Mechanical and Durability Performance of Novel Self-activating Geopolymer Mortars

The research project aimed to explore the feasibility of activating fly-ash based geopolymer by the hybridization of fly-ash (FA) with high calcium wood ash (HCWA), a by-product from timber manufacturing industry, without the addition of conventional alkaline activators and post heat treatment curing regime. The raw materials namely FA and HCWA were characterized in term of their chemical and mineralogical phases by X-ray diffraction (XRD) and X-ray fluorescence (XRF). FA was substituted by HCWA at high replacement level of 50% to 100% at 10% incremental, by binder weight. Hardened geopolymer mortars samples were subjected to water curing and tested on the age of 7, 28 and 7 days + 24 hours hydrothermal treatment. Mechanical performance of the geopolymer mortars were assessed in term of compressive, flexural strength, ultrasonic pulse velocity (UPV) and dynamic modulus. Durability properties namely water absorption, vacuum porosity and capillary absorption were also investigated. Results were positive on the viability of hybridizing FA with HCWA to produce novel self-activating geopolymer mortars as mixtures with PFA replacement level of 50% and 60% showed enhanced mechanical and durability performance at all curing ages in comparison with other HCWA-PFA geopolymer mortar mixtures. Early strength development of HCWA-PFA geopolymer mortars was mainly contributed by the combination of hydraulic reaction of HCWA and geopolymerization of FA. On prolonged curing, strength development was due to the aforementioned reactions, plus pozzolanic reaction between the reactive silica from FA and portlandite formed from HCWA, producing additional secondary C-S-H gels. This experimental program showed positive findings in incorporating highly alkaline materials i.e. HCWA (12% K2O) towards activating FA based geopolymer, thus eliminating the needs of external alkaline activator in conventional geopolymer mix design.