Cement blended

Blended cements represent about 1% of the cement shipped ia the United States. In Europe, the use of blended cement is very common. Most of the blended cement used ia the United States is Type IP and it is used ia the same appHcations as that of regular Type I or II Portland cement. 

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. 

Multi-stage preheating, pre-calciners, kiln combustion system improvements, enhancement of internal heat transfer in kiln, kiln shell loss reduction, optimize heat transfer in clinker cooler, use of waste fuels Blended cements, cogeneration... 

Bleach products, fragrances in, 78 363 Bleed-and-feed process, 9 797 Blended cements, 5 492 493, 501 Blending, cotton, 8 17. See also Mixing and blending... 

The vendor further claims that the blended cement product can be sold for approximately 60 per ton (D18214J, p. 25). 

Lange, L. Hills, C. Poole A. (1997). Effects of Carbonation on Properties of Blended and Non-Blended Cement Solidified Wasteforms. J. Hazard. Mater. 

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]. 

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... 

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. 

According to the European standard EN 197-1 [10], Portland cement and blended cements can be classified on the basis of composition and performance (strength) at 28 days. 

R. B. Polder, Simulated de-icing salt exposure of blended cement concrete - chloride penetration , Proc. 2nd International RILEM Workshop Testing and Modelling the Chloride Ingress into Concrete, C. Andrade,... 

Table 2.1 shows the ionic concentrations measured by different researchers in the pore solution of cement pastes, mortars and concretes, obtained both with Portland cements (OPC) and blended cements [4-14]. Measurements were carried out by chemical analysis of the liquid extracted under pressure, using specific pore-extraction devices. 

In non-carbonated and chloride-free concrete, the concentration of hydroxyl ions (OH ) varies from 0.1 M to 0.9 M, due to the presence of both NaOH and KOH (the latter is predominant, especially in Portland cement). Other ions, e. g. Ca and S04 , are present only in very low concentrations. Addition of blast furnace slag or fly ash to Portland cement results in a moderate reduction of ionic concentration, and thus in pH. From hydroxyl ion concentrations in Table 2.1, values of pH of 13.4-13.9 can be calculated for Portland cement, and pH values of 13.0-13.5 for blended cements. Addition of condensed silica fume in higher percentages may lead to a decrease in the pH to values to below 13 [4, 10]. 

The resistivity of concrete is an important parameter used to describe, for example, the degree of water saturation, the resistance to chloride penetration or the corrosion rate. The resistivity of concrete may have values from a few tens to many thousands of n m (Table 2.3) as a function of the water content in the concrete (relative humidity), the type of cement used (Portland or blended cements), the iv/c, the presence of chloride ions or whether the concrete is carbonated or not At early ages, the resistivity of concrete is low and considerable increases occur due to hydration of the cement AU of these factors can be rationalised on the basis of ion migration in the porous and tortuous concrete microstructure a high relative humidity increases the amount of water-filled pores (decrease of resistivity), the iv/c ratio and type of cement determine the pore volume and pore-size distribution (less but more coarse pores with pure Portland cement more but finer pores and less interconnectivity of pores with blast furnace slag or fly ash) chloride ions increase the conductivity of the pore solution and carbonation decreases it. An increased resistivity is accompanied by a reduced corrosion rate [38]. Table 2.4 shows resistivities determined for mature concrete in various chmates [39-41]. 

Other properties of concrete. For example, for concrete made with a certain type of cement, correlations between the water permeability and the diffusion coefficient of chlorides, that is between K and D, can be established. Nevertheless, when variations are made in the type of cement used, for example, changing from Portland cement to blended cement, the relationships are no longer valid (D may vary even two orders of magnitude with no corresponding variation in K [3]). 

Analogously, correlations exist between the coefficient of water permeabihty k and that of capillary absorption S, but these lose their validity if the surface of the concrete is subjected to a hydrophobic treatment, which will reduce considerably capillary absorption but not permeation. For concrete obtained with Portland cement, correlations between the coefficient of water permeability (k) and conductivity (o = 1/p) measured for a given value of relative humidity are available. On the other hand, conductivity varies greatly in concrete made with blended cements or carbonated concrete, while there is no significant change in water permeability. 

A. Larbi, A. L. A. Fraay, ]. M. Bijen, The chemistry of the pore solution of silica fume-blended cement systems , Cement and Concrete Research, 1990, 20, 506-516. 

Corrosion rate of rebars from linear polarization resistance and destructive analysis in blended cement concrete after chloride loading , 15 International Corrosion Congress, Granada, 22-27 September 2002 (CD-ROM). 

R. B. Polder, Simulated de-icing salt exposure of blended cement concrete - chloride penetration , Proc. 

Redistribution of chloride in blended cement concrete during storage in various climates , 3rd RILEM Int. Workshop Testing and Modelling Chloride Ingress into Concrete, Madrid, 9-10 September 2002, to be published. 

With regard to protection against sulfate attack, the quality of the concrete is a crucial factor a low permeability is the best defence against this type of attack, since it reduces sulfate penetration. This can be obtained by decreasing the w/c ratio and using blended cement (i. e. pozzolanic or blast furnace slag cement that reduce the calcium hydroxide content and refine the pore stracture of the matrix). Finally, the severity of the attack depends on the content of CjA and, to a lesser extent, of C4AF in the cement. Standards in different countries provide for sulfate resistant cements with a C3A content below 3-5 %. 

Prevention. In conclusion, to prevent ASR it is preferable to use non-reactive aggregate, low alkali Portland cement or blended cements with sufficient amounts of fly ash or slag, such as GEM II/B-V (> 25 % fly ash), GEM III/B or GEM III/A with >50% slag [23]. However, it should be noted that it is not always possible to avoid using reactive aggregates, because of the limits of local availability. In addition, methods for the evaluation of aggregate reactivity are sometimes difficult because... 

This is the reaction of main interest, espedally for concrete made of Portland cement, even though the carbonation of C-S-H is also possible when Ca(OH)2 becomes depleted, for instance by pozzolanic reaction in concrete made of blended cement [1]. 

CO2 concentration. The concentration of carbon dioxide in the atmosphere may vary from 0.03% in rural environments to more than 0.1% in urban environments. Comparatively high concentrations can be reached under specific exposure conditions, such as inside motor vehicle tunnels. As the CO2 content in the air increases, the carbonation rate increases. Accelerated tests carried out in the laboratory to compare the resistance to carbonation in different types of concrete show that, indicatively, one week of exposure to an atmosphere containing 4% CO2 will cause the same penetration of carbonation as a year of exposure to a normal atmosphere [8]. Some researchers suggest that with a high concentration of CO2 the porosity of carbonated concrete is higher than that obtained by exposure to a natural atmosphere, particularly if the concrete has been made with blended cement or has a high cement content However, this is controversial, since it was shown that even 100 % CO2 under increased pressure, produced the same microstructure as natural carbonation [9]. 

---