Mineralogical and chemical evolution of hydrated phases in the pozzolanic reaction of calcined paper sludge

Calcined residual paper sludge can be reused and valorized in a safe an environmental way by the construction industry. The highly reactive metakaolin produced by calcination of the paper sludge exhibits good pozzolanic properties and permits its incorporation in cement systems. The pozzolanic reaction of metakaolin in Ca(OH)2-saturated solution at 40 °C is reported in this study as a function of time, up to 1 year. The stability of hydrates phases has been evaluated according to the evolving aqueous conditions. Analytical determinations of the solid phase show early formation of C–S–H phases and a later precipitation of laminar minerals, hydrotalcite and strätlingite, at the expense of the C–S–H phases. The thermodynamic calculations confirm the experimental observations.