Cement-Poor Concrete and Grout for Use in Underground Constructions

Sammanfattning: This thesis deals with the composition and properties of cement-based materials, concrete and grout, for use as seals and for stabilizing fracture zones in underground facilities like mines and repositories for chemical and radioactive waste. It is of general character with respect to the composition and preparation of concretes and grouts but focuses on the sealing of boreholes for mineral prospection and site investigation for locating underground repositories of hazardous waste. The proposed principle of constructing concrete seals in very deep boreholes (VDH) where they are intersected by fracture zones is to cast the concrete on-site over placed clay seals to the upper end of the respective fracture zones where the next clay seal is installed. The chemical interaction between concrete and clay causes mutual degradation that must be taken into consideration. Ordinary concrete with Portland cement as binder is not suitable because of its poor chemical stability over long periods of time and because of itshigh pH, more than 12, which attacks contacting clay seals. Another drawback is that such concrete requires organic superplasticizers for achieving fluidity and they can give off organic colloids that can transport radionuclides. Inorganic fluidizers like talc and use of low-pH cement offer new possibilities to prepare suitable concretes and grouts. They are the focus of this present document. The criteria for the concrete seals are: sufficient fluidity and coherence at casting and sufficiently high bearing capacity and low compressibility for maintaining constant volume conditions under the load of the overlying series of clay and concrete. The hydraulic conductivity of the hardened concrete should be lower than that of the surrounding fracture zone. Since the concrete must perform acceptably for up to 100,000 years and the cement component will ultimately be dissolved and lost, the rest, i.e. the aggregate components, must still provide acceptable support for overlying clay and concrete seals in deep boreholes. The aggregate component must therefore be very densely packed and have a granular composition that resists erosion. The main objectives of this study are: 1) To investigate the performance of new types of cement-poor, organic-free concrete for long-term duty under normal and extreme hydrothermal conditions at varying temperatures. Properties of special importance are fluidity, mechanical strength, rate of strengthening, pH, and minimum impact on contacting seals of smectite-rich clay seals. 2) To investigate the function of new cement-based grouts proposed for sealing finely fractured rock around boreholes, tunnels and shafts where concrete seals will be cast, especially with respect to fluidity, rheological behaviour, mechanical strength, rate of strengthening, and penetrability into fine fractures. The study comprised testing of low-pH cement concretes and grouts with talc and other silicate minerals added for achieving fluidity and high strength. They were composed according to modern packingtheories for obtaining low porosity and a minimum amount of cement paste. The use of talc is firstly for decreasing the viscosity of the mixture at casting or injection and secondly for assuring that it can really contribute to the mechanical strength by chemical interaction with the cement component. The need for chemical integrity led to the choice of quartzite as main aggregate in the study. The low-pH cement used was Merit 5000 manufactured by SSAB Merox AB, Oxelösund. For one of the candidate grouts, Portland cement was used in order to get quick hardening, whilst the other contained low-pH cement for providing high strength in the course of time. The principle of composing the aggregate was to obtain a filtering function so that the risk of losing fines is minimized in the construction period when local hydraulic gradients may still be high. It was concluded from this study that theoretical modeling as well as experimental results confirm that concrete mixtures can get high packing degrees and bulkdensities if the ballast gradation is suitable and that the role of the fine fraction is thereby very important. The achieved density of the concrete is believed to be sufficiently high in order to make the concrete perform acceptably for very long periods of time even after complete loss of the cement. It was documented that talc serves to reduce the viscosity and contribute to the strength. The relatively low pH of Merit concrete (≈10) in contact with clay indicates that chemically generated destruction of either of the materials will be low. Such concrete, containing properly composed quartz-rich aggregate, and with talc added, fulfills the criteria set for its performance under both normal and high temperature conditions. It was also concluded that grout fluidity is positively affected by the hydrophobic nature of talc and negatively by thixotropic action of other silicates and by early cement hydration. The experiments highlight the role of the rheological properties and the need for proper composition of the grouts. A general conclusion from the grout flow study was that effective injection of grouts into fractures narrower than 100 µm requires that the viscosity is as low as 0.05 Pas and that high injection pressures are used.