A long-term study on structural changes in calcium aluminate silicate hydrates.

Cement production remains a major source of CO2 emissions, prompting research into alternative materials. Blended cements, which incorporate supplementary cementitious materials (SCM), offer a promising approach to reduce emissions. These SCMs often contain high levels of Al2O3 and SiO2, which influence the hydration process and the structure of calcium aluminate silicate hydrate (C-A-S-H). While C-A-S-H is a primary hydration product, the presence of SCMs introduces variability in its chemical composition. Prior research has shown that SCMs can alter the hydration kinetics and phase assemblage of cement systems. However, the long-term structural evolution of C-A-S-H under varying Al/Si ratios remains unclear. This gap motivated an investigation into how Al/Si molar ratios and equilibration times affect the formation and stability of C-A-S-H and secondary phases. Understanding these changes is critical for optimizing cement formulations and ensuring long-term durability.

This study aimed to examine the structural and chemical changes in C-A-S-H gels over extended periods. The focus was on how varying Al/Si molar ratios influence the hydration process and the formation of secondary phases. Researchers sought to determine whether the uptake of Al into C-A-S-H reaches equilibrium over time and how this process is affected by initial Al concentrations. By analyzing gels equilibrated for up to two years, the study aimed to provide insights into the long-term behavior of C-A-S-H in blended cement systems. The investigation also aimed to clarify the role of tetrahedrally coordinated Al in stabilizing the C-A-S-H structure. This work is essential for improving the sustainability and performance of cementitious materials. The findings could inform the design of cement mixtures that balance environmental impact with structural integrity.

The study employed spectroscopic and structural analysis techniques to examine C-A-S-H gels. Researchers varied the Al/Si molar ratios and equilibration times to observe structural changes. 27Al MAS NMR spectroscopy was used to determine the coordination of Al in the gels. Thermogravimetric analysis provided insights into phase composition and stability. The study included gels with Al/Si ratios ranging from 0.001 to 0.2 and equilibration times spanning 3 months to 2 years. The presence of secondary phases such as strätlingite, katoite, and Al(OH)3 was monitored using these methods. Researchers also tracked how Al concentrations in C-A-S-H changed over time. This approach allowed them to assess the kinetics of Al incorporation and the formation of secondary phases under different conditions.

The study found that secondary phases such as strätlingite, katoite, and calcium aluminate hydrate form at Al/Si ratios ≥ 0.03. These phases limit the uptake of Al into C-A-S-H. At higher Al concentrations, the amount of secondary phases decreased over time as more Al was incorporated into C-A-S-H. At low Al contents, Al concentrations decreased strongly with time, suggesting a slow equilibration process. In contrast, at higher Al contents, no significant change in Al concentrations was observed, indicating faster equilibration. 27Al NMR studies revealed that tetrahedrally coordinated Al was incorporated into C-A-S-H and its amount increased with the Al content in the solution. The presence of tetrahedral Al suggests a more stable C-A-S-H structure. These findings highlight the dynamic nature of C-A-S-H formation and the influence of Al/Si ratios on hydration processes.

The study's findings suggest that the structural evolution of C-A-S-H is closely tied to the Al/Si molar ratio and equilibration time. At higher Al concentrations, secondary phases form but decrease as more Al is incorporated into C-A-S-H. The presence of tetrahedrally coordinated Al indicates a more stable C-A-S-H structure. The study also found that equilibration time affects the rate at which Al is taken up into C-A-S-H. At low Al contents, equilibration is slower, while at higher contents, equilibrium is reached more rapidly. These observations align with the authors' hypothesis that Al/Si ratios significantly influence hydration kinetics and phase assemblage. The results provide a clearer understanding of how SCMs affect the long-term stability of C-A-S-H. This information can guide the development of more sustainable cement formulations. The study's implications are specific to the structural behavior of C-A-S-H under varying Al/Si conditions.