Experimental Study on the Application of Recycled Concrete Waste Powder in Alkali-Activated Foamed Concrete

The alkali-activated cementitious material was prepared by partially replacing slag with recycled concrete powder (RCP). The influence of RCP substitution rates (10%, 20%, 30%, 40%, and 50% mass fraction) on the performance of alkali-activated slag-RCP-based (AASR) foamed concrete was studied. The fluidity, water absorption, softening coefficient, compressive strength, flexural strength, drying shrinkage, thermal conductivity, and frost resistance of AASR foamed concrete were studied. The results show that the fluidity and softening coefficient of AASR foamed concrete decreases with the increase in RCP content, and the fluidity range is 230-270 mm. Due to the porous structure of the RCP, the water absorption of AASR increases. With the increase in the curing age, the strength of AASR foamed concrete increases. The addition of RCP reduced the mechanical properties of AASR foamed concrete. Compared with the control group, the compressive strength of AASR50 decreased by 66.7% at 28 days, and the flexural strength decreased by 61.5%. However, the 28 d compressive strength of AASR foamed concrete under all RCP replacement rates still meets the standard value (0.6 MPa). The addition of RCP effectively reduces the thermal conductivity of the AASR foamed concrete, and when the RCP content is 50%, the thermal conductivity is lowest, 0.119 W/(m·K); the drying shrinkage of the AASR foamed concrete can be improved by adding RCP, and the drying shrinkage value is lowest when the RCP is 30%, which is 14.7% lower than that of the control group. The frost resistance of AASR foamed concrete decreases with the increase in the RCP content. When the recycled micropowder content is 20-50% and after 25 freeze-thaw cycles, AASR foamed concrete has reached freeze-thaw damage.