Long-term mechanical properties of different fly ash geopolymers

Geopolymer concrete is a sustainable construction material with the potential to act as a replacement for portland-cement (PC) concretes. A detailed investigation of the mechanical properties of four different fly ash geopolymer concretes was carried out up to 1 year of age. Compressive, flexural, and splitting tensile strengths, elastic modulus, and Poisson's ratio of four geopolymer concretes at 1 year ranged between 28 and 88 MPa (4.06 and 12.76 ksi), 3.92 and 6.3 MPa (0.568 and 0.914 ksi), 1.86 and 4.72 MPa (0.27 and 0.684 ksi), 10.3 and 29 GPa (1493.5 and 4205 ksi), and 0.16 and 0.28, respectively. The results show an increase in performance observed between 90 and 365 days for all concretes depending on the fly ash properties. Tarong displayed the highest increase while Gladstone had the least, although Gladstone did display the best performance throughout. The nature of the gel matrix formed, in terms of uniformity and compactness, was observed to determine the mechanical properties. The nature of the interfacial transition zone formed between coarse aggregate and mortar and its density was observed to govern the tensile strength. An increase in porosity and microcracks was seen to negatively affect the compactness of the gel matrix, which in turn affected the elastic modulus.