Alternative cementitious materials are finely divided materiasl that replace or supplement the use of portland cement. Their use reduces the cost and/or improves one or more technical properties of concrete. These materials include fly ash, ground granulated blast furnace slag, condensed silica fume, limestone dust, cement kiln dust, and natural or manufactured pozzolans. The use of these cementitious materials in blended cements offers advantages such as increased cement plant capacity, reduced fuel consumption, lower greenhouse gas emissions, control of alkali-silica reactivity, or improved durability. These advantages vary with the type of alternative cementitious material.

Fly ash is a combustion by-product generated in coal-burning power plants. It is a fine particulate residue removed by a dust collection system. Approximately 40 million tons of fly ash are produced annually in the United States. Fly ash particles are spherical particles ranging from 1-150 mm in diameter. ASTM C618 categorizes fly ash as either Class C or Class F based on the origin of the coal used and the resulting fly ash chemical composition. Class F is a low-calcium fly ash and is pozzolanic, while Class C fly ash exhibits both pozzolanic and cementitious properties because of its high calcium content. The use of fly ash provides improved workability, increased long term compressive strength, reduced heat of hydration, decreased costs and increased resistance to alkali-silica reaction, and sulfate resistance (Class F only) when compared to unblended portland cement.

Ground granulated blast furnace slag is produced in a blast furnace where iron ore is converted into iron. This slag forms when the silica and alumina compounds of the iron ore combine with the calcium of the fluxing stone (limestone and dolomite). The newly formed slag floats on the liquid iron and is drawn off from a notch at the top of the hearth while the liquid iron flows from a hole at the bottom of the hearth. These reactions take place at temperatures ranging from 1300-1600^oC, so the slag is conveyed to a pit where it is cooled. The United States produces approximately 14 million metric tons of blast furnace slag annually (NSA, 1988). The conditions of the cooling process determine the type of blast furnace slag: air-cooled, foamed, water granulated, or pelletized. Of these types, ground granulated blast furnace slag is both cementitious and pozzolanic. Ground granulated blast furnace slag is a replacement of portland cement and provides several advantages such as improved workability, reduced heat of hydration, decreased costs increased resistance to alkali-silica reaction, and sulfate resistance and increased compressive and flexural strength when compared to unblended portland cement.

Condensed silica fume is a by-product of the smelting process in the silicon metal and ferrosilicon industry. Silica fume is produced when SiO vapors, produced from the reduction of quartz to silicon, condense. In the United States, approximately 100 thousand tons of silica fume is generated annually (Mehta, 1989). Silica fume particles are spherical with an average diameter of 1-mm and contain approximately 90% silicon dioxide with traces of iron, magnesium, and alkali oxides. When compared to portland cement, fly ash, or ground granulated blast furnace slag, silica fume is much finer. The addition of small amounts of silica fume (2-5%) increase workability while large amounts of silica fume (>7%) decrease workability, increase compressive strength, decrease permeability and provide resistance to sulfate attack and alkali-silica reaction.

Limestone functions as a supplementary cementing material when it is finely ground with clinker into portland cement. Limestone quality should have at least 75% calcium carbonate by mass, a clay content less than 1.2% by mass, and an organic content less than 0.2% by mass. There are several advantages to using limestone in portland cement such as reduced energy consumption and reduced CO₂ emissions. Additional cost savings are realized if limestone is available in close proximity to the site. In portland cements with high C₃A (tricalcium aluminate) contents, the carbonate from the limestone will react with the C₃A during hydration and may increase strength gain and resistance to sulfate attack.

Approximately 14 million tons of cement kiln dust are generated yearly in the United States with 9 million tons being reused into clinker manufacturing and 5 million tons discarded (Detwiler et al., 1996). Cement kiln dust varies as the raw material, clinker, and type of operation varies; however, it consists of unreacted raw feed, partially calcined feed and clinker dust, free lime, alkali sulfate salts, and other volatile compounds. After the alkalis are removed, the cement kiln dust can be blended with clinker to produce acceptable cement, and cement kiln dust can be added to portland cement with other materials such as slag and fly ash.

Other natural pozzolans exist such as volcanic ash, opaline chert and shales, tuffs, and diatomaceous earths. These materials originate from volcanic eruptions and have raw or calcined natural material. These natural pozzolans have large internal surface areas and vary depending on the type of magma from which they originate. Calcined kaolinite is a processed natural pozzolan, which is highly reactive in the presence of lime upon hydration. By including calcined kaolinite in portland cement, increased compressive strengths and decreased permeability may result (Caldarone and Gruber, 1995).

Alternative cementing materials can directly replace a portion of portland cement. These materials can be used alone or blended with other alternative cementing materials to produce a cement or concrete with properties different than those resulting from the use of portland cement. The use of alternative materials affects cement and concrete properties such as workability, hydration, compressive strength, and durability.

Information compiled by Erica Van Tassel

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