The Early Age Hydration Products and Mechanical Properties of Autoclaved Cement Paste Incorporating Supplementary Cementitious Materials

This study systematically investigated the effects of four supplementary cementitious materials (SCMs), namely ferronickel slag (FNS), lithium slag (LS), steel slag (SS), and ground granulated blast furnace slag (GBFS), on various properties of autoclaved cementitious materials. Cement pastes and mortars with 0% and 30% replacement levels were prepared to examine their impacts on early age hydration products and mechanical properties, with comparisons made to specimens under standard 28-day curing. Key findings reveal that autoclaving promoted the formation of tobermorite, crystalline calcium aluminosilicate hydrate (CASH), gypsum and hydrogarnet, with the latter two phases potentially compromising concrete durability. Autoclave curing significantly enhanced SCM reactivity, as evidenced by thermogravimetric analysis: the mass loss below 200 °C (mainly from C–S–H gels decomposition) in SCM-incorporated pastes ranged from 87.0% (SS) to 104.6% (GBFS) of the control value, while the portlandite (Ca(OH)2) content decreased to between 47.7% (LS) and 82.4% (GBFS) of the control. Among the SCMs studied, autoclaving exhibited the most pronounced activation effect on LS, which also showed superior potential as a GBFS alternative in autoclaved concrete products. However, the low CaO content and acidic nature of LS limited its use to low replacement levels unless supplementary sources of alkalinity and CaO were provided.