Testing of industrial products - Aggregates for construction

The Total Sulfur test method

The principle of the reference method

In the reference method for the determination of total sulfur used in the cross-testing experiment reported here, the laboratory sample is reduced to a sub-sample of mass not less than the mass specified in Table 1 appropriate to the nominal size of the aggregate. This sub-sample is then reduced by a stepwise process involving crushing, grinding and reduction, to produce a sub-sample of approximately 20g of material that passes a 0.125mm sieve. Approximately 1g of this material is taken for the test.


Table 1. Minimum mass of preliminary sub-sample.
Nominal maximum size of aggregate
mm
Minimum mass of sub-sample
kg
6350
4535
31.515
22.4 or less5

Note that the draft European Standard method (Draft prEN 1744-1 Tests for chemical properties of aggregates. Part 1 Chemical analysis. November, 1994.) refers to the 1g sub-sample as the test portion. It would be more in accordance with other aggregate test methods if the sub-sample of uncrushed aggregate (whose mass complies with Table 1) were to be called the test portion, and if the 1g of powdered aggregate were to be called a specimen. This preferred terminology is used in this report. Note also that the European Standard method should state that the crushing, grinding and reduction process should follow requirements for such processes given in the European Standard for sample reduction (Proposed prEN 932-2 Tests for general properties of aggregates; Part 2 Methods for reducing laboratory samples to test portions. CEN/TC 154/TG 5 committee paper N 188. June, 1994.).

The analysis involves three steps. In the first, the weighed specimen is treated with bromine and concentrated nitric acid (density 1.42) to convert any sulfur compounds to sulfates, and the nitric acid is removed by evaporation with concentrated hydrochloric acid (density 1.19). The second step is intended to remove iron, aluminium and other Group 111 elements that might otherwise contaminate the final barium sulfate precipitate. The solution is made just alkaline (using methyl red indicator) with ammonium hydroxide solution, causing the unwanted constituents to precipitate so they can be removed, together with the insoluble residue. As a precaution against losing sulfates, the precipitate is re-dissolved in the acid, and precipitated a second time with ammonia. In the third step, the filtrates from the second step are combined and treated with barium chloride solution and the barium sulfate precipitate is matured, filtered, ignited and finally weighed.

The result of the test is reported as the percentage of sulfur, by mass, in the aggregate.

The proposed CEN reference method

Participants in the cross-testing experiment were sent copies of the method identical to the method in prEN 1744-1 (Draft prEN 1744-1 Tests for chemical properties of aggregates. Part 1 Chemical analysis. November, 1994.), the draft giving test methods for chemical properties of aggregates that was circulated for CEN Enquiry in 1994. According to this draft, the method outlined above is to be the reference method, in European Standards, for determining the total sulfur content of aggregates.

The method has been in use for many years, although it is questionable whether the second step is really necessary: any improvement in accuracy that it gives is likely to be out-weighed by the additional variability that it introduces. Several of the participants commented critically on the method, saying that it is time-consuming, labour-intensive, and includes the handling of toxic substances.

One participant pointed out that the method does not mention that the specimen should be dried to constant mass before it is weighed (although the method does specify a limit on the temperature of drying, if drying is considered necessary by the analyst).