Ceramics and Hydraulic Cements

Hydraulic cements are another class of technologically important materials. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed this way set into a hard mass that has sufficient compression strength and can be used as stmctural materials. Their structure is generally noncrystalline. 

Hydraulic cements are excellent examples of accelerated chemical bonding. Hydrogen bonds are formed in these materials by chemical reaction when water is added to the powders. These bonds are distinct from the bonds in ceramics in which high temperature interparticle diffusion leads to consolidation of powders. 

Portland cement is the most common hydraulic cement. It is formed by clinkering a mixture of powders of limestone, sand, iron oxide and other additives at a very high temperature ( 1500°C). It is mixed with water to form hydrated bonding phases 

The preparation of calcium aluminate cements is similar. Here, instead of calcium and silica, calcium and alumina react with water to form hydrated calcium aluminate [2] as the bonding phase. The initial strength gain for this material is faster than that for Portland cement. 

The main distinction between ceramics and cements is thus how they are produced. Objects that go through intense heat treatment for their consolidation are ceramics, while those formed by chemical reaction at room temperature are cements. 

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Another major distinction between ceramics and hydraulic cements is the porosity. Ceramics are made dense unless their application requires some degree of porosity. Hydraulic cements, however, are inherently porous. Porosity is <1 vol% for the best ceramics, but typically 15-20 vol% for cements. Ceramics tolerate very high temperatures, and are corrosion resistant in a wide range of pH, while cements are made for use at ambient temperatures and are affected by high temperature as well as acidic environment. Compared to cements, ceramics are more expensive thus, cement is produced in high volume while ceramics, except few products such as bricks, are specialty products. 

The distinctions made between ceramics and hydraulic cements do not cover the many products that have been produced by materials research in the last 50 years. Some of such products are made by partial heat treatment first and then set like cements. There exist products that are made like cements, but exhibit a structure like that of ceramics, because the bonding mechanism in them is covalent and ionic. They have much higher... 

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