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Technical note: Sulfurous strategies

Murray Reid. This note was first published in GE’s February 2002 edition.

Sulfur compounds in soils, rocks and fill materials can cause problems in civil engineering works because they attack materials such as concrete and steel. The attack is mainly due to sulfate ions in solution, and assessment procedures for the sulfate content of soils and groundwater have been available in BRE Digest 363 and the Specification for Highway Works (SHW) or a number of years.

More recently, the role of reduced sulfur compounds such as pyrite has been appreciated, particularly where it is present as clusters of fine-grained crystals invisible to the naked eye (Figure 1). Oxidation of these compounds can generate large quantities of sulfate, which then cause further attack on construction materials. Oxidation of pyrite was implicated in the thaumasite form of sulfate attack on concrete bridge foundations on the M5 motorway, and the latest BRE guidance on concrete in aggressive ground, Special Digest 14, includes assessment procedures for pyritic ground.

The test methods which are generally prescribed for sulfur compounds are the water-soluble and acid-soluble sulfate tests in BS1377: Part 3: 1990 and the total sulfur test in BS1047: 1983. In these tests, sulfur is brought into solution as sulfate and determined gravimetrically by precipitation as barium sulfate. This is a lengthy procedure with poor precision, which is subject to interference from substances such as organic matter and iron oxides.

There is no standard direct method for reduced sulfur compounds. BS1377 recommends that, if pyrite is thought to be present, it should be determined as the difference between total sulfur by BS1047 and acidsoluble sulfate by BS1377. methods involve precipitation of barium sulfate and suffer from poor precision and reproducibility. The estimate of reduced sulfur will therefore be subject to large potential errors. In practice, very few analytical laboratories use the barium sulfate precipitation method or the BS1047 acid digestion procedure for total sulfur. A variety of in-house methods are used, leading to variation in results for the same material tested by different laboratories.

Transport Research Laboratory (TRL) and the University of Sheffield have carried out a research project for the Highways Agency to develop better test methods for sulfur compounds and to develop assessment procedures for reduced sulfur compounds in backfill to structures.

The results have now been published as TRL Report 447 GE January 2002 news) and are summarised in this article. Throughout the project, liaison was maintained with the Thaumasite Expert Group and BRE, and the test methods and assessment procedures are compatible with those given in BRE Special Digest 1. The new test methods given in TRL Report 447 are included in the list of recommended test methods in Table Al of Special Digest 1. Together, the two documents give a comprehensive guide to test methods and assessment procedures for situations where sulfate attack on construction materials is likely to occur.

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