In contrast to the hydration of C 3 S in water, the relationship between the storage modulus and the heat of hydration can not be modeled with a single exponential function. We fitted the data with a piecewise exponential function containing two exponential functions. The complex relationship between storage modulus and heat suggests that different microstructural mechanisms control the strength build-up in C 3 A, alkali salt systems. The details of a multi-step stiffening reaction need to be investigated in future studies.