Investigations into the composition of arkalite

Paving/flooring material used extensively in parts of St. John’s Cathedral, Brisbane, is pitting and spalling in a number of places. This material is basically a reconstituted stone/cement product that was specified by a British architect and introduced into Australia for the Cathedral in 1904. By establishing the composition of this man-made product it is hoped to find, or manufacture locally, a compositionally similar product that is a satisfactory match to the existing floor.

This is a very unusual, composite, man-made product. Most of it has been derived from waste slag materials produced within a blast-furnace ladle during iron smelting operations. The glass has clearly been added at a later stage and the quartz appears as a possible impurity introduced during manufacture.

There are no clear indications as to how this material was produced. The layering and surface finish suggests some form of hot-pressing or hot-rolling after mixing with the carbonaceous and probably gypsiferous matrix. Nearly all the components are basically anhydrous and the majority of slag minerals are seriously silica-deficient. This provides some evidence as to why there has been no hydration of the glass nor the presence of an alkali-silica reaction within what was thought to be a cementitious matrix.

Duplication is likely to be tricky even though all the ingredients can be readily obtained. Crushing and blending is straight-forward but the moulding process is not understood. To produce the strength for a paving product it would be necessary to add a flux such as gypsum. This could explain the formation of jarosite (an iron sulphate formed by reaction with both the freely available iron and a calcium sulphate). The source of the potassium is unclear. Steam could possibly provide the driving mechanism for the required reactions to solidify the material (similar to the manufacture of man-made sandstone bricks) but it is more likely that it would have to be baked.