Expansive cements

Fig. 9.18 Effect of free CaO content on the K type cement expansion, (according to [93])...

In clinkers and cements, expansive hydration of free lime produces popcorn or cauliflowerlike crystals of calcium hydroxide, called epezite, surrounding the original particle of free lime (see Brown and Swayze, 1938). Formation of epezite via air slaking normally causes clinker disintegration. 

Numerous construction products are formulated from asphalt and coal tar for such applications as driveway sealers, cutback asphalts, flashing cements, conerete primers, concrete cold mixes, roof cements, expansion joint fillers, patch liquids, waterproofing liquid-applied membranes, and pipeline eoatings. All these produets are likely to contain... 

Odler, L, and Colan-Subauste, J. (1999) Investigations on cement expansion associated with ettringite formation. Cement and Concrete Research 29,731-735. 

Expansive cements Expansive cements, in addition, Portland clinker, expansive clinker and are designed to offset the effect of the constituent materials. 

Heng,ShengTao Tasek Cement expansion is vital to Malaysian growth. — In Rock products 1977/128 — 138. 

Calcium chloride also has an effect on the hydration of various other cementitious systems such as pozzolanic cements, slag cements, expansive cements, high alumina cement, g q)sum, rapid hardening cement, etc. (See, for example. Refs. 1 and 40.)... 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

Thermal Fatigue. Cemented carbide tools sometimes exhibit a series of cracks perpendicular to the tool edge when appHed in intermpted cutting conditions such as milling. These thermal cracks are caused by the alternating expansion and contraction of the tool surface as it heats while cutting... 

In addition to chemical analysis a number of physical and mechanical properties are employed to determine cemented carbide quaUty. Standard test methods employed by the iadustry for abrasive wear resistance, apparent grain size, apparent porosity, coercive force, compressive strength, density, fracture toughness, hardness, linear thermal expansion, magnetic permeabiUty, microstmcture, Poisson s ratio, transverse mpture strength, and Young s modulus are set forth by ASTM/ANSI and the ISO. 

In North America, a special, high conductivity, low permeability, "hot-pressed" carbon brick is utilized almost exclusively for hearth walls. Because of their relatively small size and special, heat setting resin cement, and because the brick is installed tightly against the cooled jacket or stave, differential thermal expansion can be accommodated without refractory cracking and effective cooling can be maintained. Additionally, the wall thickness is generally smaller than 1 m, which promotes the easy formation of a protective skull of frozen materials on its hot face. Thus hearth wall problems and breakouts because of carbon wall refractory failure are virtually nonexistent. 

Other Phases in Portland and Special Cements. In cements free lime, CaO, and periclase, MgO, hydrate to the hydroxides. The in situ reactions of larger particles of these phases can be rather slow and may not occur until the cement has hardened. These reactions then can cause deleterious expansions and even dismption of the concrete and the quantities of free CaO and MgO have to be limited. The soundness of the cement can be tested by the autoclave expansion test of Portiand cement ASTM C151 (24). 

The expansive component C A SI in Type K expansive cements hydrates in the presence of excess sulfate and lime to form ettringite is... 

ASTM C845 Type E-I (K) expansive cement manufactured ia the United States usually depends on aluminate and sulfate phases that result ia more ettriagite formation duriag hydration than ia normal Portland cements. Type K contains an anhydrous calcium sulfoaluminate, C A SI. This cement can be made either by iategraHy burning to produce the desired phase composition, or by intergrinding a special component with ordinary Portland cement clinkers and calcium sulfate. 

Jiir-entraining cements produce concretes that protect the concrete from frost damage. They are commonly used for concrete pavements subjected to wet and freezing conditions. Cement of low alkah content may be used with certain concrete aggregates containing reactive siUca to prevent deleterious expansions. 

Veneering Investments. These are phosphate bonded and contain finely ground quart2, 2irconium oxide, and/or titanium oxide to produce highly refractory, low expansion dies of fine detail. The dies are formed within impressions taken of teeth that the dentist has prepared in anticipation of covering the front surface with an aesthetic ceramic veneer. Porcelain or ceramic powders are shaped to detail on the dies and these are fired at high (- 1000° C) temperatures to produce the veneers. The veneers are then cemented to the front surface of the previously prepared teeth. 

The coefficients of expansion of iron and cement are nearly ahke. Table 10-39 gives dimensions of cement-lined pipe. 

Interna] Insulation The practice of insulating within the vessel (as opposed to applying insulating materials on the equipment exterior) is accomplished by the use of fiber blankets and hghtweight aggregates in ceramic cements. Such construction frequently incorporates a thin, high-alloy shroud (with slip joints to allow for thermal expansion) to protect the ceramic from erosion. In many cases this design is more economical than externally insulated equipment because it allows use of less expensive lower-alloy structural materials. 

Thermal oxidizers must be built to provide the residence time and temperatures to achieve the desired destruction efficiency (DE). As such, thermal oxidizers are comparatively larger than catalytic oxidizers since their residence time is two to four times greater. Historical designs of thermal oxidizers were comprised of carbon steel for the outer shell and castable refractory or brick as the thermal liner (a refractory is like a cement, which is put on the inside of the rector shell to act as a thermal insulation barrier). Modern units are designed and built using ceramic fiber insulation on the inside, which is a lightweight material, and has a relatively long life. Old refractory would tend to fail over a period of years by attrition of expansion and contraction. 

Air pockets or bubbles are left on the surface of all concrete. Good vibration and placing techniques will reduce their number but not eliminate them. Many air pockets have a small opening on the surface in relation to their size. Paints will not penetrate into such holes, with the result that air or solvent is trapped and subsequent expansion will cause the coating to blister. In addition, some air pockets are covered with a thin layer of cement that also has no strength and will cause loss of adhesion. 

These cements have marked creep characteristics and flow under pressure even when fully set. In this they contrast markedly with the rigid phosphate cements (Wilson Lewis, 1980). This plastic behaviour explains why such cements provide a good seal despite a high setting shrinkage and thermal expansion of 35 x 10 °C" (Civjan Brauer, 1964). 

A series of test methods and procedures have been developed to measure these phenomena [67]. Cracks should be avoided because these increase the permeability of the cement. The expansion of the cement, without the formation of macro-fissures, depends on the time at which the expanding... 

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