Features of the formation of crystalline hydrosulfoaluminates during hydration of stoichiometric mixtures of calcium aluminates with calcium sulfate

• Published in: Construction & building materials 2023-08, Vol.393, p.132102, Article 132102
• Main Authors: Samchenko, Svetlana V., Korshunov, Andrey V.
• Format: Article
• Language: English
• Subjects: Calcium aluminates; Calcium sulfate; Ettringite; Extent of conversion; Growth and morphology of crystals; Hydration; Monosulfoaluminate
• ISSN: 0950-0618; 1879-0526
• DOI: 10.1016/j.conbuildmat.2023.132102

•A decrease in the basicity of aluminates in the C3A–C12A7–CA–CA2 series causes a decrease in the yield of ettringite and an increase in the yield of monosulfoaluminate when interacting with calcium sulfate.•The formation and growth of ettringite primary crystals occurs in the Al(OH)3 gel matrix.•An increase in pH in the range of 10–13 contributes to a decrease in the size and an increase in the number of primary crystals of ettringite.•The morphological types of ettringite crystals depend on the basicity of the initial calcium aluminate. Calcium aluminates as mineral binders allow to produce cements and concretes with high early strength, low porosity, increased heat resistance, frost resistance and corrosion resistance compared to materials based on Portland cement. An important structure-forming process in aluminate cements is formation of hydrosulfoaluminate phases (ettringite, monosulfoaluminate). Finding out the parameters that allow to control this process is of exceptional importance for construction practice. Hydration of various calcium aluminates C3A, C12A7, CA, and CA2 in the presence of stoichiometric amounts of various forms of calcium sulfate (CaSO4⋅2H2O, α-CaSO4⋅0.5H2O, anhydrite) was studied in order to determine the comparative ratio and the peculiarities of nucleation and growth of the hydrosulfoaluminate phase crystals. It was found that the highest yield of ettringite (∼100 wt% by 28 days) is achieved during hydration hardening of the composition C3A + 3CS¯H2, the yield is reduced in the series C12A7–CA–CA2. Monosulfoaluminate is not formed as the predominant product, its yield reaches 25 wt% during hydration of low-base aluminates. Ettringite crystals can form as a highly dispersed mass (from C3A) or prismatic (fibrous) crystals (from other aluminates). It was clearly shown for the first time that the origin and growth of ettringite crystals occurs in the matrix of aluminum hydroxide gel formed during hydration of calcium aluminates. Varying the pH of aluminate-sulfate mixtures changes the morphology, size and number of primary ettringite crystals. Lamellar hexagonal crystals of monosulfoaluminate are formed from hydroaluminates of type C4AH13-19 at low availability of calcium sulfate. The explanation of the experimental data was carried out using thermodynamic calculations. The results obtained can be used in practice to control the content, ratio and morphology of hydrosulfoaluminate phases in cement stone.