Influence of alkali solution on compressive strength of calcined clay and GGBS alkali activated mortar

Calcined clay is presently considered as aluminosilicate source that will extend the application of alkali activated materials (AAM) due to its low embodied energy and global abundance, but its strength performance when cured under ambient conditions depends largely on the calcination process and the mineralogy of the clay etc. In this paper, the density, compressive strength and ultrasonic pulse velocity of ambient-cured calcined clay is compared with that of GGBS mortar activated by sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solutions. Three groups of mixes were tested, group one and two based on varying the alkali solution to the respective binder ratio, while group three mixes are based on activation of the GGBS with different molarities of the NaOH solution from 4 M to 12 M concentration. The results shows that alkali solution to binder ratio of 0.35 gives the highest compressive strength for all the GGBS mortar mixes. Also, the effect of increasing the molarity of the NaOH solution on the strength of the mortar did not show any clear trend implying that the optimum molarity required is such that there is sufficient alkali to complete the reaction and not in excess to disrupt the geopolymerization reaction or insufficient to initiate and satisfy the reaction. The results further show that Na2SiO3 solutions enhances the strength of the GGBS mortar more than NaOH solution by enriching the silicate content of the N-A-S-H gel thereby increasing the binding effect.