Showing papers on "Calcium aluminates published in 1991"

Residual mechanical properties of heated concrete incorporating different pozzolanic materials

Abstract: This paper aims at showing the effect of high temperatures on mechanical properties of concretes in which Portland cement (PC) has been partially replaced by pozzolanic materials Three types of pozzolanic material, one natural pozzolana and two lignite fly ashes (one of low and one of high lime content) were used for the replacement Concrete specimens were tested at four temperature levels of 200, 400, 600 and 800°C without any imposed load The specimens for each of the chosen temperatures were heated under the same heating regime, so a comparison of their behaviour during heat exposure concerning their mechanical properties was possible Tests of compressive strength, splitting tensile strength and modulus of elasticity were carried out on specimens cooled slowly to room temperature 1 day, 7 days, 1 month and 3 months after heating Based on the results of this experimental work it can be said that concretes with pozzolanic materials added to the mixer in partial replacement of PC are more sensitive to exposure to fire than conventional concretes A relatively greater drop in the strength of the concretes with pozzolanic materials was found especially in the temperature area of 200°C when they were compared with conventional concrete made from PC of Greek type (10% insoluble residue) This particular sensitivity was attributed to heat deformations of hardened pastes, as all the other testing parameters concerning consistency and rate of heating remained the same for all specimens It is suggested that this is caused by the escape of the non-evaporable water that exists as combined water in hardened pastes of pozzolanic binding agents These pastes are richer in compounds such as calcium aluminates, calcium aluminate sulfates and tobermorite gel (C-S-H) which are decomposed between 110 and 150°C

The direct bonding between AlN ceramics and CaO-doped tungsten

Abstract: The bonding of AlN ceramic substrates with tungsten can only be achieved by incorporating CaO into the metallic powder. This oxide improves the sintering process of W and leads to calcium aluminates at the AlN/W interface, whose enrichment in CaO through the metal gives compositions CA, C 12 A 7 and C 3 A with decreasing eutectic temperatures and viscosity. This allows a liquid phase to diffuse at firing temperatures. A good adhesion needs: (i) a CaO content threshold to form substantial amounts of liquid phase, which depends on the amount of alumina available from the nitride surface; and (ii) temperatures above 1490°C, a eutectic temperature in the system CaO/WO 3 , because the solid Ca tungstate film simultaneously formed near the W/AlN interface may prevent Ca aluminate from diffusing when not displaced towards the metal core by means of a dissolution-precipitation mechanism.

High-temperature resistant, porous aggregates, process of producing the same and moulded parts made therefrom

Abstract: A high-temperature resistant, porous aggregate, a process for producing the same and products made therefrom are disclosed, which are useful to prepare highly effective heat insulators available as mixtures or as moulded products that have applications in the construction of industrial ovens and of high-temperature installations. The aggregate on the basis of calcium aluminates and/or magnesium aluminates and/or their mineral derivatives having one or several components contains 0.20 to 0.95 parts in volume of micropores, whereas more than 60 % of the micropores have less than 40 νm in diameter. The porous aggregate is produced by mixing the corresponding oxidic or hydroxidic finely ground raw materials with 0.1 to 4.0 parts in mass of water, in relation to one part in mass of dry raw materials. The homogeneous mixture is then treated by autoclave at temperatures higher than 125 °C, for 1 to 25 hours, the solidified mass is dried, then sintered at temperatures equal to or higher than 1000 °C for 1 to 25 hours and the sintered product is transformed into aggregates.

Polymer-cement composition

Abstract: PURPOSE:To shorten handling time by blending Portland cement with calcium aluminates each having a specified compsn. and a polymer dispersion and using the blend as a base. CONSTITUTION:Calcium aluminates represented by the formula (where (x), (y) and (z) are the mol numbers of the components, X is halogen and (x:y:z)=1:(0.8-6):(0-2) are added to Portland cement by 2-50 pts.wt. per 100 pts.wt. of the cement and a polymer dispersion contg. one or more kinds of rubber latexes or resin emulsions of a styrene-butadiene polymer, a vinyl acetate polymer, etc., as polymer components is added by 2-300 pts.wt. (expressed in terms of solid matter). They are mixed and inorg. sulfate such as anhydrous gypsum, a setting regulating agent such as tartaric acid, aggregate and a fibrous material are further added to the mixture as required. The mixture is then heated and cooled to obtain a polymer-cement compsn.

Hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates

Abstract: Water-resistant hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates. The said binder contains, in effective quantity, additives capable, when it sets, of inhibiting the formation of hydrated calcium aluminates at the expense of the crystallisation of ettringite in tabular form.