Performance of ternary blended cement mortar containing palm oil fuel ash and metakaolin

Abstract

The partial substitution of Portland cement with pozzolans in concrete greatly reduces the environmental pollution due to CO2 emission during cement production. Pozzolans equally enhance mechanical properties and guarantee the production of concrete with minimum costs. These added benefits, result in the increasing use of pozzolans as a significant innovation in the construction industry. Although palm oil fuel ash (POFA) as pozzolan improves strength and durability of concrete, it however delays early strength development due to its low pozzolanicity. Conversely, metakaolin (MK) improves early strength development but equally reduces workability and increases heat of hydration which can be detrimental to the durability of concrete. MK is also deficient in magnesium sulfate environment and at high temperatures. Thus, the scope of application of the binary blends of POFA and MK in the construction industry may be limited. However, the simultaneous use of these materials in the form of ternary blend has the potential to compensate for the deficiencies due to their synergistic interactions. Hence, this study was set out to investigate the effects of the combination of POFA and MK on the properties of cement mortar. Accordingly, a total of 17 different mortar mixtures of binary and ternary blends of POFA and MK at up to 30% replacement levels by weight, and water to binder ratio of 0.55 were used. An optimal ternary blend in terms of strength development and porosity reduction was selected for further detailed investigation. The properties of the optimal ternary assessed at its fresh state include; consistency, setting times, workability and temperature rise. While at its hardened state, compressive strength, sorptivity and microstructures were evaluated. The durability was studied in terms of resistance to sulfuric acid attack, sulfates attack and at high temperatures. The properties of the binders were also examined and their conformity to the relevant standards was confirmed. The results showed that the optimal ternary blend was 10% POFA and 10% MK. The ternary blend significantly improved the workability of mortar with minimal use of superplasticizer compared to MK binary blend. It was also discovered that while the MK binary blend increased the semi-adiabatic temperature by 7% compared to plain OPC, the ternary blend showed a reduction by 4%. Besides, the ternary blend was not only effective in offsetting the low compressive strength of POFA binary at early ages but also enhanced the long-term strength compared to MK, and POFA binary. The TGA and XRD data proved that the early strength improvement of the ternary blend was due to the high pozzolanicity of MK. Furthermore, the ternary blend exhibited superior performance over the MK binary blend and plain OPC in terms of resistance to magnesium sulphate attack and at high temperatures. Generally, the optimal ternary blend of OPC, MK and POFA showed better performance and can be used in construction particularly where the binary blends of either POFA or MK proved deficient. The combined use of POFA and MK would contribute not only to the development of environmental friendly material but also the reduction of CO2 emission.