Effects of modulus ratio and dosage of alkali-activated solution on the properties and micro-structural characteristics of alkali-activated fly ash mortars

This study investigates the properties and micro-structural characteristics of alkali-activated fly ash (AAFA) mortars. Sodium oxide (Na2O) dosages of 121 and 150 kg/m3 and liquid sodium silicate with alkaline modulus ratios (mass ratio of SiO2 to Na2O) of 1.23 and 0.8 were prepared as activators. Three alkaline solution to binder ratios of 0.35, 0.5, and 0.65 were selected to cast AAFA mortars. Compressive strength test, drying shrinkage test, water absorption test, initial surface absorption test, mercury intrusion porosimetry test (MIP), scanning electron microscopy (SEM) and X-ray diffraction analysis were conducted and their performance was discussed and compared with reference mortars produced with ordinary Portland cement (OPC). Test results show that both alkaline modulus ratio and the dosage of Na2O are two significant factors influencing the characteristics of AAFA mortars. The higher alkaline modulus ratio and dosage of Na2O are, the superior properties of AAFA mortars achieved. SEM and XRD demonstrate that the activation products of AAFA mortars are mainly amorphous alkaline aluminosilicate gels, which are attributed to the compressive strength. Under the alkaline modulus ratio of 1.23 and liquid/binder ratio of 0.5, AAFA mortars with Na2O dosage of 150 kg/m3 may be considered as the optimum mix design based on the results.