Recycle of soda waste sludge as the limestone material for Portland cement: Chemical composition, physical properties and hydration mechanism

Soda waste sludge (SWS) was discharged from the ammonia-soda process for producing sodium carbonate. It is out of effective treatment method but primarily composed of CaCO3. Limestone material is the necessary raw material for the Portland cement production, accounting for more than 80 wt%. Thus, a Portland cement in this study was produced from the SWS with different Cl- content as the substitute for limestone, and its chemical composition, physical properties, and hydration mechanism was investigated. Results demonstrated that when the Cl- in SWS was reduced to 0.26 wt% through water scrubbing, the chlorine-bearing species such as Ca10Mg0.8[(SiO4)3.4(AlO4)0.6O2Cl], C11A7·CaCl2, and Ca3(SiO4)Cl2 in cement clinkers and the 3CaO·Al2O3·CaCl2·10 H2O and Ca10(SiO4)3(SO4)3Cl2 in its hydration products vanished. And the Portland cement produced from 25.64 wt% SWS meets standard requirements for 52.5-grade cement on the flexural and compressive strengths. Its permeability coefficient was increased from 186.27 ×10−13 m/s to 8.65×10−13 m/s, and the mass loss and compressive strength loss were decreased to 6.53% and 7.33%. Also, the mineral phase composition of cement clinker were the 3CaO·SiO2, 2CaO·SiO2, 3CaO·Al2O3, and 4CaO·Al2O3·Fe2O3, the hydration products were the CSH gels, Ca(OH)2, Al(OH)3, and 3CaO·Al2O3·3CaSO4·32 H2O. This work realizes the application of SWS as the substitute for limestone, which is beneficial to the resource treatment of SWS and the reduction of limestone mining.