Long-Term Hydration Study of Blended Cement: Calcined Kaolinite–Illite Composite Clays Compared to Fly Ash

Calcined clays are a promising route to lower-carbon binders, but widescale adoption of limestone calcined clay cements (LC³) requires using low-kaolinitic resources due to the limited availability and pricing of high-grade sources. This study evaluates the long-term hydration of two locally available kaolinite-illite composite clays (kaolinite contents 18% and 13%) in binary (30% SCM) and ternary LC³-type (30% SCM + 15% limestone) binders, benchmarked against OPC and fly ash systems. Over 1 year, thermogravimetric analysis showed lower portlandite (CH) and increasing bound water in SCM systems relative to OPC, reflecting ongoing secondary hydration reactions of the SCMs. XRD/Rietveld confirmed formation of hemi- and monocarboaluminate, enhanced in LC³ versus the corresponding binaries. The degree of hydration (DoH) for clay blends exceeded OPC from 7 days onward and reached comparable hydration levels after 1 year, indicating a beneficial later-age contribution from illite. Mortar tests showed that binary clay mixes approximated the 42.5 N class target at 28 days, while all LC³ mixes exhibited lower early strength but additional strength gain from 28 to 90 days. Overall, low-grade kaolinite-illite clays combined with limestone deliver LC³ systems with hydration development and mechanical performance comparable to commonly used fly ash-based cements, supporting their feasibility as regionally sourced, low-carbon binders.