Reactions and reactivities of compounds in hydraulic cements

Solid State lonics 43 (1990) 31-35 North-Holland

REACTIONS AND REACTIVITIES OF C O M P O U N D S IN HYDRAULIC C E M E N T S H.F.W. TAYLOR Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB9 2UE, Scotland, UK Received 8 December 1989; accepted for publication 14 February 1990

The initial reactions with water of the compounds present in Portland and some other hydraulic cements are ones of proton transfer, in which the compounds act as bases. Factors affecting the reactivities of individual compounds include the charges on the oxygen atoms, other chemical or structural features, types and concentrations of defects and formation of protective layers of reaction products. Physical factors, such as particle size distribution and microstructure of the starting material, are also very important. Some hydraulic cements contain one or more constituents that act as bases and one or more that act as acids. With yet others, some or all of the initial processes are simple dissolutions.

1. Introduction Hydraulic cements are powders that set and harden through reactions with water, a n d which, if mixed with water in appropriate proportions, continue to develop strength even if stored in water after they have set. Much the most i m p o r t a n t is Portland cement, which is made by heating a mixture of limestone and clay, or other raw materials of similar bulk composition a n d sufficient reactivity, to a temperature at which partial fusion occurs. Over a century ago, LeChatelier concluded that the reactions with water took place by dissolution followed by precipitation of less soluble products. This hypothesis has subsequently been found to account satisfactorily for a wide range of data, though in some cases doubt has been expressed as to whether passage of material through a true aqueous solution phase occurs, at least in the later stages of reaction. Hydraulic cements are typically ground to a specific surface area of 3 0 0 - 5 0 0 m 2 k g - ~ a n d used at a water:solid mass ratio of 0.3-0.6. For a material to act as a hydraulic cement u n d e r these conditions, it must satisfy two conditions: it must react sufficiently rapidly with water, a n d the product must be of sufficiently low solubility and of such a microstructure that its mechanical strength, durability a n d other physical and chemical properties are adequate for the purpose for which it is to be used. Tricalcium silicate

(Ca3SiOs), which in a substituted form is the major constituent of Portland cement, meets both these requirements. O n the other hand, tricalcium aluminate (Ca3AI206), though highly reactive towards water, does not give a product with suitable properties when used alone, a n d y-dicalcium silicate (Ca2SiO4) scarcely reacts with water at ordinary temperatures. In this paper, we consider briefly the various types of initial reaction that occur when hydraulic cements are mixed with water a n d some of the factors that govern the reactivities of substances, especially calcium silicates and aluminates, that might be considered as potential hydraulic cements.

2. Physical factors affecting reactivity In comparing the hydraulic activities or reactivities of compounds, it is essential to take into account such physical factors as particle size distribution, mechanical stress, or textural features such as crystallite size, morphology or the presence of microcracks. Thus, [~-Ca2SiO4, which in substituted form is another major constituent of Portland cements, c o m m o n l y contains exsolution lamellae, in some cases composed of much more reactive substances [ 1 ]; this may greatly affect its reactivity. For a substance that forms one constituent of a mixture (as in

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H.F. W. Taylor / Compounds in hydraulic cements

Portland cement), additional factors, both physical and chemical, must also be considered, the former including, for example, the mono- or poly-crystalline nature of the grains and the extent to which the phase under consideration is encapsulated in other phases of differing reactivity. Studies on the behaviour of individual compounds are of value for predicting how those compounds will behave in a mixture, but only if these and other complicating factors are taken into account. Thus, the many studies aimed at establishing the comparative reactivities of Ca,SiO, polymorphs have given divergent results, probably because of the difficulty in separating the effects of polymorphic variation from these other factors [ 2 1.

3. Hydraulic cements acting as bases Early hypotheses concerning the anhydrous calcium silicates and aluminates attempted to relate reactivity mainly to the coordination number of the Ca2+ ion, or attributed high reactivity to irregular Ca coordination or to the presence of “holes” in the structure. As Jost and Ziemer [ 31 noted, these hypotheses were based in inadequate knowledge of the crystal structures, and were untenable. They themselves associated reactivity with the presence of Ca0 polyhedra sharing faces. However, calcium oxide can be highly reactive towards water, but the CaO, octahedra in it only share edges. A more promising approach starts from the premise that in reactions of calcium silicates or aluminates with water the initial step is the transfer of protons from water molecules to oxygen atoms in the solid, which thus acts as a base. Jeffery [4], while considering that irregularity of Ca coordination was an important factor, also noted that the high reactivity of Ca$iOs must be partly due to the presence in its structure of 02- ions linked only to Ca’+, which resulted in the formation of regions of structure similar to those in calcium oxide. Danilov [ 5 ] noted that the ability of the water molecule to transfer protons to the oxygen atom increased with decreasing electronegativities of the atoms to which the latter was attached. The transfer led to the formation of a protonated surface layer, which subsequently dissolved. Barret et al. [ 6 ] similarly postulated that the initial step in the reactions of Ca,SiOs or B-Ca2Si04 was the

protonation of some of the oxygen atoms, giving a superficially hydroxylated layer, which dissolved and was continuously regenerated as new surfaces were exposed. The reactivity of an oxygen atom towards protons will tend to increase with the magnitude of the negative charge that is localized upon it. Three major aspects of this relation will be noted: (i) The charge increases, as Danilov observed, with decreasing electronegativity of the atoms attached to the oxygen: hence, Ca,SiO,, in which some of the oxygen atoms are attached only to Ca, is more reactive than any of the Ca,SiO, polymorphs, in which all are also attached to Si. At least some of the Ca$iO, polymorphs (e.g., p-Ca2Si04) are moderately reactive, whereas Mg2Si04 is not. (ii) For compounds containing a given cation and a given compositional type of anion, such as the calcium silicates, the average charge on the oxygen atoms decreases with increasing condensation of the anion. It is thus lower in a-Casio3 (pseudowollastonite), B-Casio, (wollastonite) or the Ca3Si207 polymorphs (rankinite and kilchoanite) than in BCazSi04. The anions are Si30$- rings in a-Casio,, infinite SiO:- chains in B-CaSi03, SizO$- groups in rankinite, SiOi- and Si30y; groups in kilchoanite and only SiOj- groups in B-Ca,SiO,. This last compound is the only one of those listed that reacts significantly with water at ordinary temperatures. (iii) Oxygen atoms present in anions that are broadly similar as regards the degree of condensation will tend to carry a higher charge as the central atom of the anion becomes less electronegative. Thus, the calcium aluminates, Ca3A1206, Ca,ZA114033 and CaAl,O, are all highly reactive towards water. The anions are A160fi- rings in Ca3A1206 and three-dimensional frameworks in the other two compounds. As seen above, the calcium silicates containing condensed anions of any type, including even the smallest ( Si20q- ), are inert. On the other hand, the Ca,( PO4)z polymorphs, despite the fact that the PO:tetrahedra which they contain are uncondensed, are also inert.

H.F. W. Taylor / Compounds in hydraulic cements

4. Other factors affecting the reactivities of cements acting as bases All the major constituents of Portland cement, and the active ones of calcium aluminate cements, of which the chief is CaA1204, appear to react with water as bases. The differing reactivities of compounds reacting in this way cannot, however, be explained purely by the considerations noted above. Thus, 7CazSiO4 resembles 13-Ca28iO4 in having a structure built from Ca 2+ ions and isolated SiO 4- tetrahedra, but is virtually inert at ordinary temperatures. Such differences in reactivity may arise from subsidiary differences in crystal structure, such as that considered by Jost and Ziemer [ 3 ]. Other chemical factors likely to affect reactivity are the formation of surface layers of product and the presence of defects. Since the products of reaction of hydraulic cements are of low solubility, they tend to form initially as surface layers on the particles of starting material. The kinetics of reaction frequently show an initial fast reaction, followed by an induction period during which the rate of reaction is very low, and eventually by a renewed, more rapid reaction. For tricalcium silicate, while there is not complete agreement, many investigators have concluded that the retardation of reaction in the induction period is partly or wholly due to the protective effect of such a layer [7]. The formation of surface layers high in Fe203 possibly accounts for the decrease in reactivity with iron content that is shown by members of the Ca2(A1,Fe)205 solid-solution series and is discussed later in relation to the reactivities of slags. The effect of calcination temperature on the reactivity of calcium oxide is well known, and may be related to differences in crystal size and concentration of defects. Ca3SiO5 preparations likewise vary considerably in reactivity, even if physical factors such as particle size distribution are as far as possible kept constant. Reactivity can be influenced by the introduction of foreign ions in solid solution or by heat treatment, and in either case has been shown to be associated with the effects on types and concentrations of defects. Sakurai et al. [8 ] found that Cr substitution in Ca3SiO5 and other phases enhanced reactivity at early ages, and that it was associated with an increase in the concentration of screw dislocations and also caused the material to become a p-type-

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semiconductor. Reaction was shown to begin at points of emergence of screw dislocations and grain boundaries, and the electronic imperfections were considered to favour the release of Ca ions from the surface. Fierens and co-workers [ 9,10 ] also found that reactivity was related to the presence of electronic defects, which could be studied using thermoluminescence. Boikova and co-workers [ 1 l, 12 ] found that the reactivities of Zn substituted Ca3SiO5 preparations passed through maxima at certain Zn contents. Zn substitution also altered the polymorphic form of the Ca3SiOs, and the maxima coincided with transitions between the polymorphs and with concentrations of certain types of defects. The reactivity thus depended not so much on the amount of substituent or nature of the polymorph, as on the nature and type of defects that were introduced at compositions for which one of the polymorphs was nearing its limit of stability.

5. Blastfurnace slag Ifblastfurnace slag is quenched and ground to produce a material that is largely a finely divided glass, it has cementing properties. For brevity, we shall refer to this material as "slag". Used by itself, it does not possess significant cementing properties, but the latter are shown if a small proportion of alkali (e.g., NaOH), calcium hydroxide or Portland cement is added. The cementing action is further enhanced if calcium sulphate is present. The major oxide components of typical slags are CaO, SiO2, A1203 and MgO; for typical west European slags, the atom ratios are around 31 Ca2+, 22 Si 4+, 11 A13+ and 6 Mg 2+ per 100 O 2-, charge balance being completed by the other elements present. Although the reactivity is not purely a function of the bulk composition, it tends to increase as the latter becomes more basic, as indicated, e.g., by increase in the mass ratio of (CaO+MgO+A1203) to SiO2. The mechanism of the reaction with water is not well understood, but the increase in reactivity with basicity of composition suggests that, as with the calcium silicates discussed earlier, the initial step is the transfer of protons from water molecules to oxygen atoms of the slag, which thus acts as a base. The apparent contradiction between this conclusion and

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H.F. W. Taylor / Compounds in hydraulic cements

the fact that reaction takes place only to a significant extent in the presence of alkali can perhaps be attributed to the formation of a protective coating if the pH is below a certain value [ 13 1. The structures of slag glasses have often been discussed in terms of the classical theory of network forming and network modifying elements, according to which there is an incomplete, three-dimensional framework composed of 0, Si and other relatively electronegative elements, the charge of which is balanced by the Ca and other more electropositive elements. Consideration of the typical composition given above shows this description to be unrealistic, the ratio of (Si+ Al) to 0 being such that nothing approaching a three-dimensional (Si,Al)-0 framework can be present. The model of Dron and Brivot [ 141, according to which the network forming anions are present in straight or branched chains of various lengths, is more realistic. Experimental studies [ 151 indicate that most of the Si is present in either isolated tetrahedra or SiZO, groups, if atoms other than Si and 0 are not counted as parts of the anion. On the acid-base theory discussed earlier, one might expect reactivity to increase with the fraction of the 0 atoms bound only to Ca and a single Si, or especially, to Ca and a single Al atom.

6. Hydraulic constituents

cements containing

acidic and basic

the maintenance of a sufficiently high pH, which is ensured by alkali and calcium hydroxides released from the cement. The silicate ions ultimately enter calcium silicate or aluminosilicate hydrates. The reaction of the major constituent of the cement (substituted Ca,SiO,) is accelerated. This may be partly due to the action of the siliceous material in removing Ca 2+ ions from the system, but the main effect is probably the provision of nucleation sites for the hydrated phases produced by the reaction. In support of this view, many other fine powders, including ones that are chemically inert, alio accelerate the reaction of the Ca3Si05 [ 161. Phosphate cements provide further examples of hydraulic cements containing both acidic and basic constituents. If the strongly basic tetracalcium phosphate (Ca,(PO,),O) and a more acidic calcium phosphate, such as dicalcium phosphate ( CaHP04) are together mixed with water, they react to produce hydroxyapatite (Ca, ( P04) 30H ) [ 17,18 1. The latter is closely related structurally to the inorganic constituent of tooth or bone, and cements of this type have important potential applications in dentistry and medicine. Other phosphate cements have been used in the refractories industry and for patching roads and other applications. One of the simplest is based on a mixture of alumina and phosphoric acid [ 181, but other constituents may be used, the basic ones including oxides such as CaO or MgO and the acidic ones acid phosphates such as NH4H2P04 or Al(H,PO,),

Some hydraulic cements contain one or more constituents that react with water as bases and one or more that react as acids. Portland cement may be mixed with suitably reactive materials high in SiOl, such as natural pozzolanas, fly-ash (largely glassy and finely divided ash from coal-burning power stations ) or microsilica (condensed silica fume ) , which is a waste from the production of silicon or its alloys. The siliceous material reacts as an acid, the initial step being presumably the attachment of a hydroxyl group provided by a water molecule onto a Si atom. This is followed by the breaking of one of the existing bonds to 0 from the Si atom, and this process, repeated a sufficient number of times, leads to the breakdown of the three-dimensional framework of the siliceous material and ultimately to the passage of separate silicate ions into the solution. It requires

1191.

7.Other types of hydraulic

cements

Several materials approximating in nature to hydraulic cements set and harden through processes in which the initial step for some or all of the constituents is a simple dissolution process. Thus, calcium sulphate plasters set and harden through the dissolution of hemihydrate ( 2CaS04-Hz0 approximately) or y-CaSO, and precipitation of the less soluble gypsum (CaS04-2Hz0). In magnesium oxychloride or Sore1 cement, the starting materials are magnesium oxide and magnesium chloride, the latter normally supplied as a solution. The products depend on the composition, but are most commonly Mg,(OH)loClp-8H20andMg(OH)2 [20].Onlythe magnesium oxide acts as a base.

H.F. W. Taylor / Compounds in hydraulic cements

References [ 1 ] C.J. Chan, W.M. Kriven and J.F. Young, J. Am. Ceram. Soc. 71 (1988) 713. [2] J.P. Skalny and J.F. Young, in: 7th Internat. Congress on the Chemistry of Cement, Paris, 1980, Vol. 1 (Editions Septima, Paris, 1980) p. II-1/3. [ 3 ] K.H. Jost and B. Ziemer, Cem. Concr. Res. 14 ( 1984 ) 177. [4] J.W. Jeffery, Acta Cryst. 5 (1952) 6. [5] V.V. Danilov, in: 6th Internat. Congress on the Chemistry of Cement, Moscow, 1974, Vol. 2 (Russian with English preprints; Stroyizdat, Moscow, 1976 ) part 1, p. 73. [ 6 ] P. Barret, D. M6ndtner and D. Bertrandie, Cem. Concr. Res. 13 (1983) 728. [7] H.F.W. Taylor, P. Barret, P.W. Brown, D.D. Double, G. Frohnsdorff, V. Johansen, D. M6n6trier-Sorrentino, I. Odler, L.J. Parrott, J.M. Pommersheim, M. Regourd and J.F. Young, Mat6r. Constr. (Paris) 17 (1984) 457. [8 ] T. Sakurai, T. Sato and A. Yoshinaga, in: Proc. 5th Internat. Symp. on the Chemistry of Cement, Tokyo, 1968, Vol. 1 (Cement Assoc. of Japan, Tokyo, 1969) p. 300. [9] P. Fierens and J.-P. Verhaegen, J. Am. Ceram. Soc. 55 (1972) 309. [ 10] P. Fierens, J. Tirlocq and J.P. Verhaegen, Cem. Concr. Res. 3 (1973) 549. [ 11] A.I. Boikova, M.G. Degen, V.A. Paramonova and V.A. Sud'ina, Tsement ( 1978 ) 3. [ 12] A.I. Boikova, in: 8th Internat. Congress on the Chemistry of Cement, Rio de Janeiro, 1986, Vol. 1 (Abla Gr~fica e Editora, Rio de Janeiro, 1986) p. 19.

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[ 13] R. Kondo and S. Ueda, in: Proc. 5th Internat. Symp. on the Chemistry of Cement, Tokyo, 1968, Vol. 2 (Cement Assoc. of Japan, Tokyo, 1969) p. 203. [ 14] R. Dron and F. Brivot, in: 7th Internat. Congress on the Chemistry of Cement, Paris, 1980, Vol. 2 (Editions Septima, Paris, 1980) p. III-134. [ 15 ] H. Uchikawa, S. Uchida and S. Hanehara, in: 8th lnternat. Congress on the Chemistry of Cement, Rio de Janeiro, 1986, Vol. 4 (Abla Grafica e Editora, Rio de Janeiro, 1986) p. 245. [ 16] N. Kawada and A. Nemoto, Semento Gijutsu Nenpo 22 (1968) 124. [ 17] W.E. Brown and L.C. Chow, Cem. Res. Prog. 1986 (1987) 351. [ 18 ] P.W. Brown, in: Advances in cement manufacture and use, ed. E. Gartner (Engineering Foundation, New York, 1989) p. 67. [ 19] W. Kurdowski, C.M. George and F.P. Sorrentino, in: 8th Internat. Congress on the Chemistry of Cement, Rio de Janeiro, 1986, Vol. 1 (Abla Gr~ifica e Editora, Rio de Janeiro, 1986) p. 292. [20] D. M6n6trier-Sorrentino, P. Barret and S. Saqout, in: 8th Internat. Congress on the Chemistry of Cement, Rio de Janeiro, 1986, Vol. 4 (Abla Gr~fica e Editora, Rio de Janeiro, 1986) p. 339.