Influence of metakaolin on pH of cement paste

The influence of metakaolin (MK) additive in ordinary Portland cement (OPC) paste to its pH was studied by direct pore solution pH measurement and by XRD and thermal analysis. This research program was motivated by importance of pH for reinforcement corrosion; while the steel demands the high pH for its corrosion protection, the basalt-based reinforcement elements are damaged in the high pH. The initial hypothesis thus was to reduce the concrete pH by adding of MK. The mixtures containing 0–20% of MK were prepared and characterized after 28 days. MK presence reduced the pH due to proceeding pozzolanic reaction from 12.57 to 12.39, i.e. in terms of OH− molarity from 0.0372 mol/L to 0.0245 mol/L. The pH decrease was obviously caused by pozzolanic reaction between MK and Ca(OH)2 generated in OPC hydration; it means that blended cements are more suitable for basalt elements reinforcement than OPC. There was certain difference between TGA and XRD results indicating, that part of the present Ca(OH)2 is not crystallized in form of portlandite.