A pozzolan is defined by the American Concrete Institute as "a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties". Raw or calcined natural pozzolans include such materials as diatomaceous earths, opaline cherts and shales, tuffs, volcanic ashes and pumicites. Man-made pozzolans are typified by fly ash, the finely divided residue that results from the combustion of coal, and blast furnace slag.
Portland cement is a product produced by heating a mixture of limestone and clay or shale, or other calcareous and argillaceous materials to a fused state. The fused product, which is called clinker, is interground with a few percent, normally about 4 to 6 percent, of a retarder such as gypsum. Portland cement is mixed with aggregate such as sand and stone to form a concrete having outstanding structural properties.
Pozzolans are commonly used in concrete and are either added to the concrete separately from the Portland cement at the concrete ready-mix plant or added together with the Portland cement as a special blended cement, such as a type "IP,IS" cement. The presence of a pozzolan in concrete increases the workability and decreases the bleeding characteristics of the fresh, plastic concrete. It also improves the properties of the hardened concrete by reducing its shrinkage, reducing its permeability and ultimately increasing its strength. Early usage of pozzolan in concrete was as a substitute for part or all of the "fine" aggregate component such as sand.
Pozzolans are most often used in concrete as a partial replacement for Portland cement and the replacement of 20% or more of the cement with a pozzolan is not uncommon. Pozzolans contribute strength to concrete by reacting with the calcium hydroxide (Ca(OH).sub.2) that is liberated by the Portland cement, when it reacts with water, and forms reaction products similar to those produced by the cement-water reaction. The chemical reaction between the pozzolan and the calcium hydroxide is often referred to as the "pozzolanic reaction".
The "pozzolanic reaction" is a very slow one and the strength contributed by the pozzolan is not significant until the concrete reaches 28 to 90 days of age. This means that a concrete in which a portion of the cement has been replaced with a pozzolan will inherently have lower compressive strength up to 28 to 90 days than the Portland cement concrete from which the pozzolan concrete was designed. A part of this strength reduction can be compensated for by altering the coarse and fine aggregate content of the pozzolan Portland cement concrete.
Certain chemical admixtures which have been formulated for use in Portland cement concrete also aid in off-setting the early strength deficiency of Portland cement pozzolan concrete. Such chemicals as salts of lignosulfonic acid, salts of hydroxylated carboxylic acids, sugars and corn syrups have been commonly used in concrete containing a pozzolan, but all of these materials delay the setting time of the concrete which already has an abnormally long setting time because of the pozzolan that is present.
Calcium chloride is also frequently used as an admixture in Portland cement pozzolan concrete to accelerate its set and enhance its early strength. The amount of calcium chloride commonly used is in the range of 1 to 2% on the weight of the cement. Other chloride salts such as sodium chloride have been suggested as accelerators for Portland cement pozzolan concrete (British Pat. No. 940,692 to Fombonne). Additions of chloride ion in amounts in excess of 0.5 percent on the weight of the cement pose a corrosion problem to any metal embedded in the concrete. Also at high sodium additions (greater than 0.5 percent on the cement) problems are posed by alkali-aggregate reactions.
Another admixture for pozzolan-containing Portland cement concrete that has been sold and used successfully for several years is constituted of a mixture of calcium lignosulfonate and triethanolamine (95%-5% mixture). There is a continuing need however for alternative and/or improved admixtures.