113. Production of alumina cement

Alumina cement - bystrotverdeyushchie normally settable binder obtained by pulverizing the fired to the fusion (t = 1500-1600 ° C) or sintering (t = 1250 ° C) mixture of bauxite and limestone (lime) with predominance in the finished product of calcium aluminate.

The main mineral aluminous cement is Odnokaltsievy aluminate CaO * Al203. For aluminous cement production process of sintering finely ground and thoroughly blended feed mixture of bauxite and limestone fired in shaft or rotary kilns.

This cement is available in three grades 400, 500 and 600. When the hardening of aluminous cement in a short time, a large amount of heat. (376 kJ / kg). This leads to a significant increase in the temperature of the stone and can be useful in the management of work in winter. However, a strong increase in temperature causes cracking of concrete arrays. aluminous cement density of 3100-3300 kg / m3 bulk volume weight in the loose state of 1000-1300 kg / m3.

Concrete on alumina cement water resistant vozduhostoyki, frost-resistant. Aluminous cement is used in high-speed construction, emergency work, winter concreting. The high heat resistance of alumina cement, allows to make concrete, successfully operating at t up to 1700 ° C. Odnokaltsievy aluminate (20- 30%) is dominated by the mineral composition of clinker aluminous cement, defining the basic properties of the binder: CaO • Al2O3 (CA) In addition, it contains aluminates CA2, S12A7; dicalcium silicate C2S

114. 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.