A mixture proportioning method of geopolymer recycled concrete based on the characteristics of matrix rheology and coarse aggregate skeleton

This study aims to build a mixture proportioning method for geopolymer recycled aggregate concrete (GRAC) based on the excess mortar film thickness theory and concrete skeleton model theory. The mix design protocol is built by establishing the correlation between the workability/strength of corresponding geopolymer concrete and mechanical/rheological properties of geopolymer mortar. The GRAC specimens were prepared with varying recycled coarse aggregate (RCA) replacement ratios (0 %, 25 %, 50 %, 75 %, and 100 %), mortar matrix strength grades (M30, M40, and M50), and coarse aggregate volume fractions (0.3, 0.35, and 0.4). The fresh properties of the geopolymer paste and mortar matrix were characterized in terms of rheological behavior, while the workability of the geopolymer concrete was assessed via slump and slump flow tests. The mechanical performance was evaluated by testing the compressive strength of both geopolymer paste, mortar matrix and concrete. Test results indicate that the workability of GRAC is predominantly influenced by the rheological behavior of its mortar matrix and the coarse aggregate content. The workability indicators increase with decreasing yield stress of mortar matrix or increasing mortar film thickness. The strength contribution of coarse aggregate skeleton initially increases but subsequently decreases with increasing coarse aggregate volume fraction. When the RCA replacement ratio increases from 0 % to 100 %, the strength contribution of coarse aggregate skeleton of GRAC decreased by 33.42 %–61.76 %, 27.90 %–30.15 %, and 30.51 %–37.34 % for coarse aggregate volume fractions of 0.3, 0.35, and 0.4, respectively. The later reduction in strength is attributed to transgranular fracture through the recycled coarse aggregates, resulting from the low crushing value of RCA. A mixture proportioning method based on the excess mortar film thickness theory and concrete skeleton model theory enables the prediction of GRAC’s compressive strength within a 15 % error margin. The concrete specimens designed with this method, targeting strength grades of C30 to C50, achieved compressive strengths ranging from 32 MPa to 56.8 MPa and slump values between 190 mm and 200 mm. This study provides design methodologies and theoretical foundations for the engineering application of GRAC.