Investigation of the effect of using recycled aggregate, metakaolin, and fly ash on the performance of structural lightweight concrete

This study investigated the effects of recycled aggregate (RA) substitution and fly ash (FA) and metakaolin (MK) as cement replacements on the mechanical and durability properties of lightweight concrete produced with natural perlite aggregate (NPA), as well as their performance under high temperatures. For this purpose, NPA was replaced with RA at rates of 0%, 20%, and 40% in both coarse and fine aggregate fractions. Four different mixtures were produced for each RA content: a control mixture, 10% FA, 10% MK, and 10% FA + 10% MK. Unit weight, compressive strength, splitting tensile strength, sorptivity, water absorption and electrical resistivity tests were performed on the produced concretes. In addition, to determine the behavior of concretes under high temperatures, weight losses and compressive strength losses were determined in concrete samples subjected to temperatures of 250℃, 500℃ and 750℃, and visual evaluations were also carried out. Increasing the RA content generally reduced the mechanical properties, durability performance and high-temperature resistance of the concretes. However, extending the curing period from 28 to 90 days and the use of mineral admixtures helped to compensate for these effects. Higher RA content slightly reduced mechanical properties, but the %20RA + %10FA + %10MK mix is the most balanced option as it improves durability, uses waste materials, and reduces cement use.