Mitigating Leachability from Fly Ash Based Geopolymer Concrete Using Recycled Concrete Aggregate (RCA)

Geopolymer concrete was synthesized from coal fly ash, which contains the range of silica and alumina that is needed for the formation of geopolymer phase. Studies on the leachability of geopolymer concrete made from fly ash have shown that it leaches out elements such as arsenic (As), chromium (Cr) and selenium (Se), which are of great environmental concern due to their toxicity and high mobility in the alkaline pH range that can be found in pore solution of geopolymer concrete. The leaching of these elements in high pH environment can be reduced by the addition of calcium that aids in the formation of calcium silicate hydrates (CSH) or precipitates of calcium. In this study, recycled concrete aggregate (RCA) was used as the additional calcium source in making geopolymer concretes. Different mixes were produced with 100% natural aggregate (coarse and fine) and by replacing 10% and 50% coarse aggregate with coarse RCA. The Dutch availability test (EA NEN 7371) was utilized in investigating leachability from the geopolymer samples. The results demonstrate that the use of RCA affects the compressive strength and reduces the leachability of oxyanion elements from geopolymer concrete.