Indian Journal of Engineering & Materials Sciences Vol. 24, December 2017, pp. 499-506 Effect of water-to-powder ratios on the compressive strength and microstructure of metakaolin based geopolymers Abideng Hawa a *, Woraphot Prachasaree b & Danupon Tonnayopas c a Department of Civil Engineering, Faculty of Engineering, Princess of Naradhiwas University, AmphurMuang, Narathiwat 96000, Thailand b Department of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Received 28 July 2016; accepted 22 February 2017 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. Key words: Geopolymer, Compressive strength, Metakaolin, Oil palm ash Geopolymers or alkali activated materials have the advantage that waste materials can be used in them, either as the main component or as a partial replacement. These products have high early strength with good mechanical properties and durability 1,2 . Initially the raw materials for geopolymer synthesis were fly ash (FA), metakaolin (MK), and volcanic ash (VA) 3 , with later use of waste pozzolans such as oil palm ash (OPA) 4 , rice husk ash (RHA) 5,6 and bagasse ash (BA) 7 by partial replacement. When the main raw materials have high calcium content, the resulting material is usually not considered a geopolymer. In this current study, MK with very low calcium content was used. Geopolymers are resistant to acid solutions and have less drying shrinkage than OPC materials. Under high temperature curing of geopolymer, the binder paste contributes to the low drying shrinkage. Some researches 8,9 indicate that fly ash based geopolymer mortars with alkali activator undergo very low drying shrinkage, compared with OPC concrete. Slaty et al. 10 showed that kaolinite based geopolymers with sand filler present excellent stability with very low shrinkage, and no additional shrinkage is observed up to 180 days. The main product of geopolymerization reactions is alkali aluminosilicate, which contributes to the good mechanical properties and resistance to chemical attacks in geopolymers. However, the physical and mechanical properties of component phases do not sufficiently explain the behavior of geopolymers, instead microstructural analysis of geopolymers is necessary. The information about the structure of geopolymers obtainable by X-ray diffraction (XRD) is rather limited. Consequently, scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) were considered for additional information in this study. Some studies 11-13 have investigated the geopolymer structure by SEM and FT-IR. Generally, excessive water content in cement mortar or concrete blend reduces its final strength. However, in the case of geopolymers the effects of water-cement ratio (or water-powder ratio in the current study) are poorly known. Both the mechanical properties and the structure of MK based geopolymers may be affected by the w/p ratio. Currently, OPA is a waste by-product of the palm oil industry, remaining a problem for the agro industries in Thailand and other South East Asia. It has been estimated that the annual industrial solid OPA ————— *Corresponding author (E-mail: abideng.hawa@gmail.com)