1999-11-11_REVISION - M1977208 - Laserfiche WebLink

Home Browse Search

standpoint, approval of the amendment makes the CKD an onsite rather than an imported waste. From a technical standpoint, the cement plant must be included in the reclamation permit area in order to provide regulatory controls over the generation of the CKD, e.g., the types of fuel or chemicals used in the kilning process, which may effect the geochemistry of the CKD. [n particular, if a cement plant were to begin using alternative fuels such as wood, tires, or waste oil, the potential changes to the nature of the CKD should be evaluated. This memo discusses some of the details of CKD generation and disposal at the three Colorado cement plants. A discussion of the geochemistry of cement and CKD are provided for background. The Holnam-Boettcher operation and the Southdown-Lyons operation have completed waste characterization studies and have established groundwater monitoring programs. The Holnam- Portland operation has completed waste characterization and some groundwater monitoring, and the division is processing a TR for continued CKD disposal at the Portland plant. Based on (a) leach test results, (b) chemical analysis of pit water adjacent to one of the quarries, and (c) CKD waste handling commitments, groundwater monitoring did not seem justified at the Lyons operation. The latter point - "CKD waste handling commitments" -was most important in reaching that determination and is discussed in the following sections. A PRIMER ON PRODUCTION, CHEMISTRY, AND PROPERTIES OF CEMENT Cement is produced by burning limestone and clay at very high temperatures in an inclined rotazy kiln. It can take up to 2 hours for the raw materials to pass through the kiln depending on its length. As the mixture moves down the cylinder, it progresses through four stages of transformation. Initially, any free water is driven off, then calcination occurs by driving off bound water and carbon dioxide. After calcination, the limestone has been converted to lime (CaO). The third stage is called clinkering, where lime and decrepitated clay combine to form calcium silicates and calcium aluminates as shown in the following equation. The fourth stage in the process is the cooling of the clinker. CaCO3 + SiO~ + AhO3 + Fe~O3 + H~O(bound) + heat (limestone) (clay) ~ 3CaO•SiO~ + 2CaO•SiO, + 3CaO•AhO3 + 4CaO•A1~O~•Fe,O~ (tricalcium silicate) (dicalcium silicate) (tricalcium aluminate) (tetracalcium aluminoferrite) The compounds shown on the product side of the equation comprise 90 percent of Portland cement. When water is added the two calcium silicates, which form approximately 75 percent of cement by weight, react to produce tobetmorite gel and calcium hydroxide. Tobermorite gel is the main cementing component of cement paste. The average diameter of a grain of Portland cement as ground from the clinker is about 10 Nm. The particles of the hydration product, tobermorite gel, aze on the order of a thousandth that size. The enormous surface area of the gel (about 3 million cm'/g) results in very large attractive forces, or cementation.