Effect of Activator-to-Binder Ratio and Curing Temperature on the Properties of Low-Density Alkali-Activated Mortar Incorporating Phragmites Australis Ash

Valorizing agricultural residues as cement-free binders may reduce dependence on Portland cement. However, the performance of silica-rich biomass ashes under alkali activation remains insufficiently understood. This study explores the effects of alkali activator-to-binder ratio (Al/B), curing temperature, and age on mortars containing Phragmites australis ash (PAA) as the sole binder. Specimens with Al/B ratios of 1.0, 1.1, and 1.2 were activated with sodium silicate and 12 M sodium hydroxide (SS/SH ratio of 2.5) and cured at 20, 40, and 80 °C. Flow, density, ultrasonic pulse velocity, flexural strength, compressive strength, and dimensional stability were evaluated. Compared with the Portland-cement control, PAA mortars showed lower flow and mechanical performance, but densities remained below 1500 kg/m3 under all curing conditions. Increasing Al/B significantly increased flexural and compressive strengths at 28 days. Elevated-temperature curing significantly increased ultrasonic pulse velocity and both strengths. The maximum PAA compressive strength was 4.67 MPa at 90 days after curing at 80 °C, representing a reduction of 62%. PAA mortars showed higher drying and autogenous shrinkage with increases of 8.5–9.5% and 2.9–4.4%, respectively. However, PAA mixes resulted in lower chemical shrinkage than the control mix, by around 0.2–0.6%. The present results suggest that PAA can be used as a low-density non-structural mortar in non-load-bearing applications, subject to durability and component-scale studies.