, erosion, opening, closing, and hole-filling), and each image was digitized and converted into a binary form. After these steps were completed, the air voids in the images were identified using their own software (version 6.0), to obtain the void structure parameters, and the air-void size distribution was further analyzed and plotted using Excel. For the characterization of the microscopic pore structure, the same dried sample was broken into particles (about 5 mm). These particles were then transferred to dilatometers, and tested using an automated pore tester (PoreMaster 60, Anton Paikanta, Boynton, FL, USA), with a pressure of 0.1~31,000 psi, and the related pore-structure parameters were obtained.
For the simulation of the energy consumption of building, a representative building model was established and is shown in Figure 4. Five typical hot-summer/warm-winter zones (Changsha, Nanjing, Hangzhou, Wuhan, and Chongqing) were chosen to acquire building energy consumption. After these assumptions were set, Energy Plus software (version 9.5.0, Cond FD) was used to obtain the value of energy consumption. Compared with the concrete wall, the energy consumption of the building was converted into CO2 emissions; 1 kg of standard coal can release an energy of 29.27 MJ, which was equal to the CO2 emission coefficient of 2.46, based on the Chinese standard, GB/T 2589-2020 [57]. The structure of the sandwich wallboard is shown in Figure 4.

3. Results and Discussion

3.1. Modification of CDWRS 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.

3.2. Hardened Performance of CDWRS Recycled Foamed Concrete

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.

3.3. Microstructure of CDWRS Recycled Foamed Concrete

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 <0.2 mm in the modified foamed concrete increased. When the air-void size was 0.2–0.4 mm, the cumulative void percentage increased more slowly after the foamed concrete was modified, which reflected that the air-void content reduced. When this size was larger than 0.4 mm, the increasing rate of these pores rose more rapidly than that of the unmodified sample. These two facts indicated that a modification of the foamed concrete of 600 kg/m3 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 (<10 nm), medium capillary pores (10–50 nm), large capillary pores (50–100 mm), and large 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].

3.4. Application of CDWRS Recycled Foamed Concrete in Sandwich Wallboard

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.

4. Conclusions

In this study, construction and demolition waste residue soil (CDWRS) recycled foamed concretes were successfully prepared and modified using water glass and gypsum. Pore structure, hardened performance, and sandwich wallboard application were also investigated. The main conclusions are as follows.
(1)
Water glass and gypsum could enhance the compressive strength of CDWRS recycled foamed concrete, and generated a slight influence on thermal insulation performance.
(2)
Water glass and gypsum increased the content of small air voids and microscopic pores, but also increased the percentage of big air voids in low-density CDWRS recycled foamed concrete, due to the adverse effect on stability.
(3)
Foamed concrete with a dry density of 626~948 kg/m3, a thermal conductivity of 0.11~0.14 W/(m·K), and a 56-day compressive strength of 0.60~8.01 MPa could be obtained by adding a large content of CDWRS, water glass, and gypsum.
(4)
Utilization of CDWRS recycled foamed concrete in sandwich insulation structures could fabricate a wallboard heat transfer coefficient of 0.75~1.01 W/(m2·K) and a compressive strength of 16.5~24.6 MPa, which could save building energy consumption and reduce CO2 emissions, showing great potential in consuming CDWRS and a good environmental benefit. These facts indicate that CDWRS consumption in foamed concrete contributed to the sustainable development of cities and buildings.

Author Contributions

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

Funding

This research was funded by Ningbo Science and Technology Planning Project, grant number 2022T003, and Natural Science Foundation of Sichuan Province, grant number 2022NSFSC1135.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Conflicts of Interest

Authors Chao Jin, Tie Liu, Yanping Wu and Jun Li were employed by the Ningbo Construction Engineering Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Appearance of dried CDWRS.
Figure 1. Appearance of dried CDWRS.
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Figure 2. XRD patterns of raw materials for CDWRS recycled foamed concrete and sandwich wallboard.
Figure 2. XRD patterns of raw materials for CDWRS recycled foamed concrete and sandwich wallboard.
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Figure 3. Manufacturing process of CDWRS recycled foamed concrete and wallboard.
Figure 3. Manufacturing process of CDWRS recycled foamed concrete and wallboard.
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Figure 4. (a) Model of building using the sandwich wallboard; (b) structural form of the sandwich wallboard.
Figure 4. (a) Model of building using the sandwich wallboard; (b) structural form of the sandwich wallboard.
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Figure 5. Effect of the water glass on dry density, thermal conductivity, and compressive strength of CDWRS recycled foamed concrete. (a) Dry density and thermal conductivity; (b) compressive strength.
Figure 5. Effect of the water glass on dry density, thermal conductivity, and compressive strength of CDWRS recycled foamed concrete. (a) Dry density and thermal conductivity; (b) compressive strength.
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Figure 6. Effect of gypsum on dry density, thermal conductivity, and compressive strength of CDWRS recycled foamed concrete. (a) Dry density and thermal conductivity; (b) compressive strength.
Figure 6. Effect of gypsum on dry density, thermal conductivity, and compressive strength of CDWRS recycled foamed concrete. (a) Dry density and thermal conductivity; (b) compressive strength.
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Figure 7. Thermal conductivity of CDWRS recycled foamed concrete at various dry densities.
Figure 7. Thermal conductivity of CDWRS recycled foamed concrete at various dry densities.
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Figure 8. Compressive strength of CDWRS recycled foamed concrete at various dry densities. (a) 7-day compressive strength; (b) 28-day compressive strength; (c) 56-day compressive strength.
Figure 8. Compressive strength of CDWRS recycled foamed concrete at various dry densities. (a) 7-day compressive strength; (b) 28-day compressive strength; (c) 56-day compressive strength.
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Figure 9. Air-void structure and appearance of CDWRS recycled foamed concrete. (a) Unmodified sample with a density of 600 kg/m3; (b) modified sample with a density of 600 kg/m3; (c) unmodified sample with a density of 700 kg/m3; (d) modified sample with a density of 700 kg/m3; (e) unmodified sample with a density of 800 kg/m3; (f) modified sample with a density of 800 kg/m3; (g) unmodified sample with a density of 900 kg/m3; (h) modified sample with a density of 900 kg/m3.
Figure 9. Air-void structure and appearance of CDWRS recycled foamed concrete. (a) Unmodified sample with a density of 600 kg/m3; (b) modified sample with a density of 600 kg/m3; (c) unmodified sample with a density of 700 kg/m3; (d) modified sample with a density of 700 kg/m3; (e) unmodified sample with a density of 800 kg/m3; (f) modified sample with a density of 800 kg/m3; (g) unmodified sample with a density of 900 kg/m3; (h) modified sample with a density of 900 kg/m3.
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Figure 10. Cumulative air-void size distribution of CDWRS recycled foamed concrete at various dry densities. (a) Sample with a density of 600 kg/m3; (b) sample with a density of 700 kg/m3; (c) sample with a density of 800 kg/m3; (d) sample with a density of 900 kg/m3.
Figure 10. Cumulative air-void size distribution of CDWRS recycled foamed concrete at various dry densities. (a) Sample with a density of 600 kg/m3; (b) sample with a density of 700 kg/m3; (c) sample with a density of 800 kg/m3; (d) sample with a density of 900 kg/m3.
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Figure 11. Microscopic pore size distribution and cumulative pore volume of CDWRS recycled foamed concrete at various dry densities. (a) Sample with a density of 600 kg/m3; (b) sample with a density of 700 kg/m3; (c) sample with a density of 800 kg/m3; (d) sample with a density of 900 kg/m3.
Figure 11. Microscopic pore size distribution and cumulative pore volume of CDWRS recycled foamed concrete at various dry densities. (a) Sample with a density of 600 kg/m3; (b) sample with a density of 700 kg/m3; (c) sample with a density of 800 kg/m3; (d) sample with a density of 900 kg/m3.
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Sustainability 16 02308 g012
Figure 12. Microscopic pore volume distribution of CDWRS recycled foamed concrete at various dry densities.
Figure 12. Microscopic pore volume distribution of CDWRS recycled foamed concrete at various dry densities.
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Sustainability 16 02308 g013
Figure 13. FIIR curves of CDWRS recycled foamed concrete at various dry densities.
Figure 13. FIIR curves of CDWRS recycled foamed concrete at various dry densities.
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Figure 14. XPS scan spectra of CDWRS recycled foamed concrete at various dry densities.
Figure 14. XPS scan spectra of CDWRS recycled foamed concrete at various dry densities.
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Figure 15. XPS characteristic spectra of CDWRS recycled foamed concrete. (a) Si 2p; (b) Al 2p; (c) Ca 2p.
Figure 15. XPS characteristic spectra of CDWRS recycled foamed concrete. (a) Si 2p; (b) Al 2p; (c) Ca 2p.
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Figure 16. Energy consumption of the building using sandwich wallboard fabricated by CDWRS recycled foamed concrete.
Figure 16. Energy consumption of the building using sandwich wallboard fabricated by CDWRS recycled foamed concrete.
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Table 1. Advancements in the improvement of thermal insulation and mechanical performance of foamed concrete.
Table 1. Advancements in the improvement of thermal insulation and mechanical performance of foamed concrete.
Typical ApproachDensity
(kg/m3)		Thermal Conductivity
(W/(m·K))		Mechanical Strength (Mpa)Reference
Adding lightweight aggregatesCeramsite aggregate400–1000-0.5–6.2[16]
Expanded polystyrene304–3660.070–0.104-[17]
Biomass aggregate1200, 1500-8.2, 13.5[18]
Fiber reinforcementPolypropylene fiber16000.660–0.7107.5–10.8[19]
Glass fiber8000.129–0.1463.3–4.2[20]
Basalt fiber546–10280.130–0.3591.9–9.0[21]
Carbon fiber417-3.1[22]
Polyvinyl alcohol fiber350-1.5–1.7[23]
Polypropylene + basalt fiber700–8000.121–0.1473.1–3.5[24]
Introduction of supplementary cementing materialsBlast furnace slag3100.0860.78[25]
Fly ash252–16360.065–0.4230.42–14.5[26]
Fly ash, steel slag3500.070-[27]
Fly ash, coal gangue6282.39-[28]
Fly ash, silica fume762–781-5.16–6.52[29]
Fly ash, slag, mineral powder5070.1353.90[30]
Nanosilica-0.131–0.1730.45–0.73[31]
Usage of chemical admixtureFoam stabilizer732-7.18[32]
Water repellent542–5620.149–0.1541.02–1.33[33]
Accerator173–1810.052–0.0570.30–0.35[34]
Table 2. Chemical components of the raw materials for CDWRS recycled foamed concrete and sandwich wallboard (wt/%).
Table 2. Chemical components of the raw materials for CDWRS recycled foamed concrete and sandwich wallboard (wt/%).
MaterialsCaOSiO2Al2O3Fe2O3MgOK2O
OPC19.5465.474.312.871.420.65
Slag41.8728.3915.860.327.630.37
Fly ash3.9552.7030.845.450.602.20
CDWRS6.6260.5218.686.532.443.29
Table 3. Mix proportions of CDWRS recycled foamed concrete (kg/m3).
Table 3. Mix proportions of CDWRS recycled foamed concrete (kg/m3).
Design Dry DensityBinderFoamCDWRSWater GlassGypsum
600122211126--
60012221112617-
60012221112634-
60012221112651-
60012221112668-
600122211126512.5
600122211126515.0
600122211126517.5
6001222111265110.0
6001222111265112.5
700242231080--
800360151034--
90048017988--
700242231080517.5
800360151034517.5
90048017988517.5
Table 4. Mix proportion of CDW recycled concrete (kg/m3).
Table 4. Mix proportion of CDW recycled concrete (kg/m3).
OPCFly AshWaterCDW Recycled Coarse SandCDW Recycled Fine SandCDW Recycled Coarse AggregateWater Reducer
29029013962713782623.2
Table 5. Compressive strength and heat transfer coefficient of sandwich wallboard containing CDWRS recycled foamed concrete.
Table 5. Compressive strength and heat transfer coefficient of sandwich wallboard containing CDWRS recycled foamed concrete.
Dry Density
(kg/m3)		Surface Density of Wallboard (kg/m2)Compressive Strength
(MPa)		Heat Transfer Coefficient (W/(m2·K))
WetDry
62628616.50.860.75
68831220.80.950.83
83433623.61.030.93
94836224.61.161.01
Table 6. Carbon dioxide emissions of buildings using sandwich wallboard fabricated by CDWRS recycled foamed concrete (kg/(m2·year)).
Table 6. Carbon dioxide emissions of buildings using sandwich wallboard fabricated by CDWRS recycled foamed concrete (kg/(m2·year)).
WallChongqingWuhanNanjingChangshaHangzhou
Control68.0680.3880.9677.6574.91
Wallboard of 286 kg/m260.7771.4771.4067.0666.56
Wallboard of 312 kg/m261.0771.8671.6967.2466.82
Wallboard of 336 kg/m261.3572.1972.0467.4567.18
Wallboard of 362 kg/m261.5772.4772.3367.5967.43
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MDPI and ACS Style

Yang, F.; Yang, C.; Jin, C.; Liu, T.; Li, R.; Jiang, J.; Wu, Y.; Lu, Z.; Li, J. Pore Structure, Hardened Performance and Sandwich Wallboard Application of Construction and Demolition Waste Residue Soil Recycled Foamed Concrete. Sustainability 2024, 16, 2308. https://doi.org/10.3390/su16062308

AMA Style

Yang F, Yang C, Jin C, Liu T, Li R, Jiang J, Wu Y, Lu Z, Li J. Pore Structure, Hardened Performance and Sandwich Wallboard Application of Construction and Demolition Waste Residue Soil Recycled Foamed Concrete. Sustainability. 2024; 16(6):2308. https://doi.org/10.3390/su16062308

Chicago/Turabian Style

Yang, Fengyuan, Chenxi Yang, Chao Jin, Tie Liu, Renshuang Li, Jun Jiang, Yanping Wu, Zhongyuan Lu, and Jun Li. 2024. "Pore Structure, Hardened Performance and Sandwich Wallboard Application of Construction and Demolition Waste Residue Soil Recycled Foamed Concrete" Sustainability 16, no. 6: 2308. https://doi.org/10.3390/su16062308

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