Effect of pore volume change on the transport and poro-elastic properties of a heat-treated mortar

摘要

This paper presents both experimental results and analysis on a (W/C)=0.5 mature mortar after heat cycling of up to 400 degrees C (below the temperature range for portlandite decomposition). After such degradation process, mortar microstructure is significantly affected, with extensive dehydration of its main C-S-H gel phase. Consequences on pore network and solid skeleton morphology are also anticipated. Our main purpose here is to assess the relationship between microstructure and macroscopic properties changes after 400 degrees C maximum heat-cycling. Transport properties are related to material durability by using gas permeability as our main indicator. Mechanical performance is evaluated using several poro-elastic parameters: drained bulk modulus Kb, solid matrix bulk modulus Ks and Biot’s coefficient b. An original experiment is also presented, which allows to assess accessible pore volume evolution under hydrostatic stress cycling. Firstly, gas permeability and porosity are measured on intact samples, after initial drying at 60 degrees C, and on samples heat-cycled up to 105, 200, 300 and 400 degrees C. Gas permeability of intact material and of material heated up to 105 degrees C are insensitive to confining pressure variations, which supports the absence of significant microcracking, microstructure and morphological changes. On the opposite, a clear increase in porosity and gas permeability Kgas is observed after 200, 300 and 400 degrees C heat-cycling. Kgas of mortars cycled up to 200 degrees C and above decreases significantly and irreversibly with increasing confinement. This is attributed to micro-cracks closure and/or to partial accessible pore network collapse or closure. Secondly, poro-elastic properties and gas permeability are measured simultaneously during confinement cycling. For heat-cycling temperatures above 200 degrees C, secant drained bulk modulus Kb decreases while permeability decreases irreversibly with increasing confinement. Previous work [XT Chen, CA Davy, F Skoczylas, JF Shao, Cem Concr Res (39) pp.195-205 2009] has also shown that solid matrix bulk modulus Ks and Biot’s coefficient b both decrease with confinement, for material heat-cycled above 200 degrees C. Such evolutions are interpreted as being due to micro-crack closure and/or occluded porosity increase. Thirdly, we validate experimentally our interpretation of poro-elastic and permeability property changes, by measuring the variation in connected porosity under hydrostatic loading. To this purpose, we developed an original test which quantifies the accessible pore volume at given hydrostatic stress, by static Argon gas injection. The creation of significant irreversible occluded porosity is confirmed for mortars heat-cycled above 200 degrees C.