Study on Alkali-Activated Prefabricated Building Recycled Concrete Powder for Foamed Lightweight Soils

The advantage of a prefabricated building is its ease of construction. Concrete is one of the essential components of prefabricated buildings. A large amount of waste concrete from prefabricated buildings will be produced during the demolition of construction waste. In this paper, foamed lightweight soil is primarily made of concrete waste, a chemical activator, a foaming agent, and a foam stabilizer. The effect of the foam admixture on the wet bulk density, fluidity, dry density, water absorption, and unconfined compressive strength of the material was investigated. Microstructure and composition were measured by SEM and FTIR. The results demonstrated that the wet bulk density is 912.87 kg/m3, the fluidity is 174 mm, the water absorption is 23.16%, and the strength is 1.53 MPa, which can meet the requirements of light soil for highway embankment. When the foam content ranges from 55% to 70%, the foam proportion is increased and the material’s wet bulk density is decreased. Excessive foaming also increases the number of open pores, which reduces water absorption. At a higher foam content, there are fewer slurry components and lower strength. This demonstrates that recycled concrete powder did not participate in the reaction while acting as a skeleton in the cementitious material with a micro-aggregate effect. Slag and fly ash reacted with alkali activators and formed C-N-S(A)-H gels to provide strength. The obtained material is a construction material that can be constructed quickly and reduce post-construction settlement.


Introduction
Prefabricated buildings have the advantages of convenient construction, high construction efficiency, and low costs [1]. It has become a development trend in the construction industry [2][3][4]. For example, the penetration rate of prefabricated buildings has surpassed 70% in Sweden [5][6][7]. Additionally, as concrete is a materials for prefabricated buildings, a large amount of waste concrete building will be produced during the demolition of construction waste. So far, recycled aggregates have been widely used in civil engineering in accordance with some codes for their application of them, such as GB/T25176-2010 "Recycled fine aggregate for concrete and mortar", GB/T 25177-2010 "Recycled coarse aggregate for concrete and mortar" [8]. However, due to low activity and high porosity, recycled concrete powder has been stockpiled for the long term. As reviewed by some researchers [9,10], recycled concrete powder has been highlighted as a sustainable approach. However, when the dosage is greater than 15%, it has adverse effects on the strength, flowability, and durability of the formed specimen. Therefore, except for a small amount used for low-value products, recycled concrete powder from building waste has not yet been effectively utilized; therefore, it is of great social significance and economic benefit to solve the low activity and realize resource utilization of RCP. Currently, there are recent studies reported on the preparation of alkali-activated recycled concrete powder [11][12][13]. The potential activity of a large amount of SiO 2 and Al 2 O 3 in the recycled concrete powder can

Raw Materials
Recycled concrete powder (RCP) is produced from broken waste prefabricated b ing concrete. It is collected by a dust removal device to manufacture recycled aggre with a specific surface area of more than 350 m 2 /kg. The slag powder was S95 grade, an activity index of 96%. The fly ash was of grade Ⅱ with a low lime content. The chem compositions of recycled concrete powder, slag, and fly ash are shown in Table 1. As be seen in Figures 1 and 2, the main components of recycled concrete powder are ca silicon dioxide, and dolomite. Among them, calcite is the phase formed after carboniza while silicon dioxide and dolomite are the phases existing in natural stone and mo The medium particle size of the recycled concrete powder is 45 µm.    The alkali activator formulated in this paper is based on sodium hydroxide an dustrial water glass as raw materials. The sodium hydroxide was used to reduce the m  The alkali activator formulated in this paper is based on sodium hydroxide and industrial water glass as raw materials. The sodium hydroxide was used to reduce the modulus of the industrial water glass solution from 3.3 to 1.4. The dosage of water glass (sodium silicate) was 6%, which was formulated according to the ratio of Na 2 O to cementitious materials. The foaming agent used in this study was sodium dodecyl sulfate (YS-200), with an analytical purity level (AR), and the foam stabilizer is calcium stearate with an analytical purity level (AR).

Design of Mix Proportion
The experiments in this study were guided by the specifications JTG D30-2015 and TJG F1001-2011. It is proposed that the construction wet capacity of foam lightweight soil should be 5.0~11.0 kg/m 3 , and the flow value is 170~190 mm. This experiment began with a single-factor study to investigate the effect of foam admixture on the fluidity, dry density, compressive strength, and water absorption of foamed lightweight soil, to determine the optimal performance of recycled micronized cementitious material foamed lightweight soil. According to the specification, the ratio design of foam lightweight soil can be calculated according to the following steps. Keeping the total mass of the construction wet weight unchanged, which was measured to be 600~1000 kg/m 3 , then gradually adjusting the amount of cementitious material. The components contained 60% recycled concrete powder, 20% slag, and 20% fly ash. Water glass as an activator was measured at 6%. With water to cementitious materials ratio of 0.45 (the total amount of components is 891 kg/m 3 , 794 kg/m 3 , 694 kg/m 3 , 595 kg/m 3 , and 476 kg/m 3 , respectively). After mixing, five mix ratio test groups with a wet density of 600 kg/m 3 -1000 kg/m 3 were obtained. Calcium stearate is mixed at 2% of the powder mass. The specific mix ratio is shown in Table 2.

Preparation
First, the foaming agent is mixed with water according to the dilution ratio, and the foaming machine (FP-5) is started to prepare foam. Second, pour the weight of recycled concrete powder, slag powder, fly ash, and calcium stearate into the mixing pot and mix evenly. Third, according to the design requirements of density grade, add corresponding foam, then pour the prepared foam light soil into the corresponding test mold. Lastly, move the entire test mold into a constant temperature cement standard curing box with a relative humidity of >90% and a temperature of 20 • C ± 0.5 • C. Then, remove the formwork 24 h after sample preparation and place it in the curing box again for curing. Upon arrival, take it out for property testing. The Preparation process of foamed lightweight soil has been shown in Figure 3.

Methods
Foamed lightweight soil pastes with varying foam contents were prepared, the w/c was set as 0.45, and a cuboid specimen measuring 40 mm × 40 mm × 160 mm was formed; six samples were tested per group. The wet bulk density weight is the weight per unit volume of freshly mixed foamed lightweight soil. According to the relevant criterion of the specification requirements, its wet bulk density should be less than 1100 kg/m 3 . The fluidity was tested according to GB/T17671-2021, the dry density was tested according

Methods
Foamed lightweight soil pastes with varying foam contents were prepared, the w/c was set as 0.45, and a cuboid specimen measuring 40 mm × 40 mm × 160 mm was formed; six samples were tested per group. The wet bulk density weight is the weight per unit volume of freshly mixed foamed lightweight soil. According to the relevant criterion of the specification requirements, its wet bulk density should be less than 1100 kg/m 3 . The fluidity was tested according to GB/T17671-2021, the dry density was tested according to GB/T 5486-2008. Water absorption was tested in accordance with JG/T 266-2011, and unconfined compressive strength in accordance with CECS 249-2008.
The infrared spectrum is primarily used to examine the composition and structure of foamed lightweight soil pastes. Cary 610/670 micro-infrared spectrometer produced by Varian in the United States is used for infrared spectrum testing. Carl Zeiss' Gemini SEM 300 ZEISS field emission scanning electron microscope system was used.

Wet Bulk Density
The wet bulk density of foamed lightweight soil decreases significantly with the increase in foam admixture, as shown in Figure 4. This is due to the use of many bubbles rather than gelling materials, which increases the proportion of generated pores while decreasing the quality of the gelling components. It can be found that the measured wet bulk density is larger compared with the theoretically calculated value. This is because the foam is crushed by the slurry during mixing. The foam soil in a mold was discovered to collapse the phenomenon of F5 to a certain degree, which is due to the addition of excessive foam, resulting in the cementing component being unable to effectively lubricate the foam wall, and the ruptured foam released free water mixed into the slurry, equivalent to improving the water-cement ratio, increasing the wet bulk density of the F5 group. The infrared spectrum is primarily used to examine the composition and structure of foamed lightweight soil pastes. Cary 610/670 micro-infrared spectrometer produced by Varian in the United States is used for infrared spectrum testing. Carl Zeiss' Gemini SEM 300 ZEISS field emission scanning electron microscope system was used.

Wet Bulk Density
The wet bulk density of foamed lightweight soil decreases significantly with the increase in foam admixture, as shown in Figure 4. This is due to the use of many bubbles rather than gelling materials, which increases the proportion of generated pores while decreasing the quality of the gelling components. It can be found that the measured wet bulk density is larger compared with the theoretically calculated value. This is because the foam is crushed by the slurry during mixing. The foam soil in a mold was discovered to collapse the phenomenon of F5 to a certain degree, which is due to the addition of excessive foam, resulting in the cementing component being unable to effectively lubricate the foam wall, and the ruptured foam released free water mixed into the slurry, equivalent to improving the water-cement ratio, increasing the wet bulk density of the F5 group.

Fluidity
Fluidity is an important indicator to measure whether foamed lightweight soil m the specification requirements. The fluidity should be controlled at 170~190 mm, and

Fluidity
Fluidity is an important indicator to measure whether foamed lightweight soil meets the specification requirements. The fluidity should be controlled at 170~190 mm, and the test results are shown in Figure 5. As shown in Figure 5, fluidity decreases significantly with the increase in foam content. This is because the poor fluidity of the foam necessitates slurry wrapping lubrication, and increased production results in insufficient slurry. Calcium stearate has a surface-activating effect and can enhance the liquid wall of the foam. Calcium stearate has hydrophobic properties and can combine with hydroxyl groups to form hydrophobic substances to achieve a bubble retention effect. The overall viscosity of the foam after calcium stearate pacification is large, which reduces the fluidity and requires the cementitious material to play a lubricating role. The alkali-activated slurry has the modifying effect of alkali activation, resulting in an unbalanced charge and a flocculent gel adsorbing a large amount of free water, which will make a low-fluidity slurry.

Fluidity
Fluidity is an important indicator to measure whether foam the specification requirements. The fluidity should be controlle test results are shown in Figure 5. As shown in Figure 5, fluid with the increase in foam content. This is because the poor fluid slurry wrapping lubrication, and increased production results cium stearate has a surface-activating effect and can enhance th Calcium stearate has hydrophobic properties and can combine form hydrophobic substances to achieve a bubble retention effe the foam after calcium stearate pacification is large, which re quires the cementitious material to play a lubricating role. The the modifying effect of alkali activation, resulting in an unbala lent gel adsorbing a large amount of free water, which will mak  Figure 6 shows a freshly mixed recycled powder light soil rate added to the left and an unadded sample on the right. Ma the surface of the foam stabilizer component without the additi is due to the high flow rate of slurries without foam stabilizers, a to float to the surface and burst during the stirring process. Desp stearate to this test sample, excessive foam caused the structura slurry, the deterioration of bonding performance, and failure to foams burst, the proportion of cementitious materials was inc  Figure 6 shows a freshly mixed recycled powder light soil sample with calcium stearate added to the left and an unadded sample on the right. Many bubbles accumulate on the surface of the foam stabilizer component without the addition of foam stabilizer. This is due to the high flow rate of slurries without foam stabilizers, and the foam is more likely to float to the surface and burst during the stirring process. Despite the addition of calcium stearate to this test sample, excessive foam caused the structural instability of foam in the slurry, the deterioration of bonding performance, and failure to float. In sample F5, many foams burst, the proportion of cementitious materials was increased, and the moisture was released, resulting in a return to fluidity. According to the data, only the F1 and F2 groups meet the criterion requirements.
was released, resulting in a return to fluidity. According to the data, only the F1 and groups meet the criterion requirements.

Dry Density and Water Absorption
According to the test results shown in Figure 7, the water absorption rate of foam lightweight soil gradually increases with the increase of the foam content. This is becau when the volume content of foam increases, the open pores and connecting pores on surface also increase, and its ability to absorb water is significantly improved. The increa in water absorption rate gradually accelerates as the foam content increases because wh the foam content is too high, the stability of the foam decreases, and small foams eas merge into large foams. Because of the capillary effect, an appropriate pore size is mo likely to retain adsorbed water, so the water absorption rate increases linearly when t content is high.  Figure 8 illustrates the surface morphology of samples F4 and F2, respectively. Sa ple F4 has insufficient slurry, a too-thin foam wall, and a smaller foam burst due to t addition of a large amount of foam. At the same time, some combinations produce foa with larger pore sizes, a more discrete foam distribution, and an uneven structure. The specimen's slurry and foam are in good condition, and it can be seen that the fine foam evenly distributed throughout the material, and the sample with this ratio is in better co dition. This is because the uniformity of pores improves the molding effect, and bett pore structure can improve the strength of the specimen.

Dry Density and Water Absorption
According to the test results shown in Figure 7, the water absorption rate of foamed lightweight soil gradually increases with the increase of the foam content. This is because when the volume content of foam increases, the open pores and connecting pores on its surface also increase, and its ability to absorb water is significantly improved. The increase in water absorption rate gradually accelerates as the foam content increases because when the foam content is too high, the stability of the foam decreases, and small foams easily merge into large foams. Because of the capillary effect, an appropriate pore size is more likely to retain adsorbed water, so the water absorption rate increases linearly when the content is high. was released, resulting in a return to fluidity. According to the data, only the F1 an groups meet the criterion requirements.

Dry Density and Water Absorption
According to the test results shown in Figure 7, the water absorption rate of foa lightweight soil gradually increases with the increase of the foam content. This is bec when the volume content of foam increases, the open pores and connecting pores o surface also increase, and its ability to absorb water is significantly improved. The incr in water absorption rate gradually accelerates as the foam content increases because w the foam content is too high, the stability of the foam decreases, and small foams e merge into large foams. Because of the capillary effect, an appropriate pore size is m likely to retain adsorbed water, so the water absorption rate increases linearly when content is high.  Figure 8 illustrates the surface morphology of samples F4 and F2, respectively. S ple F4 has insufficient slurry, a too-thin foam wall, and a smaller foam burst due to addition of a large amount of foam. At the same time, some combinations produce f with larger pore sizes, a more discrete foam distribution, and an uneven structure. Th specimen's slurry and foam are in good condition, and it can be seen that the fine foa evenly distributed throughout the material, and the sample with this ratio is in better dition. This is because the uniformity of pores improves the molding effect, and b pore structure can improve the strength of the specimen.  Figure 8 illustrates the surface morphology of samples F4 and F2, respectively. Sample F4 has insufficient slurry, a too-thin foam wall, and a smaller foam burst due to the addition of a large amount of foam. At the same time, some combinations produce foam with larger pore sizes, a more discrete foam distribution, and an uneven structure. The P2 specimen's slurry and foam are in good condition, and it can be seen that the fine foam is evenly distributed throughout the material, and the sample with this ratio is in better condition. This is because the uniformity of pores improves the molding effect, and better pore structure can improve the strength of the specimen.

Unconfined Compressive Strength
According to the specification, cubic test blocks with specimen sizes of 100 mm × 1 mm × 100 mm are loaded under unrestricted conditions. As illustrated in Figure 9, compressive strength gradually decreases with the increase in foam content. F1, F2, and F4 exceed the standard value of 0.6 MPa. This is due to the fact that as the foam cont increases, the amount of cementitious material decreases while the porosity increas This leads to a decrease in the internal load-bearing skeleton of the specimen and a d crease in the strength support part, resulting in a decline in strength. When the foam co tent is high, the internal foam is unstable and will burst to form large and connected por as shown in the image above. Large pores and connected pores cause uneven stress d tribution within the material, resulting in weak parts and deteriorating strength. Wh the foam content is moderate, its distribution is better, and the structure is uniform. Re tively good structural morphology can improve the strength of the material. After testi all other components can meet the criterion conditions, with the exception of the F5 grou which shows the collapse phenomenon of foamed lightweight soil in the mold.

TG Analysis
According to the TG image of F2 in the Figure 10, at the stage of 20~100 °C, with increase in age, the weight loss rate at 28 days show a certain increase compared to 3 da As the time increases, the C-A-S-H gel components increase and the amount of free wa adsorbed increases in the foamed lightweight soil. At the stage of 100~300 °C, the weig loss rate is stable and the residual free water gradually decreases. At the stage of 400~5 °C, there was no significant weight loss. However, due to the secondary hydration re

Unconfined Compressive Strength
According to the specification, cubic test blocks with specimen sizes of 100 mm × 100 mm × 100 mm are loaded under unrestricted conditions. As illustrated in Figure 9, the compressive strength gradually decreases with the increase in foam content. F1, F2, F3, and F4 exceed the standard value of 0.6 MPa. This is due to the fact that as the foam content increases, the amount of cementitious material decreases while the porosity increases. This leads to a decrease in the internal load-bearing skeleton of the specimen and a decrease in the strength support part, resulting in a decline in strength. When the foam content is high, the internal foam is unstable and will burst to form large and connected pores, as shown in the image above. Large pores and connected pores cause uneven stress distribution within the material, resulting in weak parts and deteriorating strength. When the foam content is moderate, its distribution is better, and the structure is uniform. Relatively good structural morphology can improve the strength of the material. After testing, all other components can meet the criterion conditions, with the exception of the F5 group, which shows the collapse phenomenon of foamed lightweight soil in the mold.

Unconfined Compressive Strength
According to the specification, cubic test blocks with specimen sizes of 100 mm × 100 mm × 100 mm are loaded under unrestricted conditions. As illustrated in Figure 9, the compressive strength gradually decreases with the increase in foam content. F1, F2, F3, and F4 exceed the standard value of 0.6 MPa. This is due to the fact that as the foam content increases, the amount of cementitious material decreases while the porosity increases. This leads to a decrease in the internal load-bearing skeleton of the specimen and a decrease in the strength support part, resulting in a decline in strength. When the foam content is high, the internal foam is unstable and will burst to form large and connected pores, as shown in the image above. Large pores and connected pores cause uneven stress distribution within the material, resulting in weak parts and deteriorating strength. When the foam content is moderate, its distribution is better, and the structure is uniform. Relatively good structural morphology can improve the strength of the material. After testing, all other components can meet the criterion conditions, with the exception of the F5 group, which shows the collapse phenomenon of foamed lightweight soil in the mold.

TG Analysis
According to the TG image of F2 in the Figure 10, at the stage of 20~100 °C, with the increase in age, the weight loss rate at 28 days show a certain increase compared to 3 days. As the time increases, the C-A-S-H gel components increase and the amount of free water adsorbed increases in the foamed lightweight soil. At the stage of 100~300 °C, the weight loss rate is stable and the residual free water gradually decreases. At the stage of 400~500 °C, there was no significant weight loss. However, due to the secondary hydration reac-

TG Analysis
According to the TG image of F2 in the Figure 10, at the stage of 20~100 • C, with the increase in age, the weight loss rate at 28 days show a certain increase compared to 3 days. As the time increases, the C-A-S-H gel components increase and the amount of free water adsorbed increases in the foamed lightweight soil. At the stage of 100~300 • C, the Materials 2023, 16, 4167 9 of 12 weight loss rate is stable and the residual free water gradually decreases. At the stage of 400~500 • C, there was no significant weight loss. However, due to the secondary hydration reaction between Ca(OH) 2 and the active mixture, residual Ca(OH) 2 is extremely prone to carbonization into CaCO 3 . Between 600 and 700 • C, the test sample loses a large amount of weight. It is due to the endothermic decomposition of calcite in the sample, which releases CO 2 and converts to CaO. It can be seen that there is a difference in the decomposition state between the alkali-activated cementitious material and the raw recycled concrete powders. The peak TG value of the cementitious material is around 700 • C, while the peak TG value of the raw recycled concrete powders is around 735 • C. This is becausee the calcite particles formed by carbonization of the cementitious material are very small and can be decomposed at lower temperatures.
Materials 2023, 16, x FOR PEER REVIEW tion between Ca(OH)2 and the active mixture, residual Ca(OH)2 is extremely pron bonization into CaCO3. Between 600 and 700 °C, the test sample loses a large am weight. It is due to the endothermic decomposition of calcite in the sample, which CO2 and converts to CaO. It can be seen that there is a difference in the decom state between the alkali-activated cementitious material and the raw recycled powders. The peak TG value of the cementitious material is around 700 °C, while TG value of the raw recycled concrete powders is around 735 °C. This is beca calcite particles formed by carbonization of the cementitious material are very sm can be decomposed at lower temperatures.

Phase Analysis Results of FTIR
The FTIR spectra of the foamed lightweight soil of F2 after 3 and 28 days o are shown in Figure 11. It is clear that the vibrational absorption peak of the pos cate Si-O bond at around 445 cm −1 smooths out. Around 445 cm −1 to 625 cm −1 ther flexural vibration absorption peaks of Si-O-Si or Si-O-Al of chain-shaped and lay icates. This indicates that under the action of the alkali activator, silicate polyme occurs, resulting in different degrees of product polymerization. The waveforms products overlap on the curve, so the spectrum tends to be flat. The four-coordin minum expansion vibration peak at 873 cm −1 weakens with time, and Si-O-R at weakens and shifts to a lower wave number. Similarly, the carbonate expansion v peak at 1419 cm −1 weakens and disappears, indicating further depolymerization groups with age. However, the foamed lightweight soil of the F2 change trend wa to that of pure recycled concrete powders. This shows that S-O bonds, Al-O bon C-O bonds are the main products in alkali-activated materials systems. Althou S(A)-H gels covered the original gels, the structure and shape of gels are similar, bonding energy becomes stronger.

Microstructure Results of SEM
SEM images of the foamed lightweight soil of F2 were observed at different ages. As shown in Figure 12a, the microscopic images of the 3-day-old pastes show more cracks in the surface. This might occur because of the low activity of recycled concrete powders. Simultaneously, the alkali activation reaction of recycled concrete powder, slag, and fly ash cannot be completely reacted in the early stages, which leads to the low strength of foamed lightweight soil. As Figure 12b shows, it is difficult to find wide cracks in the surface of 28-day-old paste. This indicated that in the alkali activating system, the hydration of the foamed lightweight soil can effectively fill gaps between early hydration products. Significantly, the prolongation of time not only improves compactness but also improves the strength of foamed lightweight soil. Therefore, the hydration of alkali-activated foamed lightweight soils is a continuous process and cannot fully react in the early stages.

Conclusions
The process of foamed lightweight soil preparation using alkali-activated recycled concrete powder has been proposed, and the effect of foam content on the material is revealed in terms of strength, microstructure, and other parameters. Based on our experimental results, the following conclusions and recommendations can be drawn: 1. As alkali-activated raw materials, when the recycled powder, fly ash, and slag occupied 60%, 20%, and 20%, respectively, it can be used to prepare foamed lightweight soil. The wet bulk density is 912.87 kg/m 3 , the fluidity is 174 mm, the water absorption is 23.16%, and the strength is 1.53 MPa, all of which can meet the requirements for light soil for highway embankment.

Microstructure Results of SEM
SEM images of the foamed lightweight soil of F2 were observed at different ages. As shown in Figure 12a, the microscopic images of the 3-day-old pastes show more cracks in the surface. This might occur because of the low activity of recycled concrete powders. Simultaneously, the alkali activation reaction of recycled concrete powder, slag, and fly ash cannot be completely reacted in the early stages, which leads to the low strength of foamed lightweight soil. As Figure 12b shows, it is difficult to find wide cracks in the surface of 28-day-old paste. This indicated that in the alkali activating system, the hydration of the foamed lightweight soil can effectively fill gaps between early hydration products. Significantly, the prolongation of time not only improves compactness but also improves the strength of foamed lightweight soil. Therefore, the hydration of alkali-activated foamed lightweight soils is a continuous process and cannot fully react in the early stages.  Figure 11. FTIR of foamed lightweight soil.

Microstructure Results of SEM
SEM images of the foamed lightweight soil of F2 were observed at different ages. As shown in Figure 12a, the microscopic images of the 3-day-old pastes show more cracks in the surface. This might occur because of the low activity of recycled concrete powders. Simultaneously, the alkali activation reaction of recycled concrete powder, slag, and fly ash cannot be completely reacted in the early stages, which leads to the low strength of foamed lightweight soil. As Figure 12b shows, it is difficult to find wide cracks in the surface of 28-day-old paste. This indicated that in the alkali activating system, the hydration of the foamed lightweight soil can effectively fill gaps between early hydration products. Significantly, the prolongation of time not only improves compactness but also improves the strength of foamed lightweight soil. Therefore, the hydration of alkali-activated foamed lightweight soils is a continuous process and cannot fully react in the early stages.

Conclusions
The process of foamed lightweight soil preparation using alkali-activated recycled concrete powder has been proposed, and the effect of foam content on the material is revealed in terms of strength, microstructure, and other parameters. Based on our experimental results, the following conclusions and recommendations can be drawn: 1. As alkali-activated raw materials, when the recycled powder, fly ash, and slag occupied 60%, 20%, and 20%, respectively, it can be used to prepare foamed lightweight soil. The wet bulk density is 912.87 kg/m 3 , the fluidity is 174 mm, the water absorption is 23.16%, and the strength is 1.53 MPa, all of which can meet the requirements for light soil for highway embankment.

Conclusions
The process of foamed lightweight soil preparation using alkali-activated recycled concrete powder has been proposed, and the effect of foam content on the material is revealed in terms of strength, microstructure, and other parameters. Based on our experimental results, the following conclusions and recommendations can be drawn:

1.
As alkali-activated raw materials, when the recycled powder, fly ash, and slag occupied 60%, 20%, and 20%, respectively, it can be used to prepare foamed lightweight soil. The wet bulk density is 912.87 kg/m 3 , the fluidity is 174 mm, the water absorption is 23.16%, and the strength is 1.53 MPa, all of which can meet the requirements for light soil for highway embankment.

2.
The addition of an appropriate amount of foam stabilizer greatly improves the foam retention of the material. When the foam content ranges from 55% to 70%, it results in an increase in the proportion of foam and a decrease in the material's wet bulk density. When the foam content is low, the slurry does not wrap the excess foam well, and the material's fluidity decreases significantly. Too much foam increases the number of open pores and it can easily merge into large foam without stability, which reduces water absorption. At a higher foam content, there are fewer slurry components and lower strength.

3.
There are more cracks in the surface of early hydration products, as the reaction progresses, the foamed lightweight soil can effectively fill gaps between early hydration products. However, the hydration of alkali-activated foamed lightweight soils is a continuous process and cannot fully react in the early stages.

4.
Compared with the general fill soil or reinforced soil, foamed lightweight soil is convenient for construction without compaction. It is a construction material that can be constructed quickly and reduce post construction settlement, and has higher economic benefits, and is therefore worth spreading and applying.