Microstructures Portland cements

A knowledge of the relevant high-temperature phase equilibria is necessary for understanding the factors that govern acceptable bulk compositions for Portland cement clinker, the conditions under which the latter can be manufactured, and the phase composition and microstructure of the resulting material. This chapter deals with these equilibria and with the phases to which they relate, with the exception of the major clinker phases, which were described in Chapter I. Some anhydrous phases primarily of interest in relation to other types of cement are also considered here. Principles underlying the preparation of anhydrous silicate, aluminate and other high-temperature phases are outlined. 

Harrisson ct til. (H4.H49) represented the results of X-ray microanalyses of individual spot analyses in all parts of the microstructure other than unreacted clinker grains on plots of Al/Ca ratio against Si/Ca ratio and of Mg/Ca ratio against Al/Ca ratio (Figs 9.1 and 9.2). If the analyses of the material formed in situ from the slag are excluded, the plot of Al/Ca against Si Ca is broadly similar to those obtained for pure Portland cement pastes, and may be interpreted in the same way (Section 7.2.5). 

Backscattered electron and X-ray images obtained in the SEM (021,U17,S68) show that the microstructures of hardened pastes of cement or CjS with pfa broadly resemble those of pure Portland cements, though, as would be expected, there is considerably less CH. Reaction rims may be seen around the pfa particles in sufficiently old pastes. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

Any attempt to understand the expansive process must thus consider the manner in which the deposition of the products affects the microstructure. The production of ettringite can cause expansion, but does not necessarily do so in a supersulphated cement (Section 9.2.9), much ettringite is formed, but there is no marked expansion. Comparable amounts of ettringite are reported to be formed when ordinary or sulphate-resisting Portland cements... 

H21 Hofmanner, F. Microstructure of Portland Cement Clinker, 48 pp., Rhcin-talcr Druckcrei und Verlag, Heerbrugg, Switzerland (1975). 

All the cements considered in this book fall into the category of hydraulic cements they set and harden as a result of chemical reactions with water, and if mixed with water in appropriate proportions continue to harden even if stored under water after they have set. Much the most important is Portland cement. Chapters I to 4 of the present work deal mainly with the chemistry of manufacture of Portland cement and with the nature of the resulting product. Chapters 5 to 8 deal mainly with the processes that occur when this product is mixed with water and with the nature of the hardened material. Chapters 9 to 11 deal with the chemistry of other types of cement, of admixtures for concrete and of special uses of cements. Chapter 12 deals with chemical and microstructural aspects of concrete, including ones relevant to processes that affect its durability or limit its service life. 

The resistivity of concrete is an important parameter used to describe, for example, the degree of water saturation, the resistance to chloride penetration or the corrosion rate. The resistivity of concrete may have values from a few tens to many thousands of n m (Table 2.3) as a function of the water content in the concrete (relative humidity), the type of cement used (Portland or blended cements), the iv/c, the presence of chloride ions or whether the concrete is carbonated or not At early ages, the resistivity of concrete is low and considerable increases occur due to hydration of the cement AU of these factors can be rationalised on the basis of ion migration in the porous and tortuous concrete microstructure a high relative humidity increases the amount of water-filled pores (decrease of resistivity), the iv/c ratio and type of cement determine the pore volume and pore-size distribution (less but more coarse pores with pure Portland cement more but finer pores and less interconnectivity of pores with blast furnace slag or fly ash) chloride ions increase the conductivity of the pore solution and carbonation decreases it. An increased resistivity is accompanied by a reduced corrosion rate [38]. Table 2.4 shows resistivities determined for mature concrete in various chmates [39-41]. 

The use of polymers must be accompanied by the adoption of a greater mix of energy, so that its effects are more intense and evenly distributed in the cement matrix. The analyze of concrete microstructure is a good way to assess the behavior of mechanical properties and others properties, by the visualization of formed hydrates of Portland cement and its distribution. Research into the combination of polymers with different characteristics appears to be a noteworthy path to continue. 

Microstructure of Portland Cement Clinker, Friedrich Hofmanner (1973, Holderbank), and... 

Gouda, G.R., "Clinker Microstructure by Scanning Electron Microscope," Second Annual Conference on Cement and Clinker Microscopy, Sponsored by Gifford-Hill Co. Inc., General Portland Inc., and Southwestern Portland Cement Company, Dallas, Texas, 1980,19 pp. 

Hills, L.M., The Effect of Clinker Microstructure on Grindability, RP331, Portland Cement Association, Skokie, Illinois, 1995,124 pp. 

Shi, C. et al. (1992a) Composition of the microstructure and performance of alkali-slag and Portland cement paste, in Proceedings 9th ICCC, New Delhi, Vol. 3, pp. 298-304. 

The structure of concrete-like materials may be considered at different levels (cf. Section 2.4). The lowest level, where the required resolution of observation is of the order of a few Angstroms, is used mostly for examination of the microstructure of cement paste. The main interest is concentrated between so-called micro- and macro-levels, which means between the size of Portland cement grains or of small pores of a few micrometers in diameter, up to tens of millimetres for maximum grains of coarse aggregate and the diameters of steel bars. 

The prineiple hydration produets of slag cements are essentially similar to those found in pordand eement pastes. The microstructure of slag cement pastes is also similar to that of portland cement pastes. X-ray microanalysis has, however, shown that the C/S ratio of C-S-H product in hydrated slag cement is lower than that found in portland cement paste. 

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