Silica fume activated by NaOH and KOH in cement mortars: Rheological and mechanical study

Abstract

Ordinary Portland cement (OPC) is the most used binder in the world, applied in practically all phases of the building. The problem is that this binder is highly harmful to the environment due to its production stage, which involves clinkerization by burning limestone. In this context, the objective of this work was to evaluate the potential of using silica fume as a partial substitute for cement, however verifying its activation potential. Silica fume is widely used as a pozzolanic material. In this work, the use of the material as alkali cement activated with sodium and potassium hydroxide (NaOH and KOH) was tested. The methods used consist of producing mortars in the compositions 1:3:0.60 (OPC: natural sand: water) and replacing the OPC with silica fume (10 and 20%) in three ways: i) without activation, as a pozzolanic material; (ii) activated by NaOH; and (iii) activated by KOH. Cylindrical specimens measuring 50x100 cm were produced, containing 0, 10 and 20% silica fume replacing OPC. Properties were evaluated in the fresh state, such as depth of penetration, consistency and squeeze flow, and tests in the hardened state, such as water absorption, density, and compressive strength. The results in the fresh state indicate that the compositions containing NaOH accelerate the setting process, but without impairing the workability of the mortar before the hardening process. The results of the fresh state of the compositions containing KOH indicate that the dissolution of this material is not as efficient in the cement medium in the first minutes after mixing the material. This impairs the workability of the mortar. In the hardened state, the density and water absorption results showed the occurrence of alkaline activation of silica fume. The results of compressive strength and diametral compressive strength were better for the compositions with NaOH and KOH, due to the occurrence of alkaline activation of silica fume which provided the formation of tobermorite and zeolites. Therefore, as an improvement this work concluded that the use of hydroxides together with the pozzolanic material silica fume is a more efficient way of replacing OPC than the isolated use of pozzolan, due to the occurrence of the alkaline activation process.