Normal pozzolan

High calcium fly ash and granulated blast furnace slag are cementitious. Low calcium fly ash is termed a normal pozzolan. Condensed silica fume and rice husk ash are highly pozzolanic. Slowly cooled blast furnace slag, bottom ash, and field burnt rice husk ash are weak pozzolans. 

It was shown earlier that aggregate types do not materially affect the performance of water-reducing admixtures. This is not true for cement and mixes containing special cements require particular care. Examples here are increased retardation with low C3A cement (for example, sulfate-resistant cement) and even an almost complete reduction in expansive properties with expansive cements in the presence of water-reducing admixtures. However, pozzolans such as fly ash appear to behave normally with water-reducing admixtures. 

Chemical tests of pozzolanicity have proved of limited use for evaluation, because the 28-day strength depends much more on the w/s ratio than on this property, and there appears to be no effective substitute for direct testing of the relevant properties on mortars or, preferably, on concretes. The degree of pozzolanic activity is, however, important for the development of strength and decrease in permeability at later ages. For equal 28-day strengths, the strengths at 91 days or more normally exceed those of otherwise similar concretes made with pure Portland cements. 

Nowadays blended cements are normally used, which are obtained by intergrinding or blending Portland cement with particular mineral substances. Among these, those with the addition of pozzolanic materials or ground granulated blast furnace slag are of particular interest with regard to durabihty of reinforced concrete. 

One group of natural pozzolanas that exhibits a strong pozzolanic activity is the zeolites. These are aluminosilicates that have a three-dimensional network structure created by comer-linked SiO and AIO4 tetrahedra. The stmcture of zeolites is very open, and contains charmels and cavities that are normally occupied by alkali and alkali earth cations and loosely held water molecules. Their stmcture may be expressed by the formula... 

Very finely ground cements These are made from mixtures of cement with sand or with slag or other pozzolan type minerals that are extremely finely groimd together. Such cements can have the same physical characteristics as normal cement but with 50% less cement particularly due to their increased surface area for the chemical reaction. Even with intensive grinding they can use upto 50% less energy to fabricate than ordinary Portland cements. 

Conduction calorimetric investigations have been earried out on Portland cement in whieh natural pozzolans have been incorporated, It has been found that in the pozzolan-containing pastes, the dormant period is deereased and the normalized height of the seeond main peak is greater than that given by the reference cement. This indieates that the pozzolan aecelerates the hydration of alite. It is possible that the aceeleration effect is due to a eombination of faetors, viz., higher specifie surface area of the pozzolan, its chemical composition, the physical state of the surface, and the rate of release of the alkalis. 

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