Autoclave curing of nanosilica-modified ultra-high-performance concrete matrix: Microstructural development, mechanical performance and fracture behaviour

This study investigates the combined effect of nanosilica incorporation and autoclave curing on the hydration behaviour, microstructural development, and mechanical performance of ultra-high-performance concrete. The investigated material corresponds to a non-fibre UHPC matrix specifically designed for subsequent fibre reinforcement applications. Mixtures containing 0–7.5 wt% NS were subjected to standard curing and autoclave curing, and were characterised by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and mercury intrusion porosimetry. XRD results show that nanosilica addition under standard curing increases the amorphous content due to accelerated clinker hydration and enhanced pozzolanic activity. In contrast, autoclave curing promotes the transformation of amorphous or weakly crystalline calcium silicate hydrate into thermodynamically stable and highly ordered tobermorite. Under autoclave curing conditions, nanosilica provides a highly reactive source of SiO₂, facilitating portlandite consumption and the crystallization of stable calcium silicate hydrates. Scanning Electron Microscopy observations corroborate these findings, revealing a transition from dense but disordered gel-like hydration products in standard curing mixtures to well-defined crystalline morphologies in autoclave treated specimens. TGA results further support the mineralogical trends identified by XRD, with nanosilica and autoclave modified mixtures exhibiting reduced mass losses associated with portlandite dihydroxylation. Porosimetry analysis indicates that moderate nanosilica dosages refine the pore structure. Overall, the synergistic use of nanosilica (3.5 wt%) and autoclave results in a refined and stabilized microstructure, which explains the observed improvement in the mechanical performance (average increases of 16% in compressive strength and 51% in tensile strength) of ultra high-performance concrete. Therefore, the reported fracture properties correspond to the matrix behaviour and do not include fibre-bridging effects typically observed in fibre-reinforced UHPC systems.