Abstract
The influence of different amounts of gypsum on the hydration of a belite-rich and a ye'elimite-rich belite-calcium sulfoaluminate clinker (BCSA) was investigated. The hydration kinetics, phase assemblages and compressive strength development of cements prepared using ye’elimite/ calcium sulfate molar ratios of 1, 1.5 and 2 were studied. Besides ettringite and monosulfate, aluminium hydroxide, strätlingite, C−S−H, iron-containing siliceous hydrogarnet and hydrotalcite were present as hydration products. Increasing the amount of gypsum increased the ratio of ettringite to monosulfate formed in the cement paste, lowered the amount of pore solution, delayed the dissolution of belite and ferrite, decreased the formation of strätlingite and, in the case of the ye’elimite-rich BCSA, led to an increase in compressive strength. Increased amounts of belite in the clinker led to the formation of higher quantities of C–S–H, at the expense of strätlingite and a lower compressive strength, as belite has a lower degree of reaction than ye’elimite and due to the formation of more C–S–H and strätlingite compared to the more space-filling ettringite. The thermodynamic model established for BCSA cement hydration agrees well with the experimental data. Compressive strength directly correlated with bound water from thermogravimetric analyses and inversely correlated with the porosity calculated from thermodynamic modelling.
Highlights
Aluminium hydroxide is not calculated to be present when the gypsum content is above 2 wt.%, as the aluminium hydroxide released by the hydration of ye’elimite is used for the formation of stratlingite, according to Eq 3
The results show that the belite-calcium sulfoaluminate clinker (BCSA)-Y cement mixtures have significantly higher compressive strengths than the BCSA-B cements at all hydration times, especially at an early age
The main hydration products at an early age are ettringite in addition to amorphous aluminium hydroxide, which precipitated due to the rapid hydration of ye’elimite in the presence of gypsum
Summary
Cement manufacturing is one of the most important industries. After water, the most used material in the world is concrete, which is mainly based on Portland cement, aggregates and water. During the production of Portland cement high emissions of CO2 are released. 212 Page 2 of 17 into the atmosphere, and a high amount of energy is consumed [1, 2]. To reduce the environmental impact related to cement production, alternative binders, such as belite-calcium sulfoaluminate (BCSA) cements, have been developed. BCSA cements are considered low CO2 and eco-friendly building materials, reducing emissions by up to 30% compared to Portland cement, and lowering energy consumption [3–5]. Substantial amounts of industrial wastes can be used to prepare BCSA cements, allowing natural raw materials to be preserved. As indicated by their name, belite-calcium sulfoaluminate cement clinkers contain belite (Ca2SiO4 or C2S) and calcium sulfoaluminate (Ca4(AlO2)6-
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