Sara Bahafid

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

Advisors: Jean Sulem, Siavash Ghbaezloo, Paméla Faure, Myriam Duc, Emmanuel De Laure

In the structure of oil, geothermal and CO2 injection wells a cement sheath is placed between the casing and the rock. It plays an important role in well stability. Along a well, the temperature rises to about 25 ° C / km with the depth because of geothermal gradient. Cement paste at different depths is hydrated under different temperatures, which strongly influences its microstructure, physical and mechanical properties. Studying the effects of hydration temperature on the microstructure and poromechanical properties of the cement paste is very important for evaluating the performance of the well and analysing its integrity . the Poromechanical properties of cement are often evaluated through laboratory tests. Because of the low permeability of the cement paste, these tests are often very long. Micromechanical models and homogenization techniques are used as complementary methods for evaluating the poromechanical properties. However a consideration of the hydration temperature in these models requires a quantitative characterization of the microstructure constituents phases hydrated under different temperatures. The main objective of this project is to evaluate the effect of the hydration temperature on the porosity, the distribution of pore size, the volume fractions of the constituents of the microstructure (CSH Portlandite, etc.), the density and the intrinsic porosity of CSH. Scientific advances in the non-invasive tools for probing materials would be of pratical interest in evaluating such parameters. Our experimental program will rely on a combination of different techniques, namely, NMR relaxometry, NMR cryoporométrie the anayse X-ray diffraction, mercury porosimetry, desorption of water vapor. Particular emphasis will be given to NMR relaxometry, which has proved its effectiveness in the study of porosity and access to information concerning the distribution of pore connectivity between these pores, the density of CSH and free water in the microstructure of the hardened cement paste. This technique has considerable advantages; it will help to overcome the difficulties related to the low permeability of the cement paste, the adsorption forces that decrease the mobility of water and non-conectivity of pores. The results of these experiments will allow to develop a quantitative model for the effect of temperature hydration on the microstructure of the cement paste. These results will be used in a micromechanical model to predict poromechanical properties of hardened cement pastes hydrated under different temperatures. These predictions will be validated by conventional poromechanical experiences, namely drained, undrained and unjackated tests in a triaxial cell

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