Meeting 12 March 2002

Bill started with the chemical analysis of thaumasite, CaSiO3, CaCO3, CaSO4, 15H20. He said the first obvious point was the high water content of the formula. When the mineral is wet it goes soft (mushy) and under dry conditions it reforms into needle like crystals. It has often been incorrectly described as a metamorphic mineral. Bill said that formation of metamorphic minerals requires heat and that heating thaumasite would result in it dissolving. In fact, the optimum growing temperature is around 5C and quite cool.

Originally, thaumasite was rare mineral, the first recorded locations being Scandinavia and Texas. It was first described in 1878 (I may have noted this wrong, could be 1868 ). Bill had nothing but admiration for the accurate Victorian analysis of thaumasite as it agrees with modern results using modern automatic equipment. In fact, he reminded us to beware of trusting the modern machines as they sometimes get it quite wrong! WHAT A SURPRISE! Bill said that the name thaumasite had its roots from the Greek meaning 'wonderful' hence the 'what a surprise'.

Anyhow, Bill stated that the mineral is now as 'common as muck' and occurs in tons (metric, I presume?). Where? The answer is in concrete!

Bill in his investigation into thaumasite said that he had tried growing it in the Laboratory, or more precisely in the freezer housed in his garage at home. He started by combining the individual chemical constituents according to the empirical formula and leaving them in the freezer. Thirteen months later he had a sample of crystals that looked right but sadly were not thaumasite but another calcite based mineral. He said that it takes longer for thaumasite to form under those conditions.

Formation of thaumasite in concrete occurs in locations below soil level where the concrete is lain over clay. He said that it didn't seem to matter which type of clay but locally the problems occurs where the concrete is in contact with the Lias, London Clay and Mersea Mudstone. The problem is extremely serious for the construction industry as it is destroying subsoil foundations. Notably, bridges on the M5 motorway are undergoing remedial work because of this problem.

Bill showed us many slides of thin sections of thaumasite in concrete, plus a few schematics. What was obvious was that the concrete was being cracked parallel to the clay/concrete boundary at some distance to this interface. The cracks are being filled and most probably enlarged by the growth of thaumasite crystals. From the clay/concrete boundary, and travelling into the concrete, a gypsum rich boundary again parallel to the clay/concrete is met first, then further on the thaumasite boundary is reached. Further into the concrete an Ettringite zone is encountered.

Note: I'm sorry if my clumsy description is sending you all to sleep. It was a well presented lecture.

Bill made a short digression about the chemistry of clays. He said that London Clay wasn't a real clay and suggested we try and dissolve some in Calgon and examine the gritty nature of the small particles that remain. He added that these were too big to be described as clay. (Worth a try when I go to Sheppey as I don't collect fossil crabs, lobsters, fish skulls, & sharks' teeth, but I'd love a snake vertebra, or a croc. tooth, see you there?).

He went on to say, that the problem is severe and further studies are quietly being carried out all over the country, on the damage occurring to motorways in particular. Bill believes that the clays are providing some of the chemicals. He pointed out that pyrite in the clay and close to the boundary remains unaltered. The carbon content in thaumasite has an isotopic abundance suggesting an organic source, and thaumasite forms more rapidly with salt water. It was suggested that the carbon was sourced from exhaust emissions, and that salt from winter gritting was being transported in solution by rainfall, and hence via ground water, into the cracks where thaumasite is growing. (I presume acid rain adds another factor?)