Changes in the phase composition in OPC and blended cement mortars due to carbonation

The effect of carbonation on the phase composition in salt-contaminated (0·8% Cl^- and 1·5% SO₃) and uncontaminated OPC and blended cement mortar specimens exposed to elevated temperature is described. Changes in the phase composition were monitored using X-ray diffraction and differential thermal/thermogravimetric analyses. Results indicated the transformation of cement hydration products to calcite in OPC, while calcite and aragonite were detected in the blended cements. The quantity of calcium carbonate in the contaminated cements was more than that in the uncontaminated specimens. The enhanced carbonation in the contaminated specimens may by attributed to: (i) an increase in the alkalinity of the pore solution; and (ii) changes in the morphology of the hardened cement, brought about by the chloride-sulfate contamination.