Experimental investigation on freeze–thaw resistance of thermally activated recycled fine powder concrete

Abstract

Recycled fine powder (RFP) is a new auxiliary cementitious material with potential activation. However, prior to its applications in concrete, its durability should be examined. This study focuses on RFP as the study subject. Through the freeze–thaw test of concrete, indexes such as the appearance change, mass and relative dynamic elastic modulus loss, microstructure, and pore and bubble structures are analyzed. The freeze–thaw model is used to predicate the service life of concrete under freeze–thaw cycles. The effect of a high RFP dosage on the freeze–thaw resistance of concrete before and after thermal activation is comprehensively examined. Results show that the appearance damage increases gradually when the RFP content is more than 15 %. For RFP dosages of 45 % and 60 %, thermally activated RFP concrete exhibits better freeze–thaw resistance than inactivated RFP concrete, and the maximum number of freeze–thaw cycles that thermally activated RFP can withstand is greater than that for inactivated RFP concrete. The mass loss and relative dynamic elastic modulus loss of thermally activated RFP concrete at 60 % dosage are lower than those of inactivated RFP concrete. Microscopic analysis shows that the most probable pore size, the proportion of harmful pores and more-harmful harmful pores of the thermally activation of RFP concrete is smaller than that of the inactivated RFP concrete when the dosages are 45 % and 60 %. Under the SEM image of dosage 60 %, the gels of the thermally activation of RFP concrete are more dense. These results indicate that the thermally activation of RFP tends to refine the pore structure and improve the pore distribution. Furthermore, the average radius and spacing of bubbles are smaller when the dosage of activated RFP is 60 %, thereby improving the freeze–thaw resistance of concrete. In this study, the freeze–thaw model used in this paper that can effectively describe the freeze-thaw change and predict the service life of RFP concrete. When the RFP dosage is 60 %, the lifespan of thermally activated RFP concrete is 2.4–3 times longer than that of inactivated RFP concrete, meeting the needs for structural designs in the central and southern regions of China for 50 years. The research results provide a theoretical basis for RFP resource utilization and the determination of the actual service life of RFP concrete.