Physical and Mechanical Properties of Thermally Altered Cementitious Sealing Materials for a Nuclear Waste Repository in Tuff

Abstract

Studies were done on the physical, mechanical, and geochemical properties of candidate cementitious materials for sealing a geologic nuclear waste repository in a tuff host rock environment. One sanded cementitious grout contains substantial replacement of cement by low-calcium fly ash and silica fume, yet maintains a water-cementitious solid ratio of 0.32 for a fluid grout. The ash and fume were used to achieve a higher content more compatible with the tuff geochemistry than a plain portland cement. One possible application for such materials is fracture sealing near waste canister emplacement holes. The effects of temperatures from 15 to 300°C on the material properties were investigated. Initial compressive strengths of materials cured at 38°C for 7 to 900 days ranged from 100 to 125 MPa. Other properties investigated include bond strength (to tuff), water permeability, interfacial permeability, Young's modulus, density, porosity, expansive stress, and phase changes. Samples heated to 150°C for extended periods (28 days), either dry or hydrothermally, maintained their strength and well-bonded microstructure, while the results of heating at 300°C were mixed, with some strengths remaining high (95 to 110 MPa) and others diminishing (44 to 51 MPa). The water permeability did not increase much at 150°C but did increase at 300°C.