Effect of different amounts of barium substitution for calcium on the hydraulic activity of the high belite binary C2S-C4A3$ system

Abstract

The substitution of Ba²⁺ for Ca²⁺ in both dicalcium silicate (C₂S) and ye'elimite (C₄A₃$) has the potential to stimulate the development of early hydration activity of high belite sulfoaluminate cement (HBSAC). To relieve the emission pressure of CO₂ in the cement industry and promote the early compressive strength development of HBSAC, the C₂S-C₄A₃$ binary system with different amounts of Ba²⁺ substitution for Ca²⁺ was synthesized and investigated in this study. The SEM-EDS and XRD test methods confirmed the stoichiometric composition of Ba-bearing C₂S-C₄A₃$ binary systems. The results showed that Ba²⁺ tends to preferentially replace Ca²⁺ in C₄A₃$ compared to replacing Ca²⁺ in C₂S, resulting in more substitution amount of Ba²⁺ in C₄A₃$ mineral, as the designed substitution amount of barium increased from 25 wt.% to 55 wt.%, the stoichiometric formula of Ba-bearing C₂S was transformed from Ca_1.66Ba_0.34SiO₄ to Ca_1.45Ba_0.55SiO₄, and the stoichiometric formula of Ba-bearing C₄A₃$ was transformed from Ca_2.29Ba_1.71Al₆SO₁₆ to Ca_0.86Ba_3.14Al₆SO₁₆. While excessive amount of barium (more than 45 wt.%) resulted in the increased content of by-products, such as BaSO₄ and BaAl₂O₄, as well as the formation of C₃S. The hydration properties of each group of synthetic clinker were investigated, including compressive strength, qualitative and quantitative analyses of the hydration products, leaching solution environment, and micromorphological analysis. The hydration products including C-(A)-S-H and AH₃, as well as hydroxide (Ca(OH)₂ and Ba(OH)₂·8H₂O), which can be carbonated to form carbonates (BaCO₃ and CaCO₃) by CO₂ in the air. As the substitution amount of Ba²⁺ increased from 25 wt. % to 35 wt. %, the hydration degree of Ba-bearing α′_L-C₂S was promoted in the early stage, and the presence of C₃S may affected the increase of early hydration activity of Ba-bearing α′_L-C₂S, however, the hydration degree of α′_L-C₂S with the most amount doping of barium in A055 samples exhibited the highest hydration degree of 88.0 % after hydration for 90 days. The compressive strength of high belite C₂S-C₄A₃$ binary system of 3-day displayed a rapid increase from 8.6 MPa to 16.1 MPa, with the amount of barium increased from 25 wt. % to 55 wt. %. The compressive strength of A025 and A035 after hydration for 90 days increased steadily, reaching 35.0 MPa and 34.0 MPa respectively. However, the compressive strength of hydrated A045 and A055 samples for 90 days deteriorated and even cracked seriously for the hydrated A055 sample, which was attributed to the formation of a large amount of Ba(OH)₂·8H₂O and BaCO₃ by excessive barium amount, and leading to the expansion and broken of the matrix. Thus, the designed amount of barium substitution for calcium in the C₂S-C₄A₃$ binary system should be controlled, for a higher content of barium more than 35 wt. % could be damage to the development of the long-term hydration strength of the clinker. These results provided a theoretical basis to make it possible for the large-scale application of activated HBSAC, devoted to the sustainable development of infrastructure.