Slag Types

Evaluation of slag

Instability mechanisms

Slag Types

TRS has specific in-house techniques and skills to identify all of the products below, how best to use them and where not to use them

The processes of iron and steel manufacture give rise to various by-products, some of which are slags. The latter can be described as non-metallic products of metal production and include:

Blast Furnace Slags
Basic Steel Slags
Acid Steel Slags

Blast Furnace Slag
Blast Furnace Slag can be broadly divided into two main categories:

• 'Basic' iron-making slags which constitute virtually all current production.
• 'Hematite' blast furnace slag derived from the use of 'hematite' iron ore and generally native to the NW of England.

Blast furnace slags, where properly utilised, can be an excellent source of constructional materials, finding applications in roadstone, concrete aggregate, biological filter media, glass making, cements, etc..

In addition to work on existing and new slag based products, TRS has carried out work to utilise old, stockpiled blast furnace slags. Such work has led to the re-utilisation of several million tonnes of old slags, previously considered unusable.

Basic Steel Slag
Basic Steel Slag is the product of steel making using 'basic' fluxes, i.e. high in Ca0 and/or Mg0. Virtually all modern steel making operations are 'basic' processes, and many old steel making slags also fall into this category. Basic steel slags, both old and new, are under-utilised compared to blast furnace slags. This is partly due to problems of volumetric instability, attributable to the delayed hydration of free lime and/or free magnesia commonly present in these slags. Nonetheless, excellent products an be produced for specific markets in road making applications. However, appropriate informed processing is required to produce satisfactory, high quality products.

Acid Steel Slag
Acid Steel Slags are now relatively rare and are encountered mainly in old deposits in the UK. They contain no free Ca0 or Mg0 and are generally accepted as being volumetrically stable.

Evaluation of Slag containing fill

TRS has very extensive experience of evaluating fill which may consist of or contain, various iron and steel industry slags.

The main purpose of evaluating fill is to identify any potentially volumetrically unstable constituents. Such materials may contain a variety of slags including:

• Blast furnace slags - Fresh make is almost certainly volumetrically stable, but when weathered may sometimes contain residual pockets of unstable material.
• Basic steel making slags - commonly contain constituents likely to give rise to volumetric instability at an unpredictable future date.
• Acid steel making slags - these are almost invariably stable.

The most abundant are blast furnace slags, followed by basic steel making slags. Nowadays, acid slags are relatively uncommon and, when encountered, are usually present as only a small proportion of the fill.

Three phased approach to analysis
Phase 1
Petrology By optical microscopy or X-ray diffraction
It is most important to identify the main constituents of the fill before embarking on any accelerated expansion tests. Failure to do so may give rise to misleading expansion results. The most effective means of determining the fills constituents is by mineralogical analysis. Phase 1 may be carried out using either optical microscopy or X-ray diffraction. Commonly, TRS would recommend microscopy for mixed fill but the decision is best left to TRS after visual inspection of the material. The results from Phase 1 determine the analyses necessary in Phase 2.
Phase 2
This phase comprises a variable suite of tests, including chemical and thermo-chemical analyses. The purpose is to identify constituents associated with past expansion and potential future volumetric instability. When both blast furnace and steelmaking slag are identified in phase 1, both groups of Phase 2 tests are necessary.
Blast Furnace Slag Water soluble sulphate Acid soluble sulphate Total sulphur Thermal analysis	Basic Steel Slag Free lime (Ca0) Free magnesia (Mg0) Thermal analysis	Acid Steel Slag No further testing	Slag Mixture Water soluble sulphate Acid soluble sulphate Total sulphur Free lime (Ca0) Free magnesia (Mg0) Thermal analysis
Phase 3 TRS accelerated expansion test The TRS accelerated expansion test is a direct method of identifying potentially volumetrically unstable slags. The TRS test is unique in being appropriate to both blastfurnace and steelmaking slag and is the product of over a decade of development at Thomas Research Services. As already indicated, phases 1 and 2 should be carried out first if misleading results are to be avoided. Evaluation of Results and Reports Evaluation of the results from the three phases described is a specialist task. TRS are research consultants and not just a test house. Detailed advice can be given before an investigation commences and TRS provide a fully interpretative reporting service. It is possible to carry out all 3 phases of work simultaneously, obviously significantly reducing the timescales involved. However, this may prove to be a more costly route, for example, if not all of the procedures were necessary.

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.

Specialist consultants in slag products