Pozzolan cements

Properties of pozzolan elements good chemical resistance 

Naturally occurring pozzolans are trass (a volcanic tufaceous rock found in the Rhineland, Bavaria and Austria), santorin earth (Greece) and the Italian pozzolans (Pozzuoli near Naples). A synthetic material with similar properties is fly ash, which is produced in hard coal- and lignite-fired power stations. 

Pozzolan cements exhibit good chemical resistance. 

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Petrifix A process for solidifying aqueous wastes, converting them to a solid form suitable for landfill. Cementitious additives are used, based on the compositions used by the Romans for making Pozzolanic cements. Developed by Pec-Engineering, Paris, France. In 1979 it had been used in France and Germany. 

Mordenite and cHnoptiloHte are used in small volume in adsorbent applications including air separation and in drying and purification [24]. Natural zeolites have also found use in bulk applications as fillers in paper, in pozzolanic cements and concrete, in fertihzer and soil conditioners and as dietary supplements in animal husbandry. 

Most natural pozzolanas are of volcanic origin, though some are sedimentary. Some clays and other materials that are unsuitable for use in concrete in their natural state become usable as pozzolanas if heat treated. Both natural pozzolanas and heat-treated materials have been used with lime since ancient times, but today they are mainly used as constituents of pozzolanic cements. Several reviews are available (M81,M82,M83). 

Costa and Massazza (C44) concluded from a study of natural pozzolanas of varied types that reactivity in mixtures with CH at w/s = 2 and 40 C depends during the first 28 days on the specific surface area and at later ages on the contents of Si02 and AI2O3 in the active constituents. A comparative study of five natural pozzolanas and three low-CaO pfas in pastes with cement showed that the CH contents of the pozzolanic cements were considerably lower than those of the pfa cements at 3-60 days, but virtually the same at 90 days, the pozzolanas thus appearing to react more rapidly than the pfas at early ages but more slowly later. Determinations of the unreacted mineral admixture in pastes with CH showed that at 90 days 23-30% of the natural pozzolana had reacted, compared with 11-15% for the pfas. The similarity in CH contents suggests, however, that these values may not apply to mixtures with cement. 

Portland-Pozzolanic cement contains a material (in our case fly ash) that reacts with the calcium hydroxide produced by Portland cement to form calcium aluminosilicates to give a cement with improved properties. 

Zeolitic tuff is also utilised in the cement industry as pozzolanic addition (see Sub-sec. 5.2.2.) to portland cement. This application recalls the use of pozzolana, since the beginning of the 1900s, to obtain blended cements, able to fix the lime formed by the hydration of the calcium silicate components of the portland clinker. The utilisation of zeolitic tuff, as substitute of pozzolana, to obtain pozzolanic cements is based on both economic and technical considerations. On one hand, manufacturing blended cements allows a 40% fuel savings, without reducing the quality of the produced binder (it is to bear in mind that the mixture lime-pozzolana is itself a cement), on the other, it involves some advantages, e.g., the... 

When it is required to seal the steel casing of gas and oil wells to the walls of the borehole and to seal porous formations, a grout mixture is used. Both Portland and pozzolanic cements are used as setting agents. The latter consists of slaked lime and a pozzolan. The setting time of the pozzolanic cement is shortened by the addition of an accelerator (e.g. soda ash which is immediately causticised by the lime) and by the relatively high temperatures at depth [32.27]. 

GEM rV(pozzolanic cement) with addition of 11-55% pozzolanic materials,... 

The setting reactions for Portland cement are similar to those for the ancient pozzolanic cement. The first reaction is the hydration of C3A. This reaction is rapid, occurring within the first 4 hours, and causes the cement to set ... 

To Romans we also owe the name hydraulic cement as they defined the binders hardening under water and thanks to the reaction with water [3]. Some blended materials, in order to better define their composition, was called pozzolanic cements. 

In Table 6.4 the effective chloride ions diffusion coefficients in the pastes from various cements, determined by Page et al., are presented [191,211]. The NaCl molar solution in saturated Ca(OH)2 solution, as well as the saturated Ca(OH)2 solution were applied in two chambers of measuring stand. As can be concluded from these data, the chloride ions diffusion coefficient is ten times lower in slag cement paste and three times lower at 30 % fly ash addition. As it can be concluded, the resistance of slag and pozzolanic cements to the attack of aggressive water solutions is significantly higher. This relates undoubtedly to the much lower capillary porosity of these cement pastes. 

Masazza, F. (1998) Pozzolana and pozzolanic cements, in Lea s Chemistry of Cement and Concrete (ed. P.C.Hewlett), Arnold, London, pp. 471-631. 

Ranogajec, J., and Fisang, L. (1992) The hydration of pozzolanic cement on the basis of fired clay bricks, in Proceedings 9th ICCC, New Delhi, Vol. 6, pp. 233-239. 

Portland pozzolan cement It includes fly ash cement, since fly ash is a pozzolan, but also includes cements made from other natural or artificial pozzolans. In coimtries where volcanic ashes are available (e g. Italy, Chile, Mexico and the Philippines), these cements are often the most common form in use. 

The opposite trend (slower reaction, longer setting times, lower early strengths, lower heat of hydration) is associated with coarser grinding, higher belite content of the cement, and the addition of blastfurnace slag (slag cements) or pozzolana (pozzolanic cements, e.g., trass cement). 

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