Pozzolanic activity

Pozzolanic activity of the used fly ash sample (Table 6), determined by the test with the hydrated lime according to the mentioned standard, is for the P5 pozzolana class which means that it is convenient for the cement production as the mineral admixture [38-41]. 

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. 

Viewed as a mineral addition in concrete, microsilica is characterized by its small particle size and high pozzolanic activity. In the absence of a water-... 

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Before concluding this subject, mention is made here of two more novel approaches for using SBE. Pollard et al. (143) have reported that SBE can be used to prepare a pseudo-graphitic char suitable as a low-cost replacement for activated carbon in the stabilization/solidification of industrial wastes. In their process, they char 2 1 blends of SBE and ZnCl2 at 450°C/1 hour and then activate the material at 600°C/1 hour. The resultant hybrid material is as effective as activated carbon for fixing toxic organics and, because of its aluminosilicate framework, exhibits additional pozzolanic activity in the cement-based stabilization/solidification reactions in which these materials are used. Very recently (144), Bohling reported on... 

The interaction of zeolite-rich materials with Ca(OH)2 is of special interest, because zeolites, like other reactive aluminosilicate systems, e.g., crushed bricks, give rise to calcium silicates and aluminates, which are able to harden upon hydration in both aerial and aqueous environments. This behaviour, already known in ancient times, is typical of a volcanic, mostly glassy material, called pozzolana, which is the genetic precursor of the mentioned Neapolitan yellow tuff, widely spread in the surroundings of Naples, Italy [61]. That is why every material able to behave as pozzolana is called "pozzolanic material" and the property to react with lime is called "pozzolanic activity". 

Natural pozzolana. This is a sedimentary material, usually of piroclastic origin, that is derived from the sediment of volcanic eruptions that have produced incoherent deposits or compact deposits that have been chemically transformed with time (such as Italian pozzolana, which was used by the Romans). Pozzolanic materials may also have other origins, such as diatomaceous earth composed of the siliceous skeleton of micro-organisms. The pozzolanic activity of these materials is related to their siliceous component in the vitreous state and to their fineness. There are also pozzolanas that are obtained by calcination of natural substances. 

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

Raask, E. P. J. Street, 1978. Appearance and pozzolanic activity of pulverised fuel ash. Unpublished proceedings of a Conference on Ash Technology and Marketing. London 22-27, October 1978. 

Stabilized base Aggregate Cementitious materials - Pozzolan - Pozzolan activator Self-cementing material... 

CKD can be used as a cementitious material or a pozzolan activator in stabilized... 

LKD has potential for use as a pozzolan activator in each respective application As a cementitious material, CKD can replace or be used in combination with Portland cement... 

As a pozzolan activator, both CKD and LKD can replace or be nsed in combination with Portland cement or hydrated hme... 

The Chapelle method [6] of pozzolanic activity determination, applied in France for many years, is similar to that proposed by Massazza [5], This method consists in boiling 1 g of pozzolana with 1 g of CaO in 250 cm of distilled water, during 16 h [6]. The combined CaO content is then determined. 

The chemical composition of fly ash is the same as the composition of clays. The most important components are Si02 and AI2O3 the pozzolanic activity of material augments with increasing amount of sihca and iumina [115]. The pozzolanic activ-... 

Reactions similar to those taking place in the calcination of pure clay minerals also take place in the production of ceramic bricks and tiles in industrial kilns, and these products also exhibit pozzolanic properties if crushed to a fine powder (Ranogajec and Fisang, 1992). In experiments performed with Lower Oxford clays it was found that samples fired to 600-800°C still contained substantial amounts of residual illite, whereas in those burnt at 900-1100°C an amorphous glass phase was present that exhibited a distinct pozzolanic activity (Wild et a/., 1996b, 1997). 

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... 

He, C., Makovicky, E., and Osbaeck, G. (1996) Thermal treatment and pozzolanic activity of Na- and Ca-montmorrilonite. Applied Clay Science 10,351-368. 

Justines, H. et al. (1990) A 29Si MAS NMR stu of the pozzolanic activity of condensed sihca fiune and the hydration of di- and tricalcium silicate. A ances in Cement Research 3,111-116. 

Liebig, E., and Althaus, E. (1998) Pozzolanic activity of volcanic tuff and suevite effect of calcination. Cement and Concrete Research 28,567-575. 

Sanches de Rojas, M.I., and Frias, M. (1996) The pozzolanic activity of different materials its influence on the hydration heat in mortars. Cement and Concrete Research 26,203-213. 

Wild, S., Khatib, J.M., arrd Jones, A. (1996a) Relative strength, pozzolanic activity and cement hydration in srrperplasticized metakaolin concrete. Cement and Concrete Research 26,1537-1544. 

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