Quantitative correlations among textural characteristics of C–S–H gel and mechanical properties: Case of ternary Portland cements containing activated paper sludge and fly ash

The hydraulic activity of ternary Portland cements containing different proportions of commercial thermally activated paper sludge (APS(I)) and fly ash (FA) from coal combustion was evaluated from the textural characterization of the C–S–H gels formed during hydration. The study was accomplished by means of the specific surface area (SSA) and pore-size distribution (PSD), which were measured by the sorption isotherms of nitrogen gas and the BET method. The microstructure was characterized by scanning electron microscopy (SEM). The incorporation of such admixtures (APS and FA) clearly revealed an increase in both porosity and SSA. This effect decreased with the progress of hydration, together with a refinement of the nano-porous structure of C–S–H. In addition, high correlations were found between nano-structural characteristics of the C–S–H gel and mechanical compressive strengths.