Monitoring the Pozzolanic Effect of Fly Ash in Blended OPC Mortars by Electrical Impedance Spectroscopy

Fly ash (FA) is a pozzolanic material widely used to replace ordinary Portland cement (OPC) in mortars and concretes. The main purpose of this replacement is to improve the durability of these materials in a sustainable way from an environmental and economical perspective. Main technical advantages, such as the improvement in durability and the enhancement of mechanical properties with long-time hydration, are derived from the pozzolanic activity. Several studies aimed to evaluate the pozzolanic reaction extent have been reported by using thermogravimetric analysis (TGA), mechanical compressive strength and mercury intrusion porosimetry (MIP). Besides, other complementary non-destructive techniques that reflect the effect of FA on microstructure are of great scientific and technical interest. In this work, an alternative non-destructive method, the electrical impedance spectroscopy (EIS) followed by the equivalent circuit (EC) analysis has been applied to identify two electrical relaxations, which are strongly associated to microstructural properties. The high frequency relaxation was assigned to the electrical conductivity of the calcium silicate hydrate (C-S H) gel. Additionally, this C-S-H gel conductivity was separated into dc-conductivity and frequency-dependent conductivity, that provided information regarding the periods of early pozzolanic reaction (around 12 days) and high pozzolanic activity (around 28 days). This research provides conclusive evidence of the validity of the applied EIS-EC method on account of the significative relationships found between electrical parameters and physicochemical properties determined by TGA, compressive strength and MIP. Specifically, non-evaporable water, compressive strength and gel-pore-volume of diameter less than 10 nm exhibited significative exponential correlations with dc-electrical resistivity of the high frequency relaxation. These correlations validated the appropriate allocation of the high frequency relaxation to C-S-H gel.