Investigation on the Compressive Strength and the Microstructure of Fly Ash-Based Geopolymer Paste and Mortar

Ordinary Portland cement (OPC) production demands high energy consumption and emits high amounts of greenhouse gases. Geopolymer is an alternative binder which can potentially replace OPC. Paste and mortar is one of the important parameters affecting the compressive strength of concrete. The simultaneous study of paste and mortar under different conditions has not been mentioned by many studies. This paper presents the compressive strength and the microstructural properties of the geopolymer paste and mortar containing low-calcium fly ash (FA). Two curing methods were tested: at ambient temperature and 60 ℃ for 24 h. The ratio of alkaline solution (AAS)-to-FA was investigated at 0.40, 0.45 and 0.50 (in mass). The compressive strengths of the geopolymer paste and the geopolymer mortar were determined at 3, 7, 14, and 28 days while their microstructural properties were examined through scanning electron microscopy (SEM) and the energy dispersive X-ray spectrometry (EDS) analysis. Results from this study revealed that the compressive strength of paste and mortar increases with age. At the same time, when increasing the ratio of AAS/FA, the compressive strength of paste and mortar decreases gradually. When the sand was added and heat cured, the strength of the samples also increased. The analytical image of SEM was shown to be consistent with this intensity variation. The EDS analysis also showed that the choice of distribution in this study was reasonable.