Early age reaction of slag in composite cement: Impact of sulphates and calcite

•Dissolved anions in the pore solution from limestone and sulphates influence early age reaction of GGBS.•Distribution of hydrates plays a critical role as the aluminium uptake into hydrated phase assemblages or its concentration in the pore solution.•Combination of limestone and sulphates in ternary blends efficiently exploit aluminate consumption into hydrates and, macro-porosity to sustain slag hydration.•Calcium carbonate and sulphate affect the hydration products and affect comparison of the bound water and calcium hydroxide with degree of hydration.