Sulfate and Sulfide Analyses of Concrete

A sulfate analysis may be required to determine if sulfate has entered the concrete from the environment, if sulfate compounds (e.g., calcium sulfate hemihydrate-plaster) have been added to the concrete as an admixture, to determine cement content, or to determine reasons for unusual
setting times.

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Because most cements and aggregates do not contain sulfide sulfur, analyses for total sulfur are often an accurate measure of sulfate sulfur. Instrumental methods for sulfur, such as x-ray fluorescence or evolutiontitration, are thus generally acceptable. However, if the result appears too high for the estimated cement contribution, sulfate analyses should be performed. The difference between total sulfur and acid-soluble sulfate determinations, each calculated as SO₃, is usually a measure of sulfide sulfur. Sulfide sulfur minerals may cause popouts or rust discoloration if they are close to the surface of a concrete member.

Sulfate Procedure

Digest 5 grams of concrete, pulverized to pass a No. 30 screen, in 20 ml of 1:4 hydrochloric acid solution. Filter through a coarse-textured paper inside or over a fine-textured paper into a 400 ml beaker. Determine sulfate by the procedures of ASTM C114 or as follows: place a small piece of filter paper in the beaker and hold down with a glass rod. Bring the solution to boiling and add drop-wise, through a pipet, 20 ml of barium chloride solution (10 g per 100 ml water). Boil until the white precipitate settles, then digest below boiling in the glass-covered beaker.

Filter through a retentive filter paper, wash the paper 10 times with hot distilled water, place in a tared porcelain crucible, ignite over a flame without inflaming, then in a muffle furnace at 820°C. Weigh. Calculate SO₃ as 0.343 × weight of precipitate.

Sulfide Procedures

Sulfide may be determined by evolution of hydrogen sulfide using the procedure of ASTM C114. Alternatively, total sulfur may be determined by evolution/titration (e.g., LECO furnace) and sulfate sulfur by Sulfate Procedure. The difference between these values, both calculated as SO3, multiplied by 0.4, is sulfide sulfur.