The role of sodium and sulfate sources on the rheology and hydration of C3A polymorphs

The higher reactivity of orthorhombic C(3)A (ort-C(3)A) in sulfate-containing solutions, compared with cubic C(3)A (cb-C(3)A), was previously related to the differences in crystal structure or the sodium in the ort-C(3)A pore solution. We analyzed the hydration of cb-C(3)A (in water and NaOH solution) and Na-doped ort-C(3)A in the presence of gypsum and hemihydrate. Calorimetry, in-situ XRD, TGA, and rheological tests were conducted. NaOH accelerated the hydration of cb-C(3)A, but ort-C(3)A still presented higher ettringite formation rate and earlier sulfate depletion. Ort-C(3)A 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-C(3)A but did not significantly influence cb-C(3)A hydration. The crystallization of hemihydrate into gypsum resulted in early (<10 min) stiffing of all C(3)A-hemihydrate pastes. Overall, the higher reactivity of ort-C(3)A is related to differences in crystal structure rather than the sodium in the pore solution.

Automated summary

Experimental study revealed that the higher reactivity of orthorhombic C(3)A in sulfate-containing solutions is mainly attributed to the differences in crystal structure rather than the presence of sodium ions in the pore solution. The hydration process of C(3)A was influenced by the type of sulfate present, with hemihydrate accelerating ettringite formation and sulfate depletion in orthorhombic C(3)A. Overall, the presence of hemihydrate resulted in early stiffening of all C(3)A-hemihydrate pastes.