The hybridizations of coal fly ash and wood ash for the fabrication of low alkalinity geopolymer load bearing block cured at ambient temperature

• Load bearing masonry unit can be produced using HCWA–PFA hybrid as primary binder.
• Formation of K-A-S-H, C-S-H and C-A-S-H governs the hardening of HCWA–PFA hybrid binder.
• Hardening of HCWA–PFA hybrid ash can occur at ambient temperature.
• HCWA–PFA hybrid ash system can undergo self-sustained hardening without alkali addition.

The limited application of conventional geopolymer in the construction industry is due to the requirement of high chemical activator dosage, the consequence handling problem associated with the high alkalinity of the material, post forming treatment at elevated temperature which resulted in high manufacturing cost and a material with high embodied energy. The study prescribed herewith proposes the ash hybridization approach between high calcium wood ash (HCWA) and Class F pulverised fuel ash (PFA) and pressurized forming technique to fabricate a low alkalinity geopolymer mortar with adequate mechanical and durability performance for industrial application. A total of eight different mix designs of geopolymer mortar were fabricated with HCWA–PFA hybridization ratio of 100:0, 70:30, 60:40 and 50:50 using two different water/binder ratios of 0.30 and 0.35. The mechanical performance of the HCWA–PFA mortar block was assessed in terms of compressive strength, flexural strength and secant modulus after a given duration of curing at ambient temperature. The durability assessments performed include tests on water absorption and total porosity. The mineralogical, elemental phase and microstructural changes of the binder phase at early age and extended curing duration were also examined. Results are indicative that a hardened binder phase consisting of a combination of potassium polysialate and gehlenite framework can be produced by the hybridization of HCWA and PFA. Moreover, the hybridization of HCWA and PFA at mass ratio of 70:30, 60:40 and 50:50 can be suitably implemented for the fabrication of mortar block with adequate mechanical strength, stiffness and durability performance to be classified as load bearing masonry unit for building construction.