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ELSEVIER PII:SO958-9465(97)00062-O

Cement and Concrere Composi res 20 (1998) 87-1010 1998 Elsevier Science Ltd. All rights reservedPrinted in &eat Britain

0958-9465/98/ 19.00

Chemical Admixture-Cement Interactions:Phenomenology and Physico-chemical ConceptsCarmel Jolicoeur & Marc-Andre SimardDepartment of Chemistry, Universitk de Sherbrooke, Sherbrooke, QuC. JlK 2R1, Canada

& stractChemi cal admi xtur es are oft en used to al t er thecourse of cement hydration reactions and thepropert i es of fr esh or hardened concrete. Theadmixtures, in most cases organic compounds,c-an erf orm such funct i ons through var i ous ypesof physico-chemi cal int eracti ons w it h he hydrat-ing cement phases. To understand t heconsequences of admixture-cement interactions,and t o opt im i ze t he functi onal propert i es ofadmi xtur es, appropri ate descri pt ions of t heir modeof act i on must be developed. An overvi ew of t hel at t er s att empt ed here, draw ing mai nly rom: (1)rhe chemi str y and phenomenology of cementhydrat ion; (2) a revi ew of t he vari ous t ypes ofmol ecular processes i n w hi ch admi xt ure mol eculescan be invo l ved at he soli d-sol ut i on nt e ace; (3)selected experimental data on the influence ofw at er reducers and super-pl asti cizers hi ch il l u-str at e some basi c feat ures of admi xt ure-cementi nt eract i ons. 0 1998 Elsev i er Science Lt d. A l ltights eserved.Keywords: cement, silica fume admixture, waterreducer, superplasticizer, interaction, adsorp-tion, hydration. rheology, calorimetry.

rationalize, particularly in cementitious systemscontaining mineral additives (pozzolans orfillers).The admixture-cement interactions may infact be viewed as the reaction between twocomplex chemical systems illustrated schematic-ally in Fig. 1. Portland cements aremulti-component, multi-phasic inorganicmaterials, comprising major components (C3S,C&3, C&A, C,AF) and minor phases (CaO,CaS04.xH20, Na, KS04, etc.); blended cementsinclude additional components such as supple-mentary materials (slag, fly ash, silica fume) andfillers (limestone, rock flour, etc.). In a similarway, chemical admixtures often contain severalcomponents which are either inherent to thenature of the admixture or a result of theirmanufacturing process. For example, lignosulfo-nate water reducers are intrinsically complexmixtures of chemical compounds derived fromchemical degradation of lignin. Synthetic admix-tures, for example high-range water reducers,often contain species with a broad distributionof molecular weights, reaction by-products, orother chemicals added for a specific purpose.Hence, in order to achieve understanding of

INTRODUCTION Cement High Range Water Reducer

Modern concretes often incorporate severalchemical admixtures, each of which may inter-act with the various constituents of cements,and influence cement hydration reactions. Theoverall consequences of admixture-cementinteractions on the properties of fresh or hard- Fig 1 Schematic illustration of the interaction betweenened concrete may thus be difficult to a cement (multi-phasic mineral system) and a chemicaladmixture (multi-component organic system).87


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88 C. Jol i coeul ; M .-A. Simard

admixture-cement (and also of admixture-admixture) interactions at a level required foroptimum use of these materials, a systematicapproach is essential, drawing both on experi-mental observations and physico-chemicalconcepts. The present report was prepared asan introductory overview of the phenomenologyand physico-chemical concepts which currentlyappear relevant to cement hydration and to theinfluence of chemical admixtures on the cementhydration processes.An overview of the basic chemical aspects ofcement hydration will first be presented, afterwhich the various types and purposes of chemi-cal admixtures currently used will be recalled.The physico-chemical concepts involved inadmixture-cement interactions will then bereviewed, and selected results on the influenceof admixtures on cement hydration reactionswill be examined. The framework will bedeveloped with a generic perspective on allorganic chemical admixtures, although most ofthe results cited (new or literature) will bear onwater reducer and superplasticizer admixtures.

CEMENT HYDRATIONThe contacting of water with Portland cementtriggers a series of processes leading to hydra-tion products which, through several types ofbonding interactions, yield a dense stablematrix. An important particularity of this com-plex reaction is that the initial reactants are in apowdered form. Heterogeneous reactions ofthese powdered minerals with water involveboth solution and interface (more appropriatelyinterphase) processes. As shown by the broadknowledge basis derived from extensive cementhydration studies,le7 the formation of hydrationproducts and the development of the micro-structural features depend upon solutionprocesses, interfacial reactions (topochemicalphenomena) and, ultimately, solid-state reac-tions.Cement hydration reactions: thermodynamicaspectsThe hydration reactions of the major mineralphases of Portland cement have been wellcharacterized, and generally serve as reference,or model reactions, in discussions of cementhydration chemistry, thermodynamics, or

kinetics. It is broadly acknowledged, however,that the hydration reactions in Portland cementsystems involve coupling of the reactionsdepicted in eqns (l)-(8) and thus, lead to morecomplex processes and products. The main purephase hydration reactions have been given as?:

2C,S+6H--+C,SZH,+3CH

2C2S+4H+C&H,+CH

(1)(2)(3)(4)(5)(6)

C,AF+3CSH,+29H-+CG(A,F)S,H,,+(A,F)H,(7)

C4AF+C6(A,F)S3H32+7H+3C4(A,F)SHIZ+(A,F)H3 (8)

Some of the key data and observations per-taining to the above reaction scheme aresummarized in Table 1. Such data are usedextensively in discussions of relative propertiesof different cements, for example, to predict themaximum heat of hydration as a function ofcement composition. From the hydration reac-tions given above and the cement compositionshown in Table 1, it can be calculated that, forcommon ASTM-Type I cements (OrdinaryPortland Cements), the amount of waterrequired to fully hydrate the various mineralphases is -30 wt%, i.e. W/C = O-30.The relative reactivity of the differentmineral phases with water is usually given asC3A > C3S > C2S z C4AF (absolute reactivitiesvary considerably depending on the degree ofmetal ion substitution in the phases and theircrystal structure). Accordingly, the aluminatephases and their hydration products play animportant role in the early hydration processes.In a general simplifying sense, the early (O-l h)


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Chemi cal admixt ure-cement int eracti ons 89

Table 1. Hydration reactions for components of ordinary Portland cementComponent Typi cal ontent Hydrat ion Hydrat ion w ater AHin OPC (wt O/o) h y d r t i o nproduct ( olid) [Jk (dr y)1Wc2sGA

C FCaSO, (soluble anhydrite)CaS04 (insoluble anhydrite)CaS04.05H20Na, KS04CaOTypical OPC

5.520695I

C-S-H, CHC-S-H, CHMonosulfateEttringiteC&PH,zCaS04.2H20CaS04.2H20FHC-S-H, CHMonosulfateEttringite

0.24 -5170.21 - 2620.80 - 11442.13 - 16720.37 -4180.26 2 -2ooh0.26 - 1240.19 - lo4b- -0.32 - 1166

-0.3 _ -500

Ref. , p. 232.Ref. .Ref. , p. 30.

behavior of hydrating cements is governed byreactions of the aluminate phases, particularlyC3A; the setting and early strength developmentbehavior is mostly dependent on the hydrationof silicates, particularly C3S.Because of the high reactivity of calcium alu-minate and the undesirable properties of someof the products formed (e.g. hexagonal C-A-H), the aluminate hydration reaction is carriedout in the presence of sulfate ions. The latterprovide control of the reaction rate through theformation of mixed aluminate sulfate productsnamely ettringite (AFt) and monosulfoalumi-nate (AFm), i.e. reactions (5) and (6)respectively. Calcium sulfate added to theclinker can thus be viewed as a first chemicaladmixture used to control the nature andproperties of the aluminate hydration products.Sulfates thus play a crucial role in cementhydration, and the influence of chemical admix-tures on any process involving sulfates may beexpected to be significant.Cement hydration: kinetic and mechanisticaspectsAs noted previously, the overall process ofcement hydration and setting results from acombination of solution processes, interfacialphenomena and solid-state reactions. To helpvisualize the influence of admixtures on cementhydration, it is useful to recall the main eventsof the hydration process, and the time scaleover which they occur. A schematic representa-

tion of the evolution of a Portland cementhydration reaction with time is reproduced inFig. 2 (adapted from Kondo and Diamon).The latter identifies five distinct stages, theboundaries of which are determined by sharpvariations in a reaction parameter, typically, theheat flux measured as a function of time. Thesefive stages correspond respectively to (timesshown are approximate):

I. Initial hydration processes (O-15 min);II. Induction period or lag phase (15 min-4 h);III. Acceleration and set (4-8 h);IV. Deceleration and hardening (8-24 h);V. Curing (l-28 days).

II

1:

III

/10

IV

\

V

ime (lx)Fig. 2. Schematic cement hydration thermogram show-ing five distinct stages in the hydration process.


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90 C. Jol i coeur; M .-A . Simard

In the following examination of mechanisticaspects, we will emphasize those which may bemost influenced by the presence of admixtures,particularly those occurring in the early hydra-tion processes (e.g. O-12 h).Stage I: Init ial hydrat ion react ions (O-15 mi n)In the first instants of the cement-water mixing,part of the easily solubilized components (Na,KS04, CaS04xH20) are dissolved into theaqueous phase. Simultaneously, the hydrolysisof the most reactive surface silicates and alumi-nates releases Ca2+ and OH-, furtherincreasing the solution concentration of thesespecies. Since silicate and aluminate ions aresoluble in highly alkaline solution, their solutionconcentrations will initially increase as the solu-tion pH rises; maximum concentrations of thesespecies however remain orders of magnitudelower than those of Na, K++, Ca*+, OH- orSO:-. The most significant events occurringupon water-cement contact may thus be viewedas the wetting of the highly hygroscopic cementparticles, and the solubilization of a variety ofionic species Na+, K+, Ca*+, SO:-, OH-, bycomplete (congruent) or selective (incongruent)solubilization of the various phases present incement. Surface hydrolysis and incongruent sol-ubilization processes quickly lead to theformation of a thin layer of intermediate amor-phous hydration products. Considering therelative reactivities of aluminates and silicates,the initial gel products should consist largely ofaluminates,2 though C-S-H gels can alsoform very rapidly on C3S6Beyond the initial solubilization phenomena,the formation of any of the solid hydrationproducts depicted in eqns (l)-(8) will begoverned by nucleation processes; the lattermay occur homogeneously from the solutionphase, or heterogeneously at a solid-solutioninterface. As noted earlier, an important initialnucleation process is the formation of calciumsulfoaluminates from Ca*+, SOi- andAl(OH), ions. In the case of ettringite, thereaction product can be nucleated from ahomogeneous solution,4715 or at the &A-solu-tion interface.*2 Similarly, the earlynucleation of gypsum (G, CaS04*2H20) follow-ing dissolution of hemihydrate (H,CaS04*0*5Hz0) can be initiated through solu-tion or interface (topochemical) phenomena.The rate of these nucleation processes, particu-

larly in homogeneous nucleation, depends onlocal concentration fluctuations, and on precip-itation-redissolution rates, to achieve a criticalsize nucleus.

Following the nucleation step, the growth ofthe hydration products (crystalline or amor-phous) will proceed at a rate determined by thebulk concentration of the solution species, theavailability of water and ionic species at thereaction sites (i.e. diffusion through the solu-tion, or through the reaction boundary), theactivation energy for the molecular processesinvolved in the crystallization (e.g. desolvationof the ionic species) and statistical effects (e.g.orientational requirements).

As may be expected, the presence of organicadmixtures which can interfere with the nuclea-tion and/or the growth processes will influencethe hydration reaction rate, the reactionproducts, or both.

The actual rate of the initial hydration pro-cesses (Stage I) is readily visualized in Fig. 3.The latter illustrates the heat evolved when aunit weight of clinker, or cement (from thesame clinker), is immersed in excess water (W/C = 15) in a stirred calorimeter equipped with afast response probe. In these systems, solubiliza-tion and early hydration are extremely rapid( < 1 min). Following the early heat burst,heat evolution continues at a highly reducedrate; since, in these systems, water is in largeexcess, it cannot be rate-limiting. The heat evo-lution observed beyond the first few minutesmay thus be taken to reflect the progressivehydration of cement particles coated with theinitial hydrate phases. As evidenced by theinitial rate data in Fig. 3, after the first fewminutes, the chemical admixture will be mostlyinteracting with hydration products.In the latter part of Stage I (N 15 min), thecement particles in a paste become fully coatedwith a layer of hydrate products. This protec-tive layer hinders the diffusion of reactingspecies in and out of the reaction interphase,thus sharply reducing the rate of the variousreactions. The system enters into a period oflatency referred to as the induction, or dor-mant period.Stage I I: React ions duri ng t he inducti on peri od(15 mi n-4 h)As evidenced by a non-zero heat flux andchanges in electrical conductance,6* I7 the pro-


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Chemi cal adm ixt ure-cement int eracti ons 91

0 10 20 30 40 50 60Time (min)

Fig. 3. Heat evolved (J/g solid) upon immersion of powdered clinker or cement in stirred excess water (W/C = 15).

cesses initiated during Stage I continuethroughout the induction period. In the earlypart of Stage II, reactions of the aluminatephase will predominate. In addition to growthof ettringite needles (solution or interface) andto progressive thickening of the surface gellayer, other early phenomena may be observed,typically:r3 if S04- concentration is too low,excessive nucleation and growth of C-A-Hproducts may occur (flash set); if SOi- con-centration is too high (hemihydrate, alkalisulfates), massive nucleation and growth ofgypsum crystals (CaS04.2H20) may beobserved (false set).In the presence of adequate SOi- contentand availability, several physico-chemical pro-cesses will later contribute to the evolution ofthe cement-water system,2 namely: continuedgrowth of ettringite crystals; increased produc-tion of C-S-H gel leading to an overlay (outerlayer) of C-S-H onto the initial aluminate-richgel layer; increasing Ca2+ and OH- concentra-tions in solution; AFt -+AFm conversion whenthe available SOf is consumed; developmentof osmotic and mechanical pressures on the gellayer, as the hydration front moves inwards.These processes will determine the behavior ofthe cementitious system, particularly its rheo-logical properties and its initial setting time.

Again, it can readily be expected that theinteraction of a chemical admixture with any ofthe reactive species, or its interference with dif-fusion, nucleation and growth processes, canimpact significantly on the behavior of cementpastes and concretes during the inductionperiod.

St age I I I : Cement hydrat ion and sett ing(acceleration period, 4-8 h)Near the end of the induction period, the rateof cement hydration reactions sharply increases.The number and energy of the interactions(physical and chemical) between the growingparticles of the system also increase, rapidlyconverting the system into a stiff matrix (initialset). The transition from the dormant periodinto the acceleration stage in cement pastes isanalogous to that observed with pastes of pureC3S. In the latter case, the following effectshave been considered to explain the onset ofthe acceleration period.

(1)

(2)(3)(4)

Disruption of the hydrate protective layerby physico-chemical transformations ofthe hydrates (changes in composition(e.g. Ca2+ depletion) or structure).Break-down of the protective layer byosmotic pressure effects.Nucleation and growth of C-S-Hproducts.CH nucleation and growth.

Evidences reviewed and discussed by Tayloret al. 9 favor a mechanism wherein the integrityof the initial protective hydrate layer is alteredby phase transformation; the latter simultane-ously promotes C-S-H nucleation and growth,while exposing further unhydrated C3S.The phenomena which drive the accelerationstage of hydrating cement particles are con-sidered analogous to those occurring in C3Shydration. However, the critical conditions


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92 C. Jol i coeul ; M .-A. Simar d

required to initiate the acceleration processes incement pastes involve more complex reactionsthan in a C3S paste. In cement systems, as theinitial solubilization-hydration reaction pro-ceeds (Stages I, II), the concentrations of manyionic species are building-up simultaneously inthe pore solution; this likely leads to a super-saturation condition with respect to a variety ofhydrate products. The occurrence of coupledsaturation equilibria will provide linkagebetween the different nucleation-growth pro-cesses which could either catalyze, or inhibit,the growth of the main hydrate products. Also,the greater complexity of the protective layeron hydrating cement particles relative to C3Sparticles can provide additional mechanismsthrough which the protective layer can be dis-rupted. For example, hydration of thealuminate phases consumes large quantities ofwater [eqns (3)-(6)]; the continued formationof these hydrates at the inner surface of theprotective layer (reaction front) will involveconsiderable volume expansion and outwardmechanical pressure on this gel layer.These, and other processes which may beinvoked to explain the self-catalytic behaviorof cement hydration in the acceleration stage,may also be affected by chemical admixtures. Apotential role of admixtures may be envisagedat several levels; first, the admixtures may influ-ence the formation and properties (structure,permeability, strength) of the protective hydratelayer; second, admixtures remaining in the poresolution at the onset of the acceleration period,or admixture released during the transforma-tions of the initial hydrate phases, may furtherinfluence nucleation and growth of the hydra-tion products.

CHEMICAL ADMIXTURES FORCONCRETEChemical compounds are frequently introducedin concrete for the purpose of altering one ormore of its properties in the fresh or hardenedstate. These chemical admixtures, often foundby trial and error, have been classified accord-ing to the specific function they perform,typically: water reduction, set retardation oracceleration, fluidification, air entrainment,corrosion inhibition, washout prevention, imper-meabilization, shrinkage control, freeze-thawresistance, etc. The mode of action and relative

performances of many of these admixtures havebeen investigated in various cementitioussystems.2T2The American Concrete Institute provides a

broad classification22 in terms of air entrainingadmixtures (AEA), accelerating admixtures,water reducing (WR) and set-controlling admix-tures, admixtures for flowing concretes andmiscellaneous other admixtures (e.g. gas-form-ing, expansion-producing, permeability-reduc-ing; corrosion-inhibiting; reducing alkali-aggre-gate reaction, etc.).For specific groups of admixtures, moredetailed classifications have been proposed. Forexample, the American Society for Testing andMaterials has issued a standard specification forchemical admixtures of several product groups(Types A-G), which defines more specificallytheir functional properties:23 A, water reducing;B, retarding; C, accelerating; D, water reducingand retarding; E, water reducing and accelerat-ing; F, high-range water reducing; G, high-rangewater reducing and retarding.

Other functional sub-classifications haveoccasionally been suggested, for example, first,second and third generation high-range waterreducers, depending on their performances andsecondary properties.While the functional classifications are usefulfor application purposes, they are of limited usein formulating concrete mixes with cementshaving different compositions and reactivities,or mixes which incorporate several differentadmixtures to achieve a desired range ofproperties. The successful use of admixturesunder a broad range of conditions requires aminimal understanding of the basic concepts ofcement chemistry and admixture-cement inter-actions.In the following sections, we outline severalphysico-chemical concepts which often apply toadmixture-cement interactions; we will alsoattempt to illustrate how chemical admixturescan interfere in cement hydration processes andin particle-particle interactions. In doing so, wewill focus on three main groups of organicchemical admixtures, namely those used to con-trol:

hydration rate and setting time - retarders(e.g. sugars, hydroxyacids);concrete workability and porosity - waterreducers and superplasticizers (e.g. ligno-sulfonates, salts of polynaphthalene sulfonic


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Chemi cal admi xtur e-cement int eracti ons

acids, of polymelamine sulfonic acids, ofpolyacrylic acids, etc.);freezing and thawing behavior - airentraining agents (e.g. anionic surfactants).The physico-chemical effects examined belowmay apply, in part, to inorganic admixtures (e.g.calcium salts, phosphate or phosphonate salts),which are sometimes incorporated to alter thesetting and hardening behavior of cementpastes. However, reactions involving theseadmixtures in cementitious systems tend to bespecific; hence, inorganic admixtures are lessamenable to classification into generic groups.

BASIC CONCEPTS IN ADMIXTURE-CEMENT INTERACTIONSChemical admixtures can influence cementhydration and setting in several ways, some ofwhich have been envisaged above. An overviewof the main physico-chemical effects involved ispresented below and illustrated schematically inFig. 4. To emphasize the fact that, after the firstminutes of the hydration reactions, admixture-cement interactions are best described asinteractions between admixtures and cementhydration products, all illustrations show thesolid-solution interface with a hydrated layer.Surface adsorptionMost organic admixtures added in cementpastes exhibit an affinity towards the surface ofcement particles, or cement hydration products;this results in significant adsorption which isdepicted in Fig. 4(a) for two types of admix-tures. Organic molecules bearing chargedgroups (e.g. SO,, COO-) can interact with theparticle surface via electrostatic forces (surfacecharges of the particles and ionic groups of theadmixture molecule). Polar functional groups(e.g. OH) of organic molecules (e.g. sugars) canalso interact strongly with the highly polarhydrated phases, through electrostatic forcesand hydrogen-bonding interactions. A typicaladmixture bearing both ionic (-COO-) andpolar (-OH) groups is sodium gluconate. Withpolymeric admixtures (e.g. lignosulfonates) con-taining hydrophobic, polar and ionic groups, theadsorption results from a sum of effects andmay be further assisted by entropy gains whichoften stabilize the adsorbed state.

Admixtures containingphobic groups (aliphaticexample surfactants used

93

significant hydro-or aromatic), foras air entrainingagents, can interact with the hydrating surface

either through their polar groups, or throughtheir hydrophobic moiety, depending on theprevailing nature of the surface (hydrophobic,hydrophilic).Consequences of admixture adsorption onsurface propertiesPrecluding chemical reactions between thecement phases and the organic admixture, theadsorbed compounds will alter the surfaceproperties of the cement particle and thus itsinteractions with the solution phase, as well aswith other cement particles. Typically, adsorbedanionic surfactants (e.g. AEA) and polymers(e.g. WR) will convey a net negative electricalcharge to the particle surface, i.e. negative zetapotential). This will induce repulsion betweenneighboring cement particles and thus contri-bute to increased dispersion [Fig. 4(b)]. In thecase of high molecular weight polymers, physi-cal interference (steric) will lead to additionalshort-range repulsive forces. Thus, both theelectrostatic and steric forces contribute tothe fluidification of the cement paste.24725 Thesteric effects can be seen to play an importantrole, since low molecular weight dispersantsusually exhibit weak water reduction and lowpaste fluidification properties. In general, therelative importance of electrostatic and stericeffects in particle-particle repulsion will dependon both the chemical nature of the polymer(composition, structure) and its molecularweight.In the case of AEA, adsorption of the sur-factant molecules at the cement-solutioninterface can induce molecular assemblies suchas found in lipid layers (or bilayers) andmicelles. These assemblies lead to formation offilms at air-solution, or solid-solution interfa-ces, schematically depicted in Fig. 4(c).Chemical processes in adsorption ofadmixturesBecause of the high reactivity of the variousmineral phases of cements, organic admixturescan participate in, or interfere with several dis-tinct chemical phenomena.


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(a)

I tCement Hydrate products

C. Jol i coeul ; M -A . Simard

\ Charged organicmolecule(e.g. WR admixture)

3Anionic slnfactante.g. air ntrainingagent)

Film formation at solid-solution(or air-soltion) nterface

Ioncomplexant

Cd)Steric and electrostaticparticle-particle epulsion

@I

Selectiveblocking ofreactive surface sites

Cement hydrationphases modifiedy admixture ntercalation

Fig. 4. Schematic illustration of various physico-chemical effects which may occur upon interaction of chemical admixtureswith cement particles at the interface (interphase) with the pore solution. Particle surfaces are shown with a hydrate layer,depicting the situation prevailing very shortly after contact between cement and water: (a) adsorption of organic molecules at thecement-solution interface; (b) particle-particle repulsion due to electrostatic forces (repulsion between like charges) and stericrepulsive forces; (c) layered molecular organization (films) resulting from adsorption of surfactants at solid-solution interfaces;(d) preferential adsorption of organic admixtures on specific surface sites; (e) complexation and solubilization of ionic species;(f) hydrate crystal nucleation and growth inhibition by adsorbed organic admixture; (g) intercalation of the admixture in thecement hydrate products with structural alteration.


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Chemi cal admi xtur e-cement int eracti ons 95

First, specific surface reactions can occur atthe most reactive sites of the cement particlesurface; this is illustrated schematically in Fig.4(d), with organic molecules chemically adsor-bed (reacted) at specific surface sites. As notedearlier, SOi- ions act as a specific admixturereacting preferentially with aluminate phasesexposed on the particle surface. Sulfonatedsuperplasticizers (e.g. polynaphthalene sulfo-nates [PNS]) have also been shown to interactmore specifically with aluminate phases,26-30competing with SO: ions. As evidenced by therate-controlling effect of SOi- ions on thehydration of C3A, specific cement-admixtureinteractions can exert a profound influence onthe cement hydration rate from the very begin-ning of the hydration process.

Many organic admixtures, for example sugarsand hydroxy carboxylic acids, can help solubilizeionic species (e.g. Ca2+, SiOz-, A~(OH)L~),through association or complexation, as illus-trated schematically in Fig. 4(e). On the onehand, complexation reactions can accelerate dis-solution processes and initial reaction rates (e.g.sugar adsorption on C3A3). On the other hand,complexation allows higher concentrations ofthe ionic species in the solution phase, whichcan delay the precipitation of insoluble hydrates[e.g. Ca(OH)2, C-S-H]. However, complexationreactions are usually stoichiometric, so theirinfluence vanishes once the admixture has beenconsumed.Whether the adsorption of admixtures occursin a specific, or in a non-specific way, it is likelyto influence the cement hydration processes.The presence of organic molecules at the solid-solution interface can inhibit crystal nucleationand growth [Fig. 4(f)]. Adsorption occurring ona nucleation center may prevent the nucleusfrom achieving a minimum critical size. On theother hand, the growth of hydrate products inthe presence of sorbed admixtures may result instructural alteration by intercalation depicted inFig. 4(g), and/or changes in the morphology ofthe hydrate particles.30332 It is noteworthy, how-ever, that adsorption of either lignosulfonate,sugar or gluconate does not appear to retardthe formation of ettringite.33CHEMICAL ADMIXTURE-CEMENTINTERACTIONS: SELECTED EXAMPLESThe manifestations of admixture-cement inter-actions have been extensively investigated in

dilute slurries and pastes of cements, or of pureC3S, C3A, etc. phases.477V24-36Selected experi-mental observations are presented below whichillustrate the consequences of admixture-cement interactions on the hydration reaction,on the rheological behavior and on the settingof cementitious systems. The data shown per-tain mostly to PNS superplasticizers and will bediscussed in relation to the concepts outlinedabove on cement hydration and on the behaviorof organic molecules at reactive solid-solutioninterfaces.Initial admixture-cement interactions Stage I)The adsorption of a PNS superplasticizer onOPC, and OPC with 8% silica fume (OPC-SF)and silica fume (20 m2/g), is illustrated in Fig. 5.The data were obtained 10 min after initial con-tact of the material with the solution containingthe admixture. The quantities adsorbed werenormalized to unit BET (N2) surface area ofthe dry solids (mg/m2) and plotted as a functionof the free PNS concentration ([PNS],,,,) insolution at the time of the measurement.

Under the conditions of the experiment (nearneutral pH, negative surface potential), silica(silica fume or powdered quartz) adsorbs rela-tively low amounts of PNS, independently ofthe solution concentration; the adsorption datafor the cements, normalized at unit surfacearea, show 5-10 fold higher PNS adsorptionvalues, the latter depending on the solutionconcentration of the admixture. It is noteworthythat the normalized adsorption of the cement

2 5 I

Fig. 5. Adsorption of PNS measured at 10 min in pastesof cements (OPC, OPC+8% SF) and silica fume (20 m*/g) as a function of free PNS concentration in solution.Cement pastes: W/C = 0.35;42; SF paste: SF/W = 0.5,pH 13 (data derived from ref. ).


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96 C. Jol i coeul ; M -A . Simard

containing silica fume (the latter accounting fora large fraction of the surface area) is compar-able to that of the OPC. This has beenattributed to the influence of Ca2 ions whichpromote PNS adsorption on silica and on otherinert minerals.37-40As noted in the preceding section, theadsorption of PNS (and many other chemicaladmixtures3) occurs preferentially onto the alu-minate phases. It has also been reported todepend on the alkali content (Na2S04, K2SO4)of cements; in the presence of these salts, theamount of adsorbed PNS is reduced.4

The consequences of initial adsorption of thePNS admixture can be seen in various ways,namely in paste rheological properties and inthe early hydration rates, as illustrated in Fig. 6and Fig. 7.The fluidity of cement pastes (same pastes asin Fig. 5) and of a paste of ground quartz(mean diameter 8 ,um; W/Q = 0.38) is illustratedin Fig. 6 as a function of the total amount ofPNS added to the system. Due to the repulsiveforces (electrostatic, steric) induced betweenparticles by adsorbed PNS molecules [Fig. 4(b)],the fluidity of all systems increases markedly inthe presence of the admixture. Clearly, how-ever, much greater amounts are required toachieve high fluidity in cementitious systems,relative to inert systems (quartz); this is consist-ent with the adsorption data shown in Fig. 5. Asmentioned above for PNS adsorption, thepresence of Ca2+ ions (CaC12) has a markedeffect on the fluidity of pastes of silica minerals(in this case, quartz). Also, a greater amount of

IOPC

/

0 0.2 0.4 0.6 0.8 1[PNS] (h)

Fig. 6. Fluidity (l/q) values obtained at shear rate-95 s- for pastes of cements (as in Fig. 5) or quartz (W/Q = @38), the latter having a surface area close to that ofthe OPC cement (here, the PNS concentration shown isthe total added) (data from refs 38,42).

PNS admixture is required to achieve particle-particle repulsion and fluidification with theOPC-SF pastes, in agreement with the largersurface area of this blended cement.

Figure 7 shows the initial heat of hydrationfor several systems under conditions identical tothose of data shown in Fig. 3, i.e. heat ofimmersion at W/C = 15. Figure 7(a) illustratesthe influence of CaSO, and a PNS admixtureon the initial heat of hydration of clinker(immersion in water, or in a solution of theadmixture). For comparison, the heat evolutionobserved with a pure powdered C3A sample isalso included in Fig. 7(a). The results of theseexperiments show that polynaphthalene sulfo-nates can sharply reduce the heat generatedduring the very early hydration processes. Theinhibitory influence of PNS on the surface

--I I I

Clinker + PNS I

-10(a) O lo 2o Timlfmin, 4o So 6o

0

-IO/ -I0 10 20 30 40 50 60@) Time (min)

Fig. 7. Heat evolved upon initial (contact) hydrationwhen cementitious materials are immersed in water or insolution of various admixtures A. The arrows indicate thebeginning of the experiment (data from ref. 27). (a) SO,:calcium sulfate; [SO:-] = 0.08 g/100 g solution; SO;*/c = 1.3%. PNS: polynaphthalene sulfonate;[PNS] = 0.42 g/100 g solution; PNSK = 6.9%. PNS+SO,:same concentrations as above. (b) PNS HMW, LMW:respectively high and low molecular weight PNS; LS: lig-nosulfonates; [A] = 0.42 g/100 g solution, AJC = 6.9%.


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Chemi cal adm ixt ure-cement i nteracti ons 97

hydration of clinker may even exceed thatobserved with CaS04 [the CaS04 solution con-centration was 4 mM, while that of PNS(monomeric unit) was 16 mM]; the combinedinfluence of CaS04 and PNS is comparable tothat of PNS alone.

As shown by the data in Fig. 7(b), the inhibi-tory effect of admixtures on hydration reactionsis also marked with the Portland cement. Theeffectiveness of admixtures is further seen todepend on their chemical nature and molecularfeatures; moreover, low molecular weight(LMW) PNS appears significantly more effect-ive than the high molecular weight (HMW)analog, at the same solution concentration.Lignosulfonates (LS) also induce a major reduc-tion of the initial heat of cement hydration.

The specific behavior of admixtures reflectedby the initial heat of hydration data stronglyargues in favor of selective admixture-cementinteractions involving: (1) preferential adsorp-tion at most reactive sites [Fig. 4(d)] and (2)efficient inhibition of nucleation and growth ofthe hydrate products [Fig. 4(f)]. Variations inthe shape of the hydration thermograms of dif-ferent admixtures (e.g. comparing LS and PNS)point to further specificity in the mode of actionof these admixtures. Although the latter effectshave not yet been investigated in detail, theobservations discussed above emphasize the sig-nificance of admixture-cement interactions inthe first instants of the hydration process (StageI).Admixture-cement interactions during theinduction period Stage II)The influence of admixture-cement interactionson phenomena occurring during the inductionperiod can be seen from the time dependence(15 min-2 h) of the reaction parameters used tocharacterize Stage I effects, namely adsorption,rheology and heat evolution.Fig. 8 illustrates the decrease of free PNSsuper-plasticizer in the solution phase of pastes(OPC, OPC-SF) as a function of time. Theadsorption data plotted in this way, i.e. frac-tional concentration of free PNS, furtherillustrate the importance of admixture adsorp-tion during the first few minutes of thehydration reaction. Beyond the rapid initialadsorption (to saturate the most reactivephases), the admixture uptake by the hydratingcement continues at a reduced rate. At that

0 5OPC + 0.8% PNS -

OPC-SF + 0.8% PNS

0.0 40 30 60 90time (mill)

Fig. 8. Evolution of the solution concentration of PNSin pastes of OPC and OPC-SF cements as a function oftime (W/C = 0.35; [PNS],,,,, = 0.8 wt%).42

stage, continued adsorption occurs, mainly dueto the growth of new hydrate particles.The progressive consumption of the PNSadmixture impacts on the reaction rates andrheological properties of the hydrating pastes.

This is readily seen from the evolution of pastefluidity data as a function of time in Fig. 9. Asthe solution concentration of free admixturedecreases, the fluidity of the cement pastes pro-gressively decreases with time. While slump lossis expected from the evolution of the cementhydration reactions (see below), the fluiditywould readily be maintained by a periodic re-dosage of small amounts of PNS into the paste.In a more detailed study of cement paste rheol-ogy as a function of time, temperature and PNSconcentration, fluidity measured by mini-slumpdata was found qualitatively correlated to[PNSI,,,,, beyond a certain threshold value ofthe latter. This threshold was found to depend

87 -- OPC + 0.8% PNS6 --

carmel98 yusuf

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