Dicalcium Silicate

In certain blast-furnace slags, small amounts of β dicalcium silicate can be present. Sometimes, during cooling, this substance can turn into the γ form and cause surrounding material to fracture if there is enough inversion. This results in the slag breaking apart over several days, a process known as fragmentation or "falling." To ensure that weakened batches of slag are identified early, a test is required when using the slags as aggregates. In 2004, a European Standard with an ultraviolet lamp test replaced BS 1047, which imposed chemical limits on air-cooled blast-furnace slag, to detect falling slags.

To determine the most appropriate test for blast-furnace slag, a review of the literature was conducted. In the 1960s, a belief emerged in Britain that β dicalcium silicate that did not invert during slag cooling could fall apart destructively at a later date, known as "late falling." This belief has influenced attitudes to slag specifications in Britain, but not elsewhere. However, upon examination, it appears that this belief was based on misinterpretations and has never been conclusively proven.

The ultraviolet test is a suitable method for rapidly identifying weakened slags. The test can be performed in accordance with BS EN 1744-1.