Cements needle

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

The apparatus shown in Fig. 3.5 was used, and was filled completely with cement paste. Admixture additions were made by injecting with a hypodermic needle through the rubber cap. Pastes prepared in this way had air contents of less than 0.6% by volume, were free from any premature stiffening tendencies and were homogeneous. 

CiiA CaFj and jet cements, including the effects of various admixtures. A wide variety of methods was used. For the two cements mentioned above, hydrated in pastes at 20 C, the main product at 3 h was ettringite. At this stage, 40-45% of the C,iA7 CaF2 had reacted. SEM showed needles of ettringite, 0.5-1.0 pm thick, growing from the particles of clinker and... 

Solubility of fillers is also important in dental cements and biomaterials. Unlike in construction cements, the cost of the end product is not a major issue in dental cements, but purity, biocompatibility, and performance of the final product is very important. Each filler component has its own role in modifying the behavior of the final product. As we shall see later in Chapter 18, wollastonite improves the flexural strength and toughness of the dental cements because the grains of wollastonite have elongated needle shape (acicular). Hydroxyapatite is added to provide biocompatibiUty to the product. Attributes of these fillers govern the final properties of the products and hence it is necessary that, not only one understands detailed properties of the binder components, but also that of the additives and filler components. For this reason, this chapter is devoted to describe the genesis and properties of the binder and important filler components. 

Allen [36] mounted the powder directly into clear cement, dispersing it by using sweeping strokes of a needle and spreading the film on a microscope slide to dry. Lenz [37] embedded particles in solid medium and examined slices of the medium. 

Portland cement (containing gypsum) solidifies after 1 to 3 hours. The needle-shaped ettringite crystals recrystallize forming larger needles which bond the cement particles together and solidify the crust. 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

The product forms after 5 or 6 hours as a microscopic forest of long crystalline needles that lock together to solidify the cement. Later, the calcium silicates react with water to harden the cement. For example ... 

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