Effect of retarders / water reducers on slump loss in superplasticized concrete

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Effect of retarders / water reducers on slump loss in superplasticized

concrete

Ramachandran, V. S.

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EFFECT OF RETARDERS/WATER REDUCERS ON

SLUMP LOSS IN SUPERPLASTICIZED CONCRETE

by V. S. Ramachandran

Reprinted f r o m

Developments in the U s e of Superplasticizers

American Concrete Institute Special Publication 68, 1981 p. 393

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407 J? I rj

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SOMMAIRE

L'adjonction d e p l a s t i f i a n t s a u bCton a m e l i o r e s o n ouvrabiliti. pour quelques h e u r e s s e u l e m e n t . Etude d e s effets de diffC- r e n t e s q u a n t i t i . ~ dladjuvants t e l s que le lignosulfonate d e Ca, l a s a c c h a r o s e , l e gloconate de Na, l'acide c i t r i q u e , l ' a c i d e s a l i c y l i q u e , l'heptonate d e Na e t le boroheptonate d e Na, s u r llouvrabilitC. Etude d e l'influence s u r l e t e m p s d e p r i s e e t l a

r e s i s t a n c e d a n s l e s m o r t i e r s .

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Effect of RetardersIWater

Reducers on Slump Loss in

Superplasticized Concrete

By

V. S.

Ramachandran

Synopsis: Superplasticized concrete loses its workability within a few hours. The effect of different amounts of admixtures, such as Ca-lignosulfonate, sucrose, Na-gluconate, citric acid, salicylic acid, Na-heptonate and Na-boroheptonate, on the slump loss of concrete containing sulfonated melamine formaldehyde is reported. Of these admixtures, Na-gluconate proved to be the best retarder of slump loss; the influence on setting times and strengthdevelopment in mortars was also examined.

Keywords: admixtures; calorimeters; compressive strength; con- cretes ; hydration; mix proportioning; p l a s t i c i z e r s ; retarders ;

s e t t i n g (hardening) ; slump t e s t s ; water-reducing agents; workabi 1 i t y .

394

Rarnachandran

V.S. Ramachaadran, who holds B.Sc., M.Sc., and D-Phil., degrees from Mysore, Banaras, and Calcutta Universities respectively, is actively engaged in research on clay mineralogy and lime and cement chemistry. Be is Head of the Building Materials Section, Division of Building Research, National Research Council of Canada. _,

The advent of superplasticizers has made it possible to produce concrete with high workability but no reduction in

strength. Within a feu minutes of the addition of a superplastici- i zer concrete begins to flow easily and becomes self-leveling, 1 r e m a i n s c o h e s i v e , a n d d o e s n o t b l e e d o r s e g r e g a t e . Fromaninitial

1

slump of about 50 mm, superplasticized concrete attains a slump

(a measure of workability) in excess of about 200 mm. This increase is only transient, however, and is generally not maintained beyond a period of about 30 to 60 min. Consequently, there is a great reduction in the workability of concrete in the interval between mixing and placing. In ready-mix operations, therefore, it is suggested that the superplasticizer should be added at the point of discharge of concrete, although there are several practical

problems associated with this procedure (1,2).

Factors that affect slump loss in concrete include initial slump value, type and amount of superplasticizer added, type and amount of cement, time of addition of superplasticizer, humidity, temperature, mixing criteria, and the presence of other admixtures in the mix [3-6). The rate of slump loss can be decreased by adding a higher than normal dosage of superplasticizer, by adding the superplasticizer at different times, or by including some type of retarder in the formulation. Inclusion of a retarder in small amounts seems to offer advantages such as economy and better control at the point of mixing.

This paper describes the effect of different amounts of

retarders/water-reducing admixtures on the slump loss in concrete

containing sulfonated melamine formaldehyde, and includes an assessment of the resulting rate of hydration, compressive strength, and setting properties of the mortars.

EXPERIMENTAL

I

Materials

Normal low-alkali portland cement with the following composition was used in the investigation:

Effect

on Slump Loss

395

Chemical Analysis Constituent % by weight A1203 Fe203 CaO Ignition Loss 0.50 Na20 K2° Insoluble Compound Composition C3S 51.4 C2S 20.3 C3A 12.7 2 2

It had a Blaine surface area of 299 m /kg (2990 cm fg) and showed 0.05% autoclave expansion.

The types of admixture that were used in two dosages in combination with 0.3% sulfonated melamine formaldehyde (SMF) are listed in Table 1.

Method

Compressive strengths of the mortar cubes (cement:sand ratio= 1:2.75 and w/c = 0.47), cured for 1, 3, 7, 28, and 90 days, were determined by ASTM 109-75 method.

Rate of heat development during hydration was determined by a conduction calorimeter, and rate of hydration was followed by a differential scanning calorimeter (DSC).

Time of initial setting of mortar mixes was determined accor- ding to ASTM test method C403-77, using the Proctor needle.

Slump values were determined using the ASTM C-143-78 method. Each 20.1 kg batch of concrete mix contained 3 kg cement, 6 kg sand, 9.6 kg graded coarse aggregate and 1.5 kg water.

The reference concrete was made by mixing it for 3 min, filling the cone, and determining the slump, 5 min in all. Determination of slump for concrete containing a retarder also took 5 min; the aqueous solution replaced the water used formaking

396

Ramachandran

the reference concrete. Concrete containing SMF was made as follows: initially, the concrete mix was made with water (mixed for 3 min) and let stand for 3 min more; SMF was added and the concrete mixed for 1 min and then poured into the cone. This operation took 9 min. Concrete containing SMF and retarder was made by initially adding the aqueous solution of the retarder and repeating the operation as described for that containing SMF.

RESULTS AND DISCUSSION

Sulfonated Melamine Formaldehyde (SMF)

The influence of SMF on the hydration of cement can be

followed by a conduction calorimeter (Fig. 1). A broad hump with a ! maximum intensity at about 7 h for the reference cement paste

(paste containing no SMF) represents hydration of the C3S component '

of portland cement. After an induction or dormant period of about 2 h, hydration of C3S is initiated and continues beyond 20 h, as is evident from the non-return of the curve to the base line; addition of 0.3 to 2% SMF does not seem to influence this dormant period. The rate of hydration of C3S, however, is affected, as can be seen from the lower rates of heat development (Fig. 1). At 2% addition the rate of heat development at 7 h is decreased by about 50%. Adsorption-desorption isotherms of SMF on hydrating C3S and

portland cement show that SMF is irreversibly adsorbed todifferent extents (7). An adsorption complex (formed by the reaction of C3S or cement with SMF and H20) formed on the unhydrated surfaces is capable of retarding hydration.

The retarding action of SMF is reflected in slightly delayed setting times of mortars. The reference mortar exhibits an initial setting time of 44 h, which is extended to 42 and 63 h in mortars containing 0.3 and 0.6% SMF, respectively (Fig. 2). DSC curves of initially set mortars show small endothermal peaks at about 100°C, representing C-S-H and/or ettringite phases, followed by another endothermal effect of larger intensity with a peak between 450 and 500°C (Fig. 3). This peak is due to decomposition of Ca(OH)2

formed by the hydration of C3S. Both the reference mortar andthat containing SMF show endothermal effects in this region, indicating some hydration of C3S at the time of setting of the cement paste. Setting represents a matrix having a particular value ofresistance

(arbitrary) to needle penetration. The endothermal effect of the sample containing 0.3% SMF is less intense than that of the reference mortar (Fig. 3). In other words, a smaller amount of hydrated product formed in the presence of SMF offers the same resistance to the Proctor needle as does the reference mortar containing about twice the amount of hydrated C3S. This indicates that mortar containing SMF is better dispersed and forms a more compact network.

Effect

on Slump Loss

397

Addition of 0.3% SMF to cement increases mortar strength substantially at all periods up to 90 days (Table 2). For example, at 28 and 90 days the percentage increase in strength of mortars containing SMF over that of the reference mortar is about 26 and

, 22%, respectively. These higher strengths are attributed to good

mixing and better compaction.

The slump values of the reference concrete and that containing

0.3% SMF are shown in Fig. 4. The reference concrete has aninitial

value of about 57 mm, gradually decreasing to about 13 mm in 2 h.

The slump of concrete to which SMF has been added with the mixing water is increased to about 89 mm, but it is reduced to about 32 mm

in 2 h. The slump value can be as high as 178 mm for concrete to which SMF has been added a few minutes after mixing with water,but this high value decreases rapidly to about 51 mm in 1 2 0 min.

Increase i n slump value can be explained by adsorption of SMF by

hydrating cement particles, followed by mutual repulsion similar to

the phenomenon occurring in cements containing anionic admixtures. Addition of SW with the mixing water does not promote high worka- bility because most of the superplasticizer reacts chemically with hydrating aluminate components of cement (7). Thus, only a negli- gible amount of SMF is left in the aqueous phase to disperse the

silicate phase in cement. The addition of SMF a few minutes after mixing with water allows rapid hydration of the aluminate phases of cement and hydrated products. Adsorbing less SMF than the unhydra- ted phase, the hydrated products leave a substantial amount of free SMF in the aqueous phase to disperse the calcium silicate phase. Similar observations have been made in cements containing

lignosulfonates (8-10).

Retarders/Water-Reducing Admixtures

Representative data are given regarding the influence of retarders and water-reducing admixtures on slump loss, setting, hydration and strength development in concrete and mortar. Figures 5 to 8 show the influence of sucrose, citric acid,

Na-heptonate, Na-boroheptonate, Ca-lignosulfonate and Na-gluconate on the slump values of concrete. Addition of sucrose or citric acid does not result in an increase in the initial slump of concrete (Figs. 4 and 5). Incorporation of Na-heptonate or Na-boroheptonate is effective in increasing the initial slump value from 57 mm (Fig. 4) to about 89 to 95 mm (Fig. 6). The addition of Ca-lignosulfonate and Na-gluconate gives substantially higher values of slump, 127 mm (Fig. 7) and 140 mm (Fig. 8),

respectively. Although these values are more than double that of the reference concrete, they are less than those obtained withSMF.

Irrespective of the type of retarding admixture, all slump values decrease to about 38 to 51 mm kn about 2 h. Although it is recognized that complicated factors are involved in the slump loss phenomena, accelerated formation of ettringite by the reaction of C3A with gypsum is an important cause (11).

398

Ramachandran

Initial setting times of mortars containing various admixtures '

are shown in Fig. 2. All increase the setting times, the more effective admixtures being Ca-lignosulfonate (0.4%), sucrose

(0.05-0.1%) , Na-gluconate (0.1-0.2%) and citric acid (0.1%)

.

The

most efficient, however, is Na-gluconate (0.2%), which increases 4

setting time from 44 to 154 h. This is also reflected in the effect of Na-gluconate on the hydration of cement (Fig. 9). Addition of Na-gluconate not only increases the induction period

for the hydration of C3S but also shifts the maximum rate of heat 4

development for C3S from 7 to 12 h. The DSC curves of the mortars

containing various admixtures show that all cements have hydrated

to some extent at the time of initial set (Fig. 3). Just as with

SMF, the peak intensity for Ca(OH)2 is reduced in the presence of retarding admixtures. Such reduction is particularly significant in cements containing 0.2% Na-gluconate.

Development of compressive strength in mortars containing

various admixtures is indicated in Table 2. The one-day strength !

of mortars containing the retarding admixtures is generally lower _I

than that of the reference specimen, the least strength developed

by the mortar containing 0.2% Na-gluconate. This is to beexpected

1

because it is the best retarder of rate of hydration. As hydration progresses, strength of mortars containing these admixtures

generally exceeds that of the reference mortar, but at 90 days those containing Ca-lignosulfonate and citric acid exhibit much

lower strengths. Lower strengths in the presence oflignosulfonate

1

may be due to entrainment of air, which causes higher porosity.

The presence of Na-gluconate (0.1 to 0.2%), however, results in

I

about 15% higher strength than that of the reference specimen. Generally, retarders are known to impart low early strength and higher strength at 28 days; at longer periods of hydration there is a possibility that hydration products are formed by slower rates of diffusion and precipitation and that this results in their

relatively more uniform distribution in the interstitial spaces

among the cement grains. An increase in the total interparticle

bond area may thus be responsible for better strengths.

Sulfonated Melamine Formaldehyde +

Retarders/Water-Reducing Admixtures

Figures 5 to 8 show the slump values of concrete containing

SMF admixed with sucrose, citric acid, Na-heptonate, Na-borohepto-

nate, calcium lignosulfonate and Na-gluconate. Concrete with a

combination of sucrose + SMF has the same initial slump as that

containing SMF alone, but it seems to yield lower slump values at

2 h (Fig. 5). Concrete with citric acid + SMF not only has a

higher initial slump value than the sucrose + SMF mixture but also

retains it at 2 h (Fig. 5). Both Na-heptonateandNa-boroheptonate

in combination with SMF increase initial slump and exhibit slightly higher slump at 2 h than concrete with SMF alone. The calcium

lignosulfonate + SMF mixture shows similar behaviopr (Fig. 7); but

Effect on Slump Loss

399

it retains a high slump value of about 140 mm even at 2 h. SMF concrete has a slump of only about 57 mm at 2 h (Fig. 4).

The initial setting times of retarder + SMF combinations are generally slightly greater than those with retarders only. The set-retarding effect of 0.2% Na-gluconate + SMF is greater than that of the others, but although this combination effectively prevents rapid slump loss the excessive retardation effect may not be acceptable for practical applications. The 0.1% Na-gluconate +

SMF combination is also effective in retarding rate of slump loss, with initial setting time at about 8f h. This is slightly greater than that permitted by standards, but further reduction in setting time could be accomplished by decreasing the dosage to a value less than 0.1%. Additional experiments should be carried out to

discover the optimum dosage of Na-gluconate for retarding slump loss and promoting setting time to within specified limits. It is also possible that in hot weather concreting operations use of 0.1% Na-gluconate may decrease setting times to within the specified limits.

The excessive retarding effect of Na-gluconate is alsoevident in the conduction calorimetric curves shown in Fig. 9. Curves 3 and 4, representing the Na-gluconate + SMF combination, indicate extension of the induction period for C3S hydration by 10 to 14 h. The Na-gluconate-SMF combination is also different from the others in promoting the formation of a more amorphous Ca(OH)2, as evi- denced by the characteristic dual peaks between 450 and 500°C

(Fig. 3). The mortar with the Na-gluconate + SMF combinationgives higher strengths than the reference specimen from 3 to 90 days

(Table 2). Strength at 1 day is, however, lower but this can be increased by using lower dosages or by i n c o r p o r a t i n g a n a c c e l o r a t o r . Other admixture combinations containing Na-heptonate,Na-borohepto-

nate and salicylic acid also promote high strengths, but they do not retard slump loss in concrete as efficiently as does Na-gluco- nate. It appears that for retarding slump loss an admixtureshould retard the hydration reaction and have a dispersive action. Of the

I admixtures studied, Na-gluconate seems to satisfy both requirements.

! CONCLUSIONS

I

I The initial slump of concrete increases substantially with

the addition of sulfonated melamine formaldehyde, but it is reduced to a low value within about 2 h. SMF alone, in normal dosages, is only a mild retarder for setting and hydration of C3S. Mortar containing SMF yields higher compressive strengths than the reference specimen containing no SMF.

Slump loss in concrete containing SMF can be controlled to different extents by the addition of various retarders; sodium gluconate is very efficient. This may be related to its ability to act as a good retarder for C3S hydration and as a dispersant for cement particles. Mortars containing 0.1 to 0.2% gluconate

400

Ramachandran

are slow setting and exhibit low early strengths. Use of lower dosages or incorporation of accelerators may overcome these

problems. I

ACKNOWLEDGMENTS

The author wishes to acknowledge the valuable experimental

assistance of G.M. Polomark. Thanks are due also to 1

N.P. Mailvaganam, Sternson Ltd., Brantford, Ontario, for supplying

,

various admixtures.

This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

REFERENCES

1. Malhotra, V.M., "Superplasticizers: Their Effect on Fresh and Hardened Concrete,'' Energy, Mines and Resources, Canada, CANMET Report 79-31, 1979, 23 p.

2. ~olle~ardi; M., Corradi, M., and Valente, M., "Low Slump Loss Superplasticized Concrete. I. Influence of a Naphthalene- Sulphonated Polymer Based Superplasticizer on the Cement Hydration," Transportation, Res. Rec. No. 720, 1979, pp. 7-12. 3. Perenchio, W.F., Whiting, D.A., and Kantro, D.L., "Water

Reduction, Slump Loss and Entrained Air Void System as Influenced by Superplasticizers," Proc., Int. 1978, Smp. Superplasticizers in Concrete, Vol. 1, pp. 295-324.

4. Mailvaganam, N.P., *'Factors Influencing Slump Loss in Flowing Concrete,"

&

Superplasticizers in Concrete, American Concrete Institute, SP-62, pp. 389-403, 1979.

5. Ramachandran, V.S., "Superplasticizers in Concrete," National Research Council of Canada, Division of Building Research,

1

CBD 203, 1979.

6. Malhotra, V.M. and Malanka, D., t*Perforrnance of

Superplasticizers in Concrete: Laboratory Investigation

-

Part 1,l' Superplasticizers in Concrete, American Concrete Institute. SP-62, 1979, pp. 209-243.

7. Ramachandran, V.S., (Unpublished results).

I

8. Ramachandran, V.S., "Differential Thermal Investigation of the System Tricalcium Silicate-Calcium-Lignosulfonate-Water in the Presence of Tricalcium Aluminate and Its Hydrates," Proc.Third Int. Conf. Thermal Analysis, Davos, Switzerland, No. 2, 1971, pp. 255-267.

Effect

on Slump Loss

401

9. Bruere, G.M., 'qImportance of Mixing Sequence When Using S e t - Retarding Agents with P o r t l a n d Cement," Nature (London). V. 199, 1963, pp. 32-33.

' 10. Dodson, V.H. and Farkas, E . , "Delayed Addition of Set-Retarding Admixtures t o Portland Cement Concrete," American Soc. Test Mater., Proc., V. 64, 1964, pp. 816-826.

I

11. Meyer, L.M. and Perenchio, W.F., "Theory of Concrete Slump Loss a s Related t o t h e Use of Chemical Admixtures," Concrete I n t e r . , V. 1, 1979, pp. 36-43.

402

Ramachandran

Table 1: Types and Amounts of Retarders/Water Reducers

Admixture Retarder/Water Reducer % (based on cement) Calcium lignosulfonate 0.2 0.4 Sucrose 0.05 0.1 Sodium gluconate 0.1 0.2 Citric acid 0.05 0.1 Salicylic acid 0.05 0.1 Sodium heptonate 0.1 0.2 Sodium boroheptonate 0.1 0.2

Table 2: Compressive Strengths of Mortars ContainingVarious Admixtures

ADMIXTURE Reference SMF 0.3% Ca-Lignosulfonate (0.1%) Ca-Lignosulfonate (0.1%) + SMF Sucrose (0.05%) Sucrose (0.05%) + SMF Citric Acid (0.05%) Citric Acid (0.05%) + SMF Na-Gluconate (0.1%) Na-Gluconate (0.2%) Na-Gluconate (0.1%) + SMF Na-Gluconate (0.2%) + SMF Na-Heptonate (0.2%) Na-Heptonate (0.2%) + SMF Na-Boroheptonate (0.2%) Na-Boroheptonate (0.2%) + SMF Salicylic Acid (0.1%) Salicylic Acid (0.1%) + SMF

Compressive Strength (MPa) 1 d 3 d 7 d 2 8 d 9 0 d

- . . I N I T I A L S E T T I N G TIMES, h c a l l g l h w C N N C I - . V W P W O a W N 0 1 O a 0 -n e 1 - -m z E Z

o

=

2 REFERENCE 0

-

z z z

+

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m SMF, 0.3%

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% g N 0 SMF, 0.6% Z 3 0 D 0: r fi-LIGNOSULfWVATE 0.1% w + 0 11 + D

-

D w

b::~

5 m u C9-LIGNMULFONATE, 0. I % + SMF + m m 11 c.-LIGNORILFONATE, 0.2% + SMF w

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0 V) c.-UG~JOSULFONAT~. 0.4% + SMF D 0 Z C 11 u S U c R r n . C m -I

E:2=

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f, X + - + M-GLUCONAR. 0.1% + SMF C D D

.z

z b - G L U C O N A I E , 0.2% + SMF w m V)

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V) m = s 0

CITRIC ACID

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0

-

Z + _{tnwc KID, o.os% + SMF}

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D I - CllRfC ACID. 0.1% + SMF V) Z

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Z 0 SALICYLIC K I D 0

b:015%%

n

-

-

Z _{S U l N L l C ACIO, 0.05%} -n + SMF rn

-

11 +. IAUCYLIC ACID, 0.1% + SMF m

-

Z

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z 4 +

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D I*r+HEPIONAlE 7

1:;;

I 0 V) C No-MPTOHAE. 0.1% + SMF m Z Nrn-WPTONATE. 0.2% + SMF + + VI 1

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P Z 0 Nm-aCQOIEPTON*Tf

k::;

V) t N ~ - ~ O H E F T W t E . 0. I % + SMF N Ha-DWOHEPTONATE, 0.2% + SMF

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404

_Ramachandran

50 1 0 0 150

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450 5 0 0 5 5 0

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I I I I I CEMENT CEMENT + SMF. 0.3% Ca-LIGNOSULFONATE, 0.1% \ - - Ca-LIGNOSULFONATE, 0.1%

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+ SMF

- - -

SUCROSE. 0.05%

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_{- - -}

SUCROSE. 0.05% + SMF V

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CITRIC ACID. 0.05%

--

CITRIC ACID. 0.05% + SMF Na-HEPTONATE. 0.2%

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---

w -

Na-HEPTONATE. 0.2% + SMF Na-BOROHEPTONATE, 0.2% \ - Na-BOROHEPTONATE, 0.2% + SMF 1- -Na-GLUCONATE. 0.1% --- Na-GLUCONATE, 0.1% + SMF 1 -Na-GLUCONATE. 0.2% --- Na-GLUCONATE. 0.2% + SMF I

-

-- -I I I A I I I 5 0 1 0 0 1 5 0 4 5 0 5 0 0 5 5 0 F I G U R E 3 D I F F E R E N T I A L S C A N N I N G C A L O R I M E T R I C C U R V E S ( A T T I M E OF SET) OF C E M E N T M O R T A R S C O N T A I N I N G V A R I O U S A D M I X T U R E S

Effect on Slump Loss

405

0-0 0 . 3 % SMF (ADDED 3 M I N

AFTER MIXI'NG W l T H WAI

a-9 0 . 3 % SMF 1WlTH M I X I N G WATER 140 0-0 0 % SMF 120 I I I I I 0 30 60 90 120 150 TIME, m i n FIGURE 4

EFFECT OF SULFONATED MELAMINE FORMALDEHYDE ON SLUMP LOSS 220

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6-a S U C R O S E 10.05%) . -a SUCROSE 10.05%) + SMF 0-0 C I T R I C A C l D 10.05%)

200t

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.-a C I T R I C A C I D (0.05%1 0 30 60 90 120 150 TIME. m i n FIGURE 5

EFFECT OF SUCROSE A N 0 C I T R I C A C l D AND THEIR MIXTURES W l T H SMF ON SLUMP LOSS

406

Ramachandran

A-A Na-HEPTONATE 10.2%) 220 _V-v Na-HEPTONATE 10.2%) + SMF 180 160

-

140

-

120 100 60 40

-

20 c 0 0 30 60 90 120 150 TIME. min FIGURE 6

EFFECT OF Na-HEPTONATE AND Na-BOROHEPTONATE AND THEIR MIXTURES W l T H SMF ON SLUMP LOSS

220

-

A- Ca-LIGNOSULFONATE 10.15%)

-

0-0 SMF 10.3CI w-• C a - 1 IGNDSULFONATE 10.1551

-

SMF 10.311

-

-

E 140 E

-

0' 120 80 60

-

40

-

20

-

- TIME. m i n FIGURE 7

EFFECT OF Ca-LIGNOSULFONATE AN0 I T S MIXTURE WlTH SMF ON SLUMP LOSS

Effect on Slump Loss

407

0 0 30 6 0 9 0 120 1 5 0 T I M E , m i n 1 I I I L 0-0 Na-GLUCONATE (0.05%)

-

0-0 Na-GLUCONATE (0.05%) + SMF Na-GLUCONATE LlU

-

v-v Na-GLUCONATE (0.1%) + SMF

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