Experimental Study of the Diffusion of Nitrate and Sulfate Ions through Concrete in the Case of Underground Water Reservoirs and the Impact on Their Durability

Abstract

This study addresses the issue of the efficiency of concrete mix design on the evolution of its microstructure, its physico-mechanical properties, and its behavior with respect to ionic transport under aggressive environmental conditions. Eight concrete formulations (F1 to F8), produced using local aggregates from the cities of Brazzaville (Kombé quarry) and Pointe-Noire (Louvoulou and Mboubissi quarries), were investigated. These formulations differ in the nature of the aggregates (rounded or crushed), the proportion of crushed sand, as well as the water-to-cement and gravel-to-sand ratios. The physico-mechanical properties of the concretes, particularly compressive strength, water-accessible porosity, and bulk density, were characterized at 28 and 90 days and then correlated with the diffusion kinetics of nitrate ( NO 3 − ) and sulfate ( SO 4 2 − ) ions measured over a period of 12 weeks. The results highlight a progressive increase in mechanical strength with curing age, reflecting the continuation of hydration reactions and the gradual densification of the cementitious matrix. This evolution is strongly correlated with the reduction in porosity and the increase in density, thereby confirming that porosity constitutes the essential microstructural parameter controlling the mechanical performance and durability of concrete. Furthermore, the comparative analysis shows that formulations based on crushed aggregates, particularly formulations F1 and F3, develop a more compact microstructure and a denser interfacial transition zone (ITZ) than concretes made with rounded aggregates, thus limiting preferential diffusion pathways. The diffusion tests also reveal two distinct transport behaviors depending on the nature of the ions studied. Nitrates exhibit a predominantly conservative behavior governed by diffusion, with higher concentrations observed in the most porous concretes. In contrast, sulfates display a non-conservative behavior resulting from a complex coupling between diffusion, chemical reactions, and leaching phenomena. Three transport regimes were therefore identified: a purely diffusive regime, a coupled diffusion-reaction regime, and a regime dominated by the presence of internal ion sources. Finally, the obtained results demonstrate that the durability of concrete cannot be explained solely by conventional mix design parameters, but rather results from complex interactions between microstructure, ionic transport, and chemical reactivity. This study thus highlights the relevance of a performance-based approach founded on physically interpretable parameters for predicting the long-term behavior of concrete intended for underground water reservoirs in humid and polluted environments.