Experimental study on a mortar. Temperature effects on porosity and permeability. Residual properties or direct measurements under temperature

This study is a part of a large experimental program intended to characterize the effects of increasing temperature upon the hydraulic properties of cement based materials. Initial gas permeability and porosity values of the used material, a normalized mortar with a water / cement ratio of 0.5, clearly show the material homogeneity after a drying phase at 60 °C. The initial permeability is moreover insensitive to confining pressure variation which is evidence of the absence of significant initial micro-cracking in which the flow could occur. Residual properties (at room temperature) were measured after thermal treatments at 150 or 250 °C. These heating phases lead to a clear increase in porosity and permeability being sevenfold its initial value after a 250 °C treatment. This increase in permeability comes from two distinct effects that can be experimentally highlighted: a pore widening observed with the Klinkenberg effect being lower and a micro-crack closure occurring with the increase in confining pressure. Permeability was also measured with gas injection in samples submitted to thermal loading and slightly confined (at 4 MPa). Three levels of temperatures were used: 25, 105 and 200 °C; in a first stage the permeability remained almost constant to finally significantly increased at 200 °C. At 105 °C there is a widening of pores observed with the Klinkenberg effect but not sufficient to vary the permeability. At 200 °C micro-cracks have occurred but their influence upon permeability is lower than for treated material. This is an effect of the confining pressure applied on the material during heating.