Adsorption of calcium lignosulfonate on tricalcium aluminate and its hydrates in a non-aqueous medium

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Publisher’s version / Version de l'éditeur:

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Adsorption of calcium lignosulfonate on tricalcium aluminate and its

hydrates in a non-aqueous medium

Ramachandran, V. S.; Feldman, R. F.

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Adsorption of calcium lignosulfonate

on tricalcium aluminate and its

hydrates in a non-aqueous medium

V. S.

Ramachandran and

R. F. Feldman

Division of Building Research, National Research Council of Canada

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Adsorption of' calcium lignosulfonate on

tricalcium aluminate and its hvdrates in a

non-aqueous medium

V. S. Ramachandran* and R. F. Feldman*

Division of Building Research, National Research Council of Canada

Summary

Tricalcium aluminate, hexagonal calcium aluminate and cubic aluminate hydrate were treated with calcium ligno- sulfonate, using dimethyl sulfoxide as the solvent. Adsorption

was almost nil on the C 3 A _{and the cubic phase. T h e}

hexagonal phase adsorbs irreversibly about 2.2% calcium lignosulfonate. Adsorption also results in tile formation of thinner hexagonal plates havi~zg higher s u ~ f a c e area than the untreated specimen.

Introduction

It has been recognized that small amounts of some organic chemicals used as admixtures to Portland cement influence the properties of cement in terms of water requirement, setting time, strength, shrinkage, sulfate resistance, etc. Of the organic substances _- ligilosulfonic acid and its salts are most widely used as water-reducing ancl set-retarding admixtures in concrete practice. These admixtures arc known to extend the setting time by 30 to 60%, reduce the water requirement fi-om

5

_{to 10% and increase the}

compressive strength at 28 clays by 10 to 20%. Extensive data are available on the eflcct of ligno- _- sulfonates on tllc physical, chemical ancl mechanical bchaviour of concrete. Attempts have been made to explain these eflects in terms of the influence of lignosulfonate on the individual phases of the Port- land cement.

Since the tricalcium aluminate phase (C3A)t plays a dominant role in the earlier stages of hydration more attention has been directed to a study of the influence of lignosulfonates on the hydration behaviour of C3A. By the application of various methods such as X-ray, differential thermal analysis, conduction calorimetry and electron microscopy it is generally believed that lignosulfonate has a stabiliz- ing influence on the hexagonal or acicular calcium aluminate hydrates. (l-

Adsorption studies of organic anions and cations including lignosulfonates on clays and clay minerals in aqueous solutions have yielded valuable data on *Research Officer, Building Materials Section, National Research Council of Canada, Ottawa.

?The following nomenclature used in cement chemistry will be followed where necessary: C = CaO, A = A1,0,, H = H,O.

the role of organic additions on clay properties.(g-11) This approach has been applied to study the adsorption of lignosulfonates on calcium silicates and aluminates in an aqueous m e d i ~ m . ( l ~ - ~ ~ )

It has been concluded from these results that, of the Portland cement phases, tricalcium aluminate adsorbs lignosulfonate to the maximum extent. Various other workers have used this conclusion to explain their results. Whereas in clay minerals there is no chemical reaction during adsorption measurements, there is ample evidence of hydration of the C3A phase in aqueous solutions of lignosulfonate. I n other words the adsorption data in fact refer to a mixture of adsorbent phases containing C3A, C,AH,, C,AH, ancl C,AH,. Depending on the contact period, solution/C,A ratio, the concentration of lignosulfonate and its purity, temperature, etc., the relative proportions of the above phases vary. Ol~viously the so-called adsorption isotherms of lignosulfonates in an aqueous medium on C,A are unrealistic.

I t was thought that the interference efTects due to the presence of multiple types of aluminates could be circumvented by using the inclividual aluminates as starting materials. Even then there is a possibility of interaction with water which could be nullified by carrying out adsorption of lignosulfonate in a non- aqueous medium.

I t appears that studies on the adsorption in a non-aqueous medium have not been attempted because lignosulfonate is practically insoluble in most common organic solvents.(20) After trying several common organic solvents it was found that dimethyl sulfoxide is a suitable solvent for adsorption studies on calcium aluminate and its hydrates. Techniques such as differential thermal analysis (DTA), thermogravimetric analysis (TGA)

,

surface area, electron microscopy and infrared analysis were also employed where necessary.

I n the literature on the effect of lignosulfonates on the hydration of C3A some of the conflicting results may be traced to differences in the experimental procedure and the materials. However, because of meagre information available on these details, comparison of results often becomes difficult. Hence great

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care was taken in adequately characterizing the starting materials. This will be described briefly.

I t is hoped that this study may enable explanation of some of the unresolved questions on the effect of calcium lignosulfonates and possibly of other organic substances on the C3A hydration.

EXPERIMENTAL Materials

Tricalcium aluminate of high purity was prepared by calcination of CaCO, and A1,03. The material was supplied by the Portland Cement Association Research Laboratories. The sample was ground to pass through a 200-mesh sieve and had a Blaine surface area of 4 350 sq cm per g. Free lime was scarcely detectable by the X-ray method. Only faint lines of Cl,A, were detected and this aluminate may be present in a very small quantity. Infrared analysis (IRA) showed four large and four smaller maxima in the range 10.7 to 14.4 pm for C3A(") (Figure 5). TGA showed no loss in weight and DTA showed no thermal effects.

Hexagonal calcium aluminate hydrates were prepared as follows. C3A was mixed with excess water and kept at 2°C for 5 days with continuous stirring. At the end of this period, the material was filtered at a low temperature by continuously washing with cold alcohol. The white hydration product was dried in vacuum and ground in a glove box. The resultant material is designated as the hexagonal phase. Typical characteristics of this material are discussed

later.

T h e cubic calcium aluminate hydrate (C3AH,) was prepared as follows. Thc C3A powder was mixed with water at water/C3A ratio of 8 : 1 and kept at room temperature of 21

+

1°C for 24 hr with continuous stirring. The material was then washed with alcohol and vacuum dried and identified by DTA, X-ray, IRA and TGA techniques.

Figure I . Calibration curve for the estimation o f calciutn 1ignosulJbnate by the measurement of absorbance at 3 7 5 m p .

Calcium lignosulfonate sample in the form of powder was supplied by Lignosol Chemicals Ltd, Quebec, and had the following analysis:

Yo

Moisture 4-0

Lime as C a O 9.0

Ash 17.0

Reducing bodies 4.5 (total sugars, 0-16",;)

*

Total sulfur 4.7 Methoxyl 7.2 Sulfonate (S) 3.2 p H of a 50% solutioil = 6.4 *Monosaccharides O / / o Xylose traces Arabinose traces Mannose 0.10 Glucose 0.06 Galactose traces

Polysacclzal-ides (hydrolysed to monosaccharides)

0' /o Xylose 0.06 Arabinose 0.02 Mannose 0.4.8 Glucose 0.20 Galactose 0.19

The IRA of this sample is very similar to that reported by Halstead and Hime et a1.(22,23) The spectrum shows an intense peak a t 9.6 ym for ( O H ) group, peaks at 6.25 and 6.62 pm for C-C bonds of phenyl ring, weak peaks a t 6.9 and 7.3 pm probably due to sulfur-oxygen bond and a broad band a t about 8.3 ym for the sulfonate group (Figure 5). The material shows practically no X-ray lines. The molecular weight of the sample as determined by the diffusion method ranges from 600 to 12 000 with nearly 900,; falling in the range of 4 000 to 8 000.

Dimethyl sulfoxide was of spectroquality supplied by Matheson, Colcmail and Bell with an assay 99-85

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mol

%

(min.). Since this solvent is hygroscopic, care was taken to prevent moisture contamination. Procedure

Estimation of calcium lig~zosulfonate

An isotherm for the adsor,ption of calcium lignosulfonate (CLS) in dimethyl sulfoxide (DMSO) was carried out as follows.

T o 1-000 g of each of the solid materials contained in five different stoppered polypropylene tubes 15 cc of stock solution of DMSO containing different concentrations of CLS was added and the mixture was continuously mixed for a day on rollers. (Preliminary experiments on adsorption for various periods showed that for equilibrium 24 hr was more than adequate.) After 24 hr the suspension was centrifuged and the concentration of CLS in the supernatant liquid determined by the Perkin Elmer double-beam 350 spectro- photometer at a wavelength of 375 mp. The amount of adsorption was determined from the concentration of the CLS before and after adsorption. A good straight line was obtained for the calibration of adsorption at 375 mp at different concentrations of CLS (Figure 1 ) . At higher concentrations thesolution was diluted for spectrophotometric determination. The adsorption isotherm was determined at room temperature, 21" f _1°C.

A few points on the desorption curve were obtained starting a t each stage of equilibration on the adsorption isotherm by withdrawing 10 cc of the supernatant solution and adding 10 cc of DMSO each time. I n this way the coilceiltration 01 lignosulfonate in DMSO was decreased by steps. One to three days werc allowed between points.

Differential tlzermal analysis

DTA was carried out using a DuPont 900 thermal analyser. This unit utilizes platinum holders and platinum vs. platinum-rhodium thermocouples for both differential and sample temperature measurements. The reference material was ignited a-Al,O, and the rate of heating was 20°C/min. In each experi- ment 20 mg of the sample ground to 100 mesh was packed with moderate pressure. Thermograms were obtained in air or in a flow of nitrogen. I n the thermograms the temperature on the X-axis is registered in millivolts developed by the Pt-Pt.Rh (13%) thermo- couple. O n the ordinate the differential temperature is registered a t a sensitivity of 0.02 mV/in.

Thermogravimetl-ic analysis

TGA of the samples was carried out using a sensitive Cahn balance at a heating rate of 10°C/min. The temperature is plotted on the X-axis and the loss in weight, as a percentage of the original weight, on the Y-axis. All the runs were carried out in a continuous vacuum. All the samples were compacted into a pellet before placing in the balance. Otherwise

there was a tendency for the powders to be carried out of the container during rapid dehydration in vacuum.

X-ray diffraction

X-ray diffractograms were obtained by a Hilger and Watts unit using CuKcr source. Some of the experiments were also carried out under a constant humidity of 32%.

Surface area

Surface areas were obtained with nitrogen as the adsorbate by a Numinco Orr surface-area-pore- volume analyser.

Electron microscope

Electron microscopic examination was done both by scanning electron microscope supplied by Cambridge Instruments Co. and through transmission Phillips EM-75 electroil microscope. For examination by transmission a drop of alcohol suspension of the material was put on a carbon copper grid and was then evaporated.

Inzared analysis

IRA was done with Perkin Elmer 621 spectrophoto- meter by the KBr pellet technique.

RESULTS AND DISCUSSION

If DMSO is to be used as a solvent it should not interact directly with calcium aluminate, its hydrates or CLS or it can influence the results. The starting materials, viz. C3A, hexagonal phase, C3AH6 and CLS were treated with DMSO and after a day were subjected to X-ray, DTA, TGA, I R A and electron microscopic studies. No difference between the treated and untreated material could be observed by any of these methods. Also, no perceptible difference in molecular weight of lignosulfonate is observed in aqueous or DMSO solutions.(24)

I n the hydration of C3A at normal temperature the metastable hexagonal aluminate hydrates form first and these convert ultimately to the stable cubic C3AH6. I t has been observed previously that at low temperatures of hydration of C3A the hexagonal hydrates C,AH,, and C,AH, are not easily converted to the C3AH, form.(25) An X-ray diffraction pattern of the sample formed a t 2°C after 5 days of hydration and dried to 32% relative humidity (R.H.) shows strong peaks a t 10.5A, 7.9A and 5-24A, indicative of the presence of both C,AH,, and C2AH, (Figure 2).The number of H 2 0 molecules attached to these aluminates is sensitive to drying conditions and may vary from these figures. For brevity these phases together are referred to as hexagonal phase. There is also a small peak corresponding to carbo- aluminate, the formation of which could not be avoided during hydration. C,A and C3AH6 are absent.

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HEXAGONAL PHASE

+

The hexagonal phase shows endothermal effects below 150"C(26) mainly due to the loss of molecular water (Figure 3). The endothermal peak at about 200°C is attributed to the dehydration of C2AH,. The pronounced endothermic peak at 285°C can be assigned to the partial dehydroxylation of C4AH13. The broad valley at about 530°C is due to dehydration of & ( O H ) , formed by dehydroxylation of C,AH13. The dyilamic differential calorimetric investigations of C,AH,, show four endothermal effects, two below 200°C, one at 300°C and the other at 500°C.(27) There is also an exothermal hump a t about 800°C and this is probably due to a recom- bination reaction of the products formed from the dehydrated C2AH, and C4AH13.(25) The hexagonal phase does not indicate the typical large endothermic peak for C3AH, showing that this phase is absent.

TGA of the hexagonal phase shows a considerable loss in weight up to about 300°C, beyond which the loss is very gradual (Figure 4). Distinct steps have been reported for C,AH13 carried out under the quasi-static weight loss meth0d.(~6,~7,~8) The absence of these steps is due to (a) the TGA being done under dynamic conditions, (b) constant evacuation during analysis and (c) the hexagonal phase being a mixture of C,AH,, and C,AH,. The C,AH, phase gives a continuous loss in weight and the TGA reported herein in fact represents a superposition of two thermal curves of C,AH13 and C2AH8. TGA of the hexagonal phase indicates that the sample is

I

1 i

::

:

I

_I I I I I I I I I I I

,,'

'\

\ I I I

I t

I I I \ I

I

I I

_{: I}

_I

Ca,- LIGNOSULFONATE ( A I R ) I I 0 2 4 6 8 10 M I L L I V O L T S

essentially completely hydrated. The loss in weight, although not definite evidence, suggests that the two hexagonal phases are in equimolar proportions. Infrared analysis of the hexagonal phase shows a characteristic band a t 7 to 7.3 pm typical of this phase (Figure 5).

From the above discussion it can be seen that hydration of C3A a t 2°C for 5 days yields a mixture of C2AH8 and C,AH,, with some amounts of carbo- aluminate. I n actual hydration of C3A at room temperature these hexagonal phases form as metastable products and quickly convert to the cubic hydrate. Hence the hexagonal phases together were used as the starting material for adsorption measure-

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Figure 2 ( f a c i t i g , l e f i ) . X-ray digractogrart o f hexalonal plrase alone atrd that treated w i t h lignosulfirrate.

Figure 3 ( f a c i r ~ g , right )

.

Tlrerrnograrns of C,A, hexagor:al phase, C , A H , and calcium .lignosulfonate.

Figure 4 (aboue). T G A of lrexagonal phase, hexagorral phase

+

C L S , C3.-1H, arid Ca-Iigtro~~~lfonnle (uacccurn)

.

Figtcre 5 ( r i g h t ) . IR.4 ccaues.

400 600 T E M P E R A T U R E , " C

-

I I I I I I 1 I C 3 A H 6 HEXAGONAL PHASE

+

C L S I I i I I I I I I 2. 5 3 4 5 6 . 8 10 15 W A V E L E N G T H . M I C R O N S

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3 . 0 e w "7 4 I a -1 4 ,' 2 . 0 0 4 X w L w L I- Z 0

2

1.0 m E 0 "Y n 4 "Y -1 U 0 0 0. 04 0. 0 8 0.12 0. 16 0 . 2 0 E Q U I L I B R I U M C O N C E N T R A T I O N O F C L S . %

merits tllall syntlletically prepared Figure 6. -.ddsor/~tior~-desorpliorl isotherms o f calci~lirz 1igrlos1Cforzale O I L the

hexagonal pllase.

because the former more nearly simulates the

FQctre 7. D T A of hexagorlnl phase and after t ~ e a t m e n t w i t h calcizrm

products formed in the hydration of C,A. Iigrzos~tlfor~~zte.

I n Figure 6 are given the typical adsorption isotherms of CLS (in DMSO) on the hexagonal phase. There is a rapid increase in the amount of

adsorption with concentration at low equilibrium TEMPERATURE, " C

concentrations, a decreased slope in the curve there- 0 260 458 638 806 964 -

after and a tapering off at about

2.2%.

_{The scanning}

desorption isotherms do not follow the adsorption isotherm. The irreversibility even at lower concentrations indicates that CLS added is strongly reacted as a complex with the hexagollal phase. I n desorp- tions from higher concentration there is a very slight tendency for lignosol to be drawn into the solution, indicating a possibility of some reversible physical adsorption.

W

The nature of the main and scanning curves,

(r

3 which show a further increase of irreversible com-

+

4 ponent with a small increase of concentration,

(r

suggests that this component has possibly entered interlayer positions. A small increase of pllysically adsorbed CLS, with increase in concentration changes the energy balance (by decrease in surface

free energy) allowing more CLS to enter the layered _{HEXAGONAL PHASE}

structure.

+

Ca- L IGNOSULFONATE (VAC.)

A comparison of these curves with adsorption curves of water or methanol on hydrated Portland cement shows great similarity.(29) Length change measurements (Al/l) showed that Al/Aw was much greater for the irreversible water or methanol than the adsorbate which was found to be reversible along the scanning curve. The irreversible component was correlated well with interlayer penetration by water a n d methanol into the hydrated silicates. I n adsorp-

tions from solution, complications do arise because 0 2. 0 4 . 0 6. 0 8. 0 10.

a

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expect that adsorption of CLS in aqueous solution may be different because of the possibility of water with a high dipole moment entering the interlayer positions and thus enabling more CLS to enter.

DTA of hexagonal phase containing 2.2% CLS (without any free lignosol) is given in Figure 7. A comparison with the curve for the hexagonal phase shows a difference through an unmistakable exothermal hump at about 360°C. Pure CLS indicates a clear exothermal peak at about 360°C clue to oxidation effects (Figure 3). The absence of an exotherm in the thermogram obtained in vacuum shows that lignosulfonate is responsible for this peak. The two endothermal effects for C2AH, and C,AH,, do not seem to be affected.

T G A of the hexagonal phase shows a steep loss up to about 300°C. A comparison of this curve with the hexagonal phase containing CLS shows about 1.0 to 1.5% less loss in weight in the range 75" to 500°C (Figure 4). This difference is slightly more than that expected for the loss due to 97.8% hexagonal phase and 2.2% CLS. Possibly the hexagonal phase con- tainii;g lignosulfonate is slightly more resistant to dehydration. At about 1 000°C the hexagonal phase with ligr.0~01 shows a slightly higher loss and is almost equivalent to what should be expected from 97.894 hexagonal phase

+2.2%

lignosulfonate.

The surface area of the hexagoilal phase is 1 1.06 m2/g. The sample with adsorbed lignosulfonate shows a higher area of 14.2 m2/g. This cannot be attributed to the contribution by 2.2% CLS which has a low area of only 0.69 m2/g. The higher values are clue to the dispersion of tlle hexagoilal phase by CLS or to a possible penetration of the layered structure by

N,.

A11 electron microscopic investigation shows that the hexagonal phase contaiils many thick plates (Figure 8). The lignosulfonate-treated sample, however, shows thinner plates and some curled ones con- sequent on dispersion. Ligilosulfonate by itself shows as irregular plates of different sizes, some curled up. X-ray cliffractograms show certain important differences between the hexagonal phase and that treated with CLS (Figure 2). The hexagoilal phase aluminate shows a sharp peak for C4AH13 a t 7.90A and two characteristic pealts for C2AH8 at 10.5A and 5-23A. The lignosol-treated sample containing 2.2% C I S shows a broad line for C2AH, and the line at 7.90A is iilteilsified (Figure 2). Hence it is possible that tlle CLS adsorbing on the C,AH8 disperses it. The intensification of the 7.90A peak may represent further ordering of C4AHI3. Young has reported that in the hydration of C,A in the presence of sucrose and mannose the lines due to C2AH, are absent.c30) Calcium lignosulfonate seems to influence the X-ray diffraction patterns in the C,A-H20-CLS system as evidenced by the unidenti- fied line a t 8.9A reported by Chatterji.(" The

DMSO-treated hexagonal phase does not influence the diffraction lines and is not expected to be responsible for these effects. The enhanced mechanical strength in concrete containing CLS is more than could be explained by the reduction in waterlcement ratio. There is the possibility that the increased strength is due to the higher surface area of tlle products. A change in the internal structure is also possible as evidenced by development of new diffraction lines.

The infrared ( I R ) results of hexagonal phase or that treated with calcium lignosulfonate showed no perceptible difference.

The cubic aluminate hydrate C,AH, prepared at room temperature exhibits a typical X-ray, I R , T G A and DTA behaviour reported in the literature (Figures 3, 4 and 5). DTA shows a large endothermal effect at 336°C followed by another at 500°C. Dehydration of C,AH, occurs in two stages ancl consequei~tly two endothermal effects of these have been reported by Feldman and Ramachandran, Majumdar and Roy, Young, Jones and Schwiete and

Ludwig and discussed in a recent DTA also

shows that this material is not contaminated with hexagonal phase. X-ray data show peaks at 2.81, 2.30 and 2.04A characteristic of C,AH,. No lines for C3A and hexagonal phase are present. TGA shows the sample to be completely hydrated.

Adsorption experiments on C,A and C,AH, show practically no adsorption of lignosulfonate within the accuracy of measurements. The low surface areas of C3A and C3AH, (being only 0.7 ancl 1.6 myg, respectively), the nature of the surface and absence of interlayer spaces are probably the causes for this behaviour. These results call explain why in a mixture of C,S

+

C,A, if ligilosol solution is added after hydratioil has proceeded to some extent, the hydration of C3S is inhibited. This occurs because during prellydration C,A is already converted mostly to C,AH, which adsorbs practically no ligilosol and a substantial amount of lignosol is free to inhibit hydration of C,S.

CONCLUSIONS

Dimethyl sulfoxide is a suitable solvent for calcium lignosulfonate and can be used for studies of adsorption on cementitious and other related materials. Tricalcium alumillate and tricalcium aluminate hexahydrate adsorb little if any lignosulfonate. T h e hexagonal phase containing C4AHn and C2AHn together adsorbs about 2.2% lignosulhnate. T h e sorption isotherm of lignosulfonate on the hexagonal phase is largely irreversible and suggests that a n interlayer complex of the hexagonal phase has been formed. TGA and DTA results confirm that lignosol is mainly sorbed on the hexagonal phase. X-ray diffraction results show that by treatment with lignosulfonate the C2AHn phase is dispersed and

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fix to^ 8.

( a ) tlesngor~al phase ( >: 3 5 0 0 ) .

(6) Cblcitrrrl lic.nos~d/or,ate ( x 5 4 0 )

effects on the C,AH, are also observed. Thinner hexagonal plates and higher surface area result when lignosol is sorbed on the hexagonal phase. The stabilization of the hexagoi~al phase by lignosulfonate should be explained on the basis of the formation of a complex on this phase. The present tendency of attributing adsorption of lignosulfoilate on the C,A phase should be altered to 'hydrated calcium aluminates'.

Acknowledgement

The authors thank Mr H. E. Ashton and Mr P.

J.

Sereda for useful discussions and M r G. M. Polomark

( I . ) IIesc~~orrnl pizclsc trealca' icitil calciltrn 1i.ylosulJoonate ( :< I f 0 0 0 ) .

( d ) IIesngor~al phase treated i c i t i ~ calcillrr~ li,nnosltlJorlale ( x 7 0 0 0 ) .

for experimental assistance. This paper is a contribution of the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Divisioi~.

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.

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I 4. Y O U N G , J

.

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.

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