Sodium sulphate effect on the mineralogy of ternary blended cements elaborated with activated paper sludge and fly ash

This paper presents an exhaustive study about the identification and evolution of the mineralogical hydrated phases of ternary Portland cements (TPC), containing thermally activated clay waste (paper sludge) and fly ash, exposed to sodium sulphate salt up to 180 days. The repercussion of these mineralogical changes on the microstructure and strength properties is also discussed in this work. The reaction mechanisms of sulphate ion with the paste was carried out through a microstructure study, which included the use of scanning electron microscopy (SEM), porosity and pore-size distribution and X-ray diffraction (XRD). The results showed that ternary Portland cement was stable against sodium sulphate attack at the conditions here studied. The enhancement of flexural strength was a result of the formation of non-expansive ettringite inside the pores and an alkaline activation of the pozzolanic reaction of additions promoted by the ingress of sulphate and secondary related reactions. Accordingly, the microstructure was strongly refined.