Mechanical performance and reaction mechanism of copper slag activated with sodium silicate or sodium hydroxide

Copper slag (CS) is an industrial by-product of copper metal smelting. In the present study, two alkaline activators (sodium silicate (SS) and sodium hydroxide (SH)) were used to develop different samples of alkali-activated CS cementitious material, respectively. The mechanical performance of alkali-activated CS cured with the standard and steam programs were investigated. The results indicate that the steam program was more suitable. In particular, SS was more effective than SH in improving the compressive strength, i.e., the highest 28 day compressive strength of CS activated with SS was 30.3 MPa; SH achieved only 4.7 MPa. Furthermore, the effect of SS and SH on the microscopic properties of alkali-activated CS were studied. The C–S–H in the CS activated with SS was mostly amorphous and had a higher degree of polymerization. By contrast, in the CS activated with SH, some reaction products are network structure, which was composed of plate-shaped plombierite with high crystallinity. The drastically different structures and degrees of polymerization of the reaction products resulted in different matrix densities, which affected the compressive strength.