Effect of water-to-powder ratios on the compressive strength and microstructure of metakaolin based geopolymers

Abstract

This study investigated the properties of geopolymer mortars prepared from metakaolin (MK) partially replaced with oil palm ash (OPA). The geopolymer blend had an alkaline activator (sodium silicate, sodium hydroxide, and water). The main parameters studied were compressive strength, microstructure, and sulfuric acid resistance (losses of strength and mass). Geopolymers were made varying the heat curing and the water to powder ratio (w/p set at 0.45, 0.55, and 0.65). The geopolymer samples were cured at 80°C for 0.5, 1, 2, or 4 h and kept at ambient temperature until testing. The partial substitution of MK by OPA was set at 5, 10, and 15 wt% in the mixtures. The compressive strength was measured after 2, 6, and 24 h and 7 and 28 days. The matrix structure effects were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The results revealed that the geopolymer mortar with 5% OPA and water to powder ratio 0.45 was superior to the other samples. SEM imaging indicated that it had a dense-compact matrix structure, which contributed to the highest compressive strength.