La Matrice Cimentaire

8/12/2019 La Matrice Cimentaire

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L a matrice cimentaire base de lianthydraulique, appele galementpte de ciment, correspond unassemblage de phases solides, majori-tairement des hydrates. Lenchevtre-ment de ces hydrates assure la cohsiondes matriaux, tels que mortiers et b-tons, et confre cette matrice le rledunevritable colle, lorigine de perfor-mances mcaniques parfois trs leves.Cependant, aussi compacte soit-elle, lamatrice cimentaire reste, des degrsvariables, un milieu poreux, au sein du-quel peuvent se produire des transportsde matire (solutions liquides, gaz et par-ticules collodales), des ractions chimi-ques ou des changements de phase.Ces phnomnes entranent des modifi-cations des proprits physiques et physi-cochimiques pouvant parfois affecter

srieusement la durabilit du matriau.Ainsi, lobtention de bonnes performancesmcaniques et une durabilit suffisante

ncessitent le choix dun ciment et uneformulation du matriau cimentaireadapts aux conditions environnantes.

LES CONSTITUANTS PRINCIPAUXDES CIMENTS

La norme EN 197-1[1]spcifie vingt-septtypes de ciments courants.

Le clinker

Le clinker, constituant de base des ci-ments Portland courants, est issu de lacuisson haute temprature (1 450 C),

appele clinkrisation, d'un mlanged'environ 80 % de calcaire et de 20 %d'argiles. Sa fabrication peut tre raliseselon quatre mthodes : par voie humide,

semi-humide, sche (la plus courante) etsemi-sche.

Le clinker est compos de quatre phasesminrales majeures, dites hydrauliques.Il renferme galement de faibles quantitsdalcalis (Na2O, K2O) et des traces demtaux lourds. Il peut renfermer, par ail-leurs, de faibles proportions de chaux vive(CaO) et/ou de magnsie (MgO). Cesdernires sont strictement contrles ;

leur teneur est limite par crainte de gon-flement en prsence deau. La distinctionet la caractrisation des diffrentes pha-ses du clinker peuvent tre ralises parmicroscopie optique sur sections polies(voir figure 1). La notation cimentire estindique entreparenthses : C : CaO,S:SiO2, A : Al2O3, F : F e2O3.

A la sortie du four, le clinker est finementbroy avec environ 5 % de gypse (sulfa-te de calcium) afin de rguler sa prise.Le produit ainsi obtenu est le cimentPortland. Mlang avec leau, le cimentdonne une pte de ciment compose dephases solides hydrates et dune phaseliquide interstitielle alcaline imprgnantun rseau de pores et de capillaires.

Les principales ractions dhydratationsont schmatises ci-aprs. La figure 2montre des hydrates forms dans unepte de ciment une semaine aprs gcha-ge avec un rapport eau/ciment = 0,5 (mi-crographie au microscope lectronique balayage, MEB sur fracture frache).

Par ailleurs, les ractions dhydratationsaccompagnent dun dgagement dechaleur, plus ou moins important selonla composition minralogique.

Toutes les ractions se produisent simul-tanment. Pour les deux premires, lescoefficients stchiomtriques du CSH(silicate de calcium hydrat) ne sont qu'ap-proximatifs, le CSH est peu ou pas cristal-lis et plusieurs types de CSH existent.Lhydratation du C3A, trs rapide, est r-gule par la prsence de gypse pour vi-ter un raidissement prcoce de la pte

de ciment. Celle du C3S seffectue gale-ment rapidement, ds la mise en contactavec leau. Le C2S et C4AF shydratentplus lentement. Lhydratation des grains

CIMENTS, BTONS, PLTRES, CHAUX N 890 Avril-Mai60

Au cur du ciment / Inside cement

LES COMPOSANTS DE LA MATRICE CIMENTAIRE(RAPPELS ET INTERACTIONS)

CEMENT MATRIX COMPOUNDS(REMINDERS AND INTERACTIONS) par / byNoureddine Rafa, ingnieur expert Durabilit Matriaux /Expert engineer on Material Durability, Direction Recherche & Innovation / Rechearch & Innovation Direction, Lerm (Arles) Laboratoire d'tudes etde recherches sur les matriaux / Materials Research Laboratory, Lerm (Arles)

Dans le dernier numro deCBPC (n 889, fvrier-mars 2008),notre saga des matriaux expliquait la naissance du cimentartificiel. Ltape suivante rappelle ici les diffrents composantsdu ciment et de la matrice cimentaire (clinker, ajouts, additionsminraux et adjuvants) et voque certaines interactionsentre ces composants.

In the latest issue of CBPC(N 889, February-March 2008),our saga on building materials covered the birth of artificial cements.This second stage will describe to us the different compoundsof cement and of their cementitious matrix (clinker, additives,mineral additions and admixtures) and discuss a number of reactions that occur between these compounds.

www.lasim.orgsocit de l'industrie minrale
http://www.lasim.org/http://www.lasim.org/http://www.lasim.org/


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de clinker pouvant se poursuivre pendantplusieurs mois, voire plusieurs annes.

Le dbut de prise correspond l'aug-mentation brusque de la viscosit de lapte de ciment et une lvation de satemprature. La fin de prise correspond

la transformation rgulire et progres-sive de la pte de ciment en un blocrigide. C'est le dbut du durcissement.

Ce dernier dure plus longtemps et varieen fonction de la nature du liant. Le CSHdveloppe la rsistance de la pte deciment, et reprsente environ 70 % de lamatrice cimentaire durcie. La portlandi-te participe aux rsistances trs jeunege, sa teneur est de lordre de 20 %. Lesaluminates et les sulfo-aluminates de cal-cium hydrats (ettringite et monosulfo-aluminates) participent galement aux

rsistances mcaniques, et reprsen-tent environ 10 % de la matrice durcie.

Les ajouts ou additionsminrales

Les ciments, dont le constituant princi-pal est le clinker, peuvent contenir dau-tres lments minraux, sous formedajouts au clinker ayant ou non desproprits pouzzolaniques ou hydrau-

liques. Ces mmes constituants peuventtre introduits dans les btons et mortierssous forme dadditions.

Les proprits hydrauliques signifientque le matriau a la capacit de durciret de faire prise en prsence d'eau. Les

proprits pouzzolaniques signifient quele matriau peut se combiner, en prsen-ce deau, de la chaux tempratureambiante et lui donner des qualits hy-drauliques.

Ces constituants, quand ils se substi-tuent une partie du clinker, agissent surles proprits du matriau ltat fraiset ltat durci. En effet, ils modifient leprocessus dhydratation du ciment, lanature et la structure des produits hydra-ts. Leur incorporation agit ainsi sur lou-vrabilit, la porosit, la permabilit, la

diffusivit et les rsistances mcaniquesdes matriaux cimentaires.

Par ailleurs, leur utilisation constitue unatout dune part conomique, leur cottant infrieur celui du clinker, et dautrepart environnemental. On distingue : les additions ractives : elles ragissent

avec le ciment Portland et forment deshydrates. Ce sont les additions carac-tre hydraulique latent (laitier granulde haut fourneau) ou pouzzolanique

(cendres volantes siliceuses ou calci-ques, fumes de silice), qui corres-pondent principalement des co-produits industriels. Les pouzzolanesnaturelles correspondent des produitsd'origine volcanique (riches en silice eten alumine) possdant naturellementdes proprits pouzzolaniques, ou des produits (argiles et schistes parexemple) activs thermiquement. Cesderniers, sont appels galement pouz-zolanes artificielles ;

les additions quasiment inertes (fillerscalcaires) : elles ne ragissent pasavec le ciment Portland, mais modifientles proprits des matriaux, notam-ment en raison de leur fine granu-lomtrie qui assure un rle de remplis-sage, amliorant certaines propritsdu matriau frais et durci.

Laitier de haut fourneauLe laitier de haut fourneau est un co-produit de la transformation du mineraide fer en fonte brute. Il correspond toute la matire strile extraite du minerai.Lorsque cette dernire est refroidie trsrapidement par trempage, le laitier estessentiellement vitrifi (c'est--dire noncristallis) ; appel laitier granul, il poss-de des caractristiques intressantes entant que constituant principal du ciment[1]et en tant qu'addition minrale[2].

Le laitier granul de haut fourneau est uncompos caractre hydraulique latent,c'est--dire qu'une activation est nces-saire pour qu'il dveloppe de tellesproprits. En effet, l'ajout d'eau unchantillon de laitier vitrifi n'entraneaucun durcissement du mlange.

Pour faire prise, le laitier doit subir uneactivation calcique, sulfatique ou sulfo-calcique, ou par des alcalins. Deuxexemples de ractions d'hydratation etdactivation du laitier granul sont donnsci-aprs :

En prsence de ciment Portland, le laitierpeut tre activ par la chaux que librele clinker hydrat, et par le sulfate de cal-cium (rgulateur de prise). Le caractre

Ettringite [3CaO.Al2O3.3CaS O4.32H2O]Monosulfoaluminate de calcium hydrat[CaO.Al2O3.CaS O4.12H2O]Hydrogrenat [Al2O3.3CaO.6H2O]Hydrotalcite [Mg6Al2(CO3)(OH)16.4H2O]

Laitier granul + eau C-S-Hciment portland (activation)

Laitier granul + eau C-S-H, C4AH13chaux (activation)

CIMENTS, BTONS, PLTRES, CHAUX N 890 Avril-Mai 2008

LA SAGA DES MATRIAUX /THE MATERIALS SAGA

61

2

1

4 3

30 m

Silicate tricalcique (alite) /Tricalcic silicate (alite) 3CaO.SiO2 (C3S) 50 - 70 %Silicate bicalcique (blite) /Bicalcic silicate (belite) 2CaO.SiO2 (C2S) 15 - 30 %Aluminate tricalcique /Tricalcic aluminate 3CaO.Al2O3 (C3A) 2 - 15 %Aluminoferrite ttracalcique /Tetra-calcic aluminoferrite 4CaO.Al2O3.Fe2O3 (C4AF) 5 - 15 %

1 : C3S, 2 : C2S, 3 : C3A (gris / gray), 4 : C4AF (blanc / white)

1

23

4

Figure 2 Matrice cimentaire (MEB) /Cement matrix (SEM) C3S : 2 (3CaO.SiO2) + 6H2O 3CaO.2SiO2.3H2O + 3 Ca(OH)2

C-S-H PortlanditeC2S : 2 (2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2C3A : 3CaO.Al2O3 + 6H2O 3CaO.Al2O3.6H2O Aluminate de calcium hydrate

Hydrated calcium aluminate

3CaO.Al2O3 + CaSO4.2H2O + 10H2O 3CaO.Al2O3.CaSO4,12H2O Monosulfoaluminate de CaCalcium monosulfoaluminate 3CaO.Al2O3 + 3CaSO4.2H2O + 26H2O 3CaO.Al2O3.3CaSO4.32H2O Ettringite

1 : portlandite, 2 : C-S-H, 3 : aluminate de Ca hydrat /hydrated calcium aluminate , 4 : ettringite

Figure 1 Microscopie optique en lumire rflchie sur section polie d'un clinkerOptical microscopy in reflected light on polished sections of a clincker


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hydraulique (ractivit) du laitier estessentiellement influenc par son degrde vitrification, la composition chimiqueet minralogique des particules, ainsique par leur finesse.

Degr de vitrification

Il est directement li la vitesse de refroi-dissement du laitier. Un refroidissementlent entrane la cristallisation du laitier ;sa ractivit est alors faible. Par contre,

un refroidissement rapide entrane unestructure amorphe ou vitreuse confrantau laitier ses proprits liantes. D'unefaon gnrale, le laitier granul contienten gnral au minimum 75 % de phasevitreuse.

Composition chimique

La composition chimique est galementun facteur important, car elle influencele degr de vitrification obtenu lors de latrempe, ainsi que la ractivit du verreau cours de l'hydratation. Le tableau 1prsente les principaux oxydes contenusdans les laitiers, ainsi que leur plage devariation usuelle.

Quand les laitiers sont partiellementcristalliss, les principaux minraux

forms sont la merwinite (C3MS2) et lesmllilites, qui correspondent une so-lution solide avec un ple magnsien :akermanite (C2MS), et un ple alumi-neux : ghlenite (C2AS),

Cendres volantes

Les cendres volantes sont des co-pro-duits de la combustion du charbon pulv-ris dans les centrales thermiques. Elles

sont obtenues par dpoussirage lectro-statique ou mcanique de particulespulvrulentes provenant du courant degaz des chaudires, alimentes au char-bon pulvris. Les cendres obtenues pard'autres mthodes ne doivent pas treutilises dans les ciments conformes la norme NF EN 197-1.

Les cendres volantes se prsentent gn-ralement sous forme de sphres de verresolide (voir figure 4), et peuvent tre par-tiellement ou totalement cristallises.Ces sphres peuvent tre creuses oupleines et ont un diamtre compris entreun et 100 m. Comme le laitier granul,les cendres volantes ont besoin d'treactives pour dvelopper des propritshydrauliques. Leur activation par la chauxlibre lors de l'hydratation du clinkerconduit la formation des produits sui-vants :

Les cendres volantes utilises en tant queconstituant principal du ciment[1]et en tantqu'addition minrale[3, 4]peuvent tre denature silico-alumineuse ou silicocalcique.Les premires ont des proprits pouzzo-laniques, alors que les secondes, enplus, peuvent dvelopper des propritshydrauliques.

La composition chimique moyenne descendres volantes est difficile tablirdans la mesure o ces dernires sontfortement htrognes en raison de

plusieurs facteurs lis la combustiondu charbon (type de charbon, tempra-ture de fusion, temprature et vitesse derefroidissement).

Fume de silice

La fume de silice est un coproduit indus-triel de la fabrication du silicium mtal-lique ou de divers alliages de ferrosilicium.La fume de silice est produite lors de larduction du quartz trs pur par du char-bon dans un four arc lectrique. Elleest recueillie par filtration des gaz quis'chappent lors de la combustion.

La fume de silice se prsente gnra-

lement sous forme de particules sph-riques dont le diamtre moyen est del'ordre de 0,1 m (voir figure 5). Ces parti-cules sont essentiellement vitreuses, cequi en fait un produit hautement pouzzo-lanique.

Comme le laitier de haut fourneau et lescendres volantes, la fume de silice peuttre utilise en tant que constituant princi-pal du ciment[1] et en tant qu'additionminrale[5], notamment dans la fabrica-tion des btons hautes performances(BHP). Sa raction avec la chaux libreau cours de l'hydratation du cimentPortland permet de former des silicatesde calcium hydrats (CSH).

Cendres volantes + eau

C-S-H+C4AH13chau x (activation)


Oxyde / Oxide SiO2 Al2O3 CaO MgO Fe2O3 TiO2 MnO S Na2O + K2O Cl- F-

Quantit (% massique)Quantity (mass fraction %) 27 - 40 5 - 33 30 - 50 1 - 21


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Les fumes de silice sont en gnralconstitues de 85 98 % de silice. Ellesprsentent une surface spcifique del'ordre de 20 23 m2 /g.

Pouzzolanes

Les pouzzolanes naturelles sont essen-tiellement des substances dorigine volca-nique ou des roches sdimentaires ayantune composition chimique et minralo-gique appropries. Elles sont essentiel-lement composes de silice, dalumineet de fer, et dveloppent naturellementdes proprits pouzzolaniques. Les pouz-zolanes artificielles correspondent desproduits essentiellement composs desilice, dalumine et doxydes de fer qui,activs thermiquement, dveloppent desproprits pouzzolaniques (argiles ouschistes par exemple).

Les pouzzolanes ne durcissent pas elles-mmes en prsence deau, mais ellesragissent temprature ambiante enprsence d'eau, avec lhydroxyde decalcium Ca(OH)2 libr au cours delhydratation du clinker, pour former desCSH et des aluminates de calciumhydrats. Ces composs sont semblables ceux forms lors de lhydratation desmatires hydrauliques. Elles peuventtre utilises en tant que constituantprincipal du ciment[1]et en tant qu'addi-tion minrale[6].

Fillers

Les fillers sont des produits obtenus parbroyage fin ou par pulvrisation de cer-taines roches naturelles ou non, agissantsur certaines qualits du ciment (mania-bilit, diminution de la permabilit et dela capillarit, rduction de la fissurabi-lit) grce leur granulomtrie appro-prie. Les fillers les plus utiliss en Francesont des fillers calcaires[7]. Ils sont inertescar ils nont aucune action chimique surles ciments en prsence deau. Ils jouentessentiellement un rle de remplissage,permettant doptimiser la compacit desbtons.

LES ADJUVANTS Afin damliorer les performances dumatriau ltat frais et durci, des adju-vants sont de plus en plus ajouts auxmatrices cimentaires, en trs faibles quan-tits (moins de 1 % par rapport au poids

du ciment), gnralement lors du malaxa-ge. Les adjuvants sont des produits or-ganiques ou minraux classifis par lanorme europenne EN 934-2[8]. Gn-

ralement, un adjuvant est destin uneaction principale, mais il peut prsentergalement des actions secondaires (ef-

fets secondaires). Lefficacit et les effetssecondaires peuvent varier, notammenten fonction du ciment. Les mcanismesdinteraction adjuvant-ciment (ou adju-vant-adjuvant quand plusieurs adjuvantssont utiliss) ne sont pas encore totale-ment cerns. Il est gnralement conseil-l de raliser des essais de convenan-ce dans les conditions du chantier. Bienquil soit dune grande utilit pratique,ladjuvant nest pas un remde un mau-vais dosage ou une mise en uvreincorrecte.

La classification des adjuvants est lasuivante : plastifiants rducteurs deau : ce sont

des produits base de lignosulfona-te. Doss entre 0,3 et 0,5 % du poidsde ciment, ils se fixent par adsorption la surface du ciment, provoquant unedfloculation des grains et une lubrifi-cation de la pte, amliorant ainsi lamaniabilit en rduisant la quantitdeau (5 10%) ;

superplastifiants hautement rducteursdeau : ils provoquent les mmes effetsque les plastifiants, mais avec une in-tensit plus importante permettant unerduction deau de 15 25 %. Ce sontdes produits de synthse ; les plus uti-liss sont les rsines mlamines sul-fones, les naftalne-sulfonates et lesvinyl sulfonates ;

rtenteurs deau : gnralement utilissen poudre, ils permettent de rduire leressuage et damliorer la cohsion(des btons faible dosage en ciment) ;

entraneurs dair : ils permettent la for-mation et la stabilisation de trs nom-

breuses bulles dans la matrice (diam-tre compris entre quelques microns etquelques dizaines de microns). Ilsaugmentent la rsistance au gel et

amliorent galement la plasticit etlouvrabilit ;

acclrateurs : ils augmentent la vites-se dhydratation et entranent une priseplus rapide. Deux types se distinguent :les acclrateurs de prise (alcalis,carbonates et sulfates de sodium ou

de potassium), et les acclrateurs dedurcissement (chlorure et carbonates).Les adjuvants base de chlorures sont viter dans les btons arms. Lutili-sation des acclrateurs induit uneaugmentation des rsistances initiales,mais peuvent entraner une lgrediminution des rsistances 28 jours ;

retardateurs : ils retardent plus oumoins lhydratation et le dbut de prise.Ils compensent les effets de hautetemprature et prolongent la priodeavant la prise (utiliss pour le pompa-ge des btons pour les grandes struc-

tures). Ce sont les mtaux lourdsPb, Zn, Sn, sucre, acide gluconate,tartrique.

T he cementitious hydraulic binder based matrix, also known as cement paste, corresponds to a solid phase assembly, consisting primarily of hydrates. The entangled hydrates provide adhesion of the materials, such as in mortars and concretes, and truly act as glue, and sometimes are at the origin of some very high mechanical performances. Nevertheless, no matter how com- pact it may be, the cementious matrix remains, more or less, a porous media,in which material transport (liquid solutions, gases and colloidal particles),chemical reactions, and phase variations can occur. These phenomena bring about changes in the physical and physico- chemical properties which can sometimes have a serious effect on the strength of the material.

Thus, to obtain good mechanical performances and sufficient strength requires selecting cement and a cementious material formula which are suitable for the overall conditions.

MAJOR CONSTITUENTS OF CEMENT

The EN 197-1 [1] standard specifies twenty seven traditional cement types.

CIMENTS, BTONS, PLTRES, CHAUX N 890 Avril-Mai 2008 63


Figure 5 Aspect d'une matrice contenantdes fumes de silice (photo MEB)Aspect of a matrix containing silica fume (photo SEM)

E n

gl i sh


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Clinker

Clinker, a basic constituent of ordinary Portland cements, is obtained from firing a mixture of about 80% limestone and 20% clay at high temperature (1,450 C) through a process known as clinkerisa- tion, of a mix containing 80% of limestone and 20% clays. It can be produced by four methods: wet, semi-wet, dry (most fre- quent) and semi-dry.

Clinker is composed of four major mineral phases, known as hydraulic phases.It also holds low quantities of alkalis (Na 2 O, K 2 O) and traces of heavy metals.Moreover, it sometimes holds low proportions of quicklime (CaO) and/or magne- sia (MgO). The latter are strictly monitor- ed; their content needs to be limited for

fear of swelling in the presence of water.The distinguishing properties and the characterisation of the different phases can be determined by optical microscopy on polished thin sections (see figure 1).

After being discharged from the kiln the clinker is finely ground with about 5% gypsum (calcium sulphate) in order to regulate its setting properties. The product thus obtained is Portland cement. Mixed with water, cement yields a blended cement paste constituted of three solid hydrated phases and of one alkali inter- stitial liquid phase that impregnates a network of pores and capillaries.

The major hydration reactions are set- out hereinafter. Figure 2 shows hydrates formed in a cement paste one week after mixing with a water/cement ratio of 0.5),(micrograph taken with a SEM (scanning electron microscope) on a fresh broken sample).

Besides, hydration reactions release a more or less significant amount of heat depending on the mineral composition of the cement.

All of the reactions occur simultaneous- ly. During the first two reactions, the calcium-silicate-hydrate (C-H-S) stoichime- try is only approximate, CSH does not crystallise much or not at all, and several types of CSH are present.

The very fast C 3 A hydration, is slowed by the addition of gypsum to avoid early stiffening of the cement paste. C 3 S hydration also occurs rapidly, immediately on

contact with water. C 2 S and C 4 AF hydrates more slowly. The hydration of clinker grains may carry on for several months or years.

Initial setting corresponds to the sudden increase in the cement paste's viscosity and to its increase in temperature. The final setting corresponds to the regular and progressive transformation of the cement paste into a rigid block. This is the beginning of the hardening process.The latter takes longer and changes depending on the nature of the binder.CSH increases the strength of the cement paste, and represents about 70% of the hardened cement matrix. Portlandite contributes to very early strengths; its content is about 20%. Aluminates and hydrated calcium sulfl-aluminates (ettringite and mono sulfo-aluminates) also contribute to mechanical strengths, and represent about 10% of the hardened matrix.

Mineral additionsand admixturesCements, constituted mainly of clinker,may also contain other mineral compo- nents as additives to the clinker and may or may not have pozzolanic or hydraulic properties. These same constituents can be introduced in concretes and mortars as additives.

Hydraulic properties mean that the material has the capacity to harden and to set in the presence of water. Pozzonalic properties mean that the material can combine, in the presence of water, with lime at ambient temperature and to impart hydraulic properties to it.

These constituents, when substituted for part of the clinker, act on the properties of the material both in their fresh state and hardened state. Indeed, they change the cement's hydration process and the hydrated product's structure. Their incorporation thus acts on their workability,porosity, perviousness, diffusivity and on the mechanical strengths of cementious materials.

Moreover, their uses constitute both an economic advantage, their cost being less than for clinker, and an environ- mental advantage. We generally make a distinction between: added reagents: they react with the

Portland cement and form hydrates.These include additives having latent hydraulic properties (blast furnace slag) or pozzolanic (silicate or calcareous fly-ashes, silica fume), that mainly involve industrial by-products. Natural

pozzolans involve products of volcanic origin (rich in silica and in alumina) that have natural pozzolanic properties,or thermally activated products (clays

and schists for example). The latter, are also referred to as artificial pozzolans;

nearly inert additives (limestone fillers): they do not react with the Portland cement but they modify the properties of the materials, especially due to their fine particle sizing to work as a filler thus

improving some properties of the fresh- ly hardened material

Blast furnace slag

Blast furnace slag is a by-product that results from processing iron ore to make pig iron. It includes all of the waste extract- ed from the ore. When the latter is very rapidly cooled by quenching, the result- ing slag is basically vitrified and generally referred to as granulated slag (which means non cristallised). Granulated slag shows interesting characteristics as a

major constituent of cement [1] and as a mineral additive [2].

Granulated slag is essentially a com- pound with latent hydraulic properties, i.e.that it requires to be activated to achieve these properties. Obviously, adding water to a sample of vitrified slag will not cause the mixture to harden in any way. To set,the slag must be submitted to a calcareous, sulfatic or sulfo-calcareous activation or by alkalis. Two examples of hydration reactions and granulated slag are given hereafter:

In the presence of Portland cement, the slag can be activated by the lime which is freed by the hydrated clinker and by the calcium sulphate (a set regulator) The hydraulic properties of the slag (reactivity) are primarily influenced by its degree of vitrification, by its chemical and mineralogical composition of the particles as well as by their fineness.

Degree of vitrification

The degree of vitrification is directly proportional to the cooling speed of the slag. Slow cooling leads to the crystalli- sation of the slag; its reactivity in this

Ettringite [3CaO.Al 2 O 3 .3CaSO 4 .32H 2 O] Hydrated calcium monosulfoaluminate [CaO.Al 2 O 3 .CaSO 4 .12H 2 O] Hydrogrenat [Al 2 O 3 .3CaO.6H 2 O] Hydrotalcite [Mg 6 Al 2 (CO 3 )(OH) 16 .4H 2 O

Granulated slag + water C-S-H Portland cement (activation)

Granulated slag + water

C-S-H, C 4 AH 13 lime (activation)



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case is low. On the other hand, a rapid cooling of the slag will result in an amorphous or vitreous structure that gives the slag its binding properties. Generally, the granulated slag will usually contain a minimum of 75% of vitreous phase.

Chemical compositionThe chemical composition of the slag is also an important factor given that it influ- ences the degree of vitrification achieved during quenching as well as the reactivity of the glass during hydration. Table 1shows the major oxides contained in the slag as well as the range of their typical variations. The main minerals formed,when the slags are partially crystallised,are merwinite (C 3 MS 2 ) and melilites that include a solid solution with a magnesium pole figure: akermanite (C 2 MS), and an

aluminium pole figure: gehlenite (C 2 AS).

Fly ash

Fly ash is a by-product of burning pulverised coal in coal-fired power plants.It's obtained by electrostatic or mechanical precipitation of the fine particles collected from the gas stream of the pulverised coal-fired boilers. Fly ash obtained by other methods should not be used in cements according to the NF EN 197-1 standard.

Fly ash are usually in the form of solid glass spheres (see figure 4) and can be partially or totally cristallised. These spheres can be hollow or solid and have a diameter of between one and 100 m.Similarly to the granulated slag, fly ash also requires to be activated in order to achieve hydraulic properties. Their activation by the lime liberated during the hydration of the clinker leads to the formation of the following products:

The fly-ash used both as a major constituent of cement [1]and as a mineral additive [3, 4] may be of a silico-alumina or silico-calcareous nature. The former have pozzolanic properties whereas the latter, in addition to this, can achieve hydraulic properties

The average chemical composition of fly-ash is difficult to establish to the extent

that fly-ash is highly heterogeneous due to several factors which are linked to coal combustion (type of coal, melting temperature, cooling temperature and speed).

Silica fume

Fly ash + water

C-S-H+C 4 AH 13 lime (activation)

CIMENTS, BTONS, PLTRES, CHAUX N 890 Avril-Mai 2008 65

E n

gl i sh


Clay bricksand rooftilesManufacturingand properties

The objective of this book is tooutline the present state of thescience in this industry and tech-niques in the field as clearly aspossible. The book deals withraw materials (clay in the quarry

and additives in the clay mixture), process funda-mentals (mixture preparation, product shaping, drying, firing,etc.), machines and plants, and it also reviews the impact onthe environment and the industry's search for quality.The mechanical, thermal, hydric and durability properties of heavy clay products are also examined. The perspectivesthat new technological developments offer to users, and therecent product evolutions are also reviewed. Finally, thenew European standardisation and the CE marking arebriefly described.This book is, above all, for those with a basic scientificknowledge and who are not familiar with this material, butuse it or have an interest in it: ceramic engineers, buildingengineers and technicians, architects, real estate experts,students, and anyone concerned by tiles and bricks.This book is a collective work: it was written by Dr MichelKornmann, former Technical Director of the CTMNC(Technical Centre for Construction Materials), with the helpof a number of engineers from the Technical Centre forConstruction Materials specialised in ceramics.

Size: 160 x 250 mmNumber of pages: 292 - 72 figures and 71 tables + numerous appendicesISBN : 2-904845-32-1 - Unit selling price: 45 (Excluded VAT) + shipmentSocit de l'Industrie Minrale - 17 rue St-Sverin - 75005 Paris - FranceTel. +33 (0)1 53 10 14 74 - Fax +33 (0)1 53 10 14 71Email : [email protected] order form on:www.lasim.org/docs/bdc/bdc_kornmann_en.pdf

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7/7

Precipitated silica fume is an industrial by-product of metallic silicon production or of various ferrosilicon alloys. It is produced during the reduction of very pure quartz with coal in an electric arc furnace and recovered by filtering the combustion exhaust gases.

Silica fumes usually come in the shape of very finely divided spherical particles that have an average diameter of about 0.1 m (see figure 5). These particles are basically vitreous in nature, which makes it a highly pozzolanic product.

Similarly to blast furnace slag and to fly ash, precipitated silica fumes can be used as a major constituent of cement [1] and also as a mineral additive [5] , especially for making high performance concretes (HPC). Its chemical reaction with the lime

freed during the hydration of Portland cement makes it possible to form hydrated calcium silicates.

Precipitated silica fumes usually contain 85 to 98% silica. They have a specific surface area of about 20 to 23 m 2 /g.

Pozzolana

Natural pozzolana are basically products of volcanic origins or sedimentary rocks that have the appropriate chemical and mineralogical composition. They are primarily composed of silica, alumina and iron, and are able to naturally achieve pozzolanic properties. Artificial pozzolana correspond to products essentially composed of silica, alumina and iron oxides,which thermically activated, develop pozzolanic properties (clays or schists for example).

Pozzolana do not harden on their own when mixed to water, but they do react,when mixed with water at ambient temperature, with the calcium hydroxide Ca(OH)

2 that is freed during the hydra-

tion of the clinker, to form CSH and hydrated calcium aluminates. These compounds are similar to those formed during the hydration of hydraulic materials. They may be used as major constituent of cement [1] and as a mineral addition [6] .

Fillers

Fillers are products that are obtained by fine grinding or by pulverizing rocks found in nature or not, that react because of

some of the properties of cement (workability, reduction of perviousness and capil- larity, reduction of cracking) thanks to their suitable particle sizing. The most

used fillers in France are calcareous fillers [7] . These fillers are inert given that they do not react chemically with cements in the presence of water. They basically play a filling role that allows optimis- ing the compactness properties of concretes.

ADMIXTURES In order to improve the performances of the material in its fresh and harden state,admixtures are more and more often added to the cementious matrices, in small quantities (less than 1% as com- pared to the cement weight), usually during the mixing operation. Admixtures are organic or mineral products classed under European standard EN 934-2 [8].

Generally, an admixture is designed for a main purpose, but secondary benefits can also result from its addition. The efficiency and the secondary benefits can vary, depending in particular on the cement. The admixture/cement interac- tion mechanisms (or admixture-admixture when several admixtures are used) are not as yet quite understood. Its usually recommended to carry out suitability tests for given site conditions. Although they are very useful in practice, admixtures are not to be confused as a remedy against poor mixing or implementation practices.

Admixtures are classified as follows:

water reducing plasticisers: these are lignosulfonate based products When added in quantities of between 0.3 and 0.5% by weight of cement, they bond by adsorption to the cement surface,bringing about a deflocculation of the particles and a lubrification of the paste,thus improving workability by reducing the amount of water (5 to 10%);

highly water reducing superplasticizers: they bring about the same effects as the plasticizers but with greater inten- sity and make it possible to reduce water by 15 to 25%. These are synthet- ic products; the most commonly used are sulfonated melamine resins, naph- talene sulfonate and vinyl sulfonate compounds;

water retentive agents: usually used in powder form, they make it possible to reduce bleeding and to improve cohe- sion (for concretes of low cement con- tents);

air-entraining agents: they make it possible to form and to stabilise a large number of air bubbles in the matrix (diameter between a few microns to some

tens of micron). They increase resis- tance to freezing and also improve plasticity and workability;

setting agents: they increase the hydration speed and cause the concrete to set more rapidly. There are two types of setting agents (alkalis, carbonates

and sodium or potassium sulphates),and hardening agents (chlorides, carbonates). Chloride based admixtures should be avoided in reinforced concretes. The use of setting agents results in an increase of the initial strength, but can also result in slightly lowering the 28-day strength;

retarders: they delay more or less the hydration and the setting of the cement mixtures. They compensate for the effects of high temperature and extend the period prior to setting (used for

pumping concretes for major struc- tures). They include heavy metals e.g.Pb, Zn, Sn, sugar, gluconic and tartar- ic acids.


Documents de rfrenceReference documents

[1]NF EN 197-1 (P 15-101-1) : Ciment- Part. 1 : Composition, spcifica-tions et critres de conformit deciments courants

[2]NF P 18-506 : Additions pour btonhydraulique - Laitier vitrifi moulu dehaut fourneau

[3]NF EN 450 (P 18-050) : Cendresvolantes pour bton - Dfinitions,exigences et contrle de qualit

[4]P 18-050-1 (NF EN 450-1 + A1) :Cendres volantes pour bton -Partie 1 : Dfinition, spcificationset critres de conformit

[5]P 18-502-1 et 2 (NF EN 13263-1et 2) : Fume de silice pour bton- Partie 1 : Dfinitions, exigences etcritres de conformit. Partie 2 :valuation de la conformit

[6]NF P 18-308 : Btons - Pouzzolane[7]NF P 18-508 : Additions pour bton

hydraulique - Additions calcaires.Spcification et critres de confor-mit

[8]EN 934-2 : Adjuvants pour bton,mortiers et coulis Partie 2 :

adjuvants pour bton dfinition,exigences, conformit, marquageet tiquetage.

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