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.