Chapter 11: Portland cement and other calcareous hydraulic binders: history, production and mineralogy
J. Elsen, G. Mertens and R. Snellings
Portland cement has become a cornerstone of modern society and the present-day cement and concrete industry is one of the largest consumers of industrial minerals. The historical evolution of early calcareous hydraulic binders into the present standard Portland cement is a typical example of a product-performance-optimization process driven by the gradual accumulation of empirical know-how and fundamental process understanding. The early observation that a hydraulic binder may be formed when impure limestone is burnt at temperatures above the decomposition temperature of limestone led to the development of a wide range of early hydraulic binders throughout the 18th and 19th centuries. Initially, burning at relatively low temperatures (1000–1100°C) of impure limestone resulted in the production of fast-setting natural cements and hydraulic limes. Eventually, over the course of the 19th and 20th centuries, sintering at increasingly higher burning temperatures of natural impure limestones and artificial mixes of ground limestone and clay was introduced to produce slow-setting natural cement and finally (proto-)Portland cement with superior strength development. Today, the Portland cement-production process consists of an energy-intensive, high-temperature sintering phase (1450°C) of the raw materials, followed by fast cooling and fine intergrinding of the clinker product with gypsum to produce the Portland cement. The mineralogy of the clinker phases is relatively complex. C3S and C2S show several high- and low-temperature polymorphs, whereas C3A and C4AF allow considerable compositional solid solution. The addition of water to Portland cement initiates a complex scheme of hydration reactions to form a hardened cement paste. The advent of novel analytical techniques prompted recent advances in the understanding of the structures of the hydration products and the hydration mechanism. Nevertheless many aspects of the hydration reactions remain unsolved. The necessary future developments towards less energy intensive, low-CO2 cements may take advantage of the historical knowledge acquired in the production of a wide range of alternative hydraulic binders.
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