Effects of recycled concrete powders on the rheology, setting and early age strength of cement paste

This work studies the effect of substitution rate and types of recycled concrete powders (RCPs) on the rheology over time, setting time, and 1 day compressive strength of cement paste. Two RCPs with distinct compositions, namely RCPI and RCPII, were adopted. Results showed that incorporating RCPs contributes to the hydration of cement paste in the first 5 h, and the initial zeta potential of cement paste is reduced after replacing cement by RCPs, which results in that utilizing RCPs increases the initial rheological properties of cement paste and its growth rate over time. Specifically, the paste with 30% RCPs exhibits 66.85-122.79% higher initial static yield stress than the pure cement paste, and the structuration rate in the paste with 30% RCPs is 10.26-141.03% higher than that of plain paste. Utilizing RCPs decreases the initial setting time. Utilizing RCPs is detrimental to the hydration reaction beyond about 8 h and pore structure of cement paste, the total porosity of paste with 30% RCPs at 24 h is 11.31-13.70% higher than that of plain paste, and the content of harmful and more harmful pores shows an increase of 19.30-30.43%, which results in a nearly 50% reduction in the 1 day compressive strength. Compared to the RCPI cement paste, the RCPII cement paste with the same RCP content exhibits lower initial rheology, higher growth rate of rheological parameters, lower total porosity, thus has relatively higher 1 day compressive strength.