Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate

The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems

Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (C₃A) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in C₃A dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of C₃A occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto C₃A, reducing its zeta potential to negative values at pH >12. The S and Ca <i>K</i>-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO₄^2– on the partially dissolved C₃A solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for C₃A dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca–S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon C₃A dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated C₃A and thus PC systems, which is important to better controlling the workability of fresh PC concrete

Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate

The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems

Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate

The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems

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

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.JSAN, PRM, and PJPG thanks the financial support of CAPES (Coordination for the Improvement of Higher Education Personnel) [88882.439908/2019-01]. JSAN thanks the University of Malaga (Spain), where the experiments for the characterization of the raw materials were performed. JSAN and AGdT also thank the Spanish Junta de Andalucía [P18-RT-720] research project for the research stage at the University of M´alaga (Spain) and the Graduate Program in Civil Engineering: Construction and Infrastructure (PPGCI) of the Federal University of Rio Grande do Sul (UFRGS). PRM and PJPG thank the Brazilian funding agency FAPESC. The participation APK and CEMC were sponsored by CNPq (Brazilian National Council for Scientific and Technological Development) through the research fellowships PQ2017 305530/2017- 8 and PQ2019 304756/2019-9. The in-situ XRD data collection was carried out by PRM and CEMC at Laborat´orio de Difraç˜ao de Raios-X (LDRX-UFSC). Ms. Patrícia Prates from Laborat´orio de Materiais (LabMAT-UFSC) is kindly acknowledged for the assistance in the SEM analysis