Interfacial design of nano-TiO2 modified fly ash-cement based low carbon composites

Abstract

An enhanced use of fly ash in cement composites can decrease energy consumption, reduce related CO2 emissions, and conserve natural resources. However, low activity fly ash reduces the degree of early-age hydration and early strength of cement composites. This paper presents a facile processing method to increase the reactivity of fly ash and improve the early strength of low-carbon composites. Fly ash is mixed with TiO2 nanoparticles through ball milling, and the resulting roughened particles are used as supplementary cementitious materials (20 wt%) in a cement matrix. The TiO2 nanoparticles coated on the surface of fly ash improve the interfacial interlocking between fly ash and cement, and increase the early-age pozzolanic activity of fly ash. At a nano-TiO2 composition of 1 wt% of the binder materials, the early flexural strength and compressive strength of the paste exhibit a dramatic increase (of 37.74% and 39.11%, respectively) over those of a paste without nano-TiO2. A hydration heat evaluation of the fly ash–cement pastes conducted by an isothermal calorimeter indicates that the TiO2 nanoparticles coated on the surface of fly ash increase the hydration rate and hydration extent of the early hydration reaction of fly ash–cement paste. The thermogravimetric analysis results indicate that the TiO2 nanoparticles coated on the surface of fly ash promote the reaction of fly ash with calcium hydroxide (hydration products). This study illustrates the effectiveness of the interfacial design by this facile ball-milling process in overcoming the low reactivity of class-F fly ash and improving the early strength of low-carbon composites. More importantly, the proposed strategy can be conveniently extended to other waste materials, which could potentially improve the waste recycling efficiency.