Physico-chemical investigation of CEM I cement paste degradation in carbonated water with controlled CO2 partial pressure (1.3 10-2 atm) at 50°C

摘要

In the framework of radioactive waste geological disposal, concrete structures are submitted to various combined chemical degradations (hydrolysis, carbonation and sulfate attacks mainly) due to the underground chemical conditions. Carbonation is considered to be one of the key processes and to have a major impact on the interaction between cementitious materials and clay materials. In the present context, this phenomenon is controlled by the CO2 partial pressure in the underground natural conditions of Callovo-Oxfordian clayey rock of Bure (France), which is 30 times higher than the atmospheric value. The effect of temperature is also an important factor since radioactive wastes could induce a significant temperature increase. The main goal of this study is to describe the behaviour of CEM I cement paste submitted to carbonation in aqueous solution in equilibrium with calcite and with a pCO2 equal to 1.3 kPa (1.3 10-2 atm) at 50°C. To carry out these experiments, an original device was developed. It consists of a leaching reactor, in which pH and pCO2 are maintain constant during the experiment using a CO2 flow-meter controlled by a pH regulator. Samples of cement paste (w/c = 0.4) are immersed for one to five months. Solution renewals ensure stable chemical composition of the aggressive solution. The mineralogical evolution was characterized mainly by XRD analysis, but also by SEM observations. Portlandite dissolution front informs on the degraded zone depth. The precipitation of calcite at the surface vicinity induces the pore clogging and the formation of a densified microstruture zone close to the surface. This microstructure is characterized by simultaneous coupled phenomena: decalcification (over about 1700 μm depth after 5 months), ettringite dissolution (over about 1000 μm depth after 5 months) and calcite precipitation (over about 200 μm depth after 10 months). The modelling, using a coupled reactive transport code, such as HYTEC, associated with a dedicated thermodynamic database, shows the limitation of thermodynamic approach.