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140

Fundamentals of Drilling Engineering

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of Portland Cement ~Composition main ingredient in almost all drilling cements is Portland cement, an artificial cement made by burning a blend of limestone and clay. This is the same basic type of cement used in making concrete. A slurry of Portland cement in water is ideal for use in wells because it can be pumped easily and hardens readily in an underwater environment. The name "Portland cement" was chosen by its inventor, Joseph Aspdin, because he thought the produced solid resembled a stone quarried on the Isle of Portland off the coast of England. A schematic representation of the manufacturing process for Portland cement is shown in Fig. 4.1. The oxides of calcium, aluminum, iron, and silicon react in the extreme temperature of the kiln (2,600 to 2,800°F), resulting in balls of cement clinker upon cooling. After aging in storage, the seasoned clinker is taken to the grinding mills where gypsum (CaS04-2HP) is added (1 to 3% by weight of cement) to control setting time and hardening of cement (strength). The units sold by the cement company are the barrel, which contains 376 Ibm or four 94-lbm sacks, or by U.S. tons (2,000 Ibm) and metric tonnes (2,205 Ibm). Cement chemists have found that there are four crystalline compounds in the clinker that hydrate to form or aid in the formation of a rigid structure. These are tricalcium silicate (3CaO-Si07 or C3S), dicalcium silicate (2CaO-SiO~ or C2S), tricalcium aluminate (3CaO-AI903 or C3A), and tetracalcium aluminoferrite (4CaO· Alp,· or C.AF). The hydration reaction is exothermic and generates a considerable quantity of heat, especially the hydration of C3A. The chemical equations representing the hydration of the cement compounds when they are mixed with water are as follows.

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2(3CaO· Si02) + 6HP~ 3CaO • 2SiO~ • 3Hp + 3Ca(OH)2. 2(2CaO • Si02) + 4HP~ (slow)3CaO • 2Si02 • 3Hp + Ca(OH)2. 4CaO· AIP3·