Using Metakaolin as an Enhancing Component in Supersulfated Cement: A Comprehensive Evaluation

Using metakaolin (MK) as a supplementary cementitious material (SCMs) to enhance the early strength of supersulfated cement (SSC) is a feasible strategy. The effects of 0-30 wt% MK on the hydration thermodynamics, flowability, setting time, and mechanical properties of supersulfated cement (SSC) were evaluated. Characterization techniques, including ICC, XRD, FTIR, TG-DTG, and SEM-EDS, provided insight into how MK influences the performance and microstructural evolution of metakaolin-supersulfated cement (MSSC). The results indicate that MK accelerates the early hydration rate of MSSC by providing additional nucleation sites. The optimal MK content was found to be 25 wt%, with compressive strengths reaching 18.7 MPa at 3 d and 42.4 MPa at 28 d, reaching 122.22% and 208.87% of the MK0, respectively. Additionally, the pozzolanic activity of MK facilitated the formation of ettringite (AFt) and calcium aluminosilicate hydrate (C-(A)-S-H) in MSSC. By adjusting the Ca/Si and Al/Si ratios, the C-(A)-S-H structure was optimized, resulting in a denser matrix pore structure. These findings were further supported by phase composition and microstructure analyses. However, when the MK content reached 30 wt%, its excessive addition limited its reactivity due to insufficient portlandite in MSSC, leading to reduced compressive strength. This study identifies 25 wt% MK as the optimal dosage for enhancing SSC performance, improving early strength, and maintaining environmental sustainability.