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Characteristics and Hydration of High Alumina Cement

1. About high alumina cement

High alumina cement is manufactured by sintering or fusing alumina and calcareous materials in appropriate proportions and grinding the resulting product into a fine powder. The two main elements used to make high alumina cement are limestone and bauxite. Load the two ingredients into the furnace. The stove is blasted with hot air using pulverized coal or oil. The melting process in the furnace is usually carried out at a temperature of about 1550 to 1600°C. Cement remains liquid in the furnace. Molten cement is then filled into the mold and cooled. When this molten cement is cooked, it looks like a black, fine-grained, dense rock, similar in structure and hardness to basalt. The cooled dissolving cement particles are crushed and then ground in a tube mill to a fineness of about 3000 cm2/g.

2. What are the characteristics of high alumina cement?

High alumina cement is very resistant to chemical attack, high alumina cement has a low pH value, and high alumina cement is resistant to chemical corrosion, so it is used to build water pipes, sewage pipes, factory drains, coastal buildings and factory chimneys. Aluminium cement has a high refractive index, and high-alumina cement has high durability in sulfuric acid, and the hardening performance of this high-alumina cement is fast.

3. Hydration of high alumina cement

Important reactions during the setting of high alumina cement (HAC) are the formation of monocalcium aluminate decahydrate (CAH10), dicalcium aluminate octahydrate (C2AH8) and alumina gel. These aluminates provide high strength to HAC concrete, but they are metastable and gradually convert to the more stable tricalcium alumina hexahydrate (C3AH6) and gibbsite at ambient temperature. Compositional changes can lead to loss of strength, change from hexagonal to cubic crystal form, and release of water, resulting in increased concrete porosity.

Changes in high alumina cement due to temperature, water-cement ratio and chemical environment. The composition that changes due to the loss of strength and the change in crystal form from hexagonal to cubic is called transition. Experimental evidence suggests that temperature affects decomposition in the important reactions of CAH10 to C3AH6 conversion and alumina hydrate. The higher the temperature, the faster the conversion. Experimental studies also show that the higher the water-cement ratio, the higher the conversion rate. It should be noted that this reaction releases all the water needed to continue the conversion process. Since the overall size of the grout or concrete specimen remains significantly constant, the conversion reaction will result in a decrease in solid volume and an increase in porosity.

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Characteristics and Hydration of High Alumina Cement