Portland pastes

Special purpose and blended Portland cements are manufactured essentially by the same processes as ordinary Portland cements but have specific compositional and process differences. White cements are made from raw materials of very low iron content. This type is often difficult to bum because almost the entire Hquid phase must be furnished by calcium aluminates. As a consequence of the generally lower total Hquid-phase content, high burning-zone temperatures may be necessary. Past cooling and occasionally oil sprays are needed to maintain both quaHty and color. 

C. W. Lent2, in Special Keport90, Structure of Portland Cement Paste and Concrete Highway Research Board, NRC-NAS, Washington, D.C., 1966. 

Impeller, SFW 4 Pasted valve tag 18V x27i/ (face) 470 X 700. 3-170 Insert sleeve Portland cement 94 1506 —325 mesh... 

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

A cement paste phantom was prepared from white Portland cement. A thin sheet of copper was placed in the top of the specimen at the time of casting to produce a synthetic crack 300-pm wide, 8-mm long and 3-4-mm deep. The sheet was removed the following day. The specimens were 10 x 10 x 10 mm. At the time of demolding, the specimen was slightly damaged and two cracks formed on the corners of the specimen. The thicknesses of these cracks was estimated to be 100 pm and 60 pm. Prior to the MRI measurement the samples were stored in water to ensure saturation. 

Hardened Portland cement contains a distribution of pore and capillary sizes, depending on the initial water-cement ratio and the maturity of the paste. 

The following points are relevant to the reactions occurring in pastes of Portland cement containing normal proportions of C3A, C3S, C2S, C4AF and gypsum in the presence of calcium chloride. 

Fig. 5.7 Effect of various calcium salts on the time of setting of Portland cement pastes.

In order for normal set to occur in Portland cement paste, mortar, or concrete, calcium sulfate must be present in the cement-water system. In today s cements, most of the calcium sulfate introduced into the system as a component of the cement, can be present in one or more forms gypsum (CaS04.2H20), hemihydrate (CaS04.1/2... 

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

The air-bubble generating and stabilizing process requires a minimum paste consistency. Silica fume particles are smaller than those of Portland cement and addition of silica fume therefore increases the fine fraction of the particles. The higher fraction of smaller particles then increases the surface area causing a greater binding of the water in the mix. This removes the water required for the bubble-generating process. 

Tishmack, J. K. 1999. Characterization of High-Calcium Fly Ash and Its Influence on Ettringite Formation in Portland Cement Pastes. PhD dissertation, School of Civil Engineering, Purdue University, West Lafayette, IN. 

Kundu, S., Kavalakatt, S.S., Pal, A. et al. (2004) Removal of arsenic using hardened paste of Portland cement batch adsorption and column study. Water Research, 38(17), 3780-90. 

California Portland Cenent (Arizona Portland) mill to, az 6-dry Mine 1 with PH/PC coal-fired 2 kllna Inactive In 1990. Past use 2 x2" 10X of energy from T0F TDF since 1986 No References 1 and 2... 

Southdown, Inc. Southwestern Portland Cement Co. Fairborn, OH 1 dry kiln PH FF coal fired. Past permitted use Whole 36" 10-15X use was successful and are renewing alternate fuels permit tires were slid, not rolled. Into feed end of kiln. 

Hardened Portland cement contains a distribution of pore and capillary sizes, depending on the initial water-cement ratio and the maturity of the paste. A typical distribution of pore radii in the hardened cement paste of concrete was shown in Fig. 1.40 which indicated that the majority of pores he in the region of 0.05 and 1.0 pm diameter and it is through these pores that water passes by applied pressure or capillary rise, as shown in Fig. 4.5 (a). 

Portland cement pastes. Influence of composition on volume constancy and salt resistance. Ind. Eng. Chem., 26 1049-1060. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

Preparation of Hydrated Silicates. The hydrated silicate specimens used were all in the paste form—that is, mixtures of one of the calcium silicates with a limited amount of water to form a slurry, which sets and hardens as portland cement itself does. These pastes were prepared by the vacuum mixing procedure described by Powers, Copeland, Hayes, and Mann (23), adapted so that the temperature of the mix upon removal from the mixer was the temperature at which the specimen was to be hydrated. The 5° specimens were made by starting with an ice-water mixture the-50° specimens by starting with preheated water. A manostat was incorporated into the pumping system to prevent the pressure from dropping below the vapor pressure of water at the desired final temperature. This was especially important for the 50° mixes, to prevent excessive cooling. 

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