What Are Pozzolans? What Is Pozzolan Cement?

The classes of pozzolans most likely to be available are classes F and C fly ash and silica fume. Class N may be considered at those sites where a source of natural pozzolan is available.

(a) Regulations governing use of fly ash. The Solid Waste Disposal Act, Section 6002, as amended by the
Resource Conservation and Recovery Act of 1976, requires all agencies using Federal funds in construction to allow the use of fly ash in the concrete unless such use can be shown to be technically improper.

The basis of this regulation is both energy savings and waste disposal, since most fly ash in use today is the result of the burning of coal for electrical power.

(b) General. The use of pozzolan should be considered coincident with the consideration of the types of
available cements. Portland cement to be used alone should always be considered in the specifications as well as blended hydraulic cements or the combination of portland cement with slag cement or pozzolan unless one or the latter is determined to be technically improper.

Classes F and C fly ash are generally accepted on all Corps of Engineers’ (CE) civil works projects, and their use should be allowed in all specifications unless there are technical reasons not to do so.

(c) Class F pozzolan. Class F pozzolan is a fly ash usually obtained from burning anthracite or bituminous coal
and is the class of fly ash that has been most commonly used to date. It must contain at least 70.0 percent of
Si02 + Al203 + Fe203 by chemical analysis.

(d) Class C pozzolan. Class C pozzolan is a fly ash that is usually obtained from the burning of lignite or
subbituminous coal. It must contain at least 50.0 percent of Si02 + Al203 + Fe203 .

(e) Other considerations. Class C fly ashes often contain considerably more alkalies than do Class F fly
ashes. However, when use of either class in applications where alkali-aggregate reaction is likely, the optional
available alkali requirement of ASTM C 618 (CRD-C 255) should be specified. Use of Class F fly ash in replacement of portland cement results in reduction of heat of hydration of the cementitious materials at early ages.

Use of Class C fly ash in the same proportions usually results in substantially less reduction in heat of hydration. An analysis of the importance of this effect should be made if Class C fly ash is being considered for use in a mass concrete application.

Class F fly ash generally increases resistance to sulfate attack. However, if the portland cement is of high
C3A content, the amount of improvement may not be sufficient so that the combined cementitious materials are equivalent to a Type II or a Type V portland cement. This can be determined by testing according to ASTM C 1012 (CRD-C 211).

Class C fly ashes are quite variable in their performance in sulfate environments, and their performance
should always be verified by testing with the portland cement intended for use. Both Class F and Class C fly ashes have been found to delay for initial and final set. This retarding action should be taken into consideration if important to the structure.

Most Class C and Class F fly ashes are capable of reducing the expansion from the alkalisilica reaction. Use of an effective fly ash may eliminate the need to specify low-alkali cement when a reactive aggregate is used.

The effectiveness of the fly ash must be verified by ASTM C 441 (CRD-C 257). For additional information, see Appendixes D and E.

(f) Class N pozzolan. Class N is raw or calcined natural pozzolans such as some diatomaceous earths, opaline cherts, tuffs, and volcanic ashes such as pumicite.

(g) Silica fume. Silica fume is a pozzolan. It is a byproduct of the manufacture of silicon or silicon alloys.
The material is considerably more expensive than other pozzolans. Properties of silica fume vary with the type of silicon or silicon alloy produced, but in general, a silica fume is a very finely divided product and consequently is used in concrete in different proportions and for different applications than are the more conventional pozzolans discussed in the previous paragraphs.

Applications for which silica fume is used are in the production of concrete having very high strengths, high abrasion resistance, very low permeability, and increased aggregate bond strength.

However, certain precautions should be taken when specifying silica-fume concretes. Use of silica fume
produces a sticky paste and an increased water demand for equal slump. These characteristics are normally counteracted by using high-range water-reducing admixtures (HRWRA) to achieve the required slump. This
combination, together with an air-entraining admixture, may cause a coarse air-void system.

The higher water demand for silica-fume concrete greatly reduces or eliminates bleeding, which in turn tends to increase the likelihood of plastic shrinkage cracking. Therefore, steps should be taken as early as possible to minimize moisture loss, and the curing period should be increased over that required for conventional concrete.

Related post


Post a Comment