Elsevier

Cement and Concrete Research

Volume 115, January 2019, Pages 460-471
Cement and Concrete Research

Effects of severe heating and rehydration on poro-mechanical properties of a mortar

https://doi.org/10.1016/j.cemconres.2018.09.020Get rights and content

Abstract

A normalized mortar was severely heated at different temperatures up to 600 °C that led to a strong material degradation linked to its loss of water and to the Portlandite decomposition. These heating were followed by pure water rehydration as these operations have proven to be efficient as regards the recovery in the transfer properties. The present study is based on the main material's poromechanical property measurements with gas as the pressurized porous fluid. They are useful to indicate, after heating, the strong increase of the mortar's skeleton compressibility and of its expansion due to internal pressure. These phenomena are due to heating micro-cracking, pore widening and to a material fractioning highlighted with the increase in its Biot's coefficient. Unambiguously, rehydration led to a visible recovery of the mortar's poromechanical properties that is linked to newly formed hydrates.

Introduction

This experimental study aims at understanding and analysing the effects of temperature and rehydration on poromechanical property changes of a normalized mortar. This is a part of large research program on coupled effects of temperature, mechanics and hydraulics on the degradation of cement-based materials, conducted in our laboratory [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. As referred in [11], in the case of fire accident or radioactive waste storage, the observed degradation due to temperature occurs in the following stages: decomposition of the AFm/Aft [[13], [14], [15], [16]], progressive decomposition of the C-S-H over a wide range of temperatures [13,17,18], and the decomposition of Portlandite [18]. The decomposition of C-S-H and Portlandite are accompanied by the loss of chemically bounded water, both of which are known to cause strong micro-cracking and progressive de-saturation of the material, modifying its matrix and skeleton structure, and thus its compressibility [11]. The decision to focus on effect of temperature on poromechanical properties was related to their recognized importance, with respect to an evaluation of the material's durability. However, effects of high temperature on poromechanical behaviour have been scarcely identified experimentally, except, to our knowledge, in [5,7,19].

Poromechanical method is a useful tool to describe and explain the evolutions and degradations of porous media at the microstructure scale (i.e. changes in morphology) or at the macroscopic scale of a structure. For instance, experimental identification of the poromechanical properties of rocks and cementitious materials is widely available in the literature [8,[20], [21], [22], [23], [24], [25], [26], [27], [28], [29]]. In particular, M. Lion et al. [30] have validated a linear and isotropic poroelastic model, and identified the effect of heating up to 250 °C upon the poro-elastic properties of a limestone. All tests have been performed using ethanol as a neutral saturating fluid and poromechanical residual values (after heating/cooling cycle of 250 °C) were identified for both drained and undrained cases to indicate a slight micro-cracking of the material skeleton and solid matrix, although not observed during subsequent SEM analysis at the micrometric scale. M. Sibai et al. [31] have shown that the solid matrix compressibility, Ks, may be considered constant for a Fontainebleau sandstone, even after a heating/cooling cycle up to 800 °C. Changes in the skeleton compressibility, Kb, for heating cycle up to 800 °C do not appear significant either, when recorded after the initial nonlinear phase (i.e. that due to crack closure under confinement) such as in [30,32]. Yet, for cement-based materials, Ulm et al. [22], Coussy [23], Dormieux et al. [33] and Ghabezloo et al. [24] have proposed strong arguments in favor of the Theory of Porous Media (TPM) for cement-based materials and, into this frame, our contribution provides additional experimental arguments. In particular, Skoczylas et al. [7] identified experimentally the effects of heat-treatment and confinement upon drained modulus Kb, solid matrix bulk modulus Ks and Biot's coefficient by drained or change in pore pressure tests under different confining pressure levels (ranging from 12 to 25 MPa). In this research, the thermal degradation of the solid skeleton due to various heat-induced micro-cracks was identified but the temperature treatment was limited to 400 °C i.e. below the level of Portlandite decomposition. The present study is carried out up to 600 °C of heating i.e. therefore more complete and indicative than the previous one. It can also be usefully compared to the effects of heating on transfer properties conducted on the same material [34].

In addition, the above mentioned heat-induced cracks are likely to facilitate the transport of aggressive agents (liquids or gases) into the materials, thereby accelerating its degradation and strongly reducing its durability [35,36]. In such a context, it also can be of great interest to study the rehydration effect on a thermally deteriorated cementitious material, with the aim of restoring a certain degree of its initial durability [[37], [38], [39], [40]]. Many studies have shown that cementitious materials have the autogenous ability to heal cracks [[41], [42], [43], [44]], by rehydration. This mechanism has been attributed to various different phenomena: delayed hydration in the presence of anhydride cement [[43], [44], [45]], neo-formation of ettringite [46], C-S-H [[45], [46], [47]], Portlandite [47], and calcite [43,[48], [49], [50]]. However, there are no strict rules governing all types of self-healing as they depend on curing time, crack opening and the material's age as well as its composition.

The material's improvement in performance resulting from rehydration in the presence of water has often been quantified in mechanical terms, and many studies, based on this topic, can be found in the literature. Although restoration of the material's compressive strength and/or flexural stiffness have been observed by Jacobsen et al. [51], Granger et al. [52] and Ferrara et al. [53], in the case of calcite precipitation during self-healing the durability of concretes or mortars is often evaluated in terms of their transport properties, among which permeability and porosity play a vital role. Previous studies aiming at evaluating the durability of cementitious materials after rehydration have been mainly performed on the penetration of water, and on the water permeability of materials under low confining pressure [43,[48], [49], [50],54]. From the point of view of gas transport and the structure healing, Pei et al. [11,12] have used gas (or water) permeability measurements, which were simultaneously carried out with mechanical loading (confining pressure in particular), to evaluate the beneficial effects of self-healing through the rehydration of strongly heated mortars. The transfer properties: gas permeability and gas porosity have been assessed. Overall, from the research, heating up to 700 °C can strongly degrade the material, with an increase in permeability of nearly 3 orders of magnitude, and an increase in porosity of approximately 30%–40%. The material weakening, whose hydrates have lost most of their bound water, leads to a considerable change in permeability under confining pressure, and to strong, irreversible effects during unloading. This irreversibility is attributed to crack closure and to the collapse of pores, which do not recover their initial state following such unloading. The impact of self-healing is spectacular, since the neo-formed hydrates return the permeability/porosity to a value close to its level prior to heating, and the confining pressure sensitivity is strongly reduced, whatever the level of heating (up to 700 °C). The pore size, which was observed through the Klinkenberg effect, was considerably reduced. The complementary micro CT tests also showed that the rehydration (re)creates a large proportion of the Portlandite which was decomposed during heating, and that a new C-S-H structure was formed.

Based on the mentioned interpretation, a logical following of this work was the poromechanical confirmation of this phenomenon. In such a context, the originality of our contribution is to identify the degree of degradation of heat-treated and after rehydration of a cementitious material, by means of poromechanical properties measured with gas. This paper is organized so as to explain accurately how thermal damage and rehydration degree are identified from a simplified poromechanical approach (Section 2); then an original experimental methodology is adopted (Section 3). The use of gas as an interstitial fluid can avoid the reaction between water (or ethanol [12]) on the de-hydrated cement paste and can also saturate the pore network in a relative short time.

Section snippets

Poromechanical behaviour and experiments associated

Most studies on the poromechanical behaviour of rocks or cementitious materials are based on works initially conducted by Biot [57] for fully-fluid saturated porous media. Both mechanical stress Σ and internal fluid pressure P contribute to loading. For an elastic porous medium, the effective stress tensor Σ is defined as follows:Σ=Σ+bP1

This effective stress is supposed to be the only argument that controls the stress–strain relationship. b is known as the Biot's coefficient of the material.

A

Materials used

The present study was carried out on a normalized mortar made from Leucate sand and CEM II/B-M (LL-S) 32.5 cement. The respective cement and mortar compositions are indicated in Table 1, Table 2. It should be noted that this mortar has already been extensively studied in our laboratory, and been described in several publications dealing with the influence of temperature on its mechanical and transfer properties [1,2,[4], [5], [6], [7],[9], [10], [11], [12]]. The present research relates to a

Effect of the heating on Kb

In Fig. 3 below can be found the global behaviour under confining pressure of samples heated up to 600 °C (105 °C reference state, 200, 400, 500 and 600 °C) and containing the unloading phase. As it was already observed for porosity and permeability [11], there is now a high effect of temperature on the material compressibility except at 200 °C as the (200 °C) samples behave virtually like the (105 °C) ones [10]. This phenomenon was previously observed on the Young modulus, which remains almost

Conclusions

This present work is the continuity of a larger study, on severely heated mortars, which focused on the change in their transport and poromechanical properties before and after rehydration with pure liquid water. The global objective was to bring a complementary approach on the material degradation and/or recovery (after rehydration) in comparison to more classical ones often based on Young's modulus, strength and connected porosity measurements. The interest of such kind of measurements is

Acknowledgements

The authors are grateful to Th. Dubois for technical expertise, and the China Scholarship Council (CSC) for the financial support.

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