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High alumina cement


High alumina cement developed to resist the sulfate attacks. Then it was used as a rapid hardening cement. High alumina cement produced from limestone or chalk and bauxite. The bauxite consists of hydrated alumina, oxides of iron and titanium with a small amount of silica. The raw material will be crushed and heated to a temperature of 1600 c, then it is cooled and grounded. The high alumina cement is considered more costly compared to rapid hardening cement (type III). 


Figure 1

The table no:1 shows the typical values of oxide composition for high alumina cement.

Table 1

The high alumina cement is considered good in resisting sulfate attack. The good resistance of sulfate attack attributed to the absence of Ca(HO)2 in the product of hydration. However, lean mixes are less resistant to sulfate attack, and the chemical resistance decreases drastically after conversion (crystalline re-arrangement). High alumina cement gains almost 80% of its strength within 24 hours and some time with 6 to 8 hours. The rapid hardening of cement produces a high rate of heat development. The high rate of heat development is 2.5 times the heat developed by rapid hardening cement type III. 

The rapid hardening of high alumina cement does not accompany by rapid setting. In fact, high alumina cement is a slow setting, but the final setting occurs rapidly. The setting time of high alumina cement is affected by the addition of plaster, lime, portland cement, and HAC. The mixing of cement and high alumina cement may lead to a flash setting. 
The conversion of HAC may result in increasing the paste porosity. The porosity increase by the increase of w/c and the decrease of temperature. Even with low w/c, the conversion of HAC increase the paste porosity, which decreases the resistance of chemical attack. Therefore HAC is no longer used for structural works below or above the ground. But it is considered as valuable repairing material for the short term.

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