Cements continued hydraulic

The permeability of concrete is a guide to its durability (Section 1.5.2) but it can also be relevant to the design of structures which are intended to withstand a hydraulic head of water or other liquid. Extreme porosity is usually due to continuous passages in the concrete, due to poor compaction or cracks which can be minimized by the use of water-reducing admixtures to give increased workability whilst maintaining a low water-cement ratio. 

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. 

A rather broad definition proposed by the ACI 549.2R-04 for ferrocement is A type of thin wall reinforced concrete constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small wire diameter mesh. The mesh may be made of metallic or other suitable materials. In this definition all kinds of non-metallic meshes are included, as well as fabrics and mats made with non-metallic fibres that are treated as textile reinforcement. 

The essentials of modern cement manufacture have been followed for the last 200 years (Lea 1970). A calcium source (usually limestone) and an aluminosilicate source (usually shale or clay) are mixed together and fired at high temperatures (1300 to 1500°C) to produce calcium silicates, aluminates and alumino-ferrites. The calcium silicates hydrate on contact with water to produce CSH gels which are the chief binding agents in the cement. Cements containing CSH gel (including pozzolanic cements) are termed hydraulic because they set and harden on reaction with water and once set, continue to harden if placed underwater (Taylor 1990). 

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