Influence of supplementary cementitious materials on ITZ characteristics of recycled concrete

The poor performance of recycled aggregate concrete (RAC) limits its application on wide filed. This paper attempted to study the influence of fly ash (FA), slag powder (SP) and silica fume (SF) on the performance of RAC from a microscopic point of view. The compressive strength and chloride resistance were evaluated when FA, SP and SF were used. Then the evolution of ITZ performance with different admixtures was observed by backscattering scanning electron (BSE) microscopy image, Mercury intrusion porosimetry (MIP), microhardness and so forth. Results indicate that, (1) the supplementary cementitious materials (SCMs) can improve the compressive strength and chloride penetration resistance of RAC, with a certain dosage range for different SCMs; (2) the SCMs can refine the pore structure of ITZs for RAC, increase the microhardness and modify the phases around ITZs; (3) the three SCMs both fill the cement paste voids and promote the compactness of ITZ structure by reacting with calcium hydroxide to generate CSH gels, which in turn enhance the macroscopic performance of RAC. The results of this research can provide a meaningful reference for further study of SCMs to improve the ITZs structure of RAC.