Alkali-Activated Concrete Based on Natural Volcanic Pozzolan: Chemical Resistance to Sulfate Attack

In this article, a comparative analysis of the sulfate resistance (Na2SO4 and MgSO4) of an alkali-activated binary concrete (AABC) (70% natural volcanic pozzolan and 30% granulated blast-furnace slag) and a concrete based on ordinary portland cement (OPC) is performed. The AABC and OPC concrete were cured for 28 days prior to their immersion in solutions of sulfates (5% by weight) for a period of up to 730 days. Dimensional changes, compressive strength, absorption, and porosity properties were evaluated at different test ages. Additionally, the level of concrete deterioration was monitored by visual inspection, scanning electron microscopy, and X-ray diffraction (DRX). The results show that the AABC presents resistance to sulfate attack greater than that of OPC. The percentage of expansion to 730-day exposure for AABC was 0.0985% in Na2SO4 with a compressive strength index of −0.58%; in MgSO4, these values were 0.1471% and −20.85%, respectively. AABC complies with the expansion limit specified by ASTM C1012 for the most severe class of exposure (S3). This finding allows for the classification of the AABC as a high-Na2SO4-resistant concrete.