Ionic conductive and photocatalytic properties of cementitious materials: calcium silicate hydrate and calcium aluminoferrite

Abstract

Few studies have reported the ionic conductive and photoactive properties of cementitious materials, owing to the compositional and structural variations of cement paste. The present report focuses on calcium silicate hydrate (C-S-H) and tetracalcium aluminoferrite (C₄AF), prepared individually from pure constituent elements. C-S-H is an ionic conductor with a conductivity ranging from 10⁻⁴ to 10⁻³ S cm⁻¹ in humidified air at temperatures between 60 and 100 °C. The hydroxide ions present in the mesopores of C-S-H as basic O–H groups were determined to be the major charge carriers, and the faradaic efficiency for the water oxidation reaction was estimated to be approximately 100%. C₄AF is a mixed ionic n-type electronic conductor, particularly at high oxygen partial pressures. The conductivity of this compound was several times lower than that of C-S-H under the same humidity and temperature conditions, not only because of the bulk resistance but also the grain boundary resistance. C₄AF and the hydrates exhibited photovoltaic and photocatalytic responses for water splitting and methylene blue degradation, respectively. These results indicate the important contribution of C-S-H and hydrated C₄AF to the electrochemical and photochemical properties of cement paste.