High Alumina Cement (HAC) is also known as calcium aluminate cement (CAC) or aluminous cement, is composed of calcium aluminates. High Alumina Cement was first manufactured by Lafarge, the cement producer, and became available in the UK in 1925.
It has good resistance to be resistant to chemical attacks which are required in marine construction work. It is able to gain high early strength which allows faster construction work. High alumina cement mostly used to manufacture structural concrete such as pre-cast beams.
High Alumina Cement Manufacturing (IS 6452 : 1989):
High alumina cement is manufactured by sintering or fusing a mixture in suitable proportions, of alumina and calcareous materials and grinding the resultant product to a fine powder. There are two main elements used to make high alumina cement are limestone and bauxite. These two ingredients were charged into the furnace.
The furnace is fired with pulverized coal or oil with a hot air blast. The fusion process in the furnace generally takes place at a temperature of about 1550-1600°C. The cement is maintained in a liquid state in the furnace. Then after the molten cement is filled in moulds and cooled.
These castings are known as pigs. After cooking this molten cement it looks like a dark, fine gey compact rock resembling the structure and hardness of basalt rock. The pigs of fused cement, after cooling are crushed and then ground in tube mills to a fineness of about 3000 sq. cm/gm.
Characteristics of High Alumina Cement:
- It is very resistant to chemical attacks.
- The pH level is low.
- High resistance to chemical corrosion, due to which it is used for the construction of water pipes, sewage pipes, factory drains, coastal constructions, and factory chimneys.
- This cement has a high refractive index.
- It has high durability in sulphuric acid.
- Hardening property of this cement is fast.
Hydration of High Alumina Cement:
The important reaction during the setting of the high alumina cement (HAC) is the formation of monocalcium aluminate decahydrate (CAH10), dicalcium aluminate octahydrate (C2AH8) and alumina gel (Ahn).
These aluminates provide high strength to HAC concrete but they are metastable and at normal temperature convert gradually to tricalcium alumina hexahydrate (C3AH6) and gibbsite which are more stable.
The composition change may result in a loss of strength and by a change in crystal form from hexagonal to cubical form with the release of water which results in increased porosity of concrete.
Changes that take place depending on the temperature, water/cement ratio, and chemical environment. The composition changes by loss of strength and change in crystal form from hexagonal to cubic shape are known as conversion.
Experimental evidence suggests that in the important reaction of the conversion from CAH10 to C3AH6 and alumina hydrate, temperature affects the decomposition.
The higher is the temperature, the faster the rate of conversion. Experimental studies have also shown that the higher the water/ Cement ratio, the greater is the rate of conversion. Hydration and conversion can be shown as follows:
It should be noted that this reaction liberates all the water needed for the conversion process to continue. The conversion reaction will result in a reduction in the volume of the solids and an increase in the porosity since the overall dimensions of specimens of cement paste or concrete remain sensibly constant.
Read More: What Is Cement And History Of Cement
High Alumina Cement Concrete:
The high alumina cements concrete first used in the United Kingdom in 1925. Afterward, it is used in France where it had been developed earlier to make concrete resistant to chemical attack, particularly in marine conditions. The high alumina cement is able to develop a high early strength offers advantages in structural use.
However, its high cost redistricts extensive use of high alumina cement for structural purposes. All the same during the 1930’s many structures were built.
European countries using high alumina cement. Following the collapse of two roof beams in a school at Stepney in the U.K. in February 1974, the Building Research Establishment U. K. started field studies and laboratory tests to establish the degree of risk likely in buildings with precast prestressed concrete beams made with high alumina cement.
The results of the BRE investigations are summarized below:
1. Measurements of the degree of conversion of the concrete used in the buildings indicated that high alumina cement concrete reaches a high level of conversion within a few years. In this concrete sample taken from beam indicates that some concrete suffered a substantial loss of strength when compared to one day strength on which the design was earlier based,
2. Long term laboratory tests have shown that:
(a) If concrete with a free water/cement ratio less than 0.4 is stored in water at 18°C throughout its initial curing period and its subsequent life, it took five years to reach minimum strength and this minimum will not be appreciably less than the strength at one day.
(b) It is come to know that if this concrete is stored in water at 38 C temperature and day after at 18 C, it converts rapidly to high limit and reaches a minimum strength in about 3 months which is very substantially less than the strength at one day.
c) if the concrete is stored in water at 18 °C for a long period (up to 8 years) and is immersed in continuous storage at 38 °C it will rapidly convert and lose strength to the minimum level, reached for since the temperature at 38°C.
d) Since the temperature at 38 °C represents an upper limit of what is likely to be reached during curing of these sections or in a normally heated building. It is suggested that the design should be based on the minimum strength at this temperature.
e) Highly converted high alumina cement concrete is vulnerable to the chemical attack in the presence of long term wetness and a chemically aggressive agent, which ma serious risk for concretes with greater water/cement ratio.
One of the most advantages of high alumina cement concrete is the very high rate of strength development. By this method, 20% of strength is achieved by curing of day. It is also capable to achieve substantial strength even at 6 to 8 hours.
High Alumina Cement Price:
High alumina cement price is 150 dollars to 250 dollar per Ton.
Uses of High Alumina Cement:
- It is mostly used in marine construction and sewer infrastructure due to its early strength gaining property.
- It is also extensively used in making refractor concrete where it has to deal with high temperatures.
- It is widely used for the manufacturing of concrete and pre-cast concrete beams.
Read More: Which Cement Is Best For House Construction
Advantages of High Alumina Cement:
The advantages of High Alumina Cement are as follows,
- It has more setting time and therefore it required more time for mixing and placing.
- This cement resistance to chemical action is good to compare to others.
- It can withstand high temperatures.
- In cold weather conditions, frost action is reduced due to heat evolved is more during setting.
- It is highly reactive and has a very high compressive strength.
- Good resistance to fire.
Disadvantages of High Alumina Cement:
Following are the disadvantages of high alumina cement,
- Due to it requires high temperatures involved in making its manufacturing cost of cement is also high.
- Cement has a fineness of less than 2250 cm2/gram, which is very fine. Cement can damage the human eye and mouth so high precaution is required.
- It produces high temperatures during the setting, it cannot be used in mass concreting works.
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