Leaching by water is a degradation mechanism for concrete structures such as dams and tunnels. This phenomenon becomes more alarming for structures designed to last several hundred of years, which is the case for radioactive waste depositories. Several investigations on old concrete structures have shown that water deteriorates concrete (Badouix 2000) and increases its porosity (Fujiwara et al. 1992). The phenomenon of leaching has been studied in several laboratories to identify its consequences on the physical and mechanical properties of cement-based materials. In contact with a reservoir of water with an ionic composition different from that of the cement paste pore solution (typically lower), various ions, and particularly calcium and hydroxyl ions, diffuse from the pore solution towards the water reservoir. These transfers involve a chemical disequilibrium and consequently a dissolution or precipitation of solid phases (Adenot & Buil 1992). Following the ranking of their solubility products, portlandite (CH) is the first hydrate to be dissolved followed by monosulphoaluminate (AFm) and then ettringite (AFt). Lastly, C-S-H is decalcified and transformed into a silica gel with minimal mechanical properties. Thus, the leached zone is characterized by a succession of dissolution-precipitation fronts between zones in chemical equilibrium, as shown in Fig. 1. The dissolution of the hydrates in cement-based materials involves an increase in porosity (Tognazzi 1998, Matte & Moranville 1999), a decrease in the elastic modulus and in the compressive strength (Carde 1996, Gérard 1998, Kamali 1999 & Le Bellégo 2001), an increase in material ductility (Carde 1996), a reduction in the fracture energy (Le Bellégo 2001), and a reduction in the friction coefficient as obtained by a triaxial compression test under drained conditions (Heukamp 2001).
The elastic or Young's modulus is one of the most important and critical properties of concrete material and structural mechanical design. The quantification of the reduction in this property due to the dissolution of one or more hydrates is not easy to measure. However, the NIST 3D microstructure models are well adapted for this type of study. These models have been used to quantify the dissolution effect of portlandite (CH), ettringite (AFt) and monosulfoaluminate (AFm), on the elastic modulus. The modelling has then been validated using one experimental result. Different cement pastes were simulated to study effect of the water-to-cement ratio, w/c, and silica fume addition. Empirical formulas were developed to relate the decrease of the elastic modulus to the increase in capillary porosity during leaching.
Figure 1. Leaching zones in a portland cement paste(w/c=0.4) leached by pure water at pH=7 and T=20 °C, according to Adenot (1992) ; C-S-Hd = decalcified C-S-H.
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