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structures made with Aspdins cement still stand today,none the worse for wear.

The important point is that concrete (the system ofcement paste binding together aggregate particles intoan artificial stone body) is inherently reactive and, givenits particular individual structure and exposure conditions,that reactivity will result in either excellent durability orpoor durability. In both the long and short terms, it is thechemistry that makes the difference in concrete perma-nence. Examples of typical exposure agents that affectdurability are: Moisture and ground water; Temperature cycles; Deicing salts; Marine environment; and Carbon dioxide and acidic air pollutants

(NOx and SOx ).Depending upon the composition of the concrete and

the exposure conditions, a variety of possible chemicalreactions may deteriorate concrete. Sometimes, how-ever, the enemy is not some outside element, but rather,the seeds of destruction may be innocently containedwithin the concrete itself.

Moisture, or water, plays the most important role inthe setting, strength development, and eventual deterio-ration of concrete. Portland cement hardens because ofa chemical process called hydration. This means that thesilicate and aluminate minerals in portland cement reactand combine with water to produce the glue that holdstogether the aggregate that we call concrete. Further-more, in a more general fashion, these cements are calledhydraulic cements. Hydraulic simply means thecapability to harden under water. Portland cement isonly one type of hydraulic cement, and it should benoted that C 1157-00, the ASTM performance specifi-cation for cement, is called the Standard PerformanceSpecification for Hydraulic Cement, and includes bothportland and blended types.

As Duff Abrams emphasized in his seminal paperDesign of Concrete Mixtures, however, presented at theDecember 1918 meeting of the Portland Cement Associa-tion and then published as the Lewis Institute Bulletin 1,the water-cement ratio (w/c) dictates the strength of

BY VAGN C. JOHANSEN, WALDEMAR A. KLEMM, AND PETER C. TAYLOR

Point of view

hy does chemistry matter? It matters becausechemistry controls the life span of concrete.

Chemistry explains why cement hardens and the inter-action between cement and its environment. We willdiscuss the basic inorganic chemistry of cement andconcrete under service conditions. Of course, there areother types of chemical processes that occur whenmixing concrete, such as arcane aspects of organicchemistry and surface chemistry. These are left out, notbecause these subjects are unimportant, but becausethey are not the main focus of this article.

The first principle to understand is that, in a broad

sense, concrete is thermodynamically unstable. Whencement paste is exposed to the earths atmosphere, itbegins to deteriorate, which is a form of chemicalcorrosion. The paste will react to exposure to acid rainor just the normal amount of carbon dioxide in the air.This reaction causes surfaces to etch and carbonate, andthen, the calcium silicate hydrates that give concrete itsstrength will be converted back to calcium carbonate,silica gel, and alumina gel.

This is one aspect of chemistry, but chemistry is notonly thermodynamics, it is also kinetics. In other words,concrete has the potential to change, but how fast willthat happen? Concrete made carefully with the rightmaterials in the proper proportions, and developing theoptimum microstructure by adequate curing, can last formany hundreds, or even thousands, of years. An exampleof this is the Pantheon in Rome, expertly constructed witha pozzolanic Roman Cement, which so far has lasted forover 2000 years.

When Joseph Aspdin patented his Portland Cementin 1824, it was titled An Improvement in the Modes ofProducing Artificial Stone. Concrete was considered tobe artificial stone having the appearance, strength, anddurability of the real thing. Some of the early concrete

W

The opinions expressed in this point of view article maynot necessarily be those of the American Concrete Institute.Reader comment is invited.

/ MARCH 2002 85

concrete. Although the size and grading of the aggregateand the quantity of cement influence the quantity of waterrequired to produce a workable mixture, the amount ofwater in a mixture controls the concretes strength. There-fore, one should use the smallest quantity of water thatwill produce plastic, or workable, concrete. Water may,depending upon the quality and nature of the aggregateused and the concrete curing history, trigger such otherdeleterious events as alkali-silica reactivity (ASR) or delayedettringite formation (DEF). Water may also function asa transport medium for potentially aggressive species,such as sulfate, to enter the system. These chemicalreactions will be covered in greater detail further inthis article.

Temperature affects the rate of chemical reactions,and a general rule of thumb is that a chemical reactionrate doubles for every 10 C increase in temperature.Thus, temperature influences the rate of both concretesetting and hardening. Additionally, cycles in tempera-ture may result in freezing-and-thawing distress if theconcrete lacks sufficient air entrainment. Curing concreteabove certain critical temperatures may lead to theexpansion and cracking associated with DEF.

Several external environmental factors may initiatedestructive chemical reactions in concrete, particularlyconcrete with a more open porosity (due to an elevatedw/c). Some of these factors include chloride-containingdeicing salts used to treat roadways in winter, salt sprayor tidal exposure to seawater, and sulfate-containingsoil or ground water. Chlorides can slowly diffuse intoconcrete and, in the presence of moisture and oxygen,will initiate corrosion of reinforcing steel. The oxidationof iron to produce iron oxide is a chemical processthat yields a large volume of oxidation product thatnot only structurally weakens the metal, but also producesinternal localized pressure that can cause severecracking of the concrete cover. Once cracking begins,more of the concrete surface is exposed to furtherchemical attack.

A vulnerable part of the concrete is the cementpaste. Although concrete is composed of 10 to 15%by mass of portland cement, it becomes the focus ofaggressive outside chemical agents like atmosphericcarbon dioxide (CO2 ) and acidic gases that dissolve inmoisture to produce acid rain. Cement paste is highlyalkaline, with a pH greater than 12.5. This high pH isdue to the presence of the calcium hydroxide hydrationproducts and the lesser amounts of alkali (sodium andpotassium) salts. Under ideal carbonation conditions(50 to 70% relative humidity and an exposed pastesurface), the hydrated lime (calcium hydroxide) constit-uent reacts with CO

2 to form calcium carbonate, whichis the same mineral as calcite or limestone. As thisprocess slowly progresses, perhaps even at rates of onlya millimeter or less per year, the pH is gradually loweredand a finely crystalline calcium carbonate replaces thehydration products.

Acid rain is aggressive and can more rapidly etch andcorrode exposed surfaces, eventually destroying thehydrated cement minerals that provide strength and

durability. One example of this destructive behavior isthe limestone obelisk in New York Citys Central Park. Ithad survived thousands of years in the Egyptian desert,remaining in almost pristine condition, but now afteronly decades of exposure to acidic gases in the citysatmosphere, the hieroglyphic inscriptions have all butdisappeared from view. Although limestone isnt concrete,the overall effects of acid rain are quite similar.

During the design of a concrete mixture, chemistrymust be considered. For instance, if the concrete isplaced where it may be exposed to aggressive environ-ments such as chlorides or sulfates, a different ASTMcement type will be selected. Type I is ordinary portlandcement, whereas Type II cement provides moderateresistance to sulfate attack, and Type V cement providesmuch greater resistance to sulfates. There are nowmany ASTM standard specifications, ASTM standard testmethods, ACI guidelines, and other recommendations forthe engineering professional on how to design concretemixtures, and all of these are based upon fundamentalknowledge of the chemistry of cement and concrete(Weaver 1978).

During the past 10 to 20 years, field experience hasshown that not only mixture design, but also the curingof concrete is very important. Curing, of course, is theprocess that provides sufficient moisture and thermalenergy to promote the hydration process. Curingconditions control strength development and thermalcracking; therefore, they have a significant impact onthe durability of concrete. A recent example of theimportance of curing and temperature control is concretedegradation from DEF. As will be discussed later, ettringiteformation is a normal and useful event as portland cementbegins to set. If its formation is greatly delayed (days ormonths after concrete hardening), however, it can causeserious durability problems.

Portland cement contains calcium silicates and calcium

aluminates formed by a sequence of thermal and chemicalprocesses, including decomposition of limestone; reactionwith other quarried materials such as clay, iron ore, andsand; partial fusion of these ingredients; and finally, theformation of hard, rounded nodules called clinker. All ofthis occurs at temperatures reaching about 1450 C ina cement plants rotary kiln. After cooling, the clinkeris ground together with approximately 5% gypsum(calcium sulfate dihydrate) to a flourlike fineness,producing the final product, portland cement (Kosmatkaand Panarese 1994).

At this stage, we will review some elementary cementchemistry. The present knowledge of the chemicalcomposition of portland cement and what happens to itwhen it is mixed with water was first disclosed in 1887by the French chemist Henry Le Chatelier (1905). In hisdoctoral thesis, he correctly identified the major cementminerals as tricalcium silicate, dicalcium silicate, andtricalcium aluminate. In 1915, scientists at the Geo-physical Laboratory in Washington, D.C., were studyingthe high-temperature phase relationships of the ternary

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system CaO-SiO2-Al2O3. Among the mineral phasesinvestigated were, of course, tricalcium silicate,dicalcium silicate, and tricalcium aluminate. In theprocess of publishing the complex triangle-shapedphase diagram, certain simplifying abbreviations forthe chemical compositions of each mineral phase wereinvented. For example, tricalcium silicate, Ca3 SiO5, couldalso be written as the combined sequence of the twooxides, such as 3CaOSiO2. The investigators, Rankin andWright, then used the shorthand notation of CaO = C;SiO2 = S; and Al2O3 = A. Accordingly, 3CaOSiO2 could bewritten as C3S. In similar fashion, dicalcium silicate,2CaOSiO2, became C2S, and tricalcium aluminate,3CaOAl2O3, became C3 A (Bogue and Steinour 1961).

This notation was so convenient and useful that, infuture publications over the following years, a shorthandnotation for other oxides was similarly introduced. Thus,Fe2O3 = F; MgO = M; H2O = H; Na2O = N; and K2O = K. Theadditional S problem of sulfur trioxide, SO3, was simplyresolved by indicating it as (S-bar). Other names forcement minerals or their hydrates concurrently enteredthe language and are commonly used today. For approxi-mately 100 years, the tricalcium silicate and dicalciumsilicate minerals, in their somewhat impure compositionas they crystallize in clinker, have been called alite andbelite, respectively. Others are called by their equivalentmineralogical name, such as periclase for MgO crystalsin cement, portlandite for calcium hydroxide (CH inshorthand notation), and ettringite for the calciumaluminosilicate hydrate (C3A3C32H). The formula forettringite in cement chemists notation appears to becomplicated, but it is simplified compared to a conventionalchemical formula, {Ca6[Al(OH)6]224H 2O}(3SO4)(2H2O).

The most rapid reaction that occurs when mixing

cement and water is the hydration of tricalcium aluminate(C3 A). Entirely by itself, C3A and water will quickly formcalcium aluminate hydrates such as C4 AH13 and C2 AH8.This can occur so rapidly that the concrete may stiffenwithin minutes and become entirely unworkable becauseof the heat emitted. This condition is called flash set.In the nineteenth century, when cement developedstrength slowly because it was coarsely ground andimperfectly reacted, flash set was not a problem. Even-tually, with the introduction of rotary kilns, a morescientific proportioning of raw material ingredients, andmuch finer clinker grinding, the addition of gypsum(CH

2 ) to the cement eliminated flash-setting problems.Chemically the C3 A, the gypsum, and the water would

form a protective coating of calcium sulfoaluminatehydrate (ettringite) over the exposed tricalcium aluminatesurfaces that would remain for several hours. The C3 Ahydration reactivates as the initial setting begins,consuming the sulfate and forming more ettringite. Ifthe portland cement is an ASTM Type I, however, itprobably contains over 8% C3 A, which is a much greaterpercentage than the sulfate present. When all of the sulfatehas been combined into ettringite, the excess C3 Acontinues to hydrate, and then begins to remove sulfatefrom some of the ettringite (trisulfate) to form another

stable calcium sulfoaluminate compound called themonosulfate, C3 AC H12 (Eq. (1) and (2)). A fourthmajor mineral in cement, the iron-containing ferritephase, or tetracalcium aluminoferrite (C4 AF) also hydrates,although much more slowly, to form chemically similartrisulfate and monosulfate compounds, in which iron(Fe2O3 ) replaces a portion of the aluminum (Al2O3 )(Steinour, 1958). Cement chemists generically call thesethe AFt (aluminate-ferrite-trisubstituted) and AFm(aluminate-ferrite-monosubstituted) phases, respectively.

C3 A + 3C H2 + 26H C6 A 3 H32 (ettringite) (1)

2C3 A + C6 A 3 H32 + 4H 3C4 A H12(monosulfoaluminate) (2)

The major strength development of concrete, how-ever, results from the hydration of the calcium silicatephases (C3S and C2S). Both of these calcium silicatescombine with water to form the gel-like calcium silicatehydrate, or C-S-H (Eq. (3) and (4)).

2C3 S + 6H C-S-H + 3CH (3)

2C2 S + 4H C-S-H + CH (4)

When concrete hardens due to hydration of the cement

paste portion, the total volume of the hydration productsis smaller than the original volume of portland cementand water. As a result of these hydration reactions andthe decrease in paste volume, a system of capillary poresis created (Fig. 1). This porosity in concrete governs itsdegree of susceptibility to various chemical factors havinga profound influence on its durability.

The reactions of greatest interest in the cement pasteare those between the components of the pore solution,and the solid phases present, which are the original cementminerals, their hydration products and, possibly, suscep-tible aggregate surfaces, in the case of ASR. This meansthat, as time passes, some of the solid phases dissolve andnew phases precipitate. The transport in solution of thevarious substances participating in the reactions controlsthe rate at which the chemical reactions take place.

In order for water to be available, it has to be able toenter the concrete and move through the paste structure,where it becomes part of the pore solution within thecapillary pore system. The pore solution contains variouschemical compounds that have dissolved from solidmaterials. These are primarily hydroxyl ions (OH) andalkali ions (K+ and Na+), as well as lesser amounts ofcalcium, silicate, aluminate, and sulfate ions. Ions fromexternal sources, such as deicing salts, may includesodium and chloride. This is the point where the impor-tance of the paste microstructure, in the form of thecapillary pore system, is evident. If cracks in the aggre-gate are disregarded, only the paste will take part in thetransport of reactive ions into a concrete system. In sucha system, there are three possibilities for water movement: Through the capillary system; Through the hydration products (primarily C-S-H

gel); and Through cracks in the paste structure.

/ MARCH 2002 87

If the concrete is moisture-saturated, the pore solutionwill fill both the capillary pore system and the cracks inthe paste. The connectivity of these systems is criticalin controlling how far and how fast solutions can moveinto the concrete. For concrete produced with a w/cof 0.6 and higher, the capillary system will be continuous(percolating system), and aggressive reactants will easilymove through the concrete mass. With decreasing w/c,however, the pore system becomes isolated into smallerand smaller unconnected clusters of pores. These smallclusters close to the surface may become saturated withsolution and dissolved reactants. Any further movementthrough the paste has to occur through the C-S-H gel orthrough the hydration products.

The rate of liquid transport through the C-S-H phaseis on the order of 1000 times slower than that occurringthrough the capillary pores. Therefore, even if the capillarypores account for only 1% of the total transport crosssection, they will still provide 90% of the solution transport.Any chemical reactions that depend solely on transportthrough C-S-H are negligible for all practical purposes.If liquid-filled cracks are present, the movement of asolution will be proportional to the number and size ofcracks. Additionally, cracks will also provide shortcutsbetween pore clusters and thereby expose deeper layersof concrete to the penetrating solution. With sufficientcracking, the cracks themselves may form a connectedsystem percolating the concrete and allowing dissolvedreactants to move into the system more quickly.

The most common type of chemical attack on concreteresults from exposure to soils or ground water containingelevated sulfate contents. This malady is a commonoccurrence in the western United States, and has beenknown for many years. Production of ASTM Type II andType V cements is particularly intended to provideresistance to deleterious sulfate exposure. The mechanismof sulfate attack is relatively simple. If cement containsan elevated amount of C3A, a substantial amount of thecalcium monosulfoaluminate (AFm) phase will form duringhydration. This substance is reactive, and if additionalsulfate from an outside source such as soil or groundwater penetrates the concrete, the monosulfate will readilyreact with it and convert back to the AFt phase, orettringite. The conversion of the monosulfate phase intoettringite will result in a significant volumetric increaseand be disruptive to the concrete. If this is allowed tocontinue, the concrete will eventually be destroyed.

Concrete made at a high w/c and subjected to sodiumsulfate-containing ground water can be damaged byanother mechanism. Studies conducted in California haveshown that, even with the use of ASTM Type II and Type Vcement types, if water is able to percolate through theconcrete, the wetting and drying on the surfaces exposedto air result in serious deterioration. Damage is mainlyfrom the repeated crystallization of alkali sulfates andcarbonates during the drying cycles (Stark 1989).

Cracks in concrete may develop for physical reasons

such as drying shrinkage or mechanical loading. Localchemical reactions in the concrete, however, may also

result in expansion, a buildup of internal pressure, andthen cracking. Concrete is a brittle material and there-fore can only expand to a limited degree before cracking.In broad terms, the observed expansion is equal to thesum of the crack widths. It is not possible to determinethe cause of expansion and cracking from the appearanceof the crack pattern on the surface of the concrete. Interiorconcrete samples must be examined microscopically,chemically, or both, to determine the root cause of theinternal expansion. In field concrete exposed to theelements, there are two basic modes of expansion: The aggregate can expand relative to the cement

paste; and The cement paste can expand relative to the aggregate.

It follows from physical considerations that, in acomposite system consisting of expanding particles in amatrix, cracks are formed in the matrix radiating awayfrom the particles. Expansion of particles in a hardenedpaste, such as aggregate particles undergoing ASR, causesthe particles to crack and the crack to extend outwardinto the surrounding paste. A particle cracking, whenexpanded from the surface, is actually a fairly commonexperience. When ice cubes are dropped into a drink anda familiar crackling sound is heardthat is the sound ofthe ice being heated on the outside, expanding, and thencracking. The expansion at the surface causes the innerpart of the particle to be under tensile stress, and itcracks from the inside outward.

Shrinkage of cement paste is a common phenomenonrelated to hydration, and from a cracking point of view,is equivalent to the expansion of aggregate particles.Consider, however, what would happen if the pasteactually expands relative to the aggregate particles. Whencement paste expands relative to the aggregate particles,as it has in DEF, gaps open up around the particles. Thisconcept is actually a little counterintuitive, in that onemight expect the expanding paste to actually crush theparticle rather than create a gap around it. This is similarto the question of whether a hole that has been drilled

Fig. 1: Relative volumes of cement, water, and hydration productsbefore (left) and after (right) hydration

Water

Unhydrated cement

Capillarypores

Gel

Unhydrated cement

88 MARCH 2002 /

through a piece of metal will become smaller or largerin diameter as the metal is heated and expands.

One way to understand the phenomenon is to considerconcrete made with aggregate that does not expand, andconduct the following imaginary experiment (Fig. 2). First,assume that the concrete, including both paste andaggregate, expands, say 20%, in all directions. There is nodistortion, no cracking, and the concrete is just somewhatlarger. Next, since the aggregate particles were not expanding,shrink them back to their original size. What has happened?The particles now rattle around in holes that are 20%larger than the individual particles. Thus, gaps haveactually formed around the aggregate particles and,furthermore, these gaps are proportional to particle size.

The chemical reaction called ASR takes place between

the highly alkaline (very high pH) pore solution andreactive siliceous portions of some aggregate particles.The large amount of hydroxyl (OH) ions present in thepore solution, due to a high alkali concentration (potassiumand sodium), dissolve the reactive silica on the aggregatesurfaces to form an alkali silicate gel. Although any formof silica can react with alkali hydroxides in theory, it isthe siliceous rocks such as opal, greywacke, some chert,and glassy volcanic materials that appear to be the mostreactive. Reactive siliceous aggregates will form alkali-silica gel starting at the surface of the aggregate and movinginwards. Tensile stresses build up during the reaction,causing the aggregate particles and the surroundingpaste to crack. In severe cases, the cracks will inter-connect and lead to weakening of the concrete. Theweakening is only due to the cracks; the paste betweenthe cracks maintains its composition and strength.

Denser polycrystalline rocks, such as granites, will reactmuch more slowly. The chemical reaction will occur atthose heterogeneous areas of grain boundaries. In suchcases, only a minimal degree of reaction may be neededto cause cracking, but only meager amounts of gel willform. In ASR distress, since each internal fracture in concretecreates an empty space, the alkali-silica reaction causescorresponding incremental volume increases (Helmuth1993). The resulting visual evidence of the reaction is themap cracking observed on the concrete surface.

ASR is an example of a chemical reaction in which

the aggregate portion of the concrete plays a role in the

deterioration mechanism. Reactionsrelated to sulfate are a group ofreactions that involve only thecement paste. DEF reactions that areassociated with concrete exposureto high temperatures during curing,in systems containing normal-sulfate-content cements, have beena hot button topic of discussionfor some time. DEF results froma chemical reaction, or sets ofreactions, that are still not wellunderstood at the present time,but the diagnosis is relatively

simple; deterioration results from paste expansion inthe affected concrete.

In DEF-affected concrete, the AFt phase, or ettringite,is usually observed. But ettringite in concrete is notunique to DEF. Ettringite is a normal hydration productformed by the chemical reaction between the aluminatephases of cement, water, and calcium sulfate (gypsum),as mentioned previously. The formation of ettringitetakes place in the paste and is uniformly distributed.Within mature concrete exposed to moist conditions,ettringite is usually found in pores and cracks. This isnot an indication of damage, but rather the result ofa normal recrystallization process known as Ostwaldripening. This means that small crystals have a highersolubility than large crystals, and when concrete becomewater-saturated to a certain degree, the small crystalswithin the paste dissolve in the pore liquid and subse-quently recrystallize as larger crystals in any availablespaces, such as cracks and pores. Ostwald ripening isa general chemical principle, and calcium hydroxidecrystallization behaves in a similar fashion.

Regarding DEF, however, a high concrete temperatureat an early age is a very important parameter. At certaintemperatures, generally above 70 C, and more frequentlyabove 80 C, ettringite becomes unstable because itssolubility increases. This temperature is strongly depen-dent upon the alkali content and other compositionalfactors of the cement that are less well understood.Where the components of the ettringite go after itsdecomposition is not clear. Portions of ettringite maybe consumed by the C-S-H or may stay in solution. Thisissue is the subject of much scientific research anddiscussion at present (Tennis et al. 1999). What is clear,however, is that the chemistry matters, even if we donot fully understand the precise sequence of reactions.

One sign of paste expansion is the presence of voidsor cracks around the aggregate particles, as explainedpreviously. Usually, ettringite fills these gaps. There are alsoexamples of paste expansion showing empty gaps, however.

The following is one possible explanation of theobserved paste expansion related to DEF, in whichsulfate dissolved in the pore liquid reacts with theanhydrous and hydrated aluminate particles in thehardened cement paste. Johansen and Thaulow (1999)discussed this previously. Hardened paste, mortar, andheat-treated as well as normally cured concrete are allplaces where one can find unhydrated clinker particles

Fig. 2: Expansion mind experimenta) a matrix containing aggregate particles;b) the whole system is expanded by 20%; and c) the aggregate particles are returned totheir original size, leaving voids around them

(a)

(b) (c)

/ MARCH 2002 89

ACI member VVVVVagn C. Johagn C. Johagn C. Johagn C. Johagn C. Johananananansensensensensen is a SeniorPrincipal at Construction TechnologyLaboratories (CTL), Inc. He has more than20 years of experience with cementchemistry and cement manufacture, and10 years of experience with concretematerials and durability-related work.Previously, he worked in Denmark for IdornConsult, and with F.L. Smith & Co.

WWWWWalalalalaldemdemdemdemdemar Aar Aar Aar Aar A. Kl. Kl. Kl. Kl. Klemmemmemmemmemm is an AffiliatedConsultant with CTL, Inc. He has 30 yearsof experience with the U.S. cement industryin plant process, chemistry, research, anddevelopmental activities. He has authorednumerous technical reports and scientificpapers on clinkering chemistry, cementhydration, admixture research, cementmanufacturing, and environmentalproblems. He is a member of ASTM.

ACI member PPPPPeteteteteter C. er C. er C. er C. er C. TTTTTaaaaaylorylorylorylorylor has beenat CTL, Inc., since 1997, and is a SeniorEngineer. He is a graduate in CivilEngineering from the University of CapeTown, South Africa, and has 18 yearsexperience in consulting and research. Heis a member of ACI Committees 232, FlyAsh and Natural Pozzolans in Concrete,and 236, Material Science of Concrete.

in various amounts. As mentioned previously, abovecertain temperatures ettringite is unstable, and theprimary hydrated aluminate phase is the calciummonosulfoaluminate (AFm). Therefore, after cooling toroom temperature following heat treatment, concretewill contain anhydrous aluminate particles, anhydrousaluminate particles with AFm, and AFm phases. Duringthe passage of time and moist curing of such concrete,these particles will continue to react with sulfate in thepore solution. Both ettringite and AFm phases form,depending upon the composition of the pore solution.The sulfate, liberated from the C-S-H that had initiallyabsorbed it during the heat treatment, maintains thesulfate concentration of the pore solution.

The hardened paste confines the reacting particlesand the volume of the AFm and ettringite (AFt) formedwill result in development of localized pressure. Crystalsunder pressure have a higher solubility than what theywould have when not under pressure. When more andmore AFm and AFt phases are formed on the reactingparticles, the pressure will increase and, therefore, sowill their solubility. If the solubility increases to a levelcorresponding with the actual concentration in the poresolution, the crystal growth stops and the pressure willact locally on the particle and its surroundings. In thisway, the reacting particle can act as a local pressurecenter. This will cause stress to build up in the surroundingpaste as sort of a sphere of influence around the particle.If the pressure created is larger than the tensile strengthof the paste, the paste will crack or yield. If the reactingparticles are sufficiently close to each other, mass volu-metric expansion will result.

Returning to the original question, why does chemistry

matter? The answer can be summarized as follows:chemistry matters because concrete composition andperformance are based upon a variety of chemicalreactions that range from the original setting andhardening of the portland cement constituent to theeventual desired engineering properties. The durabilityof concrete depends on chemical processes developingout of cement and aggregate compositional factors,curing conditions, and exposure to a variety of envi-ronmental effects. The chemical reactions that occurduring the hydration of the clinker minerals determinethe concrete microstructure. The hardened concreteis chemically reactive given the right conditions, asshown in the examples. Therefore, it is essential todesign concrete mixtures properly and erect structuresin a way to control or adequately compensate forchemical reactivity.

Bogue, R. H. and Steinour, H. H., Origin of the Special Chemical

Symbols Used by Cement Chemists, Journal of the PCA Research &Development Laboratories, V. 3, No. 3, Sept. 1961, pp. 20-21.

Helmuth, R., Alkali-Silica Reactivity: An Overview of Research,SHRP-C-342, Strategic Highway Research Program, National ResearchCouncil, Washington, D.C., 1993.

Johansen, V. and Thaulow, N., Heat Curing and Late Formation of

Ettringite, Ettringite: The Sometimes Host of Destruction, SP-177,B. Erlin, ed., American Concrete Institute, Farmington Hills, Mich.,1999, pp. 47-64.

Kosmatka, S. H. and Panarese, W. C., Design and Control ofConcrete Mixtures, PCA Engineering Bulletin EB001.13T, PortlandCement Association, Skokie, Ill., 1994.

Le Chatelier, H., Experimental Researches on the Constitution ofHydraulic Mortars, Translated by J. L. Mack, McGraw Publishing Co.,New York, N.Y., 1905.

Stark, D., Durability of Concrete in Sulfate-Rich Soils, PCAResearch and Development Bulletin RD097, Portland CementAssociation, Skokie, Ill., 1989.

Steinour, H. H., The Setting of Portland Cement: A Review ofTheory, Performance and Control, PCA Research DepartmentBulletin 98, Portland Cement Association, Chicago, Ill., Nov. 1958.

Tennis, P. D.; Bhattacharja, S.; Klemm, W. A.; and Miller, F. M.,Assessing the Distribution of Sulfate in Portland Cement andClinker and Its Influence on Expansion in Mortar, Cement, Concrete,and Aggregates, V. 21, No. 2, Dec. 1999, pp. 212-216.

Weaver, W. S., Committee C-1 on CementSeventy-Five Years ofAchievement, Cement StandardsEvolution and Trends, ASTM STP663, P. K. Mehta, ed., American Society for Testing and Materials,Philadelphia, Pa., 1978, pp. 3-15.

Received and reviewed under Institute publication policies.