John Bensted

Bio: John Bensted is an academic researcher from Birkbeck, University of London. The author has contributed to research in topics: Cement & Portland cement. The author has an hindex of 14, co-authored 79 publications receiving 1459 citations.

Structure and Performance of Cements

Abstract: 1. Cement Manufacture 2. Composition of Cement Phases 3. The Hydration of Portland Cement 4. Calcium Aluminate Cements 5. Properties of Concrete with Mineral and Chemical Admixtures 6. Special Cements 7. Developments with Oilwell Cements 8. Gypsum in Cements 9. Alkali-Silica Reaction in Concrete 10. Delayed Ettringite Formation 11. Chloride-Corrosion in Cementitious Systems 12. Blastfurnace Cements 13. Properties and Applications of Natural Pozzolanas 14. Pulverised Fuel Ash as a Cement Extender 15. Metakaolin as a Pollolanic Addition to Concrete 16. Condensed Silica Fume as a Cement Extender 17. Cement-Based Composite Micro-Structures 18. X-Ray Powder Diffraction Analysis of Cements 19. Electron Microscopy of Cements 20. Electrical Monitoring Methods in Cement Science 21. Nuclear Magnetic Resonance Spectroscopy and Magnetic Resonance Imaging Studies of Cements and Cement-Based Materials 22. The Use of Synchroton Sources in the Study of Cement Materials

Thaumasite — background and nature in deterioration of cements, mortars and concretes

Abstract: Thaumasite has been shown to form at low temperatures, particularly 0–5 °C, as a non-binding calcium carbonate silicate sulphate hydrate under conditions of destructive sulphate attack. Formation of thaumasite arises generally from calcium silicate hydrate CSH and Ca2+, CO2−3, SO2−3, CO2 and water, or from ettringite in the presence of CSH, carbonate and/or carbon dioxide and water. It basically resembles a carbonated ettringite, with which it has often been confused in the past. Conversion of the main cementitious binder CSH into the non-binder thaumasite is a destructive form of sulphate attack. Greater awareness of the potential problems that thaumasite can cause has arisen with the increased use of limestone fillers in cements, the common employment of limestone aggregates in concrete and the introduction of Portland limestone cements, together with the realisation that structural foundations of buildings are, on average, below ambient temperature and (more often than not with on- and above-ground construction) are within the optimum temperature range for thaumasite to be formed. Instances have been found in specific studies of large quantities of thaumasite being formed in foundation concretes with no evidence for any structural damage above ground level. It is important to be aware of the propensity of thaumasite to form at low temperatures for mix designs, so as not to encourage any destructive sulphate attack by thaumasite to arise. This means utilising low water cement ratios for workable mortars and concretes, so as to give reduced permeability. This will prevent, or at least suitably hinder, ingress of destructive ions and water, so that the potential for destructive sulphate attack by formation of thaumasite is not actually realised in practice.

Hydration of Portland Cement

Abstract: Publisher Summary This chapter highlights the investigations of the hydration processes taking place with Portland cements, which have continued to assume increased importance in recent times. Trends toward the employment of more advanced and sophisticated construction techniques in different types of situations, and also the ability to deal effectively with problems encountered during usage, necessitate a more detailed understanding of the various facets of hydration in Portland cements. To understand the chemistry of Portland cement hydration, it is necessary to consider the hydration processes of the components of Portland cement clinker along with the effects of the gypsum added during the grinding stage. Tricalcium silicate is the major cementitious component of most Portland cements and hydrates steadily with a moderate evolution of heat of hydration. The majority of the hydration reaction has taken place within 28 days and is to a large degree effectively complete after about one year.

Thaumasite––direct, woodfordite and other possible formation routes

Abstract: Two main formation routes for thaumasite exist below 15 °C. One is the direct route from C–S–H reacting with appropriate carbonate, sulfate, Ca2+ ions and excess water. The other route is the woodfordite route from ettringite reacting with C–S–H, carbonate, Ca2+ ions and excess water, in which thaumasite arises through the intermediate formation of the solid solution woodfordite. The woodfordite route for thaumasite formation appears to be relatively quicker (although still slow) than the direct route, presumably because with the former the ettringite already has the octahedral [M(OH)6] units that can facilitate the critical change from [Al(OH)6]3− to [Si(OH)6]2− groupings. Both routes are mutually dependent on each other. The presence of magnesium salts can modify the path to thaumasite formation. High pressure might be able to stabilise [Si(OH)6]2− groupings and allow thaumasite to become formed above 15 °C. This possibility is discussed.

Conversion of calcium aluminate cement hydrates re-examined with synchrotron energy-dispersive diffraction

Abstract: Previous studies [l-41 on calcium aluminate cement (CAC), formerly known as high alumina cement (HAC), have shown that on reaction with water the initial hydrate formed at lower temperatures, par- ticularly below 15 "C, is the metastable, hexagonal hydrate CAHlo (see Table I for cement nomen- clature system) CA