Effect of combined activator of Ca(OH)2 and Na2CO3 on workability and compressive strength of alkali-activated ferronickel slag system

The utilization of ferronickel slag into the alkali-activated system is promising to mitigate environmental pollution. Due to slower reaction rate and satisfactory strength, a combined activator of Ca(OH)2 and Na2CO3 (molar ratio of 1: 1) was used to investigate the effect of combined activator content (i.e. 3, 5, 7 and 9% Na2O equivalent) on the workability, water requirement of normal consistency, setting time, soundness and compressive strength of alkali-activated ferronickel slag (AFS) system. X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and mercury intrusion porosimetry were used to characterize the reaction products and pore structure. Results indicate that the workability of AFS system decreases with the increase of Na2O equivalent, while the compressive strength first increases and then decreases. The reaction process is accelerated with Na2O equivalent from 3% to 7%, thereby promoting the formation of C-(A)-S-H gel, calcite and hydrotalcite that can improve the compressive strength. Whereas an increasing porosity due to microcracks is observed at 9% Na2O equivalent, thus resulting in the decrease of compressive strength. Considering the workability and compressive strength, the optimal Na2O equivalent in AFS system is 5%.