Arthur Harrisson - Clinker Microscopy

Arthur Harrisson - Clinker microscopy 

Arthur Harrisson trained as a geologist and worked for the British Geological Survey for five years before joining the Research Department of Blue Circle Cement. There he developed an expertise in cement chemistry and cement clinker microscopy. He has since occupied the positions of plant chemist, quality and environmental officer and chief chemist For Rugby Cement (now CEMEX) and Buxton Lime and Cement (part of Tarmac).

Since 2010 he has operated a microscope laboratory in North Wales where he has examined clinkers from all around the World.

Arthur also uses his experience with cement raw materials to assess the quality of potential reserves and to assist in Due Diligence operations prior to major sales but this note refers to the examination of clinker by optical microscopy. This is a very brief summary of the possibilities for operational and quality improvements using regular microscopic examination. Further information can be found at the website www.arthurharrisson.com , in the monthly articles written for the International Cement Review or chapter 4 in the latest, 5th edition of Lea's Chemistry of Cement and Concrete.

Capabilities

Clinker Microscopy

Examination of samples of cement clinker. A typical examination encompasses the following:

• Examination of polished section of the clinker and recording under the headings in the form below.

The crystal sizes and shapes provide evidence of the burning conditions in the kiln.

The type of belite present depends among other things on the maximum temperatures reached and the cooling regime.

Clusters of crystals frequently indicate the efficiency of raw meal preparation.

The form of C₃A may give evidence of possible reducing conditions.

Corroded alite may mean very slow cooling or reducing conditions depending on other features.

On slow cooling, pinheads of belite can appear in the matrix (C₃A and C₄AF).

Magnesium oxide has a number of influences on clinker including the formation of periclase (MgO). It also influences the polymorph of alite which survives cooling and this may have an effect on its reactivity.

• Crystal point counting and size distributions

Polished section of clinker with blue alite, brown belite and white or grey matrix.

The quantity of each clinker phase as calculated by the Bogue formula does not give an accurate measure of the crystal proportions actually present in a clinker. This is because it takes no account of the minor constituents of the clinker which act as substitutes for CaO or SiO₂ in the alite and belite crystals. In combination with the shapes, relative positions in the clinker nodules, the extent to which the crystals are clustered and the size distribution, a fuller picture of the conditions through which the kiln feed has travelled can be provided.


Using software provided by Petrog a polished section of clinker is moved under the microscope objective on a grid pattern. In point counting the crystal beneath the cross wires is logged and the stage moves to the next position. It is alternatively possible to select the alite or belite crystal nearest to the centre of the field of view and measure it before moving to the next position. When the required number of crystals have been measured the data is transferred to a spreadsheet where the size distributions can be analysed.

The example above shows the size distribution of 100 belite crystals from a clinker. The main peak has very fine crystals related to the length of time the raw materials and fuel ash spent in the burning zone. The secondary peak above 20 microns relates to clusters of coarser crystals on the site of a coarse grain in the kiln feed. These are a feature of most clinkers at a low level, but if this peak is significantly larger than here it would demonstrate an issue with milling silica which will inhibit combination in the kiln.

Contact:
For any further information on clinker examinations or to request a quotation please contact arthur.harrisson@gmail.com