Material blended cements

Oil well cements (78) are usually made from Pordand cement clinker and may also be blended cements. The American Petroleum Institute Specification for Materials and Testing for Well Cements API Specification 10) (78) includes requirements for nine classes of oil well cements. They are specially produced for cementing the steel casing of gas and oil wells to the walls of the bore-hole and to seal porous formations (79). Under these high temperature and pressure conditions ordinary Pordand cements would not dow propedy and would set prematurely. Oil well cements are more coarsely ground than normal, and contain special retarding admixtures. 

Utilization of different blended cements leads to the highest reduction of the CO2 emissions during the cement production. This is realised by replacement of a portion of clinker in cement by granulated materials. Blast-furnace slag, fly ash with accelerators were used as additive materials. Practical decrease of the CO2 emissions from both fuel and raw material can reach a reduction by 50% or even more [5]. 

Self-Tuning Adaptive Control of Cement Raw Material Blending, by L. Keviczky, J. Hetthesy, M. Hilger, and J. Kolostori, Automatica, 14, 525 (1978). 

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. 

CO2 (molecular weight 44) that can react with a concrete produced with 300 kg/m Portland cement that we can suppose is composed by 64% of CaO (molecular weight 56) is 300 X 0.64 X 44/56 150 kg/m. In the case of blast furnace slag cement with 70 % of GGBS, the percentage of CaO is only 44%. For other blended cements, the quantity of CaO is somewhere between these two values [3]. For blended cement, hydration of pozzolanic materials or GGBS also leads to a lower Ca(OH)2 content in the hardened cement paste which may increase the carbonation... 

Materials and Methods. The Portland cement used was ternary blend cement, Cemento 5-5 , from Cemento Loma Negra, Argentina, with 300 m /kg of blaine surface. PVA was provided by Quimica Oeste, Buenos Aires, Argentina, and the hydrolysis degree is 88%. The clay was provided by Minarmco, Argentina. It was a sodic bentonite and it was used as received. 

Additives and cement replacement materials such as pulverized fuel ash (fly ash), ground granulated blast furnace slag, silica fume (micro silica) and other materials can reduce the pore size and block pores enhancing durability further. However, proper curing may be vital to get the required performance with these blended cements. 

The materials for the base and sub-base course are unbound aggregate mixtures or hydraulically bound aggregate mixtures. The aggregates may be crushed granular materials, manufactured materials from rock deposits or industrial by-products (slags) or recycled materials. The hydraulic binders, in the case of bound materials, are cement, fly ash, slag, lime, a mixture of some of them or factory-blended hydraulic binders for road use. 

Table 7.1 summarizes the typical composition and properties of a representative series of mineral additions used in blended cements. Binders based on granulated blast furnace slag and related materials are discussed in mote detail in Chapter 8, and those based on natural and artificial pozzolanas in Chapter 9. Cements containing calcium caibonate ate discussed in section 2.2.14. 

The specific gravity of microsilica is around 2.20. Because of its very small particle size and high specific surface area, the bulk unit weight of the loose material is very low, and ranges typically between 240 and 300 kg/m. To lower the transportation costs, it is common to ship the material in a predensified form, by which the unit weight may be increased to 540-600 kg/m. It is even more convenient to transport and handle the material in the form of a slurry with a high solid content. Blended cements, in which microsihca is combined with Portland clinker and calcium sulfate, are also commercially available. A comparison of different forms of microsilica indicates that densification of this material decreases its chemical reactivity (Sanches de Rojas et al, 1999). 

Ambroise, J., Maximilien, S., and Pera, J. (1994) Properties of metakaolin blended cements. Advances in Cement Based Materials 1,161-168. 

The response of blended cements to sulfate attack may be distinctly improved by a caibonation of the surface of the material prior to exposure to sulfates (Sersale et al 1997). Improved resistance may also be achieved by adding finely dispersed calcium carbonate to the original mix. In its presence monocalcium caiboaluminate (C3A.CC.Hj2) is formed instead of monosulfate, and this phase does not convert to ettringite in the course of sulfate attack (Piasta et al, 1997). 

The thermal decomposition of hydrated composite cements is similar but not identical to that of plain Portland cement pastes (Ztirz et al 1986). Here also the amount of combined water declines sharply with increasing temperature, and may drop to near-zero values upon exposure to 1000°C (Fig. 23.2). The original amount of fiee calcium hydroxide in pastes made with blended cements is lower, and the amount of calcium oxide formed in the decomposition process dechnes even further or may even drop to zero values, owing to solid-state reactions that may take place in the material at high temperatures (Fig. 23.3). 

As an alternative to Portland cement, alkali-activated blended cements (see section 8.5 and 9.1.5) have also been suggested as a binder in wood-based materials (Lin et al, 1994). The hardening of these binders is little affected by the quality of the wood employed. 

White blended cements It is used to produce white clinker and white supplementary materials such as high-purity metakaolin. 

The clinoptilohte-rich natrual zeolite has been compared to Portland cement with respect to the radiation attenuahon properties [lOKl]. It was concluded that special cate shorrld be taken when using this type of natiual zeolite as blend material in cements because of the natiual radioachve content of clinoptilohte-rich natural zeolite, even in trace level, and the poorer X-ray attenuahon compared with Portland cement. 

Rodrigues, C.S., Ghavami, K., Stroeven, R, 2006. Porosity and water permeability of rice husk ash-blended cement composites reinforced with bamboo pulp. Journal of Materials Science, 41(21), pp. 6925-6937. 

The replacement of cement clinker by GGBS may vary from 10% up to 70% or even more for pure slag cements, and different kinds of blended cements are available. The use of GGBS as a partial replacement of Portland cement has a significant ecological effect. The deposits of industrial waste are transformed into a constituent of concrete. The emissions of dusts and gases into the atmosphere from cement production are limited and the need of natural raw materials is proportionally reduced (Bijen 1996). 

The general conclusion is that various kinds of supplementary binding materials are used to produce different blended cements, but this rapidly developing practice should be accompanied by deep knowledge and care to obtain results corresponding well with the expectations. 

The influence of fly ash, ground blast-furnace slag and other micro-fillers on the properties of high performance concrete is positive. As for ordinary concretes, these mix components densify the structure, and because of their pozzolanic properties they take part in hydration processes. Partial replacement of Portland cement by fly ash and ground slag enables a decrease in the cost of materials, improves the workability and reduces the heat of hydration. In practice, the majority of concrete structures are made with binary or ternary blended cements, which means that more than one additional binder is used with Portland cement. 

By-products such as blast furnace slag from steel production, coal ash from coal-fired power plants, and wastes such as sewage sludge and incineration ash from local municipalities are widely used as alternative raw materials (AR). Also, blended cement can be produced by grinding blast furnace slag and fly ash with the clinker as cement additives. 

Blended cements Hydraulic cements consisting essentially of an intimate and uniform blend of a number of different constituent materials commonly Portland cement, limestone, fly ash or blast furnace slag. 

Filler effect Accelerating effect of fine, inert materials on the hydration kinetics of the clinker phases in blended cements because of the presence of additional nucleation sites and the presence of a higher effective water-to-cement ratio often wrongly assigned to (early) supplementary cementitious material reactivity. 

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