From cement to hardened paste — an elektrokinetic study

A new technique for the measurement of the zeta- potential of cement is applied to Portland cement and blastfurnace slag cement. It is now possible to measure the zeta-potential of hydrating cement under realistic conditions down to w/c-ratios of about 0.7. In addition the measurements have been extended to higher hydration times up to 28 days in order to study the electrochemical and electrokinetic aspects of the setting and hardening process of cement.     

Electrically induced temperature difference and deformation in hardened cement pastes

Electromechanical effect of hardened cement paste beam is investigated in this paper. When an external electrical current is applied to the electrodes attached to opposite surfaces of a cement beam, it is found that temperature on the positive electrode is always higher than that on the negative electrode. The sign of electrically induced temperature difference is determined by the direction of applied electrical current. Electrically induced temperature difference makes the beam bend towards the surface with a higher temperature. Both electrically induced temperature difference and electroosmosis lead to electromechanical effect of hardened cement paste. Finally, electromechanical effect becomes more obvious by adding NaCl to cement paste.     

Study on the high‐temperature behavior and rehydration characteristics of hardened cement paste

The high‐temperature behavior and rehydration characteristics of the hardened cement paste and their mechanisms have been studied in this paper. X‐ray diffraction and thermogravimetry are used to establish the effect of elevated temperatures on the mineralogical changes that occurred in the hardened cement paste. The change of microstructure was characterized by scanning electron microscopy. The results showed that with the temperature increased, the compressive strength of hardened cement paste first increased and then decreased. According to micromeasurements, at 400°C, the porosity and average pore diameter of hardened cement paste increased slightly, while at 800°C, the porosity and average pore diameter of hardened cement paste increased sharply. When hardened cement paste was cured after exposing to 400°C, its pore structure and phase composition had no change, while when hardened cement paste was cured after exposing to 800°C, there are new hydration products, and its pore structure may be finer, but it cannot fully recover to the original state. Copyright © 2014 John Wiley & Sons, Ltd.     

Rapid ice formation in hardened cement paste,mortar and concrete due to supercooling

The aim of these experiments was to study the extent of rapid ice formation and length change due to supercooling of water in cementstone and its influence on the destruction of mortar and concrete. It could be shown that the initial freezing temperature decreases with increasing water content. Smaller specimens show a lower initial freezing temperature than greater ones. The expansion accompaning the abrupt temperature rise shows a considerable increase with increasing temperature rise. Immediately after abrupt temperature rise one part of the observed expansion contracts rapidly probably due to displacement of liquid in the structure of the hardened cement paste. After the end of a freezing and thawing cycle the magnitude of the remaining expansion increases with increasing value of rapid temperature rise.     

Microstructural and bulk property changes in hardened cement paste during the first drying process

This paper reports the microstructural changes and resultant bulk physical property changes in hardened cement paste (hcp) during the first desorption process. The microstructural changes and solid-phase changes were evaluated by water vapor sorption, nitrogen sorption, ultrasonic velocity, and ²⁹Si and ²⁷Al nuclear magnetic resonance. Strength, Young's modulus, and drying shrinkage were also examined. The first drying process increased the volume of macropores and decreased the volume of mesopores and interlayer spaces. Furthermore, in the first drying process globule clusters were interconnected. During the first desorption, the strength increased for samples cured at 100% to 90% RH, decreased for 90% to 40% RH, and increased again for 40% to 11% RH. This behavior is explained by both microstructural changes in hcp and C–S–H globule densification. The drying shrinkage strains during rapid drying and slow drying were compared and the effects of the microstructural changes and evaporation were separated.     

Influence of the age and drying process on pore structure and sorption isotherms of hardened cement paste

Sorption isotherms and scanning-isotherms of hardened Portland cement paste were measured. A structure model was developed to give a theoretical interpretation for the measured sorption isotherms. In particular, the causes for hysteresis between adsorption and desorption were described. Not only the capillary effect between condensation and evaporation in the pores but also the chemical water uptake, the drying method and the chemical aging have to be considered to explain the significant hysteresis. These results are the basis of our model for the structure and the porosity change of cementitious materials. The mathematical implementation of the structure model (called IBP-method [R.M. Espinosa, Sorptionsisothermen von Zementstein und Mörtel, Dissertation Hamburg University of Technology, 2004.]) will be described in a further essay [R.M. Espinosa, L. Franke, Inkbottle Pore-Method: prediction of hygroscopic water content in hardened cement paste at variable climatic conditions, Cement and Concrete Research (2006-this issue) doi:10.1016/j.cemconres.2006.06.011].     

Determining the water–cement ratio, cement content, water content and degree of hydration of hardened cement paste: Method development and validation on paste samples

We propose a new method to estimate the initial cement content, water content and free water/cement ratio (w/c) of hardened cement-based materials made with Portland cements that have unknown mixture proportions and degree of hydration. This method first quantifies the composition of the hardened cement paste, i.e. the volumetric fractions of capillary pores, hydration products and unreacted cement, using high-resolution field emission scanning electron microscopy (FE-SEM) in the backscattered electron (BSE) mode and image analysis. From the obtained data and the volumetric increase of solids during cement hydration, we compute the initial free water content and cement content, hence the free w/c ratio. The same method can also be used to calculate the degree of hydration. The proposed method has the advantage that it is quantitative and does not require comparison with calibration graphs or reference samples made with the same materials and cured to the same degree of hydration as the tested sample. This paper reports the development, assumptions and limitations of the proposed method, and preliminary results from Portland cement pastes with a range of w/c ratios (0.25–0.50) and curing ages (3–90 days). We also discuss the extension of the technique to mortars and concretes, and samples made with blended cements.     

The effect of w/c ratio and curing temperature on the permeability of hardened cement paste

The importance of concrete permeability is discussed relative to its effect on durability. Experimental studies were made of the effect of water/cement ratio and curing temperature on the porosity, pore size distribution and permeability of cement pastes. Although total porosities of samples cured at 60°C are smaller than in those cured at 27°C, the pore volume larger than 750Å radius is greater in the 60°C samples and is related to higher permeabilities also in the latter.     

Effect of freezing on the dynamic mechanical response of hardened cement paste down to −60°C

The dynamic mechanical response (the internal friction, tanδ, and the E-modulus) in the temperature range from −60°C to 0°C reveals a sharp modulus change and a shallow tanδ peak. This “capillary transition” occurs only in specimens suspected of having capillary water; its strength is both a function of moisture content and the pore structure. The E-T curve in the transition region for a number of saturated specimens is fitted by an empirical equation; the parameters of this equation are correlated with the capillary porosity, defined in the conventional manner. One of these parameters is interpreted in terms of a maximum pore size. A theory is outlined for interpreting the E-T curve in terms of thermodynamic, composite mechanics, and pore structure parameters. The discovery of the “capillary transition” has made available a new technique for investigating the various, rather complicated aspects of the process of freezing within a porous solid.     

Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Advances in room temperature curing adhesives and sealants—a review

Room temperature curing adhesives and sealants, defined here as those which cure without a mixing process under the influence of substrates or environmentally available reagents such as daylight, oxygen or moisture, include polyurethane, silicone, anaerobic adhesives, cyanoacrylate and certain acrylic types. Advances in cure chemistry, built-in adhesion promotion, and in formulating techniques have created the scope for significant new properties within all the mentioned systems. In addition, development of resins having two (or more) different functionalities, which respond to substrate and or atmospheric cure agents, has created the potential to achieve new advanced performance levels. Examples of these include compositions which give light curing through acrylic functions while having RTV (room temperature vulcanising) silicone resin chemistry polymerising under the influence of atmospheric moisture. The properties can be tailored by molecular design, formulation and choice of cure accelerators.     

Azides Part II — Mechanism of Fixation of Sulphonazido Dyes on Polyester Fibre*

Thermofixation of the reactive disperse azo dye, 2-hydroxy-5-methyl-4′-sulphonazidoazobenzene (I) containing a sulphonazido group, with polyester fibre leads to chemical bonding of the generated sulphonylnitrene with the fibre substrate. The nitrene interacts with the benzene ring of the polyester substrate through the formation of a sulphonamido link as well as through insertion followed by ring expansion of the adduct to yield the relevant N-sulphonyl-azepine. The mechanism is confirmed by means of a suitable model experiment of interaction of I with dimethyl terephthalate.     

Modeling the coupled effects of temperature and fineness of Portland cement on the hydration kinetics in cement paste

This paper revisits the coupled impacts of fineness and temperature on the kinetics of Portland cement hydration. The approach consists in i) modeling the impact of fineness on cement dissolution through the hypothesis that the surface dissolution rate of cement particles is independent of their size, in order to, in a second step, ii) model the impact of temperature on the kinetics of cement dissolution. The analysis of the experimental results shows that the effect of cement fineness on the hydration kinetics can be captured by a simple hypothesis: for any age, the reacted thickness of cement grains can be considered independent of the initial cement particle size. In addition, the analysis of the results at different temperatures shows that a constant activation energy can account for the effect of temperature on the hydration kinetics, with an Arrhenius equation applied to the kinetics of surface dissolution. The results from the model give a good agreement with the experimental results in a significant number of combinations (different Portland cements, water/cement ratio from 0.5 to 0.6, cement Blaine fineness from 3500 to 6600 cm²/g, temperature histories between 20 and 60 °C).     

Hydrodynamics of disturbed flow and erosion–corrosion. Part II — Two-phase flow study

Erosion–Corrosion in turbulent, two-phase liquid/particle flow with recirculation, after a sudden pipe expansion is studied by the application of a numerical flow model along with two different erosion models. The flow model, which is based on a two-phase flow version of a standard two-equation model of turbulence and a stochastic simulation of particle-fluid turbulence interactions, is capable of successfully predicting local values of time averaged fluid velocities and turbulent fluctuations, as well as predicting particle dispersion and particle-wall interaction. The erosion models used are that of Finnie (1960) and a modified version suggested by Bergevin (1984). The agreement of the predicted and measured hydrodynamic parameters, for flow through a sudden expansion, was satisfactory. Predictions of erosion rates using Bergevin's modified model were in good agreement with the stainless steel erosion measurements for a 2% water/sand slurry flow. The erosion–corrosion model was successful in predicting the overall pattern and rates of metal loss for carbon steel.     

Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAP_CSH decreased gradually with hydration time, and at a given hydration time the IAP_CSH was lower at higher curing temperatures.     

The early hydration of tricalcium silicate and β-dicalcium silicate in neat portland cement pastes

Quantitative X-ray diffraction analysis was used to study the early hydration of the tricalcium silicate and β-dicalcium silicate phases in neat Portland cement pastes. There is an initial ‘dormant’ period during which these phases hydrate only very slowly. In the case of the Ca₃SiO₅ phase in the cement used, this ‘dormant’ period lasts 3–4 hours. Of the Ca₃SiO₅ originally present 5% hydrated in 5 h; 22% in 10 h; 34% in 15 h; 45% in 24 h; and 63% in 72 h. No conclusive evidence of any β-dicalcium silicate hydration during the seventy-two hour period investigated was found.     

A gas-solid reaction model — Variable temperature,pressure and structural parameters

Diffusion and reactions in a porous pellet are treated as transient, nonisothermal, nonisobaric processes. Continuity and energy balance equations are solved simultaneously for temperature and concentration profiles in the pellet. Conductive, convective as well as radiative heat transfer are included. The ‘dusty gas’ flux model is used to describe the transport of diffusing gases. Viscous, bulk and concentration gradient terms have been included. Structural changes with reaction are accounted by considering the effect of changing grain radius on porosity and pore diameter. The model predictions match conversion trends for carbon gasification over a temperature range of 800 to 1100°C investigated experimentally.