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Article

Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades

by
Xiaofang Tong
1,2,3,
Yurong Yan
1,
Yujuan Tang
1,
Fei Xu
1,
Miao Liu
1,2 and
Yongfan Gong
2,*
1
School of Civil Engineering, Yangzhou Polytechnic University, Yangzhou 225009, China
2
College of Civil Engineering and Transportation, Yangzhou University, Yangzhou 225127, China
3
Jiangsu Engineering Research Center for Construction and Demolition Waste Recycling Technology, Yangzhou 225009, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(18), 3395; https://doi.org/10.3390/buildings15183395
Submission received: 19 August 2025 / Revised: 12 September 2025 / Accepted: 16 September 2025 / Published: 19 September 2025
(This article belongs to the Special Issue Advanced Cementitious Materials: Integrating Nanotechnology, Sustainability, and Intelligent Design)
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Abstract

Foamed concrete has been widely applied in construction engineering; however, the performance requirements vary across different structural components. Its production typically involves a substantial consumption of cement, which imposes both environmental and economic burdens. Therefore, this study examined foamed concrete with dry apparent densities of 500–1000 kg/m3, in which cement was partially replaced (0–30%) by recycled powder from construction and demolition waste. Macroscopic performance was evaluated through drying shrinkage, compressive strength, softening coefficient, carbonation coefficient, and thermal conductivity, while microstructural analysis was conducted to clarify the underlying mechanisms. The results indicate that the internal composition of the recycled powder primarily consists of SiO2, CaCO3, and C-S-H gel. When recycled powder is used to replace cement, the microstructure of the resulting paste gradually deteriorates compared to that of the control group without recycled powder, and a significant amount of inert SiO2 is introduced. As the replacement ratio of recycled powder increases, the compressive strength of foamed concrete across various density grades exhibits a gradual decline. Notably, when the replacement ratio reaches 30%, the reduction in mechanical performance becomes more substantial. However, the incorporation of recycled powder can effectively mitigate the drying shrinkage of foamed concrete. Moreover, the incorporation of recycled powder exerts minimal influence on the thermal conductivity and porosity of foamed concrete, demonstrating its favorable compatibility and potential for application in foamed concrete systems.

1. Introduction

China’s building operational energy consumption ranks third globally in terms of total energy use. Therefore, reducing building energy consumption is of great significance for alleviating the energy crisis and mitigating environmental burdens [1]. Throughout the entire life cycle of a building, energy consumption during the operational phase far exceeds that of the construction phase [2]. Consequently, optimizing the thermal performance of building components, such as walls, to reduce heat transfer has become an effective approach to lowering energy consumption. Foamed concrete, as a lightweight cementitious material, is characterized by a highly porous internal structure, which imparts low dry density and excellent thermal insulation properties. These advantages have led to its widespread application in construction engineering [3,4,5]. However, conventional foamed concrete typically relies on ordinary Portland cement as the primary binder [6]. The production of cement is associated with high energy consumption and significant CO2 emissions, making it one of the major sources of carbon emissions in the construction industry [7,8]. To promote green and low-carbon development in the construction sector, there is an urgent need to develop alternative cementitious materials that can replace traditional cement.
Meanwhile, the rapid growth of China’s construction industry has resulted in the generation of a vast quantity of construction and demolition waste. According to relevant statistics, the annual output of construction and demolition waste has reached as high as 3 billion tons [9]. Traditionally, this waste has been disposed of through open dumping or landfilling, practices that have caused significant environmental degradation [10,11]. Therefore, there is an urgent need to promote the recycling and resource utilization of construction and demolition waste [12]. Currently, scholars have developed various technologies for the resource recovery of construction and demolition waste, with one prevalent method involving the crushing of construction and demolition waste debris into recycled aggregates to partially or fully replace natural aggregates in concrete [13,14,15]. While this approach plays a vital role in advancing the utilization of construction and demolition waste, recycled aggregates inherently possess certain limitations. Unlike natural aggregates, recycled aggregates often retain adhered old mortar on their surfaces, creating a more complex interfacial transition zone, which results in higher water absorption and diminished particle strength [16,17]. These shortcomings can undermine the overall performance of concrete, especially at high replacement ratios, potentially impairing the workability of fresh concrete and subsequently reducing its mechanical properties and long-term durability [18,19,20]. Consequently, researchers have developed novel approaches for the resource utilization of construction and demolition waste by directly processing it through crushing and grinding to produce recycled powder [21,22,23]. Investigations have revealed that this recycled powder predominantly consists of silico-aluminous materials, indicating substantial potential for practical applications. Incorporating recycled powder as a partial replacement for cement in foamed concrete production is expected to generate considerable social and economic benefits.
Wu et al. [24] reported that the activity indices of three types of recycled powders all exceeded 70%, thereby satisfying the Chinese standard requirements for the activity index of conventional supplementary cementitious materials such as fly ash. Moreover, when these recycled powders were used to partially replace cement in mortar at varying replacement levels, a gradual decrease in compressive strength was observed with increasing replacement ratios. Similarly, Yao et al. [25] demonstrated that the compressive strength of engineered cementitious composites diminished as the replacement ratio of different recycled powders increased. Additionally, Chen et al. [26] investigated the effects of high-volume recycled powder replacement on the mechanical properties of foamed concrete and found that compressive strength gradually decreased with increasing replacement ratios. Yang et al. [27] studied the performance of recycled brick powder in foamed concrete preparation, demonstrating that recycled brick powder exhibits characteristics of a supplementary cementitious material. When the recycled brick powder replacement ratio is below 15%, its impact on the compressive strength of foamed concrete is minimal. Chindaprasirt et al. [28] examined A16-grade foamed concrete and observed that the incorporation of fly ash and admixtures can effectively mitigate drying shrinkage. Wang et al. [29] replaced cement with fly ash in foamed concrete and found that both early-age and later-age compressive strengths declined with increasing fly ash content, primarily due to the limited pozzolanic activity of fly ash at ambient temperature, whereby its role is mainly that of a filler.
Existing studies have primarily focused on the application of recycled powder in foamed concrete with a single dry apparent density grade. However, systematic investigations into the performance of foamed concrete incorporating recycled powder across a range of dry density levels remain limited. Given that foamed concrete can be applied to various structural components within buildings, the required mechanical properties may vary depending on the specific application. Therefore, it is necessary to investigate the suitability of recycled powder in foamed concrete across different dry apparent density grades, in order to meet diverse engineering requirements. This study investigates the performance of foamed concrete with dry apparent densities ranging from 500 to 1000 kg/m3, with a primary focus on the influence of recycled powder replacement ratios. The performance of recycled powder and the resulting foamed concrete is evaluated through comprehensive assessments of microstructural characteristics, mechanical properties, thermal performance, and durability. The aim of this work is to broaden the application of recycled powder as a supplementary cementitious material in cement-based systems.

2. Materials and Experiments

2.1. Raw Materials

In this study, the recycled powder was obtained from the dust removal system during the production of recycled aggregates from construction and demolition waste, and was provided by Huimin Recycling Resources Co., Ltd. (Yangzhou, China) as shown in Figure 1. The collected waste powder was further ground using a ball mill to obtain the recycled powder used in the experiments. In addition to the recycled powder, the raw materials included ordinary Portland cement, fly ash, a foaming agent, and a foam stabilizer. The cement used in this study was P·O 42.5 ordinary Portland cement, and Class II fly ash was employed. The foaming agent was a Legao™ 2258 composite type, and calcium stearate (C36H70CaO4), supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), was used as the foam stabilizer.
Macroscopically, the recycled powder appears light yellow in color, which is distinctly different from the gray-black appearance typically observed in cement and fly ash. This coloration is presumed to be related to the presence of a large amount of natural river sand in the construction and demolition waste. To further analyze the microstructural characteristics of the raw materials, scanning electron microscopy was employed, as shown in Figure 2. The fly ash exhibits a typical spherical particle morphology, whereas both cement and recycled powder primarily display irregular shapes. Notably, the recycled powder shares certain morphological similarities with cement at the microscale, characterized by large particles with smaller particles adhering to their surfaces. However, hydration products such as C-S-H gel are visibly attached to the surfaces of recycled powder particles, and the particle boundaries are more irregular, which may adversely affect the flowability of the paste system.
Furthermore, considering that fineness is a critical factor when using recycled powder as a supplementary cementitious material, the particle size distributions of cement, fly ash, and recycled powder were analyzed. As shown in Figure 3, the particle sizes of all three materials are generally below 110 μm. Among them, cement exhibits the finest particles, with a median diameter (D50) of 20.24 μm, indicating relatively high fineness. In contrast, the D50 of fly ash and recycled powder are 24.99 μm and 28.06 μm, respectively, with recycled powder having the coarsest particles. The relatively larger particle size of the recycled powder suggests a potential skeletal filling effect when used as a partial cement replacement, which may enhance the structural stability of the matrix. However, it could also negatively affect the workability and compactness of the mixture.
In addition, the chemical composition of the raw materials was characterized through XRD analysis. The results revealed that the main crystalline phases in cement are C3S, C2S, C3A, and C4AF. In comparison, fly ash is predominantly composed of mullite and quartz, and displays a broad hump in the 2θ range of 20°~30°, indicative of a significant amorphous phase, which is widely recognized as the primary source of its pozzolanic reactivity [30]. The recycled powder exhibited pronounced diffraction peaks, particularly a strong peak corresponding to SiO2, suggesting a high silica content. This is likely attributed to the presence of natural fine aggregates, such as river sand, within the construction and demolition waste. Moreover, Ca(OH)2 and CaCO3 were also detected. Furthermore, FTIR analysis revealed distinct absorption peaks at 875 cm−1 and 1417 cm−1, corresponding to the out-of-plane bending vibration (ν2) and symmetric stretching vibration (ν3) of CO32−, respectively, indicating the presence of CaCO3 [31]. An absorption peak at 798 cm−1 was assigned to the out-of-plane bending vibration of the Si-O bond, characteristic of SiO2 [32,33], which is consistent with the XRD results, further confirming the presence of both SiO2 and CaCO3 in the recycled powder. Additionally, a peak observed at 969 cm−1 was attributed to the stretching vibration of the Si-O bond [34], indicative of the presence of C-S-H gel. This suggests that the recycled powder may possess certain latent hydraulic or pozzolanic activity.
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Figure 4. Mineral composition of cement, fly ash, and recycled powder.
Figure 4. Mineral composition of cement, fly ash, and recycled powder.
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2.2. Mix Proportion and Preparation

This study focuses on the influence of recycled powder on the performance of foamed concrete with varying dry apparent densities. Considering the diverse density requirements of foamed concrete in practical engineering applications, six representative grades were selected for investigation: 500 kg/m3, 600 kg/m3, 700 kg/m3, 800 kg/m3, 900 kg/m3, and 1000 kg/m3. The objective is to comprehensively evaluate the suitability of recycled powder in foamed concrete across different density levels. Recycled powder replacement ratios were set at 0%, 10%, 20%, and 30%. Table 1 presents the mix proportions of the recycled powder foamed concrete. The notations A05-0RP, A05-10RP, A05-20RP, and A05-30RP correspond to foamed concrete with a target dry density grade of A05, in which 0%, 10%, 20%, and 30% of the cement is replaced by recycled powder, respectively. Similarly, A10-0RP, A10-10RP, A10-20RP, and A10-30RP represent foamed concrete of density grade A10 with recycled powder replacement rates of 0%, 10%, 20%, and 30%, respectively. In addition, A05-10RP, A06-10RP, A07-10RP, A08-10RP, A09-10RP, and A10-10RP denote foamed concrete samples with a fixed 10% cement replacement by recycled powder across various density grades ranging from A05 to A10.
Figure 5 illustrates the preparation process of the recycled powder foamed concrete. First, cement, recycled powder, fly ash, and foam stabilizer are added into a mixer according to the designed proportions and dry-mixed for 30 s to ensure uniform blending of the dry components. Subsequently, a predetermined amount of water is added, and mixing is continued for approximately 90 s to obtain a homogeneous slurry. Meanwhile, the foaming agent is mixed with water at a mass ratio of 1:60 and stirred thoroughly to generate stable foam. The freshly prepared foam is then promptly introduced into the slurry, followed by an additional 120 s of mixing to ensure uniform dispersion of the foam within the matrix. The resulting foamed slurry is cast into molds of various dimensions, vibrated to eliminate air pockets, and covered with plastic film for ambient curing for 24 h. After demolding, the specimens were subsequently transferred to a standard curing chamber maintained at 20 ± 2 °C with a relative humidity of at least 95%, where they were cured until reaching the designated testing age.

2.3. Compressive Strength, Softening Coefficient, and Carbonation Coefficient

The mechanical properties serve as essential indicators for assessing the performance of foamed concrete. In this study, the compressive strength was determined on 100 × 100 × 100 mm cubic specimens after 28 d of standard curing. The loading rate during the compressive test was set at 250 N/s [35]. Additionally, the softening coefficient and carbonation coefficient of recycled powder foamed concrete were measured in accordance with the Chinese standard GB/T 43487-2023 to assess its durability.

2.4. Thermal Conductivity, Porosity, and Drying Shrinkage

Foamed concrete exhibits excellent thermal insulation performance due to its inherently low thermal conductivity. Accordingly, this study examined the effect of recycled powder replacement ratio on the thermal conductivity of foamed concrete, using thin slab specimens with dimensions of 300 × 300 × 30 mm. Porosity is one of the key factors affecting the thermal performance of foamed concrete. The pore structure of foamed concrete primarily consists of macroscopic air voids formed by foaming and microscopic pores within the pore walls. Given the small size and complex morphology of the microscopic pores, they are difficult to characterize comprehensively using conventional methods. Therefore, this study focuses on the quantitative analysis of macroscopic pores. Specifically, specimens were cut perpendicular to the molding surface, and their surfaces were cleaned. The samples were then observed under an optical microscope at 20× magnification. The captured microscopic images were processed using image analysis software by converting them into binary (black-and-white) format, allowing for the calculation of macroscopic porosity.
In addition, the drying shrinkage behavior of the recycled powder foamed concrete was evaluated using prismatic specimens of 40 × 40 × 160 mm. The drying shrinkage tests were conducted in accordance with the Chinese standard GB/T 43487-2023 and relevant literature [27,35].

2.5. Micro-Characteristics Determination

This study employs multiple microscopic characterization techniques to analyze the hydration products of recycled powder foamed concrete (paste), thereby providing a deeper understanding of the effects of recycled powder on the microstructural properties of the matrix. Scanning electron microscopy (SEM) was used to observe and analyze the microstructure of the recycled powder and the corresponding foamed concrete. X-ray diffraction (XRD) analysis was performed to identify the mineralogical composition of the recycled powder foamed concrete, with a scanning range of 5° to 65° (2θ). Additionally, Fourier transform infrared spectroscopy (FTIR) was conducted to characterize the chemical compounds present in the recycled powder foamed concrete within a spectral range of 400 cm−1 to 2000 cm−1.

3. Results and Discussions

3.1. Drying Shrinkage of Recycled Powder Foamed Concrete

Foamed concrete exhibits a relatively high porosity, which results in greater shrinkage compared to conventional concrete. This section investigates the effect of recycled powder replacement ratio on the drying shrinkage behavior of foamed concrete across different density grades. Figure 6 illustrates the drying shrinkage rates of recycled powder foamed concrete. The results indicate that the drying shrinkage of recycled powder foamed concrete increases rapidly in the early stages and gradually levels off in the later stages. Within the first 28 d, a rapid rise in drying shrinkage is observed. For example, the 28-day drying shrinkage of A05-0RP, A05-10RP, A05-20RP, and A05-30RP reached 85.14%, 86.09%, 88.35%, and 86.98% of their respective maximum values. Similarly, the 28-day drying shrinkage of A07-0RP, A07-10RP, A07-20RP, and A07-30RP reached 89.29%, 88.90%, 89.88%, and 92.20%, respectively. For A10-0RP, A10-10RP, A10-20RP, and A10-30RP, the corresponding values were 93.23%, 96.61%, 90.27%, and 92.59% of the maximum shrinkage, respectively. However, after 28 d, the rate of drying shrinkage significantly decreases and tends to stabilize.
Figure 6 also indicates that the drying shrinkage of foamed concrete decreases progressively with the increase in recycled powder replacement ratio. For instance, the maximum drying shrinkage of A05-10RP, A05-20RP, and A05-30RP decreased by 7.64%, 12.84%, and 14.01%, respectively, compared to A05-0RP. The maximum drying shrinkage of A07-10RP, A07-20RP, and A07-30RP decreased by 3.49%, 6.67%, and 13.49%, respectively, compared to A07-0RP. The maximum drying shrinkage of A10-10RP, A10-20RP, and A10-30RP decreased by 11.28%, 15.04%, and 28.95%, respectively, compared to A10-0RP. This reduction can be attributed to the high content of inert substances in recycled powder, as shown in Figure 4, which do not participate in the early hydration reactions of cementitious systems [36]. As a result, the incorporation of recycled powder reduces the formation of hydration products, thereby decreasing drying shrinkage. In addition, due to the favorable micro-filler effect and relatively high microhardness of recycled powder, its incorporation can effectively resist shrinkage-induced stresses, further contributing to the reduction in drying shrinkage [21].
There is a strong correlation between the drying shrinkage of recycled powder foamed concrete and its density grade. As the density grade increases, the drying shrinkage gradually decreases. For instance, at a curing age of 60 d, the maximum drying shrinkage of A06-0RP, A07-0RP, A08-0RP, A09-0RP, and A10-0RP decreased by 18.96%, 22.94%, 54.20%, 59.63%, and 60.95%, respectively, compared to A05-0RP. Similarly, the maximum drying shrinkage of A06-30RP, A07-30RP, A08-30RP, A09-30RP, and A10-30RP was reduced by 20.52%, 22.47%, 56.47%, 63.64%, and 67.74%, respectively, relative to A05-30RP. This trend can be attributed to two main factors. First, since foamed concrete contains a certain amount of water within the foam, higher-density grades correspond to a lower foam content per unit volume, and thus less water is introduced. As a result, the overall water-to-cement ratio of the system is reduced, thereby suppressing the development of drying shrinkage. Second, higher-density foamed concrete exhibits fewer interconnected pores and macropores, and it generally has fewer internal defects. This leads to greater internal restraint during drying, resulting in reduced shrinkage.

3.2. Compressive Strength and Softening Coefficient of Recycled Powder Foamed Concrete

Figure 7 shows the effect of recycled powder replacement ratio on the compressive strength of foamed concrete. As the replacement ratio increases, the compressive strength of foamed concrete across all density grades shows a clear downward trend. For instance, the 28-day compressive strengths of A05-10RP, A05-20RP, and A05-30RP are reduced by 8.10%, 25.55%, and 41.74%, respectively, compared to A05-0RP. The 28-day compressive strengths of A07-10RP, A07-20RP, and A07-30RP decrease by 13.68%, 17.89%, and 34.11%, respectively, relative to A07-0RP. Similarly, the 28-day compressive strengths of A10-10RP, A10-20RP, and A10-30RP are reduced by 19.82%, 30.33%, and 52.38%, respectively, compared to A10-0RP.
This deterioration in mechanical performance is primarily attributed to the reduced overall degree of hydration in the matrix caused by the incorporation of recycled powder, which in turn leads to a decrease in the formation of hydration products and compromises the microstructural density. This interpretation is supported by the XRD and FTIR results discussed below, which confirm that higher recycled powder content results in lower formation of hydration products such as Ca(OH)2, thereby providing microstructural evidence for the observed decline in compressive strength.
Figure 7 presents the softening coefficient of recycled powder foamed concrete. With the increase in the replacement ratio of recycled powder, the softening coefficient of foamed concrete exhibits a decreasing trend. Notably, when the replacement level is 10%, the reduction in the softening coefficient is relatively limited. For example, the softening coefficient of A05-10RP is 2.50% lower than that of A05-0RP; A07-10RP shows a 1.20% decrease compared to A07-0RP; and A10-10RP shows a 1.12% decrease relative to A10-0RP. However, when the replacement ratio reaches 30%, the reduction becomes more significant. Specifically, the softening coefficient of A05-30RP is reduced by 11.25% compared to A05-0RP; A07-30RP decreases by 8.43% compared to A07-0RP; and A10-30RP is 8.99% lower than A10-0RP. This pronounced decline may be attributed to the reduction in hydration products when a high proportion of cement is replaced by recycled powder, which compromises the matrix’s resistance to water ingress. This inference is further supported by the SEM images shown in Figure 12, where the microstructure at a 30% replacement level becomes more porous and contains visible cracks.
To investigate the influence of recycled powder replacement rate on the variation of mechanical properties of foamed concrete across different density grades, regression fitting was performed on the compressive strength of recycled powder foamed concrete with varying dry apparent densities under the same replacement level. The results are presented in Figure 8.
As shown in the figure, when no recycled powder is incorporated, the compressive strength of foamed concrete exhibits a clear exponential increase with the rise in density grade. This indicates that in a pure cement system, the enhancement of dry apparent density promotes the filling effect of cement hydration products and the compactness of the microstructure, thereby improving the load-bearing capacity of the foamed concrete. However, as the replacement ratio of recycled powder increases, the rate of compressive strength growth with increasing density grade gradually slows down. Notably, at a 30% replacement level, the relationship between compressive strength and density grade approaches linearity, and the magnitude of compressive strength improvement significantly decreases. This phenomenon may be attributed to the fact that recycled powder, as a supplementary cementitious material partially replacing cement, provides a physical filling effect that can improve the microstructure to some extent. However, due to its relatively low reactivity, it is difficult to generate sufficient hydration products to support strength development. Especially in high-density grade foamed concrete, where the cement content is higher and theoretically conducive to achieving greater strength, the introduction of a high dosage of recycled powder diminishes this advantage, thereby limiting the further increase of compressive strength.

3.3. Carbonation Coefficient of Recycled Powder Foamed Concrete

Figure 9 shows the carbonation coefficient of recycled powder foamed concrete. The results indicate that the variation trend of the carbonation coefficient is consistent with that of the softening coefficient, showing an overall decreasing tendency that gradually diminishes with increasing recycled powder replacement ratio. Notably, when the replacement ratio reaches 30%, the carbonation coefficient significantly decreases, suggesting that a high content of recycled powder partially compromises the carbonation resistance of the foamed concrete. In addition, the carbonation coefficient exhibits a positive correlation with the density grade, meaning that higher density grades correspond to larger carbonation coefficients. For example, under a constant replacement ratio of 30%, the carbonation coefficients of foamed concrete with density grades A05, A07, and A10 are 0.71, 0.72, and 0.78, respectively, showing an increasing trend with rising dry density. This indicates that the compactness of the matrix has a certain influence on the carbonation behavior.

3.4. Thermal Conductivity and Porosity of Recycled Powder Foamed Concrete

The porosity and thermal conductivity of recycled powder foamed concrete are shown in Figure 10. Overall, the influence of recycled powder replacement ratio on the thermal performance of foamed concrete is relatively limited. At the same density grade, the thermal conductivity exhibits only minor fluctuations with varying recycled powder content. For example, the thermal conductivities of A05-0RP, A05-10RP, A05-20RP, and A05-30RP are 0.115, 0.117, 0.114, and 0.112 W/(m·K), respectively; the thermal conductivities of A10-0RP, A10-10RP, A10-20RP, and A10-30RP are 0.252, 0.244, 0.249, and 0.245 W/(m·K), respectively. However, the differences in thermal conductivity among recycled powder foamed concretes of different density grades are more pronounced, with thermal conductivity increasing as the density grade rises.
This is primarily because low-density foamed concrete contains a greater volume of pores, whose thermal conductivity is significantly lower than that of the solid phase, thereby effectively inhibiting heat transfer. Notably, when the dry apparent density falls below the A08 grade, a marked decrease in thermal conductivity is observed, indicating a significant improvement in the thermal insulation performance of foamed concrete below this density threshold. Furthermore, the variation in porosity shown in Figure 10b corroborates this trend, as the overall porosity of recycled powder foamed concrete increases when the density grade is below A08.
Moreover, as shown in Figure 10b, the replacement ratio of recycled powder has no significant impact on the porosity of foamed concrete. Although some fluctuations are observed, the variations are relatively minor and follow a similar trend to that of thermal conductivity. These findings indicate that the incorporation of recycled powder has a limited effect on both thermal conductivity and porosity, suggesting good compatibility of recycled powder as a partial cement replacement in foamed concrete production. However, the porosity varies substantially across different density grades. For example, the porosities of A05-30RP, A06-30RP, A07-30RP, A08-30RP, A09-30RP, and A10-30RP are 65.88%, 56.44%, 46.94%, 34.17%, 25.70%, and 17.43%, respectively. This variation is primarily due to the strong correlation between the amount of foam introduced and the target density, which leads to differences in pore structure among the various density grades.

3.5. Micro-Properties of Cementitious Composites with Recycled Powder

Figure 11 illustrates the effect of recycled powder replacement ratio on the mineral composition of the cement paste. As shown in the figure, the primary crystalline phases identified across all groups are Ca(OH)2, SiO2, and CaCO3. With increasing replacement ratios of recycled powder, the diffraction peaks corresponding to SiO2 and CaCO3 become more pronounced, indicating an elevated content of these phases within the matrix. This trend is primarily attributed to the high SiO2 content present in the recycled powder itself, as shown in Figure 3. Conversely, the diffraction peak intensity of Ca(OH)2 gradually decreases with increasing recycled powder content, suggesting a reduction in its concentration within the paste. This decline can be attributed to two main factors. First, the replacement of cement by recycled powder results in a dilution effect, thereby reducing the amount of hydration products formed. Second, the Ca(OH)2 present in the matrix reacts with the reactive SiO2 in the recycled powder to form additional C-S-H gel, further consuming Ca(OH)2 within the system [37].
Furthermore, the influence of recycled powder incorporation on the mineral composition of the matrix was investigated through FTIR. C-S-H gel is a typical amorphous material with a layered silicate structure and exerts a significant influence on the compressive strength and durability of cement-based materials. Therefore, the formation and structure of C-S-H gel are commonly used as indicators of the degree of cement hydration. FTIR detects the microstructural characteristics of C-S-H gel by analyzing changes in the [H2SiO4]2− tetrahedral structure [38,39]. As shown in Figure 11, a distinct absorption peak appears near 960 cm−1, corresponding to the stretching vibration of Si-O bonds, which is indicative of C-S-H gel formation resulting from cement hydration. With increasing recycled powder content, this absorption peak shifts toward a higher wavenumber. This shift reflects the progressive polymerization of dissolved [H2SiO4]2− species during cement hydration, leading to the formation of silicate tetrahedra with higher degrees of polymerization in the C-S-H gel. This phenomenon may be attributed to the fine particle size of the recycled powder and the presence of residual hydration products, which can serve as nucleation sites between cement particles during the early stages of hydration [40,41].
Figure 12 illustrates the effect of recycled powder replacement ratio on the microstructure of cement paste. As shown in the figure, the microstructure of the control group without recycled powder appears relatively dense, with the formation of flocculent C-S-H gel observed within the matrix. When the replacement ratio of recycled powder reaches 10%, the microstructure remains largely unchanged, indicating that at low replacement levels, the incorporation of recycled powder has minimal impact on the microstructural development of the system. In addition, several unreacted spherical fly ash particles are still observed dispersed throughout the matrix, suggesting that the fly ash exhibits limited pozzolanic reactivity under the given conditions and primarily functions as a micro-filler. With the further increase in the replacement ratio of recycled powder, the microstructure exhibits a progressive trend toward loosening. This is particularly evident when the replacement ratio exceeds 20%, at which point a noticeable increase in porosity and the emergence of microcracks can be observed. This phenomenon is likely attributed to the reduced cement content, leading to a lower production of hydration products, thereby compromising the compactness and structural integrity of the paste and ultimately hindering the improvement of its macroscopic mechanical properties. Additionally, residual recycled powder particles that have not fully participated in the reaction can be found within the matrix. Although these inert particles may provide some degree of physical filling, their low reactivity makes it difficult to compensate for the microstructure performance loss caused by the reduced formation of hydration products. Therefore, when the replacement ratio of recycled powder reaches 30%, the compressive strength of foamed concrete exhibits a significant reduction.
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Figure 12. SEM results of paste containing RP (28 d).
Figure 12. SEM results of paste containing RP (28 d).
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Based on the combined results of macroscopic performance tests and microstructural analyses, the incorporation of recycled powder leads to the deterioration of the matrix microstructure, consequently causing a decline in the mechanical properties of foamed concrete. However, when the replacement ratio is controlled below 10%, its adverse effects on both the microstructure and macroscopic performance of foamed concrete are relatively limited. This can be mainly attributed to the fact that, at this replacement level, the recycled powder can provide a certain micro-aggregate filling effect. Therefore, it is recommended that the optimal replacement ratio of recycled powder be kept below 10%.
In addition, this study was compared with the findings of other scholars. Yang et al. [35] demonstrated that both recycled concrete powder and recycled mortar powder exerted adverse effects on the compressive strength of foamed concrete, while producing only minor variations in thermal conductivity, a trend that is generally consistent with the findings of this study. Similarly, Xiao et al. [42] reported a decline in compressive strength with the incorporation of recycled powder, and the overall strength values were lower than those obtained in the present work. In contrast, Chen et al. [26] found that when the replacement level of recycled cement concrete powder was 20%, the compressive strength of foamed concrete improved; however, further increases in replacement ratio led to a subsequent reduction in compressive strength. Moreover, Xiao et al. [42] indicated that the incorporation of recycled powder had little influence on the dry apparent density of foamed concrete, which remained within the range of 0.557–0.584 g/cm3. Consistently, the present study also revealed that the dry apparent density of foamed concrete across different density grades exhibited no significant variation after the incorporation of recycled powder, with all values meeting the requirements of the Chinese standard JC/T 1062-2022.
Nevertheless, this study has certain limitations, such as the lack of systematic evaluation of environmental benefits and the full life-cycle assessment, which are important references for the broader application of recycled powder in cement-based materials. In addition, at higher recycled powder replacement ratios, the performance deterioration is particularly pronounced, and further research is needed to explore technical pathways for optimizing performance under high replacement conditions.

4. Conclusions

This study investigated the microstructural properties of recycled powder and its application as a partial cement replacement in the preparation of foamed concrete. Additionally, the effects of recycled powder replacement ratios on the macro-scale properties of foamed concrete across different density grades were examined. Based on the experimental results, discussions, and analysis, the following conclusions can be drawn:
(1)
The recycled powder exhibits an irregular microstructure, with large particles frequently covered by smaller adhered particles. Its median particle size is larger than that of both cement and fly ash, indicating a coarser particle size distribution. Mineralogical analysis reveals the presence of C-S-H gel along with a significant amount of inert components such as SiO2 and CaCO3. When used as a partial replacement for cement, the microstructure of the cementitious matrix gradually deteriorates.
(2)
With the increase in recycled powder replacement ratio, the compressive strength of foamed concrete gradually decreases, with a particularly pronounced reduction observed at a 30% replacement level. In addition, the incorporation of recycled powder also leads to a decline in both the softening coefficient and carbonation resistance of foamed concrete.
(3)
The incorporation of recycled powder effectively mitigates the drying shrinkage of foamed concrete, with shrinkage progressively decreasing as the replacement ratio increases. Notably, high-density recycled powder foamed concrete demonstrates enhanced resistance to shrinkage. Furthermore, the addition of recycled powder exerts a limited influence on both the porosity and thermal conductivity of foamed concrete, indicating good compatibility regarding thermal performance and pore structure stability.

Author Contributions

X.T.: Visualization, Validation, Resources, Methodology, Writing—Original Draft. Y.Y.: Methodology, Formal analysis, Data curation, Writing—Review and Editing. Y.T.: Formal analysis, Investigation, Writing—Review and Editing, Validation. F.X.: Resources, Data curation, Methodology. M.L.: Investigation, Formal analysis. Y.G.: Investigation, Conceptualization, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Jiangsu Visiting Scholar Program for Vocational College Teachers, the Young and Middle-aged Academic Leader Program of Yangzhou Polytechnic University, and the Open Project of Jiangsu Engineering Research Center for Construction and Demolition Waste Recycling Technology (JSGCYJZX-2024-01, 02).

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The source and stockpile diagram of recycled powder.
Figure 1. The source and stockpile diagram of recycled powder.
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Figure 2. Microstructural morphology of cement, fly ash, and recycled powder.
Figure 2. Microstructural morphology of cement, fly ash, and recycled powder.
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Figure 3. Size distribution of cement, fly ash, and recycled powder.
Figure 3. Size distribution of cement, fly ash, and recycled powder.
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Figure 5. The preparation process for recycled powder foamed concrete.
Figure 5. The preparation process for recycled powder foamed concrete.
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Figure 6. Drying shrinkage of recycled powder foam concrete.
Figure 6. Drying shrinkage of recycled powder foam concrete.
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Figure 7. Compressive strength and softening coefficient of recycled powder foam concrete.
Figure 7. Compressive strength and softening coefficient of recycled powder foam concrete.
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Figure 8. Fitting of compressive strength of recycled powder foam concrete.
Figure 8. Fitting of compressive strength of recycled powder foam concrete.
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Figure 9. Carbonation coefficient of recycled powder foam concrete.
Figure 9. Carbonation coefficient of recycled powder foam concrete.
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Figure 10. Thermal conductivity and porosity of recycled powder foam concrete.
Figure 10. Thermal conductivity and porosity of recycled powder foam concrete.
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Figure 11. XRD and FTIR results of paste containing RP (28 d).
Figure 11. XRD and FTIR results of paste containing RP (28 d).
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Table 1. Mix proportion of recycled powder foamed concrete (kg/m3).
Table 1. Mix proportion of recycled powder foamed concrete (kg/m3).
Cement
(kg)		Fly Ash
(kg)		Recycled Powder
(kg)		Water
(kg)		Calcium Stearate
(kg)		Foam
(m3)		Dry Apparent Density
(kg/m3)
A05-0RP412.8045.870229.333.440.932525.2
A05-10RP371.5245.8741.28229.333.440.932518.3
A05-20RP330.2445.8782.56229.333.440.932520.4
A05-30RP288.9645.87123.84229.333.440.932515.6
A06-0RP483.7553.750268.754.030.834619.3
A06-10RP435.3753.7548.38268.754.030.834620.8
A06-20RP387.0053.7596.75268.754.030.834613.8
A06-30RP338.6253.75145.13268.754.030.834615.3
A07-0RP548.8560.980304.924.570.744720.3
A07-10RP493.9660.9854.89304.924.570.744715.5
A07-20RP439.0860.98109.77304.924.570.744717.6
A07-30RP384.1960.98164.66304.924.570.744711.8
A08-0RP585.9065.100325.504.880.693820.7
A08-10RP527.3165.1058.59325.504.880.693819.2
A08-20RP468.7265.10117.18325.504.880.693813.2
A08-30RP410.1365.10175.77325.504.880.693809.6
A09-0RP659.5973.290366.445.500.592918.4
A09-10RP593.6373.2965.96366.445.500.592909.8
A09-20RP527.6773.29131.92366.445.500.592912.6
A09-30RP461.7173.29197.88366.445.500.592908.4
A10-0RP720.4580.050400.256.000.5081008.9
A10-10RP648.4180.0572.04400.256.000.5081003.2
A10-20RP576.3680.05144.09400.256.000.508998.3
A10-30RP504.3180.05216.14400.256.000.5081008.9
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MDPI and ACS Style

Tong, X.; Yan, Y.; Tang, Y.; Xu, F.; Liu, M.; Gong, Y. Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades. Buildings 2025, 15, 3395. https://doi.org/10.3390/buildings15183395

AMA Style

Tong X, Yan Y, Tang Y, Xu F, Liu M, Gong Y. Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades. Buildings. 2025; 15(18):3395. https://doi.org/10.3390/buildings15183395

Chicago/Turabian Style

Tong, Xiaofang, Yurong Yan, Yujuan Tang, Fei Xu, Miao Liu, and Yongfan Gong. 2025. "Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades" Buildings 15, no. 18: 3395. https://doi.org/10.3390/buildings15183395

APA Style

Tong, X., Yan, Y., Tang, Y., Xu, F., Liu, M., & Gong, Y. (2025). Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades. Buildings, 15(18), 3395. https://doi.org/10.3390/buildings15183395

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