Magnesium sulfate soundness testing

What does magnesium sulfate soundness testing measure?

It measures how well an aggregate resists breaking down when salt repeatedly crystallises in its pores. Specimens are soaked in a saturated magnesium sulfate solution and then oven dried, over and over, so that salt crystals grow inside the pore structure and force the particles apart. The proportion of the aggregate that breaks down is the result. It stands as a proxy for how the material will cope with weathering and freeze-thaw in service.

Which standard is the magnesium sulfate test carried out to?

It is usually carried out to BS EN 1367-2, the magnesium sulfate test within the series covering thermal and weathering properties of aggregates. The method uses five cycles of immersion in saturated magnesium sulfate solution followed by oven drying, on specimens prepared from a defined size fraction. Where your specification calls up a category rather than a method, that category comes from the same family of standards.

Which size fraction of the aggregate is actually tested?

The 10 mm to 14 mm fraction. Specimens are prepared from the material passing 14 mm and retained on 10 mm, and breakdown is assessed from the proportion that ends up finer than 10 mm after the cycles. That has a practical consequence, because the bulk sample you send has to contain enough of that size band, which is not a given for a finely graded material. The standard does allow other fractions where the material demands it.

How is the soundness result expressed?

As a percentage loss of mass, reported as the MS value, where a lower number is better. A material with an MS value of 10 has lost 10% of the tested fraction over the five cycles. Specifications set a ceiling using categories such as MS18, MS25 and MS35, and Type 1 unbound mixture is commonly required to meet MS35, meaning a loss of no more than 35%.

What does a poor soundness value mean in service?

It means the aggregate is likely to break down where it is repeatedly wetted and dried or frozen and thawed, generating fines from particles that were sound when they were laid. In a subbase that shows up as loss of interlock, softening, pumping and settlement rather than as sudden failure. It matters most in positions that stay damp or sit close to the surface, and much less where the material is buried, dry and confined.

How does it compare with Los Angeles abrasion or a freeze-thaw test?

They answer different questions. Los Angeles abrasion measures resistance to mechanical fragmentation, essentially how particles cope with being knocked about during handling and compaction, while magnesium sulfate soundness measures resistance to weathering. The direct freeze-thaw test in the same family of standards is the closest equivalent, and some specifications accept it as an alternative. Type 1 specifications commonly call for the abrasion and soundness results together, because one does not substitute for the other.

When is soundness testing usually specified?

Most often when an aggregate is going into a position exposed to wetting and drying or to frost, or when the specification requires a declared durability category. Type 1 unbound mixture is the common case on highway and development work. It is also asked for on recycled and secondary materials and on sources with no test history, where there is less to fall back on than with an established quarry product.

How long does a magnesium sulfate soundness test take?

The cycling alone runs to five full cycles of immersion and oven drying, so the test occupies the laboratory for the better part of a week before sample preparation and final sieving are counted. It is slower than grading or abrasion but considerably quicker than frost heave, which is worth knowing when the two are booked together. We confirm the expected reporting date for the agreed scope before the work starts.

How big a sample do we need to send for a soundness test?

Enough to yield a good quantity of clean 10 mm to 14 mm particles after sieving, which usually means a bulk sample of several tens of kilograms rather than a single bag. The coarser and the more widely graded the material, the more you need to send. If the source turns out to be short of that size band we will say so before testing rather than after.

Does the magnesium sulfate test suit every type of aggregate?

Not entirely. The standard itself notes that the method may not be suitable for all rock types, with reservations recorded about some carbonate aggregates and some aggregates containing a high proportion of magnesium bearing material. On those materials the result can read worse than the aggregate's real performance in the ground. If you are testing a limestone or a dolomitic source and the number looks out of line with experience, it is worth discussing before the material is rejected.

Do you hold UKAS accreditation for the magnesium sulfate test?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so it covers particular methods rather than every service we provide. Ask us to confirm whether the soundness testing your specification requires is covered before you book, and we will say clearly if any part of it is not.

What do you need to quote for magnesium sulfate soundness testing?

The material and its source, the specification or category you are testing against, the number of samples, the site postcode and your preferred dates. Cost is driven by the number of samples, the preparation needed to recover the tested size fraction and whether we are sampling for you or you are delivering material. Tell us what else is in the suite and we will price the programme as a whole.