A Study on the Application of Recycled Concrete Powder in an Alkali-Activated Cementitious System

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In this paper, recycled concrete powder (RCP) was used as a supplementary cementitious material (SCM) in an alkali-activation system. The contents of RCP in the cementitious materials were 0%, 10%, 20%, 30% and 40%, respectively. The fluidity, rheological properties and mechanical properties were tested, while the effects of RCP on the hydration properties of the alkali-activated system were studied by XRD, SEM-EDS, thermogravimetric analysis and the heat of hydration. The results show that the addition of RCP improves the fluidity of alkali-activated slag cementitious materials and changes the rheological index of paste. The change is greatest when the RCP content is 30%, which is 8.5% higher than that without RCP. With the increase in RCP content, the compressive strength of alkali-activated slag cementitious materials first increases and then decreases. The optimum compressive strength was attained with an RCP of 10%. The addition of RCP has little effect on the types of alkali-activated hydration products, but increases the quantity of hydration products. Further, the inactive particles in RCP combine with hydration products to form a dense microstructure. The addition of RCP reduces the early and total hydration heat of alkali-activated slag cementitious material, and delays the emergence of the second exothermic peak after the first peak.