Hydrated Portland Cement Paste Research Articles

Effect of Multiwalled Carbon Nanotubes on Mechanical Strength of Cement Paste

This paper discusses the effect of multiwalled carbon nanotubes (CNT) on strength characteristics of hydrated portland cement paste. Various cement-CNT composites and an equivalent control mix were prepared by mixing 0.5, 0.75, and 1.0 percent CNTs (by weight of cement) to check the strength gain in portland cement composite material. Cylindrical specimens of size approximately 15.8 mm×31.6 mm were prepared for compressive strength and splitting-tensile strength tests to measure mechanical strength of composites. The specimens were tested at 7, 28, 60, 90, and 180 days of curing, and at least three samples were tested at each curing age. Results show an increase in compressive and splitting-tensile strengths in portland cement–CNT composites having CNT content of 0.5% by weight of portland cement. The increase in compressive and tensile strengths with respect to control mix were 15% and 36%, respectively, at 28 days curing age. Above 0.5% CNT content, the strength decreases. Scanning electron microscopy and transmission electron microscopy have also been carried out to check the microstructure bonding of the cement-CNT paste.

Flocculation and sedimentation in suspensions using ultrasonic wave reflection

This work was undertaken to help understand and interpret the ultrasonic wave reflection response of Portland cement paste as it transforms from a fluid-like suspension to a solid in the first hours after mixing. A high impact polystyrene buffer (delay line) was used to measure small changes in the P- and S-wave reflection coefficients. Two materials were studied: a non-hydrating colloidal alumina suspension whose microstructure was manipulated between dispersed and flocculated states by adjusting the pH and a coarse silica suspension that readily sedimented. The S-wave reflection coefficient clearly distinguished between dispersed and flocculated states. Sedimentation of particles in dispersed suspensions was distinguished using the P-wave reflection coefficient. Based on these findings, the observed P- and S-wave responses from hydrating Portland cement paste are interpreted in terms of flocculation and sedimentation processes.

A method for predicting the alkali concentrations in pore solution of hydrated slag cement paste

The alkalinity of the pore liquid in hardened cement paste or concrete is important for the long-term evaluation of alkali-silica reaction (ASR) expansion and corrosion prevention of steel bar in steel reinforced structures among others. It influences the reactivity of supplementary cementitious materials as well. This paper focuses on the alkali binding in hydrated slag cement paste and a method for predicting the alkali concentrations in the pore solution is developed. The hydration of slag cement is simulated with a computer-based model CEMHYD3D. The amount of alkalis released by the cement hydration, quantities of hydration products, and volume of the pore solution are calculated from the model outputs. A large set of experimental results reported in different literatures are used to derive the alkali-binding capacities of the hydration products and practical models are proposed based on the computation results. It was found that the hydrotalcite-like phase is a major binder of alkalis in hydrated slag cement paste, and the C–S–H has weaker alkali-binding capacity than the C–S–H in hydrated Portland cement paste. The method for predicting the alkali concentrations in the pore solution of hydrated slag cement paste is used to investigate the effects of different factors on the alkalinity of pore solution in hydrated slag cement paste.

A Coupled Nanoindentation/SEM‐EDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for C–S–H/Ca(OH)2 Nanocomposites

A low water/cement ratio (w/c=0.20) hydrated Portland cement paste was analyzed by grid‐indentation coupled with ex situ scanning electron microscope‐energy‐dispersive X‐ray spectra (SEM‐EDS) analysis at each indentation point. Because finite element and Monte‐Carlo simulations showed that the microvolumes probed by each method are of comparable size (approximately 2 μm), the mechanical information provided by nanoindentation was directly comparable to the chemical information provided by SEM‐EDS. This coupled approach provided the opportunity to determine whether the local indentation response was a result of a single‐ or a multiphase response—the latter being shown predominant in the highly concentrated w/c=0.20 hydrated cement paste. Results indicate that, in the selected microvolumes where C–S–H and nanoscale Ca(OH)2 (CH) are present, increasing fractions of CH increase the local indentation modulus (and hardness), yielding values above those reported for high‐density (HD) C–S–H. Micromechanical analyses show that C–S–H and CH are associated, not merely as a simple biphase mixture, but as an intimate nanocomposite where nanoscale CH reinforces C–S–H by partially filling the latter's gel pores. The paper discusses the mechanism of forming the C–S–H/CH nanocomposite, as well as the impact of nanocomposites on various macroscopic properties of concrete (e.g., shrinkage, expansion). On a general level, this study illustrates how a coupled nanoindentation/X‐ray microanalysis/micromechanics approach can provide otherwise inaccessible information on the nanomechanical properties of highly heterogeneous composites with intermixing at length scales smaller than the stress field in a nanoindentation experiment.

Mechanical properties of calcium silicate hydrates

The dynamic mechanical properties of compacted samples of synthetic calcium silicate hydrate (C–S–H) were determined at variable stoichiometries (C/S ratio). The stiffness and damping properties of the C–S–H systems were monitored at various increments of mass loss from 11%RH following the removal of the adsorbed and interlayer water. The changes in the storage modulus (E′) and internal friction (tan δ) were discussed in terms of the state of water present in the nanostructure of C–S–H, the evolution of the silicate structure and the interaction of calcium ions in the interlayer region. Results were compared to those for the hydrated Portland cement paste and porous glass. It was shown that the C–S–H in the hydrated Portland cement has a complex yet analogous dynamic mechanical behavior to that of the synthetic C–S–H. The response of these systems upon the removal of water was explained by a layered model for the C–S–H. A mechanistic model was proposed to describe the changes occurring at various stages in the dynamic mechanical response of C–S–H.

X-ray diffraction microtomography (XRD-CT), a novel tool for non-invasive mapping of phase development in cement materials

A recently developed synchrotron-based imaging technique, X-ray diffraction microtomography (XRD-CT), has been applied here for the first time to a complex system, the hydrating Portland cement paste, in order to monitor the evolution of microstructure and phase formation with a 3D non-invasive imaging approach. The ettringite-XRD-peak-based image reconstructions, combined with transmission microtomography (X-microCT) images, allowed to assess the ubiquitous distribution of this phase, which appears early in the hydration process and showed its preferential concentration in the relatively less compact regions of the paste. The comparison of greyscale histograms for cement pastes after 9 and 58 h from hydration showed an increase of ettringite content with age, in agreement with the quantitative Rietveld analysis of the sum patterns. By renormalizing the greyscale histograms to the relative weight fraction, as obtained from Rietveld refinements, a new technique which allows estimation of phase contents with spatial resolution has been developed. The results achievable by combining XRD-CT, X-microCT and Rietveld appear very promising to provide experimental snapshots of the cement hydration process to be compared with results obtained from computer simulations.

Roughness of fracture surfaces and compressive strength of hydrated cement pastes

A new type of roughness number Rn o is formulated as a possible analytical tool for surface studies using confocal microscopy. The formulation accommodates fractal dimension and both of the boundary length scales limiting the fractal region of the fracture surface under investigation. Besides the number Rn o other roughness characteristics are discussed and their effectiveness in the field of cementitious materials is tested. 3D-surface profile SP and surface roughness SR parameters designed for surface 3D-analysis were calculated for fracture surfaces of hydrated Portland cement pastes with different values of water-to-cement ratio. The surface profile SP parameters monotonically increased with water-to-cement ratio and monotonically decreased with compressive strength. A short discussion of possible reasons for such a behavior is presented.

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C–S–H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C–S–H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C–S–H in hydrated Portland cement pastes. It is found that the relation of Na + between the moles bound in C–S–H and its concentration in the pore solution is linear, while the binding of K + in C–S–H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C–S–H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.

Dimensional change and elastic behavior of layered silicates and Portland cement paste

Abstract The role of water in hydrated Portland cement paste (hpc) is germane to understanding the nature of nanostructure – property relationships of the material. The irreversible dimensional changes of hpc and phase pure C–S–H that occur on wetting and drying are dissimilar to those observed for other silicate minerals of interest to cement science. This irreversibility in hpc is also observed for the modulus of elasticity parameter. Length change, mass change and modulus of elasticity isotherms (including drying–wetting cycles) were determined for specimens of hpc, Ca-montmorillonite and 1.4 nm tobermorite. Length change and modulus of elasticity versus mass loss curves were also obtained for phase pure C–S–H (C/S = 0.8, 1.0 and 1.5). All the isotherms exhibit significant irreversible behavior. Similarities and differences in the nature and character of the isotherms and the relevance of the C–S–H data are discussed. Inferences are made with respect to the nanostructural nature of hpc, its dimensional response in aqueous media and the correspondence in behavior of synthetic C–S–H and that formed in hpc. It is apparent that hpc has unique characteristics that are responsible for stability.

Some Investigations for Establishing Suitability of Watson’s Strength-Porosity Model for Concrete

Abstract Micro-structural characteristics of concrete were determined with the help of a helium gas pycnometer and a mercury intrusion porosimeter. Details of the methodology adopted for these studies are presented in this technical note. Results in the form of ratio of solids to the pore volume were plotted with respect to the compressive strength of concrete and suitability of Watson’s strength-porosity model for concrete was evaluated. In addition to this, based on the study attempts have been made to recommend the value of parameters for concrete that appear in Watson’s strength-porosity model, which otherwise is applicable to hydrated portland cement pastes only.

Diffusion of water and methanol vapour in hydrated Portland cement paste

Equations are developed to describe the combined effect of diffusion and adsorption of water and methanol vapour in hydrated cement paste. The dynamic equation of flow is obtained by equating the space derivative of the spreading pressure of the adsorbed film to a resistive force equal to the product of the coefficient of resistance and the velocity of the film. Qualitative agreement is found for the diffusion of water and methanol vapour through cement paste when the BET and Langmuir equations respectively are used for adsorption. The diffusion of methanol vapour is underestimated due to intercalation and chemical interaction effects. The validity of Fick's law is discussed.

Prediction of chloride ingress and binding in cement paste

This paper summarizes recent work on an analytical model for predicting the ingress rate of chlorides in cement-based materials. An integral part of this is a thermodynamic model for predicting the phase equilibria in hydrated Portland cement. The model’s ability to predict chloride binding in Portland cement pastes at any content of chloride, alkalis, sulfates and carbonate was verified experimentally and found to be equally valid when applied to other data in the literature. The thermodynamic model for predicting the phase equilibria in hydrated Portland cement was introduced into an existing Finite Difference Model for the ingress of chlorides into concrete which takes into account its multi-component nature. The “composite theory” was then used to predict the diffusivity of each ion based on the phase assemblage present in the hydrated Portland cement paste. Agreement was found between profiles for the Cl/Ca ratio predicted by the model and those determined experimentally on 0.45 water/powder ratio Portland cement pastes exposed to 650 mM NaCl for 70 days. This confirms the assumption of essentially instantaneous binding where quasi-equilibrium is established locally. This does not imply steady state diffusion however. It simply implies that incremental increases in the concentration of diffusing ions in the pore solution will rapidly re-equilibrate with the hydrates present locally, where, the greater the ratio of bound to free ions, the greater the buffering effect which slows down the rate of ingress. In the case of chlorides, this buffering effect is greatest at high contents of AFm (AFm is aluminium ferrite compounds with a single (mono) formula unit CaX.) phase and low alkali metal contents.

Binding of chloride and alkalis in Portland cement systems

A thermodynamic model for describing the binding of chloride and alkalis in hydrated Portland cement pastes has been developed. The model is based on the phase rule, which for cement pastes in aggressive marine environment predicts multivariant conditions, even at constant temperature and pressure. The effect of the chloride and alkalis has been quantified by experiments on cement pastes prepared from white Portland cements containing 4% and 12% C 3A, and a grey Portland cement containing 7% C 3A. One weight percent calcite was added to all cements. The pastes prepared at w/s ratio of 0.70 were stored in solutions of different Cl (CaCl 2) and Na (NaOH) concentrations. When equilibrium was reached, the mineralogy of the pastes was investigated by EDS analysis on the SEM. A well-defined distribution of chloride was found between the pore solution, the C-S-H phase, and an AFm solid solution phase consisting of Friedel's salt and monocarbonate. Partition coefficients varied as a function of iron and alkali contents. The lower content of alkalis in WPC results in higher chloride contents in the C-S-H phase. High alkali contents result in higher chloride concentrations in the pore solution.

On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine

Recent work on the strength-enhancing mechanism of triisopropanolamine (TIPA) suggested that TIPA enhances the mechanical properties of mortar and concrete by acting on the interfacial transition zone (ITZ) between paste and sand or aggregate rather than improving the properties of the hydrated binder. This paper presents compressive strength data for 10 Portland cements tested as cement paste as well as two different kinds of mortar after 28 days hydration, so that these two mechanisms could be compared directly. The average strength improvement with TIPA was 10% in the hydrated portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism.

The use of compacts for durability investigation of cement-based materials

Compacted specimens of ground hydrated Portland cement paste, brucite, gypsum, sodium chloride and table sugar were immersed in various solvents. The volume stability of the compacts was compared to that of non-compacted materials in the form of insitu hydrated Portland cement paste, single crystals of brucite and gypsum, and polycrystalline sodium chloride. Immersion in the various solvents resulted in the dissolution of the test samples. In all cases, dissolution was accompanied by an expansion of the material. The expansive behaviour of all systems and the suitability of using compacts as models for durability studies are discussed.

The early age short-term creep of hardening cement paste: load-induced hydration effects

Short-term creep and shrinkage strains were monitored at early age on hydrated Portland cement pastes prepared with two different water–cement ratios (0.35 and 0.50). Creep experiments were conducted in an environmentally controlled chamber maintained at (96 ± 2)% relative humidity. The three ages at loading investigated were 18, 24 and 30 h. The stress–strength ratio applied to the specimens at the age of loading, using a miniature loading system, was 0.30. Cement paste specimens were in the form of “T-shaped” columns with a minimum thickness value (for the web and flanges) of less than 1.2 mm. Load-induced hydration (not normally considered in creep prediction) of normal strength cement paste ( w / c =0.50) was found to occur at early times following the loading at 18 h. An analytical model was developed in order to predict the creep coefficient of normal and high strength cement pastes from early age data. The model accounts for load-induced hydration effects.

Volatility and Extractability of Strontium-85, Cesium-134, Cobalt-57, and Uranium after Heating Hardened Portland Cement Paste

The objective of this preliminary investigation is to determine the effect of heating hardened Portland cement paste (the cementitious component of concrete) in aiding the removal of common radionuclide contaminants including 137 Cs, 90 Sr, 60 Co, and U. Direct volatilization of radionuclides during heating as well as subsequent behaviors during extractions were established over a range of temperatures up to the melting point of Portland cement paste. Techniques for spiking pulverized Portland cement paste with shorter-lived radioisotopes ( 134 Cs, 35 Sr, and 57 Co) and U were developed using gamma-ray spectroscopy to measure their activity nondestructively before and after heating. An extraction procedure (employing five sequential extractions each with water, followed by 0.1 N CaCl 2 , followed by 0.2 N HCl) for pulverized but fully hydrated Portland cement paste was adapted to contrast behaviors after thermal treatments. Complete volatility of 134 Cs was observed at temperatures of 1200°C or greater. Thus, decontamination of 137 Cs from cement paste would be feasible by direct heating. However, no volatility of other radionuclides was observed, and, thus, their potential for facilitated decontamination will likely depend on their extraction behavior following thermal perturbation. Both 134 Cs and 85 Sr were readily removed from cement paste during initial water-based extractions for final temperatures <800°C. In contrast, both 57 Co and U exhibited negligible extractability into either water or 0.1 N CaCl 2 probably resulting from their precipitation as hydroxides in the highly alkaline Portland cement paste extracts where pHs were usually greater than 10. The release of both these radionuclides was delayed until sequential 0.2 N HCl extracts decreased the pH below 4. Significant patterns in the extraction behaviors of all radionuclides with temperature were observed in ranges where significant mineral alterations are known to occur.

Role of Cement Alkali Content in Acid Attack: A Pore‐Solution and Pore‐Size‐Distribution Study

The behavior of hydrated portland cement pastes in an acid‐buffered solution of acetic acid/sodium acetate (HAc/NaAc) (pH 4.5) during a period of 90 days at a temperature of 21°C has been studied. The changes that were caused by the acid ingress were monitored via analysis of the extracted pore solution. The evolution of the porosity and pore‐size‐distribution changes were evaluated by using intrusion mercury porosimetry, and those of crystalline compounds were evaluated by using X‐ray diffractometry. A similar study was conducted with an equivalent series of samples that were immersed in distilled water. The results showed the important role that cement alkali content (1.03% K2O, 0.2% Na2O) has in the mechanism of the acid attack. Interesting correlations were established between pore‐solution and porosity data.

96/03781 Effect of fly ashes with high total alkali content on the alkalinity of the pore solution of hydrated portland cement paste

96/03781 Effect of fly ashes with high total alkali content on the alkalinity of the pore solution of hydrated portland cement paste

Effect of carbonation on the nitrogen BET surface area of hardened portland cement paste

Abstract The progressive decrease in the nitrogen BET surface area of dried hydrated portland cement paste during storage in plastic vials was found to be caused by carbonation, a reaction between the C-S-H phase and carbon dioxide in the air. A strong correlation between the surface area and the amount of carbonation as measured by thermogravimetric analysis was found. Historically, nitrogen BET values of surface area have shown great variability and low reproducibility, which has limited the value of this useful technique. The results of this work suggest that carbonation may have contributed to this problem.