Concrete sulphate analysis by ion chromatography

Sulphate attack is one of the most common aggressive actions leading to the deterioration of concrete.

The large number of concrete structures that have degraded prematurely over the last forty years has shocked structural engineers who had previously been of the opinion that reinforced concrete was maintenance free.

Concrete can be affected by a chemical reaction involving sulphate that, when present in contaminated hardcore along with a source of moisture, reacts with cement present in the concrete causing it to expand and crack.

The force of the chemical reaction can displace external and sleeper walls on domestic and commercial buildings, leading to the opening of skirting board joints, the bouncing of timber floors, cracking and displacement of the brickwork as well as disruption of concrete floors and sub floors.

The physical defects described can take up to ten to 20 years before they become evident.

The correct remedial actions to sulphate attack is normally to remove all the defective concrete and contaminated hardcore and replace them - which is a disruptive and costly process.

Building insurance does not usually cover sulphate attack, as most policies tend to have a clause that exempts all forms of chemical attack.

Concrete footings and foundations can be subject to deterioration from chemical attack from acidic solutions or salts of sulphate.

The risk is increased in areas of previous mining activity where the concentrations of minerals in the soils pose an increased risk.

Rainwater absorbs carbon dioxide from the atmosphere, forming carbonic acid that decomposes the sulphides concentrated from mining waste to produce sulphates.

The sulphates react with cement within the concrete before causing gradual degradation and eventual failure of the concrete.

Sulphate attack on concrete.

Sulphate attack is the main cause of deterioration in cement based construction materials placed in the ground.

Its occurrence in the United Kingdom is potentially widespread as some 25% of the land area of England, as well as significant areas of Wales and Scotland, occupy natural sulphate-bearing strata.

In addition, brownfield sites commonly have high levels of sulphate contamination as concrete placed in clays containing pyrite (iron sulphide) are at an increased risk of attack, because of oxidation of pyrite due to construction related ground disturbance.

The formation of expansive products caused by chemical reactions among the concrete components is one of the most important processes of concrete structure damage.

Typically, these are based on alkali-aggregate reactions in addition to the sulphate-alumina reaction.

Hence, aggregates that historically would have been used now tend to be avoided because of the occurrence of such phenomena.

The aggregates, representing 70-80% of concrete volume, are important in the concrete durability - especially when they contain harmful constituents such as organic matter, chloride, sulphides, sulphates and clays.

Areas of ground with high sulphate levels require the use of sulphate resistant concrete for their footing, and this elevates the cost of construction by as much as £12 per cubic metre more than when normal concrete is used.

Water that is acid in nature increases the probability and severity of the attack, so pH testing needs to be carried out in conjunction with any chemical analysis for sulphate.

During the construction of new developments, samples are taken from footing trenches or trial pits for analysis of sulphate.

Any delay is unwelcome as this increases the construction costs, so a fast and reliable method for chemical analysis is imperative and most companies offering this type of consultancy typically offer a turn around of five to seven days.

Historically, the sulphate analysis of concrete has used gravimetric techniques to provide a quantitative value.

Ion chromatography presents a rapid, reliable and robust alternative once the sample is present in an ionic, homogenous form.

Different forms of sulphate attack.

Sulphate attack in concrete occurs when sulphate solutions, derived from either a constituent of the concrete such as the aggregate or from an external source such as ground waters, react with the calcium aluminate hydrates present in the hardened cement to form the hydrated calcium sulphoaluminate known as ettringite.

Ettringite can occupy twice the volume and inflicts serious damage on the concrete, leading to a weakening and potential failure of the affected structure.

In the last decade various cases have been attributed to the cracking of concrete with a delayed ettringite formation without a supply of sulphates from an external source.

This type of ettringite is usually associated with the steam curing of concrete and an alkali-silica reaction, but has been found in normal cured concrete without the presence of expansive reactions.

A second form of sulphate attack was identified during investigations upon concrete structures that were specifically designed to offer resistance to sulphate.

In this instance, the sulphate solutions react with calcium silicate hydrate phases in the presence of calcium carbonate ions (within the hardened cement paste) to form the mineral thaumasite.

The mineral thaumasite forms at temperatures below 15C and is a more complex salt than ettringite.

A brief case study.

During an operation to strengthen two older bridges on the M5 motorway in Gloucestershire, UK, during 1998, deterioration was found among the concrete columns located below ground level.

This was found to be exhibiting the thaumasite form of sulphate attack after investigation by the Building Research Establishment.

The foundations of a further three bridges were also investigated at this point, and subsequently found to be demonstrating similar deterioration.

Prior to the cases on the M5, few cases of the thaumasite form of the reaction had been observed.

Studies since conducted have concluded that this form of the reaction typically occurs when there is a source of sulphate (as in clay soils), wet and very cold conditions, as well as the presence of calcium carbonate in the concrete (limestone aggregate in the cases investigated).

The Department of the Environment, Transport and the Regions has long recognised the potential for sulphate attack in concrete by supporting research to provide best practice guidance on the problem.

As a consequence of the discoveries found during the work on the M5, the government minister for construction convened the Thaumasite Expert Group to review the problem.

The group's recommendations were to use low carbonate aggregates in pipeline systems and segmental tunnel linings, which was not economically viable as many manufacturing plants were located adjacent to limestone quarries.

After discussions between the working group and industry trade organisations, an optimistic view was taken upon the resistance of precast concrete components to sulphate attack, although this has yet to be established by either work in the field or laboratory investigation.

What is ion chromatography?.

Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction.

Chromatography techniques are divided up according to the physical states of the two participating phases.

The term ion exchange chromatography or ion chromatography (IC) is a subdivision of high performance liquid chromatography (HPLC).

A general definition of ion chromatography can be applied as follows: "ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector".

A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces.

For anion chromatography these are quaternary ammonium groups.

In theory ions with the same charge can be exchanged completely reversibly between the two phases.

The process of ion exchange leads to a condition of equilibrium, and the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.

Method for the determination of sulphate in samples of concrete.

5g of the dried analytical sample was weighed accurately into a clean, dry beaker before dispersion with 50ml of deionised water followed by 10ml of concentrated hydrochloric acid.

A further 50ml of deionised water was added before the beaker was covered with a watch glass and gently boiled for a period of five minutes.

The solution was allowed to cool before filtration through a Whatman 52 filter paper.

The filtered solution was diluted 1:50 with deionised water before being placed on the sample carousel of the Metrohm 788 IC filtration sample processor where it was injected into the Metrohm 761 Compact IC through a 0.2mm membrane.

The response for the peaks was recorded using a mobile phase eluent of sodium carbonate/sodium bicarbonate with the Metrosep A Supp 5 analytical column.

The calculation was carried out automatically using integration software IC Net 2.1 against a previously prepared calibration plot.

There are no external displays or switches on the instrument, and all the hardware is fully controlled via a single RS232 connection between the IC and the PC.

All the instrument parameters can be called upon with a click of the mouse.

The 761 Compact IC comprises a low-pulsation dual-piston pump, pulsation dampener, electromagnetic injection valve, two channel peristaltic pump, conductivity detector, eluent organiser as well as a data recording and processing module.

All the components that come into contact with the eluent and sample are metal-free.

The detector is the heart of every ion chromatograph.

The Metrohm detector's temperature varies by less than 0.01C and can be optimally adapted to the ambient conditions.

This outstanding temperature stability reduces interference and allows exact conductivity measurements.

The 788 IC filtration sample processor is freely programmable autosampler that integrates membrane filtration as a sample preparation step directly into a IC system.

The aim of sample filtration is to protect separation columns from contamination and blockages by very small particles.

Membrane filters with a pore size of less than 0.45mm are normally used for the filtration.

Ultra-filtration using the 788 is particularly suitable for samples with a light to medium load.

The samples are placed directly on the sample rack before being processed automatically.

The sample solution to be filtered passes over a membrane, while at the same time the filtrate is aspirated off from the rear of the membrane, before transfer to the sample injection valve where the sample loop is filled then subsequently injected onto the column.

Only a small fraction of the sample is removed as filtrate, the contaminants mainly remain in the sample stream and this prevents the membrane from becoming blocked too quickly.

The sulphate content determined in the sample of concrete analysed was found to be of the order 2.5g/l.

Conclusion of sulphate analysis of concrete using ion chromatography.

Ion chromatography as an analytical technique has seen an enormous surge in popularity, due partly to the simplicity of many of the methods as well as other factors such as market forces driving down the expenditure costs of the equipment and an improved instrument power.

The use of IC as a potential method for the analysis of sulphate in concrete presents an alternative to the gravimetric methods that are currently widely used.

Ion chromatography is a precise, durable technique that allows the user the functionality of quick analysis turnaround once the samples are in an ionic, homogenous form.

Only a very small amount of sample is required for the analysis and the quantified results obtained within a matter of minutes.

The samples once in solution can be loaded onto an autosampler and run overnight or during the day freeing the user to perform other duties.

The running costs of ion chromatography with Metrohm instruments are surprisingly low, requiring only the acquisition of chemicals required for the eluent and suppressor module as well as a clean, reliable source of deionised water.

Ion chromatography is a clean technique as all the reagents are enclosed; its robustness and reliability are assured, demonstrating precisely the reason why it is rapidly becoming the method of choice for many analysts in a wide range of different industries.