Impact of calcined clays additions on phase assemblage and mechanical strength of autoclaved low-clinker fiber cement matrices

Calcined clays are promising constituents for use in low-clinker autoclaved fiber cement production. Previous studies on the use of calcined clays in autoclaved fiber cement matrices have shown that their behavior is complex and sometimes counterintuitive. This study investigates how the composition of calcined clays affects the phase formation, flexural strength, and carbonation behavior of autoclaved fiber cement. XRD, SEM/EDS, TGA, and 27Al MAS NMR results, in combination with thermodynamic modeling, show that reactive Al2O3 from calcined clay promotes the formation of Al-substituted tobermorite during hydrothermal curing and increases flexural strength. The extent of aluminum incorporation in Al-tobermorite correlates with the reactive Al2O3 content in calcined clay. However, excessive reactive Al2O3 hinders Al-tobermorite formation, leaving clinker phases dehydrated and significantly reducing flexural strength. Accelerated carbonation fully decomposed Al-tobermorite and other Ca-containing hydrate phases, except for Si-rich hydrogarnet, which was still detected after carbonation. The carbonated autoclaved matrices exhibited a coarser pore structure but did not experience a decrease in flexural strength. These findings show that calcined clays derived from low-kaolinite clays, which contain more quartz and less reactive Al2O3, can be used at higher replacement levels in autoclaved systems. Conversely, clays with high reactive Al2O3 content should be used with care, as excessive incorporation reduces Al-tobermorite formation and hence reduces flexural strength. The use of low-kaolinite calcined clays is therefore preferential in terms of performance, cost, and availability.