Unsaturated Cement Research Articles

A study on the distribution and movement of water in unsaturated cement paste by a multi-component multi-phase lattice Boltzmann simulation

The distribution of water inside concrete is a crucial factor that determines its mechanical performance and durability. In contrast to experimental investigation, numerical models provide a new way to visualize and characterize the actual situation of water in unsaturated cementitious materials. However, the interactions between different components are quite complex and they are rarely considered so far. In this work, a multi-component multi-phase (MCMP) lattice Boltzmann model is developed to distribute moisture in partially saturated cement paste, in which the interactions between water and air are fully taken into account. The transport of water flow in unsaturated cement paste is modelled and the permeability is thus computed. The simulation results show a satisfactory agreement with experimental data, validating the proposed approach. The obtained permeability is found to decline at a decreased water saturation degree and this is attributed to a reduced number of transport paths for water flow. Moreover, as the contact angle increases from 80° to 100°, large pores become the preferred place for water to accumulate and the formed clusters are observed. According to the results, a strong correlation between pore structure (i.e., pore size and pore connectivity) and water permeability is found and discussed.

Experimental Investigations on the Soil–Water Characteristic Curve and the Deformation Behaviors of Unsaturated Cement–Stabilized Soft Clay

Experimental Investigations on the Soil–Water Characteristic Curve and the Deformation Behaviors of Unsaturated Cement–Stabilized Soft Clay

Single-Pipe Model of Oxygen Diffusion in Unsaturated Cement Pastes: Comprehensive Analysis of Surface, Knudsen, Bulk, Transition, and Water-Curtain Diffusion

Single-Pipe Model of Oxygen Diffusion in Unsaturated Cement Pastes: Comprehensive Analysis of Surface, Knudsen, Bulk, Transition, and Water-Curtain Diffusion

Microscopic modelling of gas diffusivity in unsaturated cementitious materials considering multiple diffusion regimes

Common degradations of concrete including reinforcement corrosion and carbonation are highly associated with the transport of aggressive gases. However, it is still challenging to accurately understand the gas diffusion behaviour and determine the gas diffusivity in unsaturated cementitious materials. This study proposes a pore-scale model for simulating gas diffusivity in unsaturated cement paste covering multiple gas diffusion mechanisms. The representative volume elements of colloidal calcium silicate hydrate and heterogeneous cement paste with various water saturation degrees are established, capturing the structural attributes of hierarchical cement paste and water-gas distribution with minimum surface energy. A finite difference model for diffusion considering Knudsen effect and molecular diffusion is developed to simulate the diffusion process of various gases through partially saturated cement pastes with different pore structures. Results indicate that gas diffusivity decreases with the increasing water saturation level, the evolution of which can be divided into stable, decreasing and non-diffusive stages. The gas diffusion in partially saturated cement paste is highly dependent on the pore structure and diffusive gas type. The results obtained from this study are expected to provide a deeper insight into the solution of diffusion-related durability problems and the prediction of service life of concrete.

Foam stability of 3D printable foamed concrete

This study investigates the susceptibility of precursor foam in 3D printable foamed concrete (3DP-FC), particularly in aspects of rheology. The relatively higher yield stress, film liquid withdrawal from unsaturated cement grains and the pumping process raises a concern of considerable foam degradation. Foam stability under the static and dynamic environment is investigated with a wide yield stress range of the base mix (200–1500 Pa) and foamed concrete density range (700–1400 kg/m3) is investigated in this study. No major foam instability was found in the static environment with comparison of various wet-to-dry density and porosity models, while the pumping process caused densification for the higher density 3DP-FC. Other than the gravimetric measurements, the X-ray CT scans were performed to analyse the porosity, sphericity and pore distribution changes between the pre- and post-pumped samples. This paper addresses the effect of fluidity on foam stability due to physicochemical interaction on microstructural level, and the producibility of a stable 3DP-FC with a density as low as 700 kg/m3

The dependence of capillary sorptivity and gas permeability on initial water content for unsaturated cement mortars

Capillary sorptivity and gas permeability characterizing the penetration ease of water and gas deserve attention in the current attempts to quantify durability performance for cement-based materials. To investigate their dependences on water content, disk specimens of cement mortars are prepared and thoroughly preconditioned. At ambient condition, available models could quantify the measured dependence of gas permeability except capillary sorptivity. Surprisingly, it follows another bilinear law with a transitional saturation degree, beyond which capillary condensation occurs in gel pores at equilibrium. This extraordinary disagreement is ascribed to the dynamic structure of C–S–H gel and thus mortars, which are sensitive to water removal and regain. Physically, it is also responsible to several unexpected anomalous characteristics of water absorption. The unsaturated flow theory is no longer applicable if ignoring its sensitivity to water, which is the key to approach the transport mechanisms of water and other deleterious agents in common unsaturated cement-based materials.

In situ characterisation of the depth and mass of water intrusion in unsaturated cement pastes via X-ray computed tomography

X-ray computed tomography was applied to continuously track the migration process of water in unsaturated cement pastes in situ. It was found that the process of water uptake starts with early rapid growth and a subsequent slowly increasing period, followed by a relatively stable stage. The rate of water penetration was accelerated with a decrease in slag incorporation. An analytical equation was developed to establish the relationship between the depth and the mass coefficient of capillary suction and the obtained predicted values were found to be in excellent agreement with the experimental values observed via the gravimetric method.

Pore-scale modelling of relative permeability of cementitious materials using X-ray computed microtomography images

Permeability of cementitious materials is an important durability indicator. In practice, concrete is rarely saturated. Therefore, it is essential to study permeability of unsaturated cementitious materials. This paper presents an integrated modelling approach for estimating permeability of cementitious materials over the full range of saturation. An in-house code based on multiphase and single-phase lattice Boltzmann models is developed and used to simulate the moisture distribution and fluid flow in unsaturated cement paste with various curing ages, the 3D microstructures of which are obtained from X-ray micro-CT. The water permeability and gas permeability of partially saturated cement paste are then estimated. The results indicate with decreasing water saturation water permeability decreases while gas permeability increases. Additionally, the moisture distribution and permeability of unsaturated cement paste are strongly dependent on its microstructure. Moreover, there exists a unique relationship between permeability and effective porosity. The simulation results show good agreement with experimental data.

Experimental investigation of solute transport in unsaturated cement pastes

This article is dedicated to the development of a method for acquiring experimental data on ionic diffusion in partially saturated cement-based materials. A modified “half-cell” method is adopted in order to introduce the diffusing solute without disturbing the initial water content of the material. After a certain diffusion time (5 to 10months), the concentration profiles of the tracer within the samples are measured by elemental mapping using the Laser Induced Breakdown Spectroscopy technique (LIBS). Analyses of the diffusion profiles obtained show a net decrease of the liquid-phase diffusivity with desaturation of the material, in accordance with the results in the literature. The innovation provided by this study consists in taking into account the adsorption of the tracer (Li) in the analytical processing of the diffusion profiles. This adsorption was quantified by measuring the interaction constant.

Moisture Transfer Between Unsaturated Cement Mortar and Ambient Air

The objective of this article is to provide a description of moisture transport in building materials by application of a non-linear diffusion model, in which the effective coefficient of diffusion D eff, being a function of moisture content, is implemented. This article proposes the method of determination of this coefficient through correlation of the theoretical desorption isotherms with the experimental ones. To this aim, a number of desorption isotherms at temperature T = 20°C for several narrow ranges, being adjacent to each other, and for broad ranges of the air relative humidity \({\Delta \varphi}\) were established experimentally for cement mortar samples with water/cement ratio w/c = 0.50. The new knowledge contained in this article concerns some long-lasting measurements carried out in desiccators, for both narrow and broad ranges of the air humidity changes, and evaluation of the diffusion coefficient by applying an advanced optimization algorithm. The final conclusion is that the non-linear diffusion theory well reflects the moisture transport in hygroscopic porous materials like cement mortar.

Oxygen diffusion through hydrophobic cement-based materials

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The obtained results show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.

Chloride transport in unsaturated cement-based materials

ABSTRACT The aim of this work is to model the transfer of chlorides in unsaturated cement based materials submitted to humidification—drying cycles. The developed model is multi-species and takes into account the binding isotherms of the chlorides and their convection velocity. The simulations of chloride profile carried out showed in the concrete cover an increase of 14% of free chloride concentration due to the convection and a decrease of 36% due to the chemical interactions. Experimentally, this model was validated by using imbibitions and drying tests. Finally, we have shown that the humidification—drying processes accelerate the chloride ingress and consequently the risk of corrosion.

Modelling the influence of ionic and fluid transport on rebars corrosion in unsaturated cement systems

This paper deals with numerical modelling of rebar corrosion kinetics in unsaturated concrete structures. The corrosion kinetics is investigated in terms of mechanistic coupling between reaction rates at the steel surface and the ionic transport processes in the concrete pore system. The ionic and mass transport model consists of time-dependent equations for the concentration of dissolved species, the liquid pressure and the electrical potential. The complete set of nonlinear equations is solved using the finite-volume method. The nonlinear boundary conditions dealing with corrosion are introduced at the steel-concrete interface where they are implicitly coupled with the mass transport model in the concrete structure. Both the case of free corrosion and potentiostatic polarisation are discussed in a one dimensional model.

Modeling the behavior of unsaturated cement systems exposed to aggressive chemical environments

Modeling the behavior of unsaturated cement systems exposed to aggressive chemical environments

Modeling ion and fluid transport in unsaturated cement systems in isothermal conditions

A description of ionic transport in unsaturated porous materials due to gradients in the electro–chemical potential and the moisture content is developed by averaging the relevant microscopic transport equations over a representative volume element. The complete set of equations consists of time-dependent equations for both the concentration of ionic species within the pore solution and the moisture content within the pore space. The electrostatic interactions are assumed to occur instantaneously, and the resulting electrical potential satisfies Poisson's equation. Using the homogenization technique, moisture transport due to both the liquid and vapor phases is shown to obey Richards' equation, and a precise definition of the moisture content is found. The final transport equations contain transport coefficients that can be unambiguously related to experimental quantities. The approach has the advantage of making the distinction between microscopic and bulk quantities explicit.

Modeling the behavior of unsaturated cement systems exposed to aggressive chemical environments

The main features of a numerical model that predicts the mechanisms of ionic transport in unsaturated cement systems are described. The model, called STADIUM, is divided into four parts: ionic diffusion, moisture transport, chemical reactions, and chemical damage. The diffusion of all ions present in the system is modeled by solving the extended Nernst-Planck/Poisson set of equations. The model takes into account the electrical coupling between the various ionic species in solution and chemical activity effects. The transport of water by capillary suction is described by a diffusion equation. In the model, the transport phenomena and the chemical reactions are uncoupled. Phase dissolution and precipitation phenomena are modeled through a separate chemical equilibrium code. The model considers the influence of chemically induced microstructural alterations on the transport properties of the material. A brief description of the model is presented in the first sections of this paper. Typical applications of the model are given in the last section.

Pores and cracks in cemented waste and concrete

Cemented waste and ordinary construction concrete are rigid, porous materials. Some concepts derived from percolation theory are presented together with relations between the porosity and the water/cement ratio and the degree of hydration. A large range of particle and pore sizes covering some 7 or 8 decades is typical for the products. The importance of interfaces is underlined. Limitations of the methods for pore structure characterization are shortly presented and the influence of pore solution concentrations on the BET method when based on water absorption is described. Interpretation problems and formation mechanisms for fractal systems are discussed in connection with the small angle neutron scattering method. The pore structure is important for hygroscopic water uptake in cement, and for the transport of gases and water vapour through unsaturated cement paste. It is also important for the advective or diffusive transport of solutes in water-saturated cement paste. Diffusivities obtained in various manners are discussed in qualitative terms. Flow of water through concrete barriers takes place preferentially through cracks or other coarse interconnected defects. Cracks may heal due to precipitation of for example calcium carbonate, but negative effects of the formation of impervious layers is also a possibility.