Experimental investigation of cement, Topopah Spring tuff, and water interactions at 200°C

The stability of minerals that tend to form in cementitious materials at elevated temperatures has been investigated experimentally. This information is needed to predict the effect of these materials on the performance of geologic radioactive waste repositories. Reaction of large amounts of cement used to build the repository with the surrounding rock and groundwater may cause changes in the hydrologic properties of the repository, impact metal canister and waste form corrosion rates, and alter the mobility of actinides in the subsurface. We have determined that 11 Å-tobermorite, calcite and quartz is the stable (or at least metastable) mineral assemblage at 200°C of complex experiments containing mixtures of cement, Topopah Spring tuff, diesel fuel and 3 mM NaHCO3 from analyses of the solids and solutions. Mesolite did not form in these experiments, despite predictions that it is the most stable phase. A solubility constant for 11 Å-tobermorite was calculated to be equal to 1039.4(±1.3) at 200°C for the following reaction:Ca5(SiO2)6(OH)10*0.5H2O+10H+=5Ca2++6SiO2(aq)+10.5H2O.AA−tobermorite}This work shows that reaction periods of at least 20 days are required to reach local equilibrium or steady-state.