The influence of metakaolin on the reaction of dolomite in Porland composite cement pastes

Dolomite (CaMg(CO3)2) is a promising alternative to the commonly used limestone as an SCM. The present study investigated the reaction of dolomite in various combinations with metakaolin as replacement of 40 %wt of Portland cement clinker in composite cement pastes. The microstructure and phase assemblage of the paste samples cured at 20 °C, 38 °C or 60 °C for up to 1 year was investigated with TGA, XRD, SEM-EDS and MIP. It could be shown, that in samples without any metakaolin approx. 60 %wt of the added dolomite has reacted in the cementitious system when cured at 60 °C after 1 year. At lower curing temperatures (20 °C, 38 °C) only minor amounts of dolomite reacted within 1 year. The phase assemblage of samples containing dolomite or limestone were similar after 1 year, with the exception that hydrotalcite (Mg4+xAl2(CO3)y(OH)16·4(H2O)) was detected in samples containing dolomite. The addition of metakaolin influences the system in three different, probably antagonistic, ways. First, the reaction of metakaolin delivers aluminum to the system. This can lead to a notable uptake of aluminum in the C-S-H phase and, in the presence of carbonates, to the formation of additional AFm-CO3 phases. Whether the aluminum delivered from the metakaolin reaction will also lead to the formation of additional hydrotalcite will be investigated. Second, the reaction of metakaolin results in the densification of the microstructure of the cement paste. This refinement in the pore structure could be a possible limitation of the reactions of dolomite and clinker minerals at high metakaolin additions. Third, metakaolin reacts with portlandite originating from the cement hydration. Dolomite was also reported to consume portlandite during its reaction. The lack of portlandite in metakaolin-rich systems could therefore be another possible limitation for the reaction of dolomite, which reacts notably slower than metakaolin.      «

Dolomite (CaMg(CO3)2) is a promising alternative to the commonly used limestone as an SCM. The present study investigated the reaction of dolomite in various combinations with metakaolin as replacement of 40 %wt of Portland cement clinker in composite cement pastes. The microstructure and phase assemblage of the paste samples cured at 20 °C, 38 °C or 60 °C for up to 1 year was investigated with TGA, XRD, SEM-EDS and MIP. It could be shown, that in samples without any metakaolin approx. 60...     »