Cement autoclaved

Kilchoanite C3S3 Ca,[Si0J[Si30,J Geothermal cements, autoclaving [172, 173]... 

Silica and Alumina. The manufacture of Pordand cement is predicated on the reaction of lime with siUca and alumina to form tricalcium sihcate [12168-85-3] and aluminate. However, under certain ambient conditions of compaction with sustained optimum moisture content, lime reacts very slowly to form complex mono- and dicalcium siUcates, ie, cementitious compounds (9,10). If such a moist, compact mixture of lime and siUca is subjected to steam and pressure in an autoclave, the lime—silica reaction is greatiy accelerated, and when sand and aggregate is added, materials of concrete-like hardness are produced. Limestone does not react with siUca and alumina under any circumstances, unless it is first calcined to lime, as in the case of hydrauhc lime or cement manufacture. 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

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). 

Beaudoin, J. J. Feldman, R. F. (1975). Mechanical properties of autoclaved calcium silicate systems. Cement and Concrete Research, 5 (2), 103-18. 

The binders are silica, lime, slag, or cement. The balls are somewhat dried, if necessary, and then cured in steam autoclaves. During the hydrothermal treatment lime and silica react to form hydrosilicate gels, which act as binders. 

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Fig. 8.8 Relations between compressive strength (log scale) and porosity data from various sources for Portland cement pastes cured at ordinary temperatures (open circles), hot pressed Portland cement paste (filled circle), autoclaved pastes of Portland cement, sometimes with added sulphur (filled squares) and autoclaved pastes of Portland cement with 50% (open squares) or 30"/o (cross) of added pfa. After Feldman and Beaudoin (F36).

The rest of the chapter deals with the hydration chemistry of Portland and composite cements at temperatures outside the range 15-25 C, including that of autoclave processes, and with specialized uses of cements in casing oil wells and in making very high strength materials. 

Fig. 11.6 Effects of partial replacement of cement by finely ground silica on the compressive strengths of mixes cured normally and in the autoclave. After Mcnzcl (MHO).

Understanding of the chemistry of autoclave processes is due primarily to the work of Kalousek and co-workers (K32,K59-K62). Above about I. SO C. for the time scales of a few hours that are used in practice, two features of cement hydration chemistry are added to those relevant at lower temperatures. Firstly, the hydration products tend to crystallize in the absence of reactive silica, C-S-H tends to be replaced by a structurally unrelated, crystalline phase, a-CjS hydrate. Secondly, the range of siliceous materials having effective pozzolanic properties is widened, and includes quartz and various other crystalline minerals, if sufficiently finely ground. 

Curing conditions play an important role in the composite behavior, which has been attributed primarily to the influence of curing on the matrix properties. Two curing methods that are generally used in fiber cement composites are air/moist curing (normal pressure and temperature) and autoclave curing (high pressure and temperature). 

Comments the commercially available 5% w/v chlorhexidine gluconate solution contains a nonionic surfactant to prevent precipitation and is not suitable for use in body cavities or for the disinfection of surgical instruments containing cemented glass components. Aqueous dilutions of commercially available chlorhexidine gluconate solutions may be sterilized by autoclaving. See Sections 11 and 12. 

PROPERTIES OF AUTOCLAVED CEMENT PASTE CONTAINING SCRAP FRP POWDER... 

Cement paste specimens 40x40x 160mm were molded, and subjected to autoclave cure (160°C, lOhr.) after 7 days in 20°C, 85%R.H. 

According to JIS R 5201, the autoclaved cement paste specimens were tested for flexural and compressive strengths, and their bulk specific gravity were also measured. 

The conclusion obtained from the test results on the properties of autoclaved cement paste containing scrap FRP or glass powder are summarized as follows. 

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