IntroductionThe use of calcium carbonate powder as a mineral admixture in cementitious materials can reduce CO₂ emissions from clinker calcination and fossil fuel combustion, offering a low-carbon solution for sustainable cement production. Calcium carbonate can effectively fill the pores, refine the pore size, and promote the growth and precipitation of early hydration products of cement. On the other hand, calcium carbonate can react with the aluminum phase in cement to generate calcium carbon-aluminate, improving the mechanical properties of cementitious materials. There are two main crystal types of calcium carbonate in nature, calcite and aragonite. Due to the difference in density, stiffness and crystal structure of calcite and aragonite, calcite and aragonite-type calcium carbonate have different effects on the properties of cementitious materials. This paper elucidated the effects of different crystal types of calcium carbonate on the microstructure and mechanical properties of cement pastes and calculated the various effects of calcium carbonate in cement pastes, which would provide a solid and effective theoretical basis for the application of calcite and aragonite in engineering practice.MethodsThe raw materials used in this study include Portland cement (P·I 42.5), slag, calcite and aragonite. The calcite and aragonite were prepared by wet carbonation and analyzed for composition and purity by XRD and TG. Calcite and aragonite were separately added to the cement–slag composite system to prepare two groups of ternary systems with a water-cement ratio of 0.4. The simplex-centroid mixture design method was used to optimize the relative composition design of the raw materials in the cement pastes. According to the cement pastes composition design, 20 mm×20 mm×20 mm specimens were moulded for testing compressive strength, ϕ20 mm×40 mm specimens were moulded for testing splitting tensile strength, and ϕ25 mm×25 mm specimens were moulded for pore structure analysis.The mechanical properties and pore structure of the specimens were tested after curing for 3, 7, 28 d and 90 d in standard curing conditions (temperature (20±1)℃, relative humidity ≥ 96%). The cement paste powders of corresponding age were taken for XRD and TG tests to analyse the type and content of hydration products. Finally, the nucleation effect, dilution effect, filling effect and chemical effect were quantified based on the contribution of calcite and aragonite to the sample compactness.Results and discussionThe addition of approximately 15% calcite or aragonite enhanced the compressive strength of the cement paste during early hydration. This was because calcite or aragonite promoted cement hydration, improved the early hydration degree of cement, and increased the content of hydration products such as Ca(OH)₂ and C-S-H. The compressive strength of the calcite specimen was significantly higher than that of the aragonite specimen at the same dosage. Since the density of calcite was lower than that of aragonite, the filling effect of calcite was higher than that of aragonite, and calcite had a more significant nucleation effect than aragonite, with more hydration products deposited on the surface.After 28 days of hydration, the compressive strength and splitting tensile strength of specimens mixed with calcium carbonate and slag were significantly increased. The synergistic effect of calcium carbonate and slag significantly improved the chemical reaction degree of calcium carbonate, and increased the content of calcium carboaluminate. Meanwhile, the formation of calcium carboaluminate inhibited the transformation of ettringite into calcium monosulfoaluminate, the solid phase volume of hydrated product increased, and the compactness of cement paste increased. Therefore, the mixture of slag and calcium carbonate was conducive to the continuous improvement of the mechanical properties of cement pastes.With the increase of hydration age, the toughening effect of aragonite on cement paste was gradually improved. At 90 d of hydration, the content of aragonite was 20%~30% (in mass), the content of slag was 5%~15%, the contribution of chemical effect to the cement paste compactness was 15%~21%, and the splitting tensile strength of the specimen was increased by 39% compared with that of pure cement specimens. The improvement of the tensile strength of aragonite was mainly related to the chemical effect. The calcium carboaluminate formed by the reaction strengthened the bond between aragonite and cement paste, enhancing the toughening effect of aragonite.ConclusionsThe compressive strength of calcite specimens was found to be higher than that of aragonite specimens at the same calcium carbonate content during the early stage of hydration, primarily due to the increased filling and nucleation effects exhibited by calcite compared to aragonite. Due to the toughening effect of fibrous aragonite in cement pastes, the splitting tensile strength of the aragonite system was significantly higher than that of the calcite system. With the increase of hydration age, the reaction degree of calcium carbonate and aluminum phase in the composite system increased, the chemical effect continued to increase, and the compactness of the matrix increased. The addition of calcite or aragonite and slag was beneficial to the continuous improvement of the mechanical properties of the cement pastes. The increase in calcium carboaluminate content enhanced the toughening effect of aragonite and significantly increased the splitting tensile strength of the mixed specimens.