Calcined clay

The interest in use of calcined clay as a supplementary cementitious material in the manufacture of concrete has been growing in recent years, particularly as an alternative means of reducing the embodied carbon of concrete. 

Calcination

Calcination is a process by which a mineral is heated to a high temperature, but below that at which it would melt, which causes a change in the chemical composition. Calcined clay, therefore, is a clay that has been heated and becomes reactive in the presence of Portland cement.  This is a pozzolanic reaction, similar to fly ash, where the calcined clay reacts with calcium hydroxide from the Portland cement and becomes cementitious. Each clay will have an optimal calcining temperature for maximum reactivity. 

Clays – definition 

Clays are naturally occurring materials that consist of a number of very finely-graded minerals. Their composition can vary widely depending on location which has a large effect on their chemical and physical properties. Some of the common minerals found in varying proportions in clays are montmorillonite, sepiolite, illite, and kaolinite. 

Kaolin 

One type of calcined clay that has been in use for decades is metakaolin. The properties of metakaolin are well understood especially the benefits to concrete durability. The kaolinic clays are those that have a high proportion of kaolin available to form metakaolin when calcined. Metakaolin may be combined with Portland cement clinker at the cement factory to produce cements such as CEM II/B-Q, or at the concrete plant as an addition to form an equivalent combination such as CIIB-Q. 

Other clays 

The effect of calcining other non-kaolinic clays is less well understood however three recent studies by the MPA / University of Dundee, HS2 / Arup / SCS , and the Eureka Consortium (Imperial College, University of Leeds, BGS) have shown that these clays can be highly reactive and therefore potentially useful as an addition in concrete or combined with Portland cement clinker to make lower carbon cements. 

Manufacture 

There are two methods of calcining clays. Rotary calciners are similar to cement kilns in that the material is heated to between 650 and 900 degrees C as it travels along a long rotating drum. Cement kilns can be converted to rotary calciners so existing infrastructure may be utilised. Precise temperature control is more difficult in a rotary kiln so making optimised calcination more difficult. 

Flash calciners heat the clay very rapidly and to within a narrow temperature range so are more energy efficient than a rotary calciner so can produce more reactive calcined clay. As existing infrastructure cannot be converted to a flash calciner, the initial CAPEX required is greater than the conversion of a cement plant to a rotary calciner. 

The final colour can be controlled in both manufacturing methods so that the final product is closer in colour to Portland cement than to the red of clay bricks. 

Mechanochemical 

Research has shown that clays can become reactive by using mechanochemical activation. This is where mechanical energy such as grinding or vibration is used to transform the material. This process uses significantly less energy than calcining through applied heat and so has the possibility to produce reactive clays with very low embodied carbon. 

What is allowed in standards 

Calcined clays are one the materials that are classed as “natural calcined pozzolana” in the European standard for cement, EN 197-1. This allows cement producers to make a range of cements with up to 55% of the Portland cement clinker replaced by calcined clay. There is not currently a European standard for natural calcined pozzolanas however, as the UK produces concretes with cementitious materials (combinations) mixed together at the concrete plant that are equivalent to cements, there is a British Standard, BS 8615. 

Calcined clays that conform to the requirements of BS 8615 may be used in concretes conforming to BS 8500. 

The cement types for which there is full guidance in BS 8500, including for durability, are: