Hydraulic cements comprise about 20% by volume of most concretes: their hydration products serve as a binder for the aggregate. Cement composition and fineness, along with other concrete constituents, influence fresh concrete properties such as rheology, heat evolution, setting, rates of strength development, ultimate strength, and color. The hydration products of cement (and on occasion, residual cement phases) affect concrete durability.
Cement clinker is manufactured from a finely-ground, homogenized blend of limestone, shale and iron ore sintered in a rotary kiln to temperatures of approximately 1450 ºC: clinker nodules 3-20 mm in diameter are normally produced. The nodules are subsequently ground with gypsum, which serves to control setting, to a fine powder <45 µm) to produce cement. Fig. 1 shows a representative clinker nodule.
Fig. 1. Nodule of portland cement clinker with a cut and polished face.
LeChatlier [1] first examined thin sections of clinker using the then new petrographic microscope. About a decade later, petrographic investigations of A.E. Tornebohm named four crystalline phases alite, belite, celite, felite, in addition to a supposed glassy phase. Subsequent studies of clinker have utilized both transmitted light and reflected light on polished, etched surfaces and have sought to understand the connections between microstructure and performance properties. Reflected light microscopy is still widely used today and ASTM standard test method C 1356 is used for quantitative phase abundance determination [2].
Phase composition and texture (crystal size, abundance, and distribution) of clinker result from complex interactions of raw feed particle size, feed homogenization, and the heating and cooling regime. Mill grinding affects the cement microstructure through fracturing of the calcium silicates and interstitial phase crystals and, depending upon conditions, it may alter the form of calcium sulfate added at this stage to control cement setting. These features in turn influence the cement's hydration characteristics.
Phase abundance estimates are commonly derived from the bulk chemistry using the Bogue equation. Errors in phase abundance estimates were recognized early and derive in part from the variability of clinker phase chemistry relative to the assumed compositions, and from not accounting for minor constituents [4, 5]. While this approach has served the industry well, it does not consider the textural attributes of the clinker, i.e. features such as crystal size, phase distribution, and surface area, which will influence performance. In contrast, microscopy provides a direct means to observe and quantify microstructure features. Detailed information on clinker microscopy may be found in [6, 7].