Influence of Silica Modulus and Curing Temperature on the Strength of Alkali-Activated Volcanic Ash and Limestone Powder Mortar

This present study evaluates the effect of silica modulus (M-s) and curing temperature on strengths and the microstructures of binary blended alkali-activated volcanic ash and limestone powder mortar. Mortar samples were prepared using mass ratio of combined Na2SiO3(aq)/10 M NaOH(aq) of 0.5 to 1.5 at an interval of 0.25, corresponding to M-s of 0.52, 0.72, 0.89, 1.05 and 1.18, respectively, and sole 10 M NaOH(aq). Samples were then subjected to ambient room temperature, and the oven-cured temperature was maintained from 45 to 90 degrees C at an interval of 15 degrees C for 24 h. The maximum achievable 28-day strength was 27 MPa at M-s value of 0.89 cured at 75 degrees C. Samples synthesised with the sole 10 M NaOH(aq) activator resulted in a binder with a low 28-day compressive strength (15 MPa) compared to combined usage of Na2SiO3(aq)/10 M NaOH(aq) activators. Results further revealed that curing at low temperatures (25 degrees C to 45 degrees C) does not favour strength development, whereas higher curing temperature positively enhanced strength development. More than 70% of the 28-day compressive strength could be achieved within 12 h of curing with the usage of combined Na2SiO3(aq)/10 M NaOH(aq). XRD, FTIR and SEM + EDX characterisations revealed that activation with combined Na2SiO3(aq)/10 M NaOH(aq) leads to the formation of anorthite (CaAl2Si2O8), gehlenite (CaO.Al2O3.SiO2) and albite (NaAlSi3O8) that improve the amorphosity, homogeneity and microstructural density of the binder compared to that of samples synthesised with sole 10 M NaOH(aq).

Automated summary

The study evaluated the effect of silica modulus and curing temperature on the strengths and microstructures of binary blended alkali-activated volcanic ash and limestone powder mortar. Using a combination of Na2SiO3(aq)/10 M NaOH(aq) activators resulted in higher compressive strength and improved microstructural characteristics compared to the sole use of 10 M NaOH(aq) activator. Curing at higher temperatures positively enhanced strength development, with more than 70% of the 28-day compressive strength achieved within 12 hours of curing.