Chemical and mineralogical composition of the alumina cement

The composition of aluminous cement. Aluminous cement quick-called hydraulic binder consisting mainly of calcium aluminate and nizkobsnovnyh obtained by pulverizing the fired to the melting or sintering raw mix of lime (limestone) and bauxite. According to GOST 969-77, in the calcium aluminate cement can be administered to 1% additive, not impair its quality.

Aluminous cement is increasingly used in the construction both in pure form or as a component of expanding, fast setting, refractory and other special binders. Sometimes aluminous cement is introduced to an acidic domain of 20-30% of granulated slag. This improves some of the technical properties of alumina cement (exotherm reduction, shrinkage and al.), And reduces its cost. A variety of alumina cement is also an alumina-belite cement.

The chemical composition of aluminous cement is subject to significant fluctuations. Thus, the content of basic oxides within the following ranges,%: -30-50 A1203; CaO - 35-45; SiO, -5-10; Fe203 - 5-15. The mineral composition of aluminous cements is predominantly represented nizkoosnovnymi calcium aluminate CaO-A1203 and CaO-2A1203 that and determine its properties as a rapid-binding agent. This plays a major role odnokal-tsievomu aluminate CA; number 12SaO-7A1203 and CA2 in conventional cements is low; the first of them is present in vysokoizvestkovyh cements, second - in maloizvestkovyh used in the production of concrete and refractory products. Odnokaltsievy aluminate is mixed with water, hydrate and ruetsya, hardening, it gives high strength rock. Dvenadtsatikaltsievy semialyuminat, which is sometimes represented by the formula 5SaO-ZA1203, mixed with water sets and hardens quickly. Its strength reaches a significant magnitude at the beginning and decreases over time. Odnokaltsievy dvuhalyuminat contained in conventional cements within about 20 to 30 ° / o, characterized hardening high strength, but relatively slow growing. Usually the aluminous cements contain a small amount of p-C2S, characterized, as is known, slow hardening and gehlenite 2CaO Al203-Si02, hardly reacts with water at ordinary temperatures. These components degrade the binding properties of alumina cement, and features geleyit that tying alumina h inert substance, reduces the amount of active calcium aluminate in the binder, so SiCh content "should be restricted to raw materials beyond the minimum (4-5%).

Iron oxides in the alumina cement is usually presented in the form of a solid solution of C,; A2F to C2F. Undesirable presence of CaO-Fe203 and free iron oxides, non-astringent. Finally, the alumina cement a small amount of magnesium oxide, typically in the form of magnesium spinel MgO-Al203, and sometimes periclase MgO or okermanita 2Ca0-Mg0-2SiQ2. In the formation of spinel degrades the quality of cement, as in the alumina binds inactive compound. Is undesirable as the presence of alkali and sulfur compounds, which deteriorate the properties of the cement.

Areas of use alumina cement

Aluminous cement strength (GOST 969-77) is divided into grade 400, 500 and 600, determined by the test results in compression size prisms halves 4X4X16 cm, made of low ductility of a 1: 3 (GOST 310.1-76 rev.) And tested through 3 days hardening. After 1 day of hardening cement 80-90% three-day prochnosti.Dlya hardening alumina cement are most favorable water conditions. Both air and combined air-wet storage is accompanied by a significant drop in the strength of concrete in this cement in the late periods of hardening (at 50- 60% in 10-20 years). Durability decreases sometimes willows first month hardening. GOST 969-77 not allowed to decrease the tensile strength of samples 28-sous-exact age strength compared to three-day-aged samples more than 10%. At low temperatures (5-10 ° C), aluminous cement hardens quite rapidly due to significant heat release, as indicated on ranee.Betony alumina cement are characterized by high resistance to water, frost and heat resistance. The water resistance of cement due, in particular, the lack of products of hydration calcium hydroxide, characterized by, as is known, a considerable water solubility (1.2 g / L at ordinary temperature CaO).