Testing of industrial products - Aggregates for construction

Acid-soluble Sulfate Test - Summary

This report gives the results of a cross-testing experiment on the determination of the acid-soluble sulfate content of aggregates involving 12 laboratories from 9 countries. The materials used in the experiment were a concreting sand, a recycled concrete, and a blast-furnace slag.

The results of the experiments show that the reproducibility of the test is not good enough for it to be used as the reference method for the determination of the acid-sulfate content of aggregates. This is when the determinations are to be used to check that the aggregate complies with a specification limit around 0.2% to 1% SO3. It is therefore recommended that attempts should be made to improve the reproducibility of the method by proficiency tests, robustness tests or the use of reference materials. Also by using a "belt and braces" approach with aggregates that give results close to a specification limit, in which total sulfur, acid-soluble sulfide and acid-soluble sulfate are all determined, and the results used to check that the difference "total sulfur - (sulfide + sulfate)" is within an acceptable tolerance. When testing for compliance with a specification limit of 0.2%, it may be possible to improve the precision by using a larger specimen mass, of say 5g to 10g instead of 2g.

It appears that the sample reduction procedure is a significant source of between-laboratory variation, and attempts should be made to reduce this variation by tightening-up the test method, and by using only non-ferrous grinding mills.

It is proposed that the results of determinations of acid-soluble sulfate on slags should be reported to the nearest 0.01% for repeatability checks, and to the nearest 0.1% when checking compliance with a limit of 1.0%; and for other aggregates they should be reported to the nearest 0.001% for repeatability checks, and to the nearest 0.01% when checking compliance with a limit of 0.2%.

The results of the experiment allow the precision of the determination of acid-soluble sulfate to be stated (as the percentage SO3 by mass) as:

r 1 = 0.021 + 0.200 X and R1 = 0.000 + 0.812 X

where "X" represents the average of the test results.