High Alumina Cement Testing
Presence and Condition of High Alumina Cement
Determination of Cement Type
In the early seventies, a number of problems occurred with structural members manufactured from High Alumina Cement Concrete (HAC). High Alumina Cement undergoes a crystalline re-arrangement known as conversion which can result in a loss in strength, particularly if the original water/cement ratio was not strictly controlled. In order to identify HAC concrete which may be potentially at risk and to distinguish it from concrete manufactured with ordinary Portland cement, a rapid chemical test was devised by the Building Research Station. This method is detailed in BRE Information Sheet IS 15/74.
General Notes on High Alumina Cement Concrete
HAC concrete has achieved a certain notoriety following the collapse of several buildings in the 1970's. Therefore a short explanation of the nature of HAC is presented below together with details of the test procedures employed.
High Alumina Cement is manufactured from limestone or chalk and bauxite (the ore from which aluminium is obtained). The two materials are crushed and fired together using pulverised coal as a fuel. The materials fuse together and the melt is transferred into steel pans. It is finally cooled, crushed and ground into a dark grey powder.
Considerably less is known about the compound composition of high alumina cement than Portland cement. The predominant compounds are calcium aluminates; calcium silicates account for no more than a few percent. The calcium aluminates react with water and the primary product is calcium aluminate decahydrate (CAH10) which has essentially a hexagonal crystal form. This material is unstable and changes to tricalcium aluminate hexahydrate (C3AH6) spontaneously (albeit slowly). The process occurs at room temperature and is accelerated by an increase in temperature. The conversion, as it is known, is accompanied by an increase in porosity and a decrease in strength. The extent to which this occurs is largely a function of the original water/cement ratio, i.e. the effects are minimised by a low water/cement ratio. An outstanding feature of high alumina cement is its very high rate of strength development; about eighty percent of its ultimate strength is achieved at the age of 24 hours. Unfortunately in its converted form high alumina cement becomes susceptible to chemical attack, both from sulphates and from alkalis. In order to assess the condition of high alumina cement concrete it is normal to determine the degree of conversion. Other chemical tests may be carried out if the concrete shows any sign of deterioration.
Determination of Degree of Conversion
As discussed above, the calcium aluminate decahydrate form of HAC gradually converts to the hexahydrate. The two hydrated forms can be detected and quantified by the method of differential thermal analysis. The test involves placing a small quantity of the finely ground concrete in a very small metal dish which is placed on one of a pair of thermocouples contained in an electric furnace. A small amount of an inert reference material (freshly ignited alumina powder) is placed on the reference thermocouple. The atmosphere of the furnace is flushed with nitrogen gas and the temperature gradually raised to 600°C. During the heating period the instrument monitors the temperature of the sample with reference to the inert material on the second thermocouple. The water of crystallisation of the unconverted and converted forms of HAC are lost at different temperatures and the energy required to drive off the water is registered as a lagging behind in temperature rise of the sample compared to the reference. The unconverted and converted forms of HAC therefore show two distinct troughs in the DTA Plot (for convenience they are shown as peaks in the reproduction of the plots). The ratio of the height of one to the other gives the degree of conversion expressed as a percentage. i.e.
Amount of C3AH6* / Amount of C3AH6 + CAH10** x 100 = degree of Conversion %
* converted HAC
** unconverted HAC
Accuracy of Test Results
The degree of conversion test is considered accurate to ±5%. The cement content determination for HAC has never been subjected to a precision trial, but is estimated to be accurate to ±15%.