Knut O. Kjellsen

Development of microstructures in plain cement pastes hydrated at different temperatures

Abstract Various methods have shown indirectly that insufficient time for diffusion of the hydration products and the large pores that form as a result are responsible for the reduction in strength of concretes cured at elevated temperatures. backscattered electron imaging provides a direct means of examining the uniformity of diffusion. This paper describes an examination of the developing microstructure of cement pastes hydrated at 5–50°C. In accordance with the indirect evidence developed previously, the investigation shows that low curing temperatures result in a uniform distribution of hydration products, while elevated temperatures result in a coarsened pore structure. Specimens cured under variable-temperature regimes show some features typical of both initial and final curing temperatures. Compressive strengths of companion mortar specimens are consistent with the observed pore structure of the pastes.

Reaction kinetics of portland cement mortars hydrated at different temperatures

Abstract This work is a portion of a larger study of the microstructure and properties of concrete cured at temperatures of 5° to 50°C, a range chosen to reflect temperatures encountered in field curing. This paper discusses the overall hydration reaction kinetics of a 0.5 water/cement ratio portland cement mortar at ages up to 91 days. The results are correlated with microstructural observations of companion cement paste specimens. The apparent activation energy is found to be 11.2 kcal/mol for degrees of hydration between 20 and 30%, indicating that at this stage the rate of reaction is controlled by a chemical process. Beyond about 30% hydration the apparent activation energy gradually decreases.

Pore structure of plain cement pastes hydrated at different temperatures

Abstract Concrete outside the laboratory cures at temperatures other than 20°C. This paper describes an investigation of the pore structure of plain cement pastes hydrated at 5°, 20°, and 50°C to reflect a range of temperatures encountered in practice. Parallel specimens of 0.50 water/cement ratio pastes were examined using mercury intrusion porosimetry and backscattered electron image analysis. Increases in curing temperature resulted in increased porosity, particularly for pores of radius 200–1000 Ȧ as measured by mercury intrusion, or 2500–12,500 Ȧ as measured in the backscattered electron images. The difference between the two results indicates the magnitude of the “ink bottle effect” inherent in the mercury intrusion technique. However, both methods suggest that elevated curing temperatures could have a deleterious effect on the durability of plain cement concretes.

Heat curing and post-heat curing regimes of high-performance concrete: Influence on microstructure and C-S-H composition

Abstract The present work reports a study on the effect of heat curing and post-heat curing conditions on the microstructure and C-S-H composition of a high-performance concrete. In addition, some previously studied cement paste specimens of ordinary w c ratio hydrated at various temperatures have been reanalyzed. Heat cured high-performance concrete showed a higher hollow shell porosity at later ages than a normally cured companion. Apparently the distribution of C-S-H throughout the cement paste matrix of the high-performance concretes was not significantly influenced by heat curing. However, the composition of the C-S-H phases was influenced by the curing regimes. The effect of heat curing on the microstructure appears to differ between high-performance and ordinary concretes.

Observations of microcracking in cement paste upon drying and rewetting by environmental scanning electron microscopy

Abstract This study reports some preliminary observations of microcracks in hardened high performance cement paste (i.e., cement paste of low water:binder ratio). Specimens were examined at high magnifications and at relative humidities ranging from 0 to 100% in an environmental scanning electron microscope (ESEM). Direct microcracks were observed in samples that had not been dried, indicating that microcracks are probably an intrinsic feature of high performance concrete. It was further observed that the microcracks widened upon drying and closed again upon rewetting. Some practical consequences of these findings are discussed briefly.

Resistance to chloride intrusion of concrete cured at different temperatures

This paper desribes a preliminary investigation of the ability of concrete to protect against the corrosion of reinforcing steel. Two types of tests measured the rate of chloride diffusion. An accelerated corrosion test compared the ability of the concrete to protect against the corrosion of reinforcing steel. Both test methods are described. The results of both tests indicate that at a given water-cement ratio, elevated curing temperatures reduce the ability of portland cement concrete to protect against chloride diffusion and the consequent depassivation of reinforcement. This effect is more pronounced at lower water-cement ratios. These findings should be taken into account in the construction of concrete structures for which durability is a concern.

Pore structure of cement silica fume systems: Presence of hollow-shell pores

Significant amounts of hollow-shell pores (i.e., Hadley grains) have been found in mature Portland cement-based systems containing silica fume. Hollow-shell pores were found directly by electron microscopy and were indicated by desorption isotherms. Hollow shells are an intrinsic type of pore, along with capillary pores and gel pores. They are large pores enclosed in cement gel and connected to the outside via gel and capillary pores. At low water-to-binder ratios they can be more than two orders of magnitude larger than the capillary pores, and they may constitute a porosity larger than that of capillary pores. Despite self-desiccation effects, hollow-shell pores seem to remain largely saturated with pore fluid. The smaller capillary pores dry before hollow-shell pores. The development of hollow-shell pores and their preservation at later ages in cement silica fume systems is discussed.

Backscattered electron image analysis of cement paste specimens: Specimen preparation and analytical methods

Abstract Several of our recent papers (1–4) involve the use of backscattered electron imaging of cement paste specimens. This note describes the methods used in preparing the specimens for study and in obtaining, processing, and analyzing the images. These methods are based on those described by Scrivener and Pratt (5) and Scrivener et al. (6).

Nuclear magnetic resonance characterization of high- and ultrahigh-performance concrete: Application to the study of water leaching

Abstract In the present study, we show that high-resolution 29 Si and 27 Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) can be a powerful tool for analyzing actual concrete mixes. The influence of the amount of silica fume and of the type of cement in high-performance concrete (HPC), as well as the influence of the type of silica fume and of the granular packing in ultrahigh-performance concrete (UHPC) were investigated. Significant effects on the amount and shape of C-S-H, on the incorporation of aluminum in the C-S-H structure, and on the distribution of aluminum-containing hydrates were observed. Nuclear magnetic relaxation of protons was also performed and it showed the fractal feature of the pore size distribution in UHPC and the higher amount of larger pores in HPC. The microstructure of the surface of these same formulations leached by mineral water for up to 1 year exhibits slight modifications.

Hollow-shell formation – an important mode in the hydration of Portland cement

The characteristics and quantities of hollow shells (Hadley grains) in the microstructure of Portland cement paste have been studied. It is shown that at relatively early hydration stages, the hydration largely occurs in a manner resulting in hollow shells. At later ages, the “hollow-shell hydration” mode becomes much less prominent as “inner products” are increasingly formed. The intrinsic porosity associated with hollow shells can be significant at later ages, between about 1% and 9% depending on the mix composition and curing regime. The presence of silica fume increases the amount of hollow-shell porosity considerably, while the hollow-shell porosity appears to decrease with decreasing water/binder ratio.