This study focuses on the reaction of dolomite powder in combination with metakaolin in Portland composite cement pastes. We studied paste samples cured at 20 °C, 38 °C, and 60 °C for up to 1 year. In these systems, the only magnesium-containing hydration phase of dolomite observed was hydrotalcite. Dolomite reacted notably already after 90 days when cured at 60 °C, whereas at lower curing temperatures the reaction was limited. The increased availability of aluminium due to the addition of metakaolin did not contribute to the formation of hydrotalcite. The refined pore space due to the metakaolin addition did not inhibit the hydrotalcite formation. However, the almost total absence of portlandite due to the pozzolanic reaction of the metakaolin inhibited the dolomite reaction, even in pastes with high porosity. Portlandite seems to be the driving force for the reaction as its absence is inhibiting the reaction to take place.

In this study, we investigated well-hydrated cement pastes containing dolomite and metakaolin cured at 38 °C or 60 °C, which were exposed to NaCl or CaCl2 solutions of various concentrations.We determined the chloride-binding capacity, the phase assemblage and the composition of hydration phases formed. The dolomite reaction led to the formation of hydrotalcite, which contributed considerably to the chloride binding of the pastes. When the samples were exposed to CaCl2, significantly more chlorides were bound in the hydrotalcite than when the samples were exposed to NaCl. It was shown that hydrotalcite contained a similar amount of chloride per mol compared to Friedel's salt when exposed to CaCl2. By mass balance calculations, it was shown that the hydrotalcite formed in the samples containing dolomite can contribute to the chloride binding of the cement pastes to a similar extent as the Friedel's salt formed in the samples containing limestone.

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The scarceness of high-quality limestone obliges the cement industry to consider alternative supplementary cementitious materials (SCMs) for the production of blended cements. This study investigated the potential usage of dolomite instead of limestone as an addition to Portland metakaolin cement by measuring the development of the compressive strength and phase assemblages at 5 °C, 20 °C or 38 °C. Laboratory grade materials were used to identify potential differences in the impact of the carbonate on the phase assemblages. As with limestone, a strength increase was observed when dolomite is added at temperatures >5 °C due to the formation of additional carbonate AFm phases and the stabilization of ettringite. Differences were observed in the amount and type of the carbonate AFm and AFt phases formed. Thermodynamic modelling in combination with the experimental results indicate that the dolomite and limestone affect Portland metakaolin cement in a similar way, with the reactivity being the major difference between the two carbonate sources. This indicates that with regard to the strength development up to 90 days dolomite can be used instead of limestone to replace parts of a Portland metakaolin cement.

To reduce CO2 emissions during the production of cement and to cope with increasing demands for concrete, and thereby cement, the cement industry needs to identify new supplementary cementitious materials. These new composite cements should provide, among others, a similar or improved durability of the concrete structures. This study investigated the hydrate phase assemblage in Portland cement pastes containing dolomite or a combination of dolomite and metakaolin after leaching, carbonation, and chloride exposure. The phase assemblage and phase compositions of the exposed samples and the unexposed reference samples were investigated using TGA, XRD, and SEM-EDS. The reaction of dolomite in the cement paste resulted in the formation of hydrotalcite. It was found that, unlike most other hydration phases, hydrotalcite can withstand high degrees of leaching and carbonation. When the samples were exposed to a chloride solution, the formation of a chloridecontaining hydrotalcite was observed.