Functionalized nanosilica in cementitious composites: Impact of replacement level on hydration and performance

Nanosilica (NS) is widely recognized as a highly reactive supplementary cementitious material (SCM) capable of increasing the compressive strength and reducing the permeability of cement-based composites, thus contributing to greater durability. However, its use can negatively affect the fresh-state properties and long-term strength development of these materials, often due to its high surface area and reactivity. To address these limitations, recent studies have explored the functionalization of NS surfaces with specific chemical groups to alter their behavior in cement matrices. Conventional approaches to functionalization typically require the use of chemical agents such as ammonium hydroxide and dichloromethane, raising environmental and safety concerns. This study proposes a more environmentally friendly alternative, applying direct surface functionalization using (3-aminopropyl)triethoxysilane (APTES) and a shrinkage-reducing additive (SRA) without additional chemical solvents. The objective is to evaluate how the partial replacement of Portland cement with 1% and 2% unmodified or functionalized NS (NSF), both used as supplementary cementitious material (SCM), influences the performance of cement pastes and mortars in the fresh and hardened states. A comprehensive experimental program was conducted on seven cementitious formulations. In cement pastes, hydration kinetics, portlandite (CH), and calcium silicate hydrate (C-S-H) contents were quantified by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR), while pore structure was evaluated by mercury intrusion porosimetry (MIP). Mortar samples were tested for compressive strength at 2, 7, and 28 days. NS characterization confirmed that NS functionalization with APTES and SRA was successful. The isothermal conduction calorimetry test demonstrates that NS functionalization delays cement hydration. Thermogravimetric analyses show that replacing part of the cement with NS or NSF reduces the amount of CH, as expected, due to the pozzolanic reaction. The MIP test demonstrates that, overall, the total porosity of the pastes containing NSF is lower compared to the reference paste. At 28 days, the mechanical strength of mortars A-1NSFA4 and A-1NSFA4SAR10 presents more significant results. The results demonstrate that the level of cement replacement significantly affects the composite's performance and that replacing 1% of cement with NS or NSF appears to be the optimal replacement rate.