ID TRANSPORT REACTION MODEL COUPLING MICROBIAL SUCCESSION OF SULFUR OXIDIZING MICROORGANISMS AND MORTAR REACTIVITY FOR OPC-, BFSC- AND CAC-BASED MATERIALS

摘要

In some local conditions found in sewers, biogenic sulfuric acid produced by sulfur-oxidizing microorganisms leads to the dissolution of material and to the secondary precipitation of gypsum and/or ettringite [1]. To evaluate the durability of mineral materials in such a deleterious environment, INSA-Toulouse has developed an experimental testing method (BACTest) based on the trickling of a nutritive solution at the surface of an inoculated coupon of the material [2]. This trickling solution enables the selection and the succession of neutrophilic and acidophilic sulfur-oxidizing microbial activities over time at the surface of the mineral materials. To improve understanding of the mechanisms coupling biological activity and the reactivity of cementitious based materials, a numerical model was developed using the software Aquasim. This 1D model takes into account: 1. biological activity represented by two microbial populations (neutrophiles and acidophiles). The influence of pH and salinity on sulfur-oxidizing activities was determined experimentally; 2. acid/base and ion pairing equilibrium represented by two reactions with opposite kinetics [3]; 3. dissolution and precipitation of solid phases controlled by saturation index calculated from the chemical composition of the interstitial solution. Cemdatal4 was used as a database to define the reactivity of the solid phases, except for C-S-H [4]; 4. diffusivity defined by a modified Fick's law allowing electro-neutrality to be respected [5]; 5. the local porosity depending on the local concentrations of the solid phases and, in turn, influencing the diffusion coefficients by a classical law defined for a porous medium. Laboratory experiments (～100 days of exposure) for Ordinary Portland Cement (OPC), Blast Furnace Slag Cement (BFSC) and Calcium Aluminate Cement (CAC) mortars were compared to simulated data. The model was able to simulate leaching processes and the reactivity of the cementitious matrix in the depth of the materials. Moreover, it highlighted the importance of the reactivity of the hydrated phases in the durability of materials and also the need to take the reactivity of the anhydrous phases into account in such a deleterious environment.