Construction Concrete

Natural cementitious materials have existed for very long time; however, synthetic materials were perhaps first used by Egyptians and Chinese thousands of years ago [1, 2], and the Romans used pozzolanic materials (a volcanic rock in powder form and used to make hydraulic cement) to build the Rome Coliseum. It was only until around 1824 when Joseph Aspdin, bricklayer of England, invented Portland cement by burn­ing finely ground chalk with finely divided clay until carbon dioxide was driven off. The sintered product was ground and named as Portland cement after the building stones quarried at Portland, England.

Portland type cements are calcium silicates, which when mixed with water form a paste, producing a hardened mass of valuable engineering properties. The clinker (the fused or partly fused by-product that after grinding becomes cement) chemistry is (by weight) 50-70% 3CS (alite, 3CaOSiO2), 15-30% 2CS (belite, 2CaOSiO2), 5-10% 3CA (3CaOAl2O3), and 5-15% 4CAF (ferrite, 4CaO Al2O3 Fe2O3). Concrete is a mix of cement paste and aggregates (inert granular materials such as sand, gravel, recycled concrete), which the cement binds together into a rock-like composite. Main applica­tions are for civil infrastructures such as buildings, highways, underground mass transit systems, wastewater treatment facilities, and marine structures. A compilation on materials, properties, working operations, and repair is given by Dobrowolski [3].

Approximately, 1.6 billion tons of Portland cements are produced worldwide annually with an estimated 5% generation of the CO2 emission. Global cement and concrete addi­tives (fiber and chemical additives) demand is forecast to grow 6.3% annually through 2006, driven by construction and by higher standards for concrete that require more additives per ton [4].

The US is the third largest cement producing country in the world (after China and India). The US concrete industry is the largest manufacturing sector in the United States (cement, ready-mixed concrete, concrete pipe, concrete block, precast and prestressed concrete, and related products), with over two million jobs related directly and including materials suppliers, designers, constructors, and repair and mainte­nance. The value of shipments of cement and concrete production exceeds $42 billion annually. Currently, there are a total of 115 cement manufacturing plants in the US, with about 75% of the total plants owned by only ten large companies: Lafarge North America, Inc., Holcim (US) Inc, CEMEX, SA de CV, Lehigh Cement Co., Ash Grove Cement Co., Essroc Cement Corp., Lone Star Industries Inc., RC Cement Cp., Texas Industries Inc. (TXI), and California Portland Cement Company.

Two types of manufacturing processes have become prevalent in the cement industry: “wet process” and “dry process.” Although these processes are similar in many respects, in the older “wet process,” ground raw materials are mixed with water to form a thick liquid slurry, while in the “dry process,” crushed limestone is used and raw materials are mixed together, with consequent higher energy efficiency as drying is eliminated. Figure 1 represents schematically the cement manufacturing process and the rotary kiln for clinker production.