Curing temperature: A key factor that changes the effect of TiO2 nanoparticles on mechanical properties, calcium hydroxide formation and pore structure of cement mortars

Abstract

[Abstract:] There are several studies about the effects of addition of TiO2 nanoparticles in mortar properties, but they were made at a given temperature. However, are the effects of this addition independent of curing temperature? The present research focused on the influence of TiO2 nanoparticles at three curing temperatures: 5 °C, 20 °C and 45 °C. Four different mortar mixtures were investigated: a reference mortar (without nanomaterial) and three mortars with different percentages of TiO2 (0.25%, 0.5% and 1%) by the total amount of cement for the reference mortar. For each curing temperature, the effect of TiO2 on (i) compressive and flexural strength at 7 days, (ii) calcium hydroxide (CH) formation at 1, 3 at 7 days and (iii) pore structure have been studied. The effect of TiO2 on compressive and flexural strength depends strongly on curing temperature: the higher the curing temperature, the lower the positive effect of TiO2. If this fact is not considered when curing temperature is higher than the standard (20 °C), improvements due to TiO2 substitution would not be as considerable as it is expected. TiO2 promotes CH carbonation during sample preparation. Results suggest that promotion of hydration is not the only effect of TiO2, which can contribute to enhance strength. A potential mechanism of these nanoparticles to enhance strength could be the modification of pore structure; according to 3D X-Ray microscope results, at 20 °C, TiO2 makes pores smaller, more homogeneous and spherical.