Nano-TiO2 in Hydraulic Lime–Metakaolin Mortars for Restoration Projects: Physicochemical and Mechanical Assessment

Abstract

In recent years, lime mortars mixed with artificial or natural pozzolans are commonly used in restoration applications. The aim of this work is the assessment of carbonation, pozzolanic reaction, setting time, and mechanical properties of metakaolin–lime mortars mixed with crystalline nano-titania (nT) as additive. The studied mortars consist of hydrated lime and metakaolin in 60/40 ratio (wt%) and fine aggregates of either carbonate or silicate sand. The concentration of the nano-titania is equal to 6 (wt%) of the binder. For comparison purposes, three types of mortars and pastes are designed: Without the addition of nano-titania, with nT activated or not under UV irradiation. The evaluation of the carbonation and pozzolanic reaction over a 1.5-year curing period is carried out through thermal analysis (DTA/TG), infrared spectroscopy (FTIR) and X-ray diffraction analysis (XRD). The uniaxial compression and the three-point bending tests at 28 days, 3 months, and 6 months were carried out to evaluate mechanical properties. The addition of activated nano-titania, due to an increased photocatalytic activity, accelerated the setting of the mortars, improving at the same time the mechanical properties. The plastic behavior of the lime–metakaolin mortars with activated nT was attributed to the evolution of carbonation and pozzolanic reaction.

RILEM TC 277-LHS report: a review on the mechanisms of setting and hardening of lime-based binding systems

“…Furthermore, compressive strength decreases in 3 months of curing time, due to microcracks formed by the shrinkage that took place over curing [223]. On the contrary, when carbonation preceded the pozzolanic reaction, mechanical properties such as long-term compressive strength and toughness are enhanced, because of the formation of stable monocarboaluminate phases that hindered the hydrogarnet appearance [224].…”