Workability Loss and Compressive Strength Development of Cementless Mortars Activated by Combination of Sodium Silicate and Sodium Hydroxide

To explore the significance and shortcomings of an environment-friendly binder using powder typed activators, 16 alkali-activated (AA) cementless mortars and a control ordinary Portland cement (OPC) mortar were mixed, cured under room temperature, and tested. Both fly ash (FA) and ground granulated blast-furnace slag (GGBS) as the source material were activated by a combination of sodium silicate and sodium hydroxide powders. The main variables examined were the mixing ratio of sodium oxide $\left({\mathrm{Na}}_2\mathrm{O}\right)$ of the activators to source material by weight, and the Blain fineness of the GGBS. The flow loss and compressive strength development of the mortars tested were examined according to the alkali quality coefficient explaining the silicon oxide-to-sodium oxide ratio $(\mathrm{Si}{\mathrm{O}}_2∕{\mathrm{Na}}_2\mathrm{O})$ in an alkaline activator, and the silicon oxide-to-aluminum oxide $(\mathrm{Si}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3)$ ratio and calcium content in the source material. The hydration products and microstructural characteristics of the AA pastes sampled from AA mortars were also investigated to evaluate the effect of the type and fineness of source material on the compressive strength of the AA mortar. The measured compressive strength development of the AA mortars was compared with an empirical equation for OPC concrete specified in ACI 209. The test results show that the flow loss and compressive strength development of the AA mortars are significantly dependent on the alkali quality coefficient and the fineness of the source material. Although no meaningful compressive strength develops in the FA-based AA mortars, the $28\text{day}$ compressive strength of the GGBS-based AA mortars having an alkali quality coefficient of 0.023 is comparable to that of the control OPC mortar, showing that the higher the fineness of GGBS, the higher the compressive strength of AA mortars. Scanning electron microscope image and x-ray diffraction clearly shows that the surface density of the calcium silicate hydrates gel in the GGBS-based AA pastes increased with the increase of fineness of the GGBS.