A multi-technique investigation of the effect of hydration temperature on the microstructure and mechanical properties of cement paste

Abstract : The cement hydration process and the resulting microstructure are highly dependent on the cement formulation and the hydration conditions. Particularly, the hydration temperature has a significant influence on the cement paste microstructure and its mechanical properties. This is for instance important for understanding the behaviour and properties of oil-well cements which are used to form a cement sheath between the casing and the surrounding formation for stability and sealing purposes. This cement sheath is hydrated under a progressively increasing temperature along the depth of a well due to the geothermal gradient (about 25°C/km). It results generally in a decrease of the mechanical properties and an increase of permeability along the well. The aim of the present thesis is to investigate the effect of the hydration temperature in the range of 7°C to 90°C on the microstructure of a class G cement paste and to establish the link between these temperature dependent microstructure and the elastic properties of the material. The microstructure characterization is done by combining various experimental methods, including X-Ray diffraction associated with the Rietveld analysis, thermogravimetric analysis, mercury intrusion porosimetry, porosity evaluation by freeze-drying or drying at 11% RH, Nitrogen and water vapour sorption experiments and finally 1H nuclear magnetic resonance. The mass assemblage of microstructure phases at different curing temperatures has been evaluated and showed a slight dependence on the hydration temperature. The porosity evaluations show an increase of the capillary porosity and a slight decrease of the total porosity at 28 days, resulting in a decrease of the gel porosity by increasing the hydration temperature. An analysis method has been proposed to evaluate the C-S-H saturated density and chemical composition in terms of H/S and C/S molar ratios. The C-S-H bulk density is increasing with increasing hydration temperature which explains the observed increase of the capillary porosity for higher curing temperatures. The C/S ratio and H/S ratio for both solid and saturated C-S-H are decreasing with increasing curing temperature. The provided quantitative characterization of cement paste microstructure is used in a micromechanical modelling for evaluation of the elastic properties at various hydration temperatures. Two and three-scale self-consistent micromechanical models have shown that the increase of capillary porosity with increasing hydration temperature cannot fully explain the drop of elastic properties. This is mainly due to the increased elastic properties of C-S-H being denser at higher temperature that cancel the effect of increasing capillary porosity on the overall elastic properties. Another way to fully account for the decrease of the mechanical properties of cement paste is to consider the porosity distribution inside the C-S-H in the form of two distinguished C-S-H types, High Density (HD) and Low Density (LD) C-S-H, as proposed by Tennis and Jennings (2000). This possibility is probed by a combination of various porosity evaluations: Mercury intrusion porosimetry, nitrogen adsorption and water vapour desorption and by a back calculation using micromechanical modelling. The results show that the LD intrinsic porosity is slightly increasing while the HD intrinsic porosity decreases significantly with increasing hydration temperature. The decrease of the elastic properties of cement based materials with increasing hydration temperature is therefore a combined action of the increase of capillary porosity and the changes of intrinsic C-S-H porosities
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Sara Bahafid. A multi-technique investigation of the effect of hydration temperature on the microstructure and mechanical properties of cement paste. Mechanics of materials [physics.class-ph]. Université Paris-Est, 2017. English. ⟨NNT : 2017PESC1021⟩. ⟨tel-01980576⟩

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