EXPERIMENTAL INVESTIGATION OF CEMENT, TOPOPAH SPRING TUFF, AND WATER INTERACTIONS AT 200-DEGREES-C

The stability of minerals that tend to form in cementitious materials at elevated temperatures has been investigated experimentally. This in formation is needed to predict the effect of these materials on the pe rformance of geologic radioactive waste repositories. Reaction of larg e 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 determ ined that 11 Angstrom-tobermorite, calcite and quartz is the stable (o r at least metastable) mineral assemblage at 200 degrees C of complex experiments containing mixtures of cement, Topopah Spring tuff, diesel fuel and 3 mM NaHCO3 from analyses of the solids and solutions. Mesol ite did not form in these experiments, despite predictions that it is the most stable phase. A solubility constant for 11 Angstrom-tobermori te was calculated to be equal to 10(39.4(+/-1.3))) at 200 degrees C fo r the following reaction: Ca-5(SiO2)(6)(OH)(10)0.5H(2)O + 10H(+) = 5C a(2+) + 6SiO(2(aq))+ 10.5H(2)0. 11 Angstrom - tobermorite (1) This wor k shows that reaction periods of at least 20 days are required to reac h local equilibrium or steady-state. (C) 1998 Elsevier Science Ltd. Al l rights reserved.