John Bensted

Structure and Performance of Cements

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

Hydration of Portland Cement

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 — 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.

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.

Early hydration behaviour of portland cement containing boro-, citro- and desulphogypsum

Abstract Portland cements containing boro-, citro- and desulphogypsum, respectively, were hydrated at water:cement ratio 0.5 for up to two hours and compared with a Portland cement containing high grade natural gypsum hydrated similarly. It was found that the cements containing boro- and citrogypsum produced considerably more ettringite than those with desulpho- and natural gypsum. Comparisons were made with previous investigations of Portland cements containing fluoro-, formo- and phosphogypsum, respectively, hydrated under analogous conditions. Reasons for the observed hydration behaviour of the Portland cements containing the aforementioned chemical gypsums are discussed.

Raman spectral studies of carbonation phenomena

Abstract Raman spectroscopy is a useful technique for detecting the presence of carbonation in cement compounds and in the hydration products of white Portland cement. Some application are described.

Comparative study of the efficiency of various borate compounds as set-retarders of class G oilwell cement

Abstract Retardation of setting (thickening) of Class G oilwell cement by eight different borates was studied by conduction calorimetry, particle electrophoresis and calcium hydroxide precipitation. Some borates are more efficient than others in their retardation. The most efficient borate retarders are those which appear to be capable of releasing the most B(OH) 3 or B(OH) 4 − monomer units into solution, such as disodium octaborate tetrahydrate (polybor) Na 2 B 8 O 13 4H 2 O or sodium pentaborate Na 2 B 10 O 16 10H 2 O

Effects of the clinker - gypsum grinding temperature upon early hydration of Portland cement

Abstract The temperature at which Portland clinker and gypsum are ground together to produce Portland cement is shown to influence the subsequent early hydration behaviour. Raising the grinding temperature from 90°C to 130°C and thence to 170°C has resulted in increased ettringite being present at all the hydration times examined up to two hours. The results have been interpreted on the basis of a through solution mechanism involving primarily the reaction of tricalcium aluminate from the cement clinker component with available soluble Ca 2+ and SO 4 2− ions in the aqueous medium.

Early hydration behaviour of portland cement containing tartaro- and titanogypsum

Abstract Portland cements containing tartaro- and titanogypsum were respectively hydrated for up to two hours at a water: cement ratio of 0.5. They were compared with a Portland cement containing high grade natural gypsum hydrated similarly. The cement containing tartarogypsum produced much more ettringite than those with titanogypsum and natural gypsum. Comparisons were made with previous examinations of the hydration of Portland cements containing other by-product gypsums. Reasons for the observed hydration behaviour of the Portland cements with tartaro- and titanogypsum are discussed.

Early hydration of portland cement — effects of water/cement ratio

Abstract The water/cement ratio at which Portland cement is hydrated is shown to influence the early hydration behaviour. Results have been obtained with both an ordinary Portland cement and a white Portland cement. As the water/cement ratio was progressively raised from 0.3 through 0.4 to 0.5, increased quantities of ettringite were formed at all the hydration times studied up to two hours. The results have been interpreted on the basis of a through solution mechanism for the formation of ettringite.