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This study reports on the microstructure, compressive strength, and drying shrinkage of metakaolin (MK) based geopolymers produced by partially replacing MK by oil palm ash (OPA). The OPA was used as raw material producing different molar ratios of SiO2/Al2O3 and CaO/SiO2. The geopolymer samples were cured at 80°C for 1, 2, or 4 hours and kept at ambient temperature until testing. The compressive strength was measured after 2, 6, and 24 hours and 7 and 28 days. The testing results revealed that the geopolymer with 5% OPA (SiO2 : Al2O3 = 2.88 : 1) gave the highest compressive strength. Scanning electron microscopy (SEM) indicated that the 5% OPA sample had a dense-compact matrix and less unreacted raw materials which contributed to the higher compressive strength. In the X-ray diffraction (XRD) patterns, the change of the crystalline phase after heat curing for 4 hours was easily detectable compared to the samples subjected to a shorter period of heat curing.

High early strength is the most important property of pavement repair materials to allow quick reopening to traffic. With this in mind, we have experimentally investigated geopolymers using low cost raw materials available in Thailand. The geopolymer mortar was metakaolin (MK), mixed with parawood ash (PWA, rubberwood ash) or oil palm ash (OPA) as binder agent. Rubberwood is often used as raw material for biomass power plants in Thailand, especially at latex glove factories and seafood factories, and burning rubberwood generates PWA. Both PWA and OPA are therefore low cost residual waste, locally available in mass quantities. The geopolymer samples were characterized for compressive strength, drying shrinkage, and bond strength to Portland cement mortar with slant shear test. The experimental design varied the contents of PWA and OPA and the heat curing time (1, 2 and 4 h) after hot mixture process. The hot mixture process resulted in very high early strength. In addition, we achieved high compressive strengths, low drying shrinkage, and very significant bond strength enhancement by use of the ashes.

Experimental tests were used to identify the optimum mix of metakaolin (MK) geopolymer mortars needed to achieve high early compressive strength. These tests investigated compressive strength, drying shrinkage, and sodium sulfate expansion of binary and ternary MK-based geopolymer mortar samples containing Parawood ash (PWA) and oil palm ash (OPA) in different replacement levels. The following amounts of PWA and OPA were used: 10 %, 20 %, 30 % and 5 %, 10 %, 15 % by weight of MK, respectively. Sodium hydroxide, sodium silicate and a curing temperature of 80 °C for 2 h were used as parameters to activate the geopolymerization of mortars. Thirteen geopolymer mortar formulations containing PWA and OPA were prepared, and the compressive strengths at 2 h, 6 h, 24 h, 7 days and 28 days were determined. The highest compressive strength of 62 MPa, after 2 h at 80 °C and 2 h at ambient temperature, was obtained with 5 % OPA.

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