Hamlin M. Jennings

A model for the microstructure of calcium silicate hydrate in cement paste

A model is proposed for the structure of calcium silicate hydrate (C-S-H) as it is formed during the hydration of Portland cement. One purpose of the model is to move toward an ability to evaluate the microstructure quantitatively, so that it can be related to properties on the one hand and processing on the other hand. It is a hypothesis intended to promote discussion and motivate experiments. Furthermore, the model is an attempt to rationalize disparate measurements of specific surface area reported in the literature by describing an underlying structure, which, when observed by different instruments, gives different results. It is a simplified representation of the microstructure within the size range of about 1 to 100 nm. The basic building block is a unit of C-S-H that is roughly spherical and approximately 2 nm across with a specific surface area of about 1,000 m2/g. These building blocks flocculate to form larger units. This paper describes the structure of the basic units and how they pack to form larger structures and microstructures. The model also explains a number of variant observations for such measured attributes as specific surface area, pore size, and density as determined by different techniques, as well as water content at different relative humidities.

A model for two types of calcium silicate hydrate in the microstructure of Portland cement pastes

A new physical basis for a previously published model for the structure of calcium silicate hydrate (C-S-H) as measured by nitrogen sorption is described. This refined model provides a method of predicting the density, the nitrogen accessible gel porosity, and associated surface area of C-S-H in Portland cement pastes. The basis for the model is that C-S-H forms as one of two types, high- or low-density C-S-H. This provides a promising tool for characterizing the microstructure in a way that can be applied to understanding properties of engineering significance.

Pore solution chemistry of alkali-activated ground granulated blast-furnace slag

The chemical composition and pH of the pore solution extracted from six different ground granulated blast-furnace slag (GGBFS) pastes were determined. The concentrations of Si, Ca, Al, and Mg are functions of the pH of the aqueous phase, with high pH associated with the higher concentrations of Si and Al and the lower concentrations of Ca and Mg. When GGBFS is mixed with an aqueous phase with pH higher than 11.5, the reaction is activated or accelerated. The main hydration product was identified as C-S-H, and hydrotalcite, at later stages of hydration, was observed in the pastes with an aqueous phase of a high pH. The effect of pore solution on the alkali activation of GGBFS is discussed with reference to the hydration products.

Moisture diffusion in cementitious materials Adsorption isotherms

Abstract This article describes an improvement on a previous model proposed by Bažant and Najjar, in which moisture diffusivity and moisture capacity are treated as separate parameters. These parameters are evaluated from independent test results, and are shown to depend on the water:cement ratio, curing time, temperature, and cement type. The moisture capacity is obtained as the slope of the adsorption isotherm. A mathematical model is developed and is shown to predict experimental adsorption isotherms of Portland cement paste very well. In the present form, the model is not applicable to high temperatures.

The influence of wall slip on yield stress and viscoelastic measurements of cement paste

The influence of wall slip on the shear yield stress and modulus of cement paste was investigated using a rotational rheometer with smooth-walled concentric cylinders and a vane. The results show that the concentric cylinders suffer from slip during yield stress measurements due to the formation of a water-rich layer at the walls of the cylinders. The use of a vane eliminates slip since shearing occurs within the material. Oscillatory tests were conducted to measure the viscoelastic properties of cement paste. The data for the vane and concentric cylinders are in excellent agreement at stresses below the yield point. It is difficult to determine the influence of slip for stress sweep measurements. Frequency sweep data for various materials show the general applicability of the vane method over several decades of modulus.

Hydration of alkali-activated ground granulated blast furnace slag

The hydration of ground granulated blast furnace slag (GGBFS) at 25 °C in controlled pH environments was investigated during 28 days of hydration. GGBFS was activated by NaOH, and it was found that the rate of reaction depends on the pH of the starting solution. The main product was identified as C-S-H, and, in the pastes with high pH, hydrotalcite was observed at later stages of hydration. The pH of the mixing solution should be higher than pH 11.5 to effectively activate the hydration of GGBFS. As deduced from very low electrical conductivity measurements, GGBFS pastes had very tortuous and disconnected pores. The effect of the pH of the aqueous solution on the composition, microstructure and properties of alkali-activated GGBFS pastes are also discussed.

Image analysis techniques for characterization of pore structure of cement-based materials

Abstract This paper explores several image analysis techniques that provide insight into the nature of pore structure as observed in backscattered electron images of polished sections. Image analysis techniques used in this study include sizing, two-point correlation, and fractal analyses. The spatial parameters computed by these techniques have potential for relating microstructure to material properties. Specimens include plain cement paste, pastes with silica fume, and mortars. Tests include determination of total porosity, mercury intrusion porosimetry (MIP), compression and flexural testing. The two-parameter fracture model is used to characterize fracture properties. Pore size distribution from MIP is compared to pore size distribution derived from BSE images.

On reactions between silicon and nitrogen

An attempt is made to rationalize various reaction mechanisms for the formation of Si3N4 from gaseous nitrogen and solid silicon. It is suggested that a fundamental difference betweenα- andβ-Si3N4 is that the former results from silicon complexing with molecular nitrogen and the latter results from complexing with essentially atomic nitrogen. Well established mechanisms (as well as controversial ones) are analysed in the light of this difference and are shown to be operative under various reaction conditions. The influence of some common reaction variables are interpreted with respect to their influence on either silicon or nitrogen or both.

New Methodology for Designing Self-Compacting Concrete

This paper introduces a new segregation-controlled design methodology for self-compacting concrete (SCC). The theory suggests that aggregate segregation is governed by the yield stress, viscosity, and density of the cement paste matrix. The concept of a rheological self-flow zone for concrete is also introduced, where aggregate segregation is avoided, yet the concrete has a high workability. The applicability of the theory is studied by systematically changing the rheology of the cement paste matrix of fresh concrete. The yield stress and viscosity of 3 different types of pastes incorporating silica fume and a cellulose thickening agent are measured as a function of density. Results suggest that the new segregation control design theory can be used to produce SCC.

Moisture diffusion in cementitious materials Moisture capacity and diffusivity

Abstract Based on a model by Bažant and Najjar, and using a new model for adsorption isotherms, moisture capacity and diffusivity of concrete are analyzed. The moisture capacity, obtained as a derivative of the ] adsorption isotherm, first drops as the humidity increases from zero, then levels off as a consuant, and finally again increases when the humidity approaches saturation, regardless of the age, cement type, temperature, and water:cement ratio. The well-known diffusion mechanisms,including the ordinary diffusion, Knudsen diffusion, and surface diffusion, are analyzed and the diffusion in concrete is treated as a combination of these mechanisms. An improved formula for the dependence of diffusivity on pore humidity is proposed. The improved model for moisture diffusion is found to give satisfactory diffusion profiles and long-term drying predictions. The model is suited for incorporation into finite element programs for shrinkage and creep effects in concrete structures.