The higher reactivity of orthorhombic C3A (ort-C3A) in sulfate-containing solutions, compared with cubic C3A (cb-C3A), was previously related to the differences in crystal structure or the sodium in the ort-C3A pore solution. We analyzed the hydration of cb-C3A (in water and NaOH solution) and Na-doped ort-C3A in the presence of gypsum and hemihydrate. Calorimetry, in-situ XRD, TGA, and rheological tests were conducted. NaOH accelerated the hydration of cb-C3A, but ort-C3A still presented higher ettringite formation rate and earlier sulfate depletion. Ort-C3A pastes showed 10-20 times higher viscosities and yield stresses. Replacing gypsum by hemihydrate increased the ettringite precipitation rate and anticipated the sulfate depletion of ort-C3A but did not significantly influence cb-C3A hydration. The crystallization of hemihydrate into gypsum resulted in early (<10 min) stiffing of all C3A-hemihydrate pastes. Overall, the higher reactivity of ort-C3A is related to differences in crystal structure rather than the sodium in the pore solution.
