Ion Chromatography in concrete sulphate analysis

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

The large number ofconcrete structures that have degraded prematurely over the lastforty years has shocked structural engineers who had previouslybeen of the opinion that reinforced concrete was maintenancefree.

Concrete can be affected by a chemical reaction involvingsulphate that when present in contaminated hardcore along with asource of moisture, reacts with cement present in the concretecausing it to expand and crack.

The force of the chemicalreaction can displace external and sleeper walls on domestic andcommercial buildings leading to the opening of skirting boardjoints, the bouncing of timber floors, cracking and displacementof the brickwork as well as disruption of concrete floors and subfloors.

The physical defects described can take up to 10 to 20years before they become evident.

The correct remedial actions tosulphate attack is normally to remove all the defective concreteand contaminated hardcore and replace them which is a disruptiveand costly process.

Building insurance does not usually coversulphate attack, as most policies tend to have a clause thatexempts all forms of chemical attack.

Concrete footings andfoundations can be subject to deterioration from chemical attackfrom acidic solutions or salts of sulphate.

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

Rainwater absorbscarbon dioxide from the atmosphere forming carbonic acid thatdecomposes - in situ- the sulphides concentrated from miningwaste to produce sulphates.

The sulphates react with cementwithin the concrete before causing gradual degradation andeventual failure of the concrete.

Sulphate attack is the maincause of deterioration in cement based construction materialsplaced in the ground.

Its occurrence in the United Kingdom ispotentially widespread as some 25% of the land area of England,as well as significant areas of Wales and Scotland, occupynatural bearing strata.

In addition, brownfield sites commonlyhave high levels of sulphate contamination as concrete placed inclays containing pyrite (iron sulphide) are at an increased riskof attack, because of oxidation of pyrite due to constructionrelated ground disturbance.

The formation of expansive productscaused by chemical reactions among the concrete components is oneof the most important processes of concrete structure damage.Typically, these are based on alkali-aggregate reactions inaddition to the sulphate-alumina reaction.

Hence, aggregates thathistorically would have been used now tend to be avoided becauseof the occurrence of such phenomena.

The aggregates representing70-80% of concrete volume are important in the concretedurability especially when they contain harmful constituents suchas organic matter, chloride, sulphides, sulphates and clays.Areas of ground with high sulphate levels require the use ofsulphate resistant concrete for their footing and this elevatesthe cost of construction by as much as œ12 per cubic metre morethan when normal concrete is used.

Water that is acid in natureincreases the probability and severity of the attack so pHtesting needs to be carried out in conjunction with any chemicalanalysis for sulphate.

During the construction of newdevelopments, samples are taken from footing trenches or trialpits for analysis of sulphate.

Any delay is unwelcome as thisincreases the construction costs, so a fast and reliable methodfor chemical analysis is imperative and most companies offeringthis type of consultancy typically offer a turn around of 5-7days.

Historically the sulphate analysis of concrete has usedgravimetric techniques to provide a quantitative value.

Ionchromatography presents a rapid, reliable and robust alternativeonce the sample is present in an ionic, homogenous form.

Sulphateattack in concrete occurs when sulphate solutions, derived fromeither a constituent of the concrete such as the aggregate orfrom an external source such as ground waters, react with thecalcium aluminate hydrates present in the hardened cement to formthe hydrated calcium sulfoaluminate known as ettringite.Ettringite can occupy twice the volume and inflicts seriousdamage on the concrete leading to a weakening and potentialfailure of the affected structure.

In the last decade variouscases have been attributed to the cracking of concrete with adelayed ettringite formation without a supply of sulphates froman external source.

This type of ettringite is usually associatedwith the steam curing of concrete and an alkali-silica reactionbut has been found in normal cured concrete without the presenceof expansive reactions.

A second form of sulphate attack wasidentified during investigations upon concrete structures thatwere specifically designed to offer resistance to sulphate.

Inthis instance, the sulphate solutions react with calcium silicatehydrate phases in the presence of calcium carbonate ions (withinthe hardened cement paste) to form the mineral thaumasite.

Themineral thaumasite forms at temperatures below 15deg C and is amore complex salt than ettringite.

A Brief Case Study: During anoperation to strengthen two older bridges on the M5 motorway inGloucestershire during 1998, deterioration was found among theconcrete columns located below ground level and found to beexhibiting the thaumasite form of sulphate attack afterinvestigation by the Building Research Establishment.

Thefoundations of a further three bridges were also investigated atthis point and subsequently found to be demonstrating similardeterioration.

Prior to the cases on the M5, few cases of thethaumasite form of the reaction had been previously observed.Studies since conducted have concluded that this form of thereaction typically occurs when there is a source of sulphate (asin clay soils), wet and very cold conditions as well as thepresence of calcium carbonate in the concrete (limestoneaggregate in the cases investigated).

The Department of theEnvironment, Transport and the Regions has long recognised thepotential for sulphate attack in concrete by supporting researchto provide best practice guidance on the problem.

As aconsequence of the discoveries found during the work on the M5,the government minister for construction convened the ThaumasiteExpert Group to review the problem.

Their recommendations were touse low carbonate aggregates in pipeline systems and segmentaltunnel linings, which was not economically viable, as manymanufacturing plants were located adjacent to limestone quarries.After discussions between the working group and industry tradeorganisations, an optimistic view was taken upon the resistanceof pre-cast concrete components to sulphate attack, although thishas yet to be established by either work in the field orlaboratory investigation.

What is Ion Chromatography?Chromatography is a method for separating mixtures of substancesusing two phases, one of which is stationary and the other mobilemoving in a particular direction.

Chromatography techniques aredivided up according to the physical states of the twoparticipating phases.

The term Ion Exchange Chromatography or IonChromatography (IC) is a subdivision of High Performance LiquidChromatography (HPLC).

A general definition of ion chromatographycan be applied as follows;" ion chromatography includes allrapid liquid chromatography separations of ions in columnscoupled online with detection and quantification in aflow-through detector".

A stoichiometric chemical reactionoccurs between ions in a solution and a solid substance carryingfunctional groups that can fix ions as a result of electrostaticforces.

For anion chromatography these are quaternary ammoniumgroups.

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

The process of ionexchange leads to a condition of equilibrium, the side to whichthe equilibrium lies depends on the affinity of the participatingions to the functional groups of the stationary phases.

5g of thedried analytical sample was weighed accurately into a clean, drybeaker before dispersion with 50mls of deionised water followedby 10mls of concentrated hydrochloric acid.

A further 50mls ofdeionised water was added before the beaker was covered with awatch glass and gently boiled for a period of 5 minutes.

Thesolution was allowed to cool before filtration through a Whatman52 filter paper.

The filtered solution was diluted 1:50 withdeionised water before being placed on the sample carousel of theMetrohm 788 IC Filtration Sample Processor where it was injectedinto the Metrohm 761 Compact IC through a 0.2m m membrane.

Theresponse for the peaks was recorded using a mobile phase eluentof sodium carbonate/sodium bicarbonate with the Metrosep A SUPP 5analytical column.

The calculation was carried out automaticallyusing integration software IC Net 2.1 against a previouslyprepared calibration plot.

There are no external displays orswitches on the 761 instrument, all the hardware is fullycontrolled via a single RS232 connection between the IC and thePC.

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

The 761 Compact IC comprises a low-pulsationdual-piston pump, pulsation dampner, electromagnetic injectionvalve, two channel peristaltic pump, conductivity detector,eluent organiser as well as a data recording and processingmodule.

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

The detector is the heart of every ionchromatograph.

The Metrohm detector's temperature varies byless than 0.01deg C and can be optimally adapted to the ambientconditions.

This outstanding temperature stability reducesinterference and allows exact conductivity measurements.

The 788IC Filtration Sample Processor is freely programmable autosamplerthat integrates membrane filtration as a sample preparation stepdirectly into a IC system.

The aim of sample filtration is toprotect separation columns from contamination and blockages byvery small particles.

Membrane filters with a pore size of lessthan 0.45m m are normally used for the filtration.Ultra-filtration using the 788 is particularly suitable forsamples with a light to medium load.

The samples are placeddirectly on the sample rack before being processed automatically.The sample solution to be filtered passes over a membrane, whilstat the same time the filtrate is aspirated off from the rear ofthe membrane, before transfer to the sample injection valve wherethe sample loop is filled then subsequently injected onto thecolumn.

Only a small fraction of the sample is removed asfiltrate, the contaminants mainly remain in the sample stream andthis prevents the membrane from becoming blocked too quickly.

Thesulphate content determined in the sample of concrete analysedwas found to be of the order 2.5g l-1.

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

The useof IC as a potential method for the analysis of sulphate inconcrete presents an alternative to the gravimetric methods thatare currently widely used.

Ion chromatography is a precise,durable technique that allows the user the functionality of quickanalysis turnaround once the samples are in an ionic, homogenousform.

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

The samples once in solution can be loaded onto anautosampler and run overnight or during the day freeing the userto perform other duties.

The low running costs of ionchromatography with Metrohm instruments are surprisingly lowrequiring only the acquisition of chemicals required for theeluent and suppressor module as well as a clean, reliable sourceof deionised water.

Ion chromatography is a clean technique asall the reagents are enclosed, its robustness and reliability areassured demonstrating precisely the reason why it is rapidlybecoming the method of choice for many analysts in a plethora ofdifferent industries.

The following internet sites were usedextensively as a references and can be used to obtain furtherinformation:- www.alanwood.co.uk - www.crofty.demon.co.uk -www.databases.odpm.gov.uk - www.metrohm.com -www.parliament.the-stationery-office.co.uk - www.seal-it.sk.ca