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Alkali Content of ConcreteMix Water and Aggregates

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Alkali silica reaction (ASR) is a chemicalreaction between some types of aggregateand the highly alkaline pore water present inall Portland cement concretes. When theconcrete contains sufficient moisture, alkalisand reactive aggregate the reaction producesa gel, which absorbs water and swells. The

swelling can create enough pressure to crackthe concrete and/or cause significantmovement of concrete components. If severe,the damage can affect the performance of thestructure.

CCANZ TR3 (2nd edition) describes how tominimise the risk of ASR damage to concretein new construction. One approach is tocontrol the amount of alkali added to theconcrete when it is mixed. For NormalConcrete, as defined by NZS 3104:2003, the

concrete producer is required to certify thatthe total alkali in the concrete, contributed byall constituents of the mix, does not exceed2.5kg/m3. Control of alkali content is one ofseveral approaches that may be taken forSpecial Concrete.

Traditionally, Portland cement has been

considered the main source of alkalis, andmost concrete manufacturers understand howto manage alkalis contributed by cement. It isnow recognised that other mix constituentssuch as water and aggregates can contributesignificant amounts of alkali. These aspectsare also discussed in CCANZ TR3 (2nd

edition). This Information Bulletin describeshow to measure and calculate the alkalicontributions from mix water and aggregates.It supports the information and recommenda-tions presented in CCANZ TR3 (2nd edition).

PCA

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In general chemistry, the pH scaleindicates whether a substance is an alkali oran acid and to what degree. The pH of analkali is between 14 and 7. The pH of an acidis less than 7. The pH of fresh concrete isaround 12.5, so concrete is considered highlyalkaline.

In water chemistry, the term alkalinitydescribes the ability of water to maintain itspH when exposed to chemicals, e.g. in the

natural environment or during industrialprocesses. It is largely determined by thepresence of carbondioxide, carbonic acid,

and calcium, bicarbonate, carbonate andhydroxide ions. If a water has enoughalkalinity, its pH will not change significantlyunless a large amount of acid is added to it.

Alkalinity is sometimes referred to as thebuffering capacity of water, because thechemicals that contribute to alkalinityneutralise the effect of acids by absorbinghydrogen ions to prevent significant pHchange.

In cement and concrete chemistry, alkalicontent refers to the amount of alkali metal

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elements (lithium, sodium, potassium,rubidium, cesium, francium) present in thecement or concrete in readily solublecompounds. Although some of thesecompounds may increase the pH of water

when they dissolve, not all of them will. ThepH change they produce is too small to be areliable measure of alkali content in concreteor wash water. Thus ASR is managed bydirectly controlling the alkali metal content ofthe concrete, i.e. its alkali content, not its pHor its alkalinity.

Most alkali in concrete is supplied assodium (Na) or potassium (K) ions. The otheralkali metals may not contribute significantlyto ASR damage. Alkali contents of mix

constituents such as cement and water aretherefore based on sodium and potassiumcontents. They are expressed as alkaliequivalent (Na2Oeq):

Na2Oeq (%) = Na2O (%) + 0.658 x K2O (%)

Na2O and K2O are the amounts of sodiumand potassium oxides measured by chemical

2.

analysis of the sample and expressed as aweight percent of the sample weight. The0.658 correction factor reflects the relativedifference in molecular weight between Na2Oand K2O.

To calculate the alkali content in kg/m3 ofa particular concrete mix, the alkali equivalentof each constituent is first converted to alkaliweight in kilograms by multiplying the alkaliequivalent by the weight of the constituentper cubic meter of concrete. The alkaliweights of all constituents are then addedtogether to give the total alkali content.Appendix C of CCANZ TR3 (2nd edition)explains how to calculate the alkalicontributions from individual constituents and

the total alkali content of the concrete.

Suppliers of cement, SCMs and chemicaladmixtures will be able to provide alkalicontents of their products. Measurements ofthe alkali content of water and aggregates,however, may need to be arranged by individ-ual concrete and aggregate producers. Thefollowing sections describe how this is done.

AAllkkaallii CCoonnttrriibbuuttiioonn FFrroomm WWaatteerrWhen Does It Need to be Measured?

Analyses of water from New Zealandmunicipal water supplies prior to 1995 can beaccessed from the JASPER database held bythe Institute for Environmental Science andResearch (ESR). Drinking water compositiondoes not vary greatly over time, so oldanalytical data is generally acceptable unlessthere has been a major change in the supply

since the most recent analysis. Specificchemical analysis of town supplies used forconcrete mix water should not be necessary.

Non-potable water supplies, includingrecycled water, do need to be analysed.Water that has been in contact with cementusually contains significant quantities of alkalisthat arise from the initial hydration of cement.Recycled wash water therefore will containmore alkali than potable water and its alkalicontent must be considered and added to thealkali contributed by other components. Thisis particularly important if the total concrete

alkali content is close to the maximumspecified limit.

PCA

It is strongly recommended that recycledwater be sampled and analysed over severalweeks to obtain a range of typical operatingalkali contents. The maximum value obtainedshould be used in concrete alkali calculations,in the same way as the maximum cementalkali content reported by the cement supplierwould normally be used.

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How to Measure It

Water should be analysed by acommercial chemistry laboratory that is IANZaccredited for analysing drinking water. A list

of these laboratories is available on the IANZwebsites Directory page. A laboratory that isspecifically accredited for sodium analysisshould also be capable of measuringpotassium although it will probably not beaccredited specifically for potassium.

The alkali contents measured will bedetermined by the way the water is sampled,particularly for recycled water. It is importantthat the sample taken for analysis representsthe water as it is delivered to the concrete

mixer. For recycled water this means includingsuspended solids. All sampling equipmentand containers must be clean, and should berinsed with the water before collecting thesample. The chemistry laboratory will be ableto advise how much water should be sampled,and how it should be stored and delivered tothe laboratory.

Analyses should be performed accordingto an internationally recognised Standard,such as those published by the American

Public Health Association (APHA) or similarorganisations. Analytical techniques common-ly used include atomic absorption (AA),inductively coupled plasma spectroscopy(ICP), and titration.

The laboratory must be instructed toanalyse the water sample for sodium andpotassium. If the water sample containssuspended solids (e.g. a if it is a recycledwash water) the laboratory must include thesolids in the analysis. This means the

laboratory must dissolve the solids andanalyse the resulting solution along with theliquid fraction of the sample. An aciddigestion is considered satisfactory for thispurpose. Acid digestion will also dissolve anynon-cementitious minerals in the slurry, i.e.minerals that are not soluble in alkaline poresolutions, and may therefore give a slightlyhigher result than if only the cement solids areincluded. Results based on a digestionprocess that selectively dissolves onlycementitious particles may be used if

analysis (e.g. by XRD) shows that thesuspended solids consist largely of aggregateparticles rather than cementitious materials.

3.

How to Calculate Water Alkali Contributionfrom the Analytical Results

Results from water analyses are usuallyreported as the concentration of sodium and

potassium ions (Na and K) expressed asmilligrams per litre (mg/l) or parts per million(ppm), rather than as weight percent ofsodium and potassium oxide. If specificallyrequested, the laboratory may be willing toexpress the results as alkali equivalent asdescribed below. There may be a small extrafee for doing this.

The sodium and potassium ion contentsNa and K in mg/L (or ppm) are converted toalkali equivalent by the formula:

Na2Oeq = (Na x 1.35) + (0.658 x K x 1.20)10,000

The correction factor 1.35 reflects therelative difference in molecular weightbetween sodium and sodium oxide. Thecorrection factor 1.20 reflects the relativedifference in molecular weight betweenpotassium and potassium oxide. The 10,000converts mg/L (ppm) to a percentage.

If the equivalent alkali is less than 0.02%(190 mg/L (ppm)) it shall be treated as nil andnot included in the calculation of concretealkali content. If it is 0.02% or greater it mustbe included in the calculation. ASTM C94 hasan optional maximum limit for alkali equivalentin wash water of 600 ppm (0.06%).

The total alkali contributed to the concreteby the mixing water is calculated from thealkali equivalent by the formulae:

W = Na2Oeq x C x W/C or100

W = Na2Oeq x water content100

where W = equivalent alkali contributionsmade to the concrete by the alkaliions sodium and potassium present inthe water in kg/m3.

C = Portland cement or Portland-limestone cement content of theconcrete in kg/m3.

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W/C = the water cement ratio.

Water content = water content of theconcrete in kg/m3.

4.

The alkali content of the water alreadypresent in the aggregate when batched isincluded in the alkali contribution from theaggregate.

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Where it Comes From

Aggregates can contribute alkalis from twosources: release from minerals within therock, and marine salt contamination on theexternal surfaces of aggregate particles.

The contribution of alkalis by the minerals

is described in CCANZ TR3 (2

nd

edition)Section 7.6. The phenomenon is limited torelease of alkali by the minerals nepheline andleucite. The amount of alkali that will bereleased from an aggregate in this way overthe lifetime of a structure is difficult to predict,so TR3 recommends not combining in thesame concrete alkali reactive aggregates andaggregates containing nepheline and leucite.Therefore alkalis contributed by nepheline andleucite are not included in the calculation ofconcrete alkali content.

Aggregates may be contaminated with seasalts, especially if they are derived from amarine source or stockpiled near the coast.The sodium chloride in the salt contributesreactive alkali to the fresh concrete. This alkalimust be determined by chemical analysis andincluded in the calculation of concrete alkalicontent.

The alkali content of the coarse and fineaggregates used in the concrete should be

determined at agreed intervals.

How to Measure It

Although the sodium and potassiumcontents of aggregate can be readilydetermined by chemical analysis of a rocksample, the sodium and potassium in the rockitself generally does not take part in ASR.Thus direct determination of the alkali contentof a rock sample by techniques such as Xrayfluorescence (XRF) is not appropriate fordetermining alkalis that will contribute to ASR.Instead, any of the following three approachesmay be used:

1. By Acid-soluble Chloride

Aggregates susceptible to chloridecontamination from sea salt should betested for acid-soluble chlorides to ensurethat the chloride content of the concretedoes not exceed NZS 3101:1995requirements. Rather than analysing

these aggregates both for water-solublealkalis and for acid-soluble chlorides, thereactive alkali content of the aggregatecan be estimated from the analysis of acidsoluble chlorides. New Zealandaggregates do not contain significantamounts of chloride other than as sea saltcontamination, so this approach will give areasonable estimate of the water-solublealkalis.

The standard methods for analysis of

concrete for acid soluble chlorides areASTM C1152 and BS 1881:Part 124:1988.These may also be applied to aggregates.Both these methods involve preparing acidextracts from the aggregate and analysingthe extracts by wet chemical techniques.Instrumental techniques such as XRF aremore convenient than wet chemicaltechniques because the aggregate itselfcan be analysed. Where the chlorides arepredominantly in the form of sea saltcontamination, results from XRF do not

differ significantly from wet chemicalanalyses of acid soluble or water-solublechlorides. Testing laboratories offeringXRF analysis will prepare the sample.

Because of its convenience, XRF analysisof the chloride content of the aggregatesample is the recommended method fordetermining alkalis that will contribute toASR.

2. By Water Soluble Chloride

The reactive alkali present in theaggregate can be estimated from its water

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soluble chloride content determined by BS812: Part 117. This test method involveswashing the aggregate in water to dissolvethe salt, then analysing the chloridecontent of the aggregate wash water. Two

laboratories may be needed for thisprocess: an aggregate testing laboratoryto wash the aggregate, and an analyticalchemistry laboratory to analyse thechloride content of the wash water. XRFis a suitable method of analysis. Theresults must be expressed as chloridecontent as a percentage of aggregateweight.

3. By Water-soluble Alkali

An alternative approach is to wash theaggregate in water as described in BS812: Part 117 and then measure the alkaliequivalent of the aggregate wash water(as described in the previous section)instead of the chloride content. The alkaliequivalent of the aggregate wash water isthen converted to alkali equivalent of theaggregate.

An IANZ-accredited chemistry laboratorywith experience in chemical analysis ofrocks and minerals or water (asappropriate to the approach taken) shouldperform the analysis. A list of theselaboratories is available on the IANZwebsites Directory page. The laboratoryshould be instructed to analyse the samplefor total chlorine or for sodium andpotassium as appropriate.

How to Calculate Aggregate AlkaliContribution from Analyses of the Chlorine

Content of Aggregate

Results from XRF analyses for chlorineare usually quoted as total chlorine in weightpercent. This approximates the total chloridecontent. When the total chloride ion level inthe aggregates is less than 0.005% it shall beregarded as nil. Otherwise, the reactive alkalicontributed by sodium chloride contaminationof the aggregates is calculated from theformula:

H = 0.76 x [(NF x MF) + (NC x MC)]100

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where H = reactive alkali contribution madeto the concrete by the sodiumchloride present in the aggregatesexpressed as kg/m3 sodium oxideequivalent.

NF = chloride ion content of the fineaggregates as a percentage by massof dry aggregate.

MF = fine aggregate content in kg/m3.

NC = chloride ion content of thecoarse aggregate as a percentage bymass of dry aggregate.

MC = coarse aggregate content inkg/m3.

The correction factor 0.76 is the product ofcorrection factors that reflect the relativedifference between the concentrations ofsodium and chloride ions in seawater and therelative difference in molecular weightbetween the sodium ion and its oxide.

How to Calculate Aggregate AlkaliContribution from Analyses of the Water-soluble Alkali Content of Aggregate

The reactive alkali contributed theaggregates is calculated from the formula:

H = (NF x MF) + (NC x MC)]100

where H = reactive alkali contribution madeto the concrete by the soluble alkalispresent in the aggregates expressedas kg/m3 sodium oxide equivalent.

NF = Na2Oeq of the fine aggregate as

a percentage by mass of dryaggregate.

MF = fine aggregate content in kg/m3.

NC = Na2Oeq of the coarseaggregate as a percentage by massof dry aggregate.

MC = coarse aggregate content inkg/m3.

Prepared by the NZRMCADecember 2004