Influence of nano-attapulgite on compressive strength and microstructure of recycled aggregate concrete

In order to solve the problem of waste concrete caused by demolition of old buildings, calcined nano-attapulgite (CNAT) was added into recycled aggregate concrete (RAC) to form nano-attapulgite recycled concrete. In this paper, the experiment was divided into two stages. Stage 1: The influence of activity of nano-attapulgite (NAT) which is calcined at different temperatures was analyzed by thermo-gravimetric analysis (TGA), scanning electron microscope (SEM) and X-ray diffraction (XRD) to find out the optimal activation temperature. Stage 2: The influence of different CNAT content and replacement rate of recycled coarse aggregate (RA) on the compressive strength of RAC were investigated, and the microstructure was analyzed by SEM, XRD and Barret-Joyner-Halenda (BJH) method. The results showed that when the calcination temperature is 500 °C, NAT has the highest activity; the compressive strength of RAC decreases gradually with the increase of replacement rate of RA. When the replacement rate of RA were 30 wt% and 50 wt% and the content of CNAT was 6 wt%, the compressive strength of concrete were the largest and the compressive strength is 13.2% and 16.1% higher than that without CNAT, respectively. Because of its filling effect, surface effect and pozzolanic activity, CNAT can fill the pores and cracks in the old mortar, new mortars and the interfacial transition zone (ITZ) of RAC. The harmful pores of RAC is refined, and the concrete is more dense. Above all, CNAT can increase the macroscopic mechanical properties of RAC by improving its microstructure effectively, which provides a theoretical basis for the further study of RAC.