Testing of industrial products - Aggregates for construction

The Acid-soluble Sulfate test method

The principle of the method

In the reference method for the determination of acid-soluble sulfate 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 2g 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 2g 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 2g 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 of aggregates (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 principle steps in the analytical procedure in the proposed reference method are as follows. The weighed specimen is dissolved in dilute hydrochloric acid at a temperature just below boiling and the residue is filtered off. The solution is made acidic with dilute hydrochloric acid (using methyl red indicator), and maintained at boiling point while 10mL of barium chloride solution (100g BaCl2.2H2O per litre, heated to just below boiling) is added. The barium sulfate precipitate is matured, filtered, ignited and finally weighed.

The result of the test is reported as the percentage of sulfate (SO3), by mass, in the aggregate.

The proposed CEN reference method

Participants in the cross-testing experiment were sent copies of the method in 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 acid-soluble sulfate content of aggregates.