Drying Cement Paste Research Articles

Miniature pH detection probe for in-situ monitoring of cement paste inside

A miniature pH detection probe which was fabricated from an oxidized tungsten wire electrode and a solid-contact silver-silver chloride electrode was developed and used for monitoring alkalinity of cement pastes. The pH detection probe which was embedded in wet cement paste block showed the stable potential differences assigned to pH 12–14 for at least more than 2 months. Although the potential difference of the probe showed inconsistent values in dry cement paste, the probe recovered to show the correct values after the re-wetting of cement paste. In an accelerated corrosion test of rebar co-embedded in cement paste using a pH detection probe, the probe showed the potential difference shift in acidic direction and illustrated depassivation of steel surface, accumulation of corrosion products in the paste, and crack formation of the paste. It was indicated that the pH detection probe was effective to evaluate not only alkalinity but also the deterioration of reinforced concrete.

A multi-scale framework for modelling effective gas diffusivity in dry cement paste: Combined effects of surface, Knudsen and molecular diffusion

Owing to the hierarchical pore structures of cement paste, varying gas diffusion mechanisms, i.e. surface, Knudsen and molecular diffusion, coexist. However, their combined effects have not been fully investigated. To this end, a multi-scale model which considers the combined effects is developed to simulate and analyse the effective gas diffusivity in dry cement paste. The multi-scale structures of cement paste using representative volume elements are reconstructed, capturing the structural characteristics at each scale. Afterwards, based on classical adsorption theory, random walk method and lattice Boltzmann method, the gas diffusivity at each scale is predicted and analysed. The results indicate that surface diffusion plays an important role in the gas diffusion in C-S-H, while the main diffusion regime in C-S-H gradually becomes Knudsen diffusion as the porosity increases. The surface and Knudsen diffusion cannot be ignored or simplified as molecular diffusion in the prediction of gas diffusivity in cement paste.

High-dynamic compressive and tensile strength of specimens made of cementitious materials

The strength of specimens made of cementitious materials increases with increasing loading rate. Herein, the model of Fischer et al. (CCR 58, 2014, 186–200) is revisited in the context of high-dynamic compression and extended to high-dynamic tension. The model is based on the assumptions (i) that cracking will start if the quasi-static material strength is reached, and (ii) that the high-dynamic strength gain refers to the increase of the stress during the failure process of the tested specimen. The model explains the behavior of cylindrical specimens made of dry cement paste, mortar, and concrete, subjected to high-dynamic compression. It also elucidates the performance of cylindrical specimens made of dry mortar and concrete, subjected to high-dynamic tension. It is concluded that the high-dynamic strength gain is a structural effect and that structures will be damaged if the dynamic stress exceeds the quasi-static strength, no matter how fast the stress is increased and how short the stress pulse lasts.

A Validation of the Ultrasound Wave Velocity Method to Predict Porosity of Dry and Saturated Cement Paste

An approximate theoretical validation of the measured variation of ultrasound velocity with porosity of dry and saturated cement paste is proposed using finite element analysis of cement paste assuming as a multiphase composite. Cement paste is a multiphase composite consisting of solid cement and microscopic voids filled either with air or water. The void content in cement paste is directly related to strength and durability. Experimental tests showed that ultrasound wave velocity is decreased with the increase in porosity of cement paste, where pore sizes are similar in dimension to the wavelength of the sound enabling ultrasound to be used as a potential condition assessment technique. However, the variation of ultrasound wave velocity also depends on the fluid in the voids. Several finite element simulations using two commercially available software packages were performed for both fully saturated and dry blocks of cement paste with different porosities. Then back‐calculated elastic moduli values from finite element simulations were used to compute the wave velocities of both fully saturated and dry cement paste with different porosities. The predicted ultrasound velocities with porosity for both dry and saturated cement paste are compared well with the laboratory measurements.

Multiscale modeling of drying shrinkage and creep of concrete

Creep and shrinkage are complicated time-dependent processes taking place in cementitious materials. In typical concretes, the most significant part of shrinkage is represented by drying shrinkage. Experimental data indicate that the ultimate value of drying shrinkage measured on concrete and mortar specimens is a nonlinear function of the ambient relative humidity. The nonlinear behavior is partially caused by microcracking, and partially by creep of C-S-H gel. On the other hand, drying shrinkage of hardened cement paste, measured on very thin specimens at a gradually decreasing relative humidity, was found to be a linear function of relative humidity. The aim of this paper is to explore whether the nonlinearities observed for macroscopic shrinkage can be explained by finite element simulations at lower levels, starting from the description of the drying cement paste by a viscoelastic model based on the Microprestress-Solidification Theory (MPS) with tensile cracking and with a constant shrinkage ratio.

Mesoscale Modeling of Nonlinear Influence of Ambient Relative Humidity on Drying Shrinkage of Concrete

Creep and shrinkage are complicated time-dependent processes taking place in cementitious materials. In typical concretes, the most significant part of shrinkage is represented by drying shrinkage; it is caused by a decrease in relative humidity of pores which leads to an increase in capillary tension of pore water and in the solid surface tension at pore walls.Experimental data indicate that the ultimate value of drying shrinkage measured on concrete and mortar specimens is a nonlinear function of the ambient relative humidity. The nonlinear behavior is partially caused by microcracking, and partially by creep of the cement matrix. On the other hand, drying shrinkage of hardened cement paste, measured on very thin specimens at gradually decreasing relative humidity, is found to be a linear function of relative humidity.The aim of this paper is to assess the nature of the macroscopic shrinkage computed employing finite element simulations at the meso-scale level in which the drying cement paste is described by a viscoelastic model with tensile cracking and with a constant shrinkage coefficient.

Micromechanical modelling of dry and saturated cement paste: Porosity assessment using ultrasonic waves

This article assesses the relationship between porosity and ultrasonic parameters of cement paste. It includes theoretical assessment of ultrasonic wave velocities of cement paste materials. Theoretical micromechanical models describing cement paste as a two-phase composite were detailed. Mechanical (bulk and shear moduli) and ultrasonic (longitudinal and transverse velocities) properties were evaluated. They were then, compared to the experimental ultrasonic properties measured on dry and fully water saturated samples with varying porosity First, the obtained micromechanical results showed that the correlation between acoustic velocity and porosity yielded the expected values: longitudinal and transverse velocities decrease with porosity. Secondly, the dilute inclusion model was able to represent the acoustic parameter of the cement paste only at low porosity, up to 20%. The self-consistent model under-estimated the measured ultrasonic properties for almost all porosity ranges. The Mori–Tanaka and the Kuster–Toksöz models succeeded in describing the acoustic parameters in dry and saturated states when assuming spherical shaped pores.

Relationship between ultrasonic Rayleigh wave propagation and capillary porosity in cement paste with variable water content

The relationship between Rayleigh wave velocity at ultrasonic frequencies and capillary porosity is investigated in dry and water saturated cement paste. Capillary porosity is varied by changing the water/cement (w/c) ratio. Water saturation is set at four levels – 100, 70, 30 and 0% – by drying the fully saturated specimen. Rayleigh wave (RW) wavefront and phase velocities are measured using air-coupled ultrasonic transducers as transmitter and receiver. The measurement accuracy is evaluated using a reference specimen made up of a non-dispersive material. As expected, a linear decrease of RW velocity vs. porosity is observed for the intermediate saturation states, but not for the fully and dry states, possibly due to micro-cracks induced during the drying phase and to the presence of bubbles in the specimens with low w/c ratio. The comparison of phase velocity in specimens with low and high w/c ratio shows an inversion of the porosity gradient perpendicular to the surface of measurements, which is due to segregation of cement at high w/c ratio. Finally, the correlation between RW velocity and capillary porosity is analyzed and compared with results obtained in mortar.

Evaluation of dilute inclusion model by finite element method: application to the cement paste

This paper deals with the application of the finite element method (FEM) to evaluate the dilute inclusion model, which is a micromechanical model used to determine the mechanical properties of the cement paste. The Young modulus of dry cement paste was characterised using a numerical method based on the finite element method and which considers porosity as a key parameter. The work is divided in three parts. In the first part, the methodology adopted to model the cement paste by FEM was presented. In the second part, the main equations of motion were described and both the FEM and the dilute inclusion homogenisation methods were applied to the case of dry cement paste. Finally, the results obtained by FEM and the dilute inclusion models were compared. The results show that there is a good adequacy between FEM and the model. This proves the reliability of the dilute inclusion model when estimating cement paste properties for a low range of porosity (less than 30%).

Dry Effect of Cementitious Materials on Testing Results of X-Ray CT

In this study three-dimensional X-ray computed tomography (X-ray CT) is used to investigate the testing results differences of cement paste and mortar before and after drying. It can be found that the mean gray values of paste and mortar before drying are bigger than after drying, and the impact of drying on cement paste is more serious than mortar. In addition, the porosity of non-drying cement paste and mortar is 1.10% and 0.43%, while that of drying cement paste and mortar is 1.55% and 0.70%, respectively. So, the porosity of paste and mortar markedly increases after drying process. The numbers of pores of paste and mortar sharply increase after drying process in particular between 0.01mm3 and 0.1mm3. The impact of drying on smaller pores is more serious than bigger pores.

Dry Effect of Cementitious Materials on Testing Results of X-Ray CT

In this study three-dimensional X-ray computed tomography (X-ray CT) is used to investigate the testing results differences of cement paste and mortar before and after drying. It can be found that the mean gray values of paste and mortar before drying are bigger than after drying, and the impact of drying on cement paste is more serious than mortar. In addition, the porosity of non-drying cement paste and mortar is 1.10% and 0.43%, while that of drying cement paste and mortar is 1.55% and 0.70%, respectively. So, the porosity of paste and mortar markedly increases after drying process. The numbers of pores of paste and mortar sharply increase after drying process in particular between 0.01mm3 and 0.1mm3. The impact of drying on smaller pores is more serious than bigger pores.

Electrical conductivity of drying cement paste

Previous research has shown that electrical measurements can be used to monitor moisture movement inside concrete. The interpretation of these measurements is frequently based on empirical relationships between moisture changes and electrical properties of concrete. As such, these empirical relationships can limit the application of the electrical measurements to a specific material or exposure history. To facilitate the development of a general method that is applicable to a concrete member in service, this paper characterizes the electrical conduction in cement paste subjected to drying (desorption) and moisture absorption. The paper quantifies how the electrical conductivity is dependent on the volume and connectivity of the moisture inside the pores and the conductivity of pore solution. This paper also presents a procedure to quantify the contribution of the surface (solid-pore) conduction on the overall conductivity of the cement paste. The results of this investigation contribute to the development of an embedded relative humidity sensor that can be used to monitor changes in the internal humidity of concrete during its service life.

Computational results of a model for chloride ingress in concrete including convection, drying-wetting cycles and carbonation

Over the past decades a considerable research effort has been attributed to the modelling of chloride ingress into reinforced concrete in order to predict its durability. Traditional models are based on the error-function. In the present paper computational results of an advanced model are presented, which takes environmental temperature and humidity fluctuations, chloride binding, diffusion and convection, as well as carbonation effects into account. These qualifications make the model particularly suitable for simulating drying-wetting cycles, an example of which is included. The good performance of the moisture sub-model is demonstrated in an example concerning drying cement paste. Another example deals with a simple submersion case. The examples are based on laboratory and real-life experiments with documented concrete or paste compositions and environmental conditions, as well as the eventual measured chloride profiles. In general, the computational results are in good agreement with the measurements. The paper concludes with a comparison between simulation results of the present model and the error-function model for the case of cyclic drying-wetting exposure. It appeared that the implicit negligence of the moisture fluctuations by the error-function model, forced that model to deploy strongly deviating material characteristics to reach agreement with the measured chloride profile.

Effect of the fluid in the inclusions of cement paste on the ultrasonic velocity

The durability of cement composites significantly depends on the movement of the fluids into the material through the porous system. The aqueous phase contained in the pores can cause irreversible damage from the dimensional stability viewpoint. In this sense, methods for non-destructive characterization of both, the porous structure and water content should be investigated. In this work, the effect of the fluid in the inclusions of the cement paste on the ultrasonic velocity is studied. Firstly, a theoretical analysis based on the micromechanical model, considering the microstructural information of the matrix and the fluid filling the pores, is presented. Some experimental work is made later using cement paste samples, whose porous structure is maintained dry or saturate with water. In both cases, the ultrasonic velocity is measured and compared to the one predicted by the micromechanical model. Using this technique, the ultrasonic velocity can be predicted with errors below 2% in the cases of dry or water saturated cement paste.

X-ray absorption study of drying cement paste and mortar

X-ray absorption was used to observe water evaporation with hydration time in paste and mortar specimens, with the aim of studying the influence of water/cement (w/c) ratio, presence of aggregates, curing conditions on drying during early hydration. For the samples subjected to surface drying immediately after mixing, there exists a moisture gradient within the internal part of the specimen. However, obvious top-down drying only occurs within a small zone near the surface for early age cement pastes and mortars. The evaporation rate of water is very high in the first day after casting and is drastically reduced afterwards due to the formation of a microstructure that greatly improves specimens resistance to moisture loss. Mortars reveal a slightly lower evaporation rate since the aggregate increases the length of the transport route because of a larger tortuosity. However, the effect of sealed curing is much more important than the tortuosity effect of the aggregates.

Effect of Heat Treatment on the Pore Structure and Drying Shrinkage Behavior of Hydrated Cement Paste

The effect of a short heat treatment on hydrated cement paste has been investigated by measuring the weight and length changes of specimens as they undergo various combinations of heating, drying, and resaturation. Heating a cement paste to 60°C coarsens the capillary pore system, decreases the volume of mesopores, and increases the degree of polymerization of the silicates. In addition, the saturated weight of the paste is permanently decreased by a heat treatment. This weight loss can be explained by conversion of bound hydroxyl groups into liquid water during polymerization of the C‐S‐H gel phase. These experiments help reconcile and interpret published results describing the properties of cement cured at various temperatures, the effects of a short heat treatment on cement paste, and the thermal expansion behavior of saturated and dry cement paste.

Critical examination of drying damage to cement pastes

Several methods of drying cement paste prior to determining its physical properties, for example pore size distributions, are critically studied. Phase-pure preparations of ettringite and calcium monosulfoaluminate are used as sensitive indicators of damage. Oven drying at 105°C and drying over strong desiccants, for example MgClO4, is damaging. D-drying for 2–3 h is recommended as a robust but relatively non-damaging treatment. D-drying reduces the water content of ettringite to ∼30 H2O per formula unit, but removal of its zeolitic water, ∼1·7 H2O, does not degrade the structure. Monosulfoaluminate, AFm, is reduced from 12 to 10 H2O with shrinkage of its basal spacing from ∼9·0 to ∼8·2 Å. However, D-drying of pastes from commercial calcium sulfoaluminate rich in AFm usually gives mixtures of the ∼9·0 and ∼8·2 Å phases.

Pore structure characterisation of GGBS/OPC grouts using solvent techniques

Solvent exchange has been proposed as a suitable method to dry cement paste samples for pore structure analysis and to simultaneously yield diffusion coefficients. The success of this depends on the solvent exchange process being one of simple counter diffusion. GGBS/OPC grouts have been immersed in both methanol and propan-2-ol and weight loss and length change data collected. Pore size distributions have been determined using a comparative solvent adsorption technique. The results suggest that the exchange process generates vacated pore space yielding anomalous weight and length changes to preclude the determination of diffusion coefficients. The degree of this behaviour is a function of GGBS/OPC ratio, curing and solvent. It is likely that solvent exchange alters the microstructure of the grouts such that unique representations of the pore structure are unobtainable.

The crypto-crystalline structure of C-S-H gel in cement pastes inferences from X-ray diffraction and dielectric capacitivity data

X-ray diffraction data indicate that the structure of the C-S-H gel in cement paste might be analogous to that of crystalline CH, but with a somewhat distorted unit cell, formed simply by replacing some of the Ca ions by Si ions in octahedral positions, or by octahedral voids. Due to the change in relative sizes of the ions involved, the Si ions seem to become mobile, oscillating or migrating between adjacent lattice positions, and strongly polarized, if an electric field is applied. Measurements of the capacitivity (“dielectric constant”) of cement paste have given values amounting to more than 500, or several times the value found for pure ice, or liquid water, which is about 80, and caused exclusively by the polarization of protons in the hydrogen bonds. The remarkably-high capacitivity of dry cement paste can be compared with those of other substances, for which very high dielectric constants have been observed, such as ceramic materials of the perovskite type. The property of ferro-electricity is obviously associated with oscillatory movements of certain cations within oxygen polyhedra, or even between polyhedra. The mechanism of polarization in cement gels might be of a similar type. It is further observed that, in the early reaction in cement pastes, a thermodynamically-unstable mixture of hydrate phases is formed. There are certain indications that the distribution of cations tends to equilibrate at a slow rate, possibly resulting in considerable reductions of strength in certain paste materials, chiefly those of high initial densities.

Measurement and Modeling of Porosity in Drying Cement Paste

ABSTRACTPreliminary models for predicting the performance of exposed concrete show that the porosity sub-model plays a pivotal role in linking properties with the developing gradients of cement paste microstructure that arise from drying and carbonation. The difficulties of porosity characterization are reviewed and some alternatives to traditional methods are considered. Property/porosity relationships are considered with particular reference to the volume, size-distribution and continuity of the larger pores. The factors influencing porosity under normal exposure conditions, such as cement hydration, drying and carbonation, are reviewed and the possibilities of modeling their effects are considered. Some comparisons between predicted and measured values of porosity are given. It is concluded that progress in modeling the performance of exposed concrete is likely to be linked with improvements in methods of pore structure characterization.