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Calcium-silicate hydrate (C-S-H) is the main binding agent in cement and concrete. It forms at the beginning of cement hydration, it progressively densifies as cement hardens and is ultimately responsible of concrete performances. This hydration product is a cohesive nano-scale gel, whose structure and mechanics are still poorly understood, in spite of its practical importance. Here we review some of the open questions for this fascinating material and a statistical physics approach recently developed, which allows us to investigate the gel formation under the out-of-equilibrium conditions typical of cement hydration and the role of the nano-scale structure in C-S-H mechanics upon hardening. Our approach unveils how some distinctive features of the kinetics of cement hydration can be related to changes in the morphology of the gels and elucidates the role of nano-scale mechanical heterogeneities in the hardened C-S-H.

Understanding the physical origin of creep in calcium-silicate-hydrate (C-S-H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C-S-H. Under a given shear stress, C-S-H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity., Schlumberger Limited, French Research National Agency (ANR-11-LABX-0053), French Research National Agency (ANR-11-IDEX-0001-02)

It has generally been accepted that the volume of cement hydration products is slightly smaller than the original volume of cement and water. However, this does not mean that the hydration reaction causes the hardened cement paste and concrete to contract. In fact, C-S-H shells that grow around anhydrous cement grains push the neighbors apart by crystallization pressure and thus cause the solid framework of cement paste to expand. Proposed here is a new idea—this expansion always dominates over the contraction, i.e., the hydration is, in the bulk, always expansive, while the source of all of the observed shrinkage, whether autogenous or due to external drying, is a compressive elastic strain in the solid caused by a decrease of chemical potential of pore water, with the corresponding changes in pore humidity, surface tension and disjoining pressure. From recent observations of autogenous shrinkage growing logarithmically in time over many years it follows that the growing C-S-H shells surrounding cement grains must act as diffusion barriers for water and ions, which slow down the hydration process and can extend it over many years and even decades. The new idea implies that all of the autogenous shrinkage must be caused by elastic compression (probably with no, or almost no, creep) of crystalline nano-sheets in the solid framework subjected to stresses that arise as a reaction to pore water stresses. Swelling under water immersion is explained by insufficient elastic compression when water is permanently supplied to the pores. The lecture first presents the aforementioned theory and then summarizes some recent advances in related phenomena, particularly a model for oriented damage due to alkali-silica reaction and a method for shrinkage extrapolation.

Despite their impact on longevity, serviceability, and environmental footprint of our built infrastructure, the chemo-physical origins of nanoscale properties of cementitious materials, and their link to macroscale properties still remain rather obscure. Here, we discuss a multi-scale approach that describes different aspects of physical properties of C-S-H at the nano- and meso-scales. These include dynamics of water, thermal properties and mechanical behavior of C-S-H and its effect on properties of cement paste at different scales.

Geopolymers are alumino-silicate hydrates obtained by reaction of an alumino-silicate source (e.g. metakaolin or fly ash) with alkali solution. Geopolymer-based binders are less environmentally impacting than ordinary cement, but their implementation in the construction field is still limited and requires a better understanding of the nanoscale origin of their mechanical properties. This understanding can be advanced with new simulations based on interaction-driven aggregation of nanoparticles, similar to what has happened in the last decade in the field of traditional cement science. This paper introduces a pathway to develop such a model starting from recent molecular models of geopolymers, which allow to compute the interaction potentials needed for the larger mesoscale. Interaction potential parameters are presented in this work as a function of different particle sizes, targeting experimentally-observed ranges of particle sizes and porosity. Overall, this work opens new opportunities to understand the linkage between mesostructure and engineering properties of geopolymers, with the aim of supporting their commercialisation as alternative cements and, in this way, contributing to the development of a greener economy.

In the light of the developing use of cementitious materials in safety critical, high temperature applications associated with energy industries (nuclear, oil and gas) this work looks to the nano-scale to consider the origins of macro-scale phenomena. It enables a first comparison of nano-scale results with macro-scale observations, toward the inclusion of physico-chemico-mechancial processes in models at larger scales and a move away from phenomenological models. Assuming the development of nano-scale porosity to be the principal effect of temperature at the nano-scale, comparisons are here developed between nano-scale measurements, simulations and macro-scale experimental results. The results suggest that the effect of temperature on the nano/micro mechanical properties of cementitious materials might be responsible for a significant part of the experimentally observed trends at the macro-scale, although more work is required to understand scaling of fracturing. It is concluded that modifying the nano-scale material response to temperature gradients could eventually impact the engineering performance of structures at elevated temperatures.

The C-S-H gel is the main constituent of cement, up to 70% of the final material. It is the phase that gives cohesion to the material and is mainly responsible for cement's properties, including creep. Understanding the intrinsic mechanical properties of the C-S-H gel and how it responds to applied load is, therefore, of vital importance for the design of the new generation of Portland Cement. However, the heterogeneous nature and characteristic length scale of the C-S-H gel makes an experimental determination of its properties very challenging. Therefore, atomic scale simulations are a valuable alternative to investigate the atomic scale forces and processes that govern creep and shrinkage. In this work, we study the mechanical processes that take place when the solid C-S-H is subject to a shear strain, using reactive force field molecular simulations. We have chosen two systems to model the C-S-H gel: the perfect mineral tobermorite and the glass-like C-S-H model developed by Pellenq, et. al. (Pellenq, et. al., 2009). First, we have computed the elastic properties of both models. Second, we investigate the global and local stresses generated in the systems under large deformations, with the aim to understand the atomic forces that govern the mechanical response of the structures. The obtained results help to understand the changes that happen in the C-S-H gel under load.

The archaeological complex, that was discovered in December 2003 in Piazza Nicola Amore during the excavation works for Line 1 of the underground railway system, was identified by archaeologists as a Greek age Gymnasium. It represents the final part of a long path that the ancient Greek athletes, the Lampadodromi, ran through in honour of Parthenope, the goddess symbol of Naples. Later, in the Roman age, emperor August allowed the winners of his Neapolitan Isolimpic Games, the Sebastà, to have their names engraved on its arcade marbles, where they can still be read.

Cement hydration in concrete and mortar has been studied thoroughly over the past 50 years. To fully understand hydration in concrete and predict the evolution of the hygral, thermal, and mechanical properties at the structural level, one needs to studynot only the reaction kinetics but also the development of the microstructure. Many models have been developed for this purpose, some of them looking only at the micro-scale or at the macro-scale and others tackling the fundamental nature of the issue, which can be qualified as a multiscale problem. This paper proposes a novel approach that consists of combining a cement hydration model at the microstructural level, the CEMHYD3D model, with a macroscopic hygro-thermo-chemical model, the HTC model. The coupling is performed by postprocessing the output of the CEMHYD3D model, in particular with reference to cement hydration degree, silica fume reaction degree, and amounts of evaporable water and chemically bound water in order to identify through a curve fitting routine the parameters of the HTC formulation. This approach allows the possibility of predicting concrete behavior at multiple scales based on the actual chemical and microstructural evolution, thus enhancing the capabilities of the so-called HTC-CEMHYD3D model. This paper focuses on 1) introducing the concepts behind the formulation of self-desiccation and 2) demonstrating the predictive capabilities of the coupled model using some available experimental data.

The properties of the cement paste are defined to a high extend by those of the C-S-H gel, and irreversible deformations such as creep and shrinkage are not an exception. It is believed that molecular scale processes play an important role on the stress accumulation and relaxation, and molecular dynamics simulation a great tool to identify and quantify those mechanisms. In this work, we use NEMD simulation to investigate first the effect of the temperature and shear rate on the shear strength, Then, we study the stress development on a C-S-H interface with an increasing distance between adjacent particles. We discuss the results in terms of water, and we define a limiting distance, 1 nm, at which the shear strength is lost.

Many amorphous materials display a logarithmic creep behavior, driven by the rare occurrence of complex, hardly detectable, microscopic, structural rearrangements. Following recent developments in experimental techniques and modeling, we develop here a new approach based on transition state theory and on activation energies computed from molecular simulations of shear tests. Our results predict the logarithmic creep of an amorphous, model structure of cement at the molecular and meso- scales. We investigate the interplay of cooperative processes at the different length-scales and establish connections with creep phenomena in other materials.

Recent experiments show that the nanoscale morphology of cement hydrates can be tuned via solution chemistry and curing conditions. However, it is not known to what an extent a nano-tailored morphology of cement hydrates may translate into improved macroscale properties. This question is addressed here, focussing on water-content-dependent durability properties, in particular self-desiccation and water sorption isotherms. Nanoparticle-based simulations provide the starting point to create model hydrates structures at the micrometre scale, whose formation mechanisms and resulting morphologies depend on solution chemistry and interaction forces at the nanoscale. These nanoscale mechanisms and morphologies are then used to inform a simple model of cement hydration that predicts pore size distribution, water content, internal relative humidity and thus self-desiccation and water sorption isotherms at the macroscale. The results show that the nanoscale morphology of cement hydrates has indeed an important impact on the above-mentioned durability properties, and that hydrates precipitation in current ordinary cements follows a mechanism that is intermediate between the two frequently used models of homogeneous hydrogelation and boundary nucleation and growth.

It has generally been accepted that the volume of cement hydration products is slightly smaller than the original volume of cement and water. However, this does not mean that the hydration reaction causes the hardened cement paste and concrete to contract. In fact, C-S-H shells that grow around anhydrous cement grains push the neighbors apart by crystallization pressure and thus cause the solid framework of cement paste to expand. Proposed here is a new idea—this expansion always dominates over the contraction, i.e., the hydration is, in the bulk, always expansive, while the source of all of the observed shrinkage, whether autogenous or due to external drying, is a compressive elastic strain in the solid caused by a decrease of chemical potential of pore water, with the corresponding changes in pore humidity, surface tension and disjoining pressure. From recent observations of autogenous shrinkage growing logarithmically in time over many years it follows that the growing C-S-H shells surrounding cement grains must act as diffusion barriers for water and ions, which slow down the hydration process and can extend it over many years and even decades. The new idea implies that all of the autogenous shrinkage must be caused by elastic compression (probably with no, or almost no, creep) of crystalline nano-sheets in the solid framework subjected to stresses that arise as a reaction to pore water stresses. Swelling under water immersion is explained by insufficient elastic compression when water is permanently supplied to the pores. The lecture first presents the aforementioned theory and then summarizes some recent advances in related phenomena, particularly a model for oriented damage due to alkali-silica reaction and a method for shrinkage extrapolation.

Understanding the physical origin of creep in calcium–silicate–hydrate (C–S–H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C–S–H. Under a given shear stress, C–S–H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity.

Cement paste has a complex mesoscale structure, and small changes in its pore network potentially cause large variation in measurements such as the water isotherm (also nitrogen). We deconvolute the water isotherm with the help of advanced computational techniques, hypotheses, and a re-examination of published data. The pore system is divided into four different categories, each containing water with its own physical properties. By viewing the highly interdependent roles of water in each of the pore categories as a system, new insights are gained regarding possible mechanisms that control drying, shrinkage, and creep, and experimental strategies for verification.

Calcium-silicate hydrate (C-S-H) is the main binder in cement and concrete. It starts forming from the early stages of cement hydration and it progressively densifies as cement sets. C-S-H nanoscale building blocks form a cohesive gel, whose structure and mechanics are still poorly understood, in spite of its practical importance. Here we review a statistical physics approach recently developed, which allows us to investigate the C-S-H gel formation under the out-of-equilibrium conditions typical of cement hydration. Our approach is based on colloidal particles, precipitating in the pore solution and interacting with effective forces associated to the ionic environment. We present the evolution of the space filling of C-S-H with different particle interactions and compare them with experimental data at different lime concentrations. Moreover, we discuss the structural features of C-S-H in the mesoscale in terms of the scattering intensity. The comparison of our early stage C-S-H structures with small angle neutron scattering (SANS) experiments shows that long range spatial correlations and structural heterogeneties that develop in that early stages of hydration persist also in the hardened paste.

Shrinkage can be critical for the strength and durability of drying cement pastes. Shrinkage becomes particularly severe at very low relative humidity, < 20%, which can be met in some activities involving extreme temperatures. Experiments and simulations suggest that small pores in the cement paste, with approximate thickness ≤ 1 nm, stay saturated unless the humidity drops below 20%. Here we suggest that this pore size can define two different categories of pores in the paste: pores thicker than 1 nm, where the Kelvin’s equation and the corresponding capillary (Laplace) pressure apply, and pores thinner than 1 nm, which can be considered as part of the solid skeleton if the humidity stays above 20%. We show that a continuum model, incorporating a pore-blocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above ∼ 20%. At lower humidities, we assume that (1) during adsorpion water re-enters the smallest pores throughout the entire RH range (supported by experiments and simulations) and (2) there exists a simple linear relationship between water and strain in the smallest pores. These minimal assumptions are sufficient to explain the low-humidity hysteresis of water content and strain, but the underlying mechanistic explanation is still an open question. Combining the low-humidity and high-humidity models allows capturing the entire drying and rewetting hysteresis, and provides parameters to predict the corresponding dimensional changes.

The logarithmic deviatoric creep of cement paste is a technical and scientific challenge. Transition State Theory (TST) indicates that some nanoscale mechanisms of shear deformation, associated with a specific kind of strain hardening, can explain the type of deviatoric creep observed experimentally in mature cement pastes. To test this possible explanation, we simulate the shear deformations of a colloidal model of cement hydrates at the nanoscale. Results from quasi-static simulations indicate a strain hardening analogous to that postulated by the TST approach. Additional results from oscillatory shear (fatigue) simulations show an increase of deformation with number of loading cycles that is consistent with the observed creep. These findings indicate that nanoscale simulations can improve our current understanding of the mechanisms underlying creep, with potential to go beyond the logarithmic creep and explore the onset of failure during tertiary creep.

Calcium-silicate hydrate (C-S-H) is the main binding agent in cement and concrete. It forms at the beginning of cement hydration, it progressively densifies as cement hardens and is ultimately responsible of concrete performances. This hydration product is a cohesive nano-scale gel, whose structure and mechanics are still poorly understood, in spite of its practical importance. Here we review some of the open questions for this fascinating material and a statistical physics approach recently developed, which allows us to investigate the gel formation under the out-of-equilibrium conditions typical of cement hydration and the role of the nano-scale structure in C-S-H mechanics upon hardening. Our approach unveils how some distinctive features of the kinetics of cement hydration can be related to changes in the morphology of the gels and elucidates the role of nano-scale mechanical heterogeneities in the hardened C-S-H.