Synthesis of alkali-activated uncalcined Pisha sandstone cement composites

Abstract

This work reports the utilization of natural Pisha sandstone (PS) as a new precursor to manufacture a novel alkali-activated cement. The rheological properties, compressive strength, drying shrinkage, hydration properties, CO2 emission and energy consumption of the alkali-activated uncalcined Pisha sandstone cement composites (AUPC) were investigated. AUPC mortar containing 1.0 wt% of sodium hexametaphosphate and 0.2 wt% of ligneous calcium sulfonate as plasticizers showed similar rheological properties to those of ordinary Portland cement (OPC) mortar. The compressive strength of AUPC could reach 52.7 MPa after curing at 80 °C for 12 h. The increase of activator concentration (up to 4 mol/L) and the extending of curing time (up to 12 h) significantly increased the hydration of feldspar, montmorillonite and blended slag in AUPC. This resulted in the formation of more low Al/Si ratio aluminosilicate gels, calcium silicate hydrate (C-S-H) gels, and calcium alumina silicate hydrate (C-A-S-H) gels. When the concentration of NaOH and the curing time exceeded 4 mol/L and 12 h respectively, the porosity of AUPC increased, thereby lowering the compressive strength. The 90-day drying shrinkage of the AUPC with 4 mol/L NaOH activator increased from 1.68 to 3.04 mm/m when the water-to-binder ratio increased from 0.25 to 0.45. The total CO2 emissions and energy consumption for producing AUPC were estimated to be much lower than producing OPC.