Effect of relative humidity and porosity on the logarithmic creep of the layered C–S–H minerals tobermorite and jennite
Introduction
In prefabricated elements made of autoclaved aerated concrete, tobermorite is the reaction product of cement, lime, and quartz cured at high pressure and high temperature [1,2]. Furthermore, tobermorite is formed in oil well cement when silica flour is added to reduce the CaO/SiO2 (Ca/Si) ratio to 0.8 to avoid the formation of a crystalline α-CSH under hydrothermal conditions (e.g., the temperature of 150–200 °C and confining pressure of 0.3–1.2 MPa), which cause an increase of capillary pores (10–1000 nm) and strength loss [[3], [4], [5]]. Moreover, tobermorite is also found in the microstructure of calcium silicate board which is widely used thanks to their properties of good sound insulation, lightweight, flame-retardant, and moisture resistance [2]. Besides, aluminate-enriched tobermorite was found to the key microstructure component of the Roman concrete (or opus caementicium) which is renown for its longevity [6,7].
In the last decades, 1.4 nm tobermorite and jennite have been extensively studied as natural analogs to the layered structure of calcium silicate hydrates (C–S–H) by diffraction, microscopic, and spectroscopic methods [[8], [9], [10], [11]]. First, Taylor [8] postulated that the atomic structure C–S–H exhibits a tobermorite-like structure at a low Ca/Si ratio of 0.8 or a jennite-like structure at a high Ca/Si ratio, respectively. In the latter case, a progressive silicate chain depolymerization may cause a preferential omission of Si bridging tetrahedra. Richardson [12] defined a two-fold classification of the C-S-H atomic structure based on a hybrid form of tobermorite and jennite (T/J model) and tobermorite with calcium hydroxide (T/CH model). More recently, quantitative X-ray pair distribution function analysis of synchrotron X-ray scattering data showed that the structure of the C-S-H resembled that of a defected tobermorite regardless of the Ca/Si ratio [10]. Moreover, Pellenq et al. [13] estabilished by atomistic modeling that C–S–H atomic structure is like a tobermorite crystalline layered structure with defected silicate chains where water molecules are adsorbed in interlayer regions. Interestingly, C–S–H nanostructure was found to evolve to tobermorite-like structure upon aging from a steady improvement of the layer stacking [14,15].
Water content plays a crucial role in the stability and stiffness of the layered silicate systems [16,17]. Based on Jennings model [18], water is mainly adsorbed on the C–S–H surface at RH lower than 40%, then water is presented in C–S–H gel pores at RH between 40% and 90%, and finally, water begins to fill the capillary pores at RH greater than 90%. While the water saturation of the porosity can be described as a function of RH based on the Brunauer–Emmett–Teller (BET) theory of multi-layer adsorption [[19], [20], [21]], it is worth reminding that there exist dissimilar theories on the state of water in hardened cement paste [22].
As for the effect on the C-S-H mechanical property, a steady decrease of dynamic elastic modulus of aged cement mortar was found with RH decreasing [23]. Several experiments performed at the macroscopic scale showed that the long-term creep rate of a cement paste is greater at greater RH [[24], [25], [26], [27]]. Interestingly, the short-term creep of the C–S–H gel was found to be associated with the microdiffusion of load-bearing water [28,29]. Contrarily, creep tests on micrometer beams showed that there is no significant difference between the creep of D-dried or fully saturated cement pastes [30]. Instead, the creep of re-saturated cement paste was much greater perhaps due to microcracking.
More recently, microindentation technique has been emerging to quickly assess the basic creep of cementitious materials [[31], [32], [33]]. Notably, the logarithmic creep measured on cement paste after few minutes was found to well correlate to logarithmic creep measured after decades of uniaxial compression creep for several mortars and concretes [34]. Microindentation results also confirmed that the creep rate of synthetic C–S–H and cement paste is greater at higher RH levels [[34], [35], [36]]. It was postulated that the increasing amount of interlayer water at a higher RH may increase the disjoining pressure and increase the basal spacing between the C–S–H sheets [[37], [38], [39]]. Such weakening effect of the van der Waals forces accelerates the C–S–H sliding mechanisms, which are at the source of basic creep [39]. Similarly, the microprestress-solidification (MPS) theory assumes that the viscosity of a cement paste is a decreasing function of the disjoining pressure [37]. Microindentation was also successfully applied to study the effect of porosity on the creep of secondary hydrated cement phases, such as calcium hydroxide, ettringite, and gypsum at a very low RH level of 11% [40]. Recently, Hu et al. [41] carried out microindentation tests on synthetic C–S–H with Ca/Si ratio ranging from 0.8 to 1.5 showing that the presence of portlandite crystals at higher Ca/Si ratio causes a lower creep (i.e., higher contact creep modulus). They also found that the indentation modulus and hardness of synthetic C–S–H decrease with the increase of porosity [41]. Nguyen et al. [42] studied the creep behavior of 1.4 nm tobermorite and jennite at very low RH level of 11% by creep microindentation. Their results showed that the contact creep modulus of 1.4 nm tobermorite and jennite is lower than that of a cement paste, but greater than that of synthetic C–S–H at a Ca/Si ratio of 1.2. In particular, jennite exhibited a slightly greater contact creep modulus than that of 1.4 nm tobermorite at different porosity.
This research aims at advancing the knowledge on the creep behavior of 1.4 nm tobermorite and jennite at different RH levels and porosity through creep microindentation tests. The presented results provide new knowledge on the load-induced dimensional changes of C–S–H minerals [43,44].
Section snippets
Material preparation
1.4 nm tobermorite and jennite were prepared at the National Research Council Canada through the compacting method explained in Refs. [42,45]. Based on their X-ray diffraction characterization, no other secondary phases were detected [17]. The Ca/Si ratio for jennite was about 1.5. The reactants, i.e., calcium oxide (CaO), silica (SiO2), and water were mixed in N2 purged high-density polyurethane bottles. Then the plastic bottles were laid on a rotating rack with a speed of 16 rpm. The mixture
MIP porosity
The relationship between the incremental intrusion of mercury and the pore entry size distribution is plotted in Fig. 1(a) and (b) for 1.4 nm tobermorite and jennite, respectively. Note that MIP can only estimate the inter-connected pores but not the isolated pores and it overstimates the real pore size due to the ink-bottle effect [51]. The pore entry size herein means the pore size that can be intruded by the mercury. The critical pore entry size (which is defined as the steepest slope) is
Simplified analysis of RH dependence of the contact creep compliance rate
A simplified model is herein employed to quantify the RH effect on the creep properties of jennite and 1.4 nm tobermorite. As a first-order approach, Eq. (4) is adapted to include the dependency on RH as follows:where the indentation modulus and the contact creep modulus are indicated as functions of RH. Indeed, the internal water may play a complex role in the creep mechanisms of the C–S–H microstructure. For instance, water microdiffusion may affect the
Concluding remarks
This work investigates the effects of porosity and RH on the creep behavior of C–S–H minerals, such as 1.4 nm tobermorite and jennite by microindentation. Based on the presented results, the following conclusions can be drawn:
- 1.
All the indentation mechanical properties measured by microindentation (i.e., indentation modulus M, indentation hardness H, and contact creep modulus C) decrease with the increasing porosity regardless of the RH levels;
- 2.
The indentation modulus M of 1.4 nm tobermorite and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We acknowledge the Université Laval/China Scholarship Council (UL/CSC) Joint Scholarship Program and the NSERC Discovery Grant RGPIN-2016-06077 for providing the salary to the first author in this study and the funding of this research. We would also like to thank the precious help of Dr. Jim J. Beaudoin and Dr. Rouhollah Alizadeh for the sample preparation and inspiring discussion.
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