New Insights into Pore Structure and Hydraulic Conductivity of Sodium Hydroxide Alkali-Activated Slag through Advanced Modelling

Abstract

The study of alkali-activated slag (AAS) is motivated by the need for more sustainable alternatives to Portland cement (PC) within the construction industry. Specifically, AAS offers good mechanical and chemical properties. However, the influence of the activator on its pore structure and hydraulic conductivity remains unclear. Both pore structure and hydraulic conductivity are key parameters in understanding the drying process and could potentially explain the high drying shrinkage observed so far. The present study aims to investigate the pore size distribution and hydraulic conductivity of six distinct AAS/sodium hydroxide mortar compositions, with a particular emphasis on the effect of varying the activator’s molarity and the solution-to-binder ratio (s/b). This research uses the mass variation in different relative humidity (RH) conditions from experimental tests to model the pore surface area, the pore size distribution, and the hydraulic conductivity. From the results, it emerges that increasing the molarity from 0.5 to 8 M reduces the open porosity and refines the pore structure, while increasing the s/b from 0.5 to 0.8 increases the open porosity while refining the pore structure. In addition, high molarity compositions are not suitable for testing in high RH and natural carbonation conditions due to the occurrence of deliquescence. Moreover, the main drying mechanism in AAS is water vapour transport even at high relative humidity, contrary to what was observed in the literature for PC. Finally, the hydraulic conductivity of alkali-activated slag presents a minimum of around 85% RH against the 60–70% RH for PC, causing AAS to dry faster when the relative humidity decreases from 85 to 50%.