Sodium-silicate-activated slag for acid-resistant geothermal well cements

This paper evaluates the usefulness of sodium-silicate-activated slag (SSAS) cement for completing geothermal wells containing highly concentrated H2SO4 and some CO2· Using a 20wt% sodium silicate solution (SiO2/Na2O mol ratio of 3·22) as the alkali activator, the SSAS cements autoclaved at temperatures up to 200°C displayed an outstanding compressive strength of more than 80 MPa, and a minimum water permeability of less than 3·0 × 10−5 darcy. The combination of calcium–silicate–hydrate (C–S–H) and tobermorite phases was responsible for strengthening and densifying the autoclaved cement. At 300°C, an excessive growth of wellformed tobermorite and xonotlite crystals generated an undesirable porous microstructure, causing the retrogression of strength and enhancing water permeability. Although all the phases formed in the autoclaved cements were vulnerable to reactions with H2SO4, so depositing bassanite scales as corrosion product over the cement's surfaces, the C–S–H phase played an important role in reta...