Mechanical, fatigue, and microstructural properties of rubber wood fly ash-based alkali-activated concrete

This research investigates the development of alkali-activated concrete from rubber wood fly ash (RWFA-AAC) as an environmentally friendly alternative to Portland cement. Despite the growing interest in AAC, limited research has examined the mechanical, fatigue, and microstructural characteristics of concrete synthesized from RWFA—a locally abundant, calcium-rich waste material. This research fills that gap by investigating the workability, strength, fatigue resistance, and microstructure of RWFA-based AAC using sodium silicate and 8 M sodium hydroxide as alkaline activators (AL). RWFA-AAC mixed with varying AL/RWFA ratios (0.45–0.60) was evaluated. Setting time increased proportionally with AL/RWFA ratios, with initial setting occurring between 14.50-20 minutes and final setting ranging from 29-39 minutes. Workability assessment through slump testing showed significant improvement with increasing AL/RWFA ratios, progressing from 121 mm at a 0.45 ratio to 240 mm at a 0.60 ratio, and reduced viscosity at higher activator dosages. The 28-day compressive strength peaked at 19.34 MPa (0.45 ratio), while flexural and splitting tensile strengths reached 2.95 MPa and 2.01 MPa, respectively. Fatigue testing showed that specimens with a 0.45 ratio sustained up to 83,382 cycles at 60% stress and 7,243 cycles at 80% stress—far outperforming those with higher ratios. Microstructural analysis via SEM and EDX revealed denser matrices and fewer voids in lower-ratio specimens, contributing to enhanced mechanical performance. These findings demonstrate the potential of RWFA-AAC, particularly at a 0.45 AL/RWFA ratio, to deliver mechanical strength and fatigue resistance comparable to conventional materials, while promoting circular economy practices in biomass-rich regions.