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Instability Mechanisms

Instability mechanisms in iron and steel industry slags

TRS has the required skills and tests to assess the potential for volumetric instability in any of the following materials

Blast Furnace Slag
Instability problems with blast furnace slag are relatively rare. Fresh-make (i.e. new) blast furnace slag should in fact have no instability problems. However, some older sources of blast furnace slag, after a period of weathering, may contain pockets of unstable material.

The most likely form of instability in this type of slag is when, as a result of weathering, a significant proportion of the sulphur oxidises to sulphate, often present as gypsum. This, under given conditions, may take part in chemical reactions resulting in the formation of a sulphoaluminate hydrate phase, apparently similar to that taking place in sulphate attack on concrete. The result is volumetric expansion and disruption of the mass.

Basic Steel Slag
Almost all basic steel slags contain some residual free lime (Ca0). Some may also contain residual free periclase (Mg0). Both of these, in the presence of water, will ultimately hydrate to portlandite [Ca(0H)2] and brucite [Mg(0H)2]. The formation of these hydrates is associated with massive expansion. Another common component in basic steel slag is a wustite type solid solution of Fe0, Mn0, Mg0 and Ca0. If this wustite is of the Ca0/Mg0 rich variety, then it will behave as such and is liable to hydrate and expand in the same manner. The identification of these products within basic steel slag is a specialist job requiring appropriate skills and tests.

Acid Steel Slag
These materials are unlikely to exhibit any form of volumetric instability. However, they are also fairly rare, originating from long defunct steel making processes.

Basic Refractory Materials
Basic retractories are often found in association with basic steel slag. They commonly contain a very high proportion (90+%) of periclase (Mg0). This represents the same danger of expansion as free Mg0 in steel slag, but due to the much higher proportion of Mg0 available, can give much more serious localised expansion.