Identification of concrete admixtures by differential thermal analysis in oxygen

CEMENT and CONCRETERESEARCH. Vol. 14, pp. 207-214, 1984. Printed in the USA 0008-8846/84 $3.00+00. Copyright ( c ) , 1984 Pergamon Press, Ltd.

I D E N T I F I C A T I O N OF C O N C R E T E A D M I X T U R E S BY D I F F E R E N T I A L T H E R M A L A N A L Y S I S IN O X Y G E N N.B. M i l e s t o n e Chemistry Division DSIR, P r i v a t e Bag Petone, N e w Zealand

(Refereed) (Received June 27, 1983) ABSTRACT It is often d e s i r a b l e to identify a c o n c r e t e a d m i x t u r e u n i q u e l y before its use to avoid adverse reactions. Methods such as ir or uv s p e c t r o s c o p y only b r o a d l y classify the type of a d m i x t u r e while chemical analysis is d i f f i c u l t and results are often inconclusive, e s p e c i a l l y for a d m i x t u r e s c o n t a i n i n g l i g n o s u l p h o n a t e s . The use of DTA under 250 kPa of 02 provides a t e c h n i q u e for i d e n t i f y i n g admixtures. Each a d m i x t u r e gives an unique p a t t e r n of e ~ o t h e r m i c peaks thus e n a b l i n g its i d e n t i f i c a t i o n p r o v i d e d a known sample is available. A d m i x t u r e s c o n t a i n i n g salts of h y d r o x y c a r b o x y l i c acids show exotherms up to 700°C w h i l e those w i t h free acids or sugars shown only exotherms in the range 200-400oc. Introduction Chemical admixtures are w i d e l y used in the concrete industry as aids for c o n t r o l l i n g the w o r k a b i l i t y and setting of fresh concrete. They are usually supplied as solutions suitable for adding to the m i x i n g w a t e r to enable an even d i s t r i b u t i o n throughout the concrete. A large p r o p o r t i o n of c o n c r e t e admixtures, e s p e c i a l l y the w a t e r - r e d u c i n g type, are b a s e d on s o d i u m or c a l c i u m l i g n o s u l p h o n a t e s - b y p r o d u c t s from the pulp and p a p e r industry. Commercial l i g n o s u l p h o n a t e s are usually not pure but contain a complex mixture of lignosulphonic acid salts as well as sugars, sugar acids and other h y d r o x y compounds. (i) Small proportions of these impurities (in p a r t i c u l a r sugars and sugar acids) have a m a r k e d e f f e c t on the h y d r a t i o n of cement compounds (2-6), and hence on the setting of c o n c r e t e so that their content must be closely controlled. Changes in raw m a t e r i a l sources for the admixture can made a p r o f o u n d difference in the b e h a v i o u r of an admixture

* Present address: New Zealand

Development Finance Corporation, P.O. Box 3090, Wellington, 207

208

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due to small d i f f e r e n c e s sulphonate.

in the c o m p o s i t i o n of the ligno-

C h e m i c a l a n a l y s i s of c o m p l e x m a t e r i a l s such as concrete admixtures is d i f f i c u l t and very time c o n s u m i n g so that the quality of an a d m i x t u r e is usually m e a s u r e d in terms of its effect on the p h y s i c a l p r o p e r t i e s of concrete, e.g. setting times, w o r k a b i l i t y and s t r e n g t h development. (7-8). Nevertheless, there are o c c a s i o n s w h e n it is important to know whether an a d m i x t u r e to be c u r r e n t l y used, is the same as the material that was used say six m o n t h s earlier and so avoid the extra physical t e s t i n g n e c e s s a r y to determine the o p t i m u m dose. Techniques such as IR and U V s p e c t r o s c o p y have been used (9-11) for i d e n t i f i c a t i o n p u r p o s e s and can serve to identify the class to which an a d m i x t u r e belongs, but they are not suitable for positive i d e n t i f i c a t i o n of an individual admixture, which is often n e c e s s a r y if adverse results have been obtained. Gel chromatography, (12) thin layer chromatography(13) and high pressure liquid c h r o m a t o g r a p h y ( 1 4 ) have been used to charac£erize admixtures but all of these methods are still unable to identify an a d m i x t u r e uniquely. This p a p e r d e s c r i b e s the use of DTA in oxygen to identify concrete a d m i x t u r e s both rapidly and uniquely. Experimental T w e n t y - o n e samples of w a t e r - r e d u c i n g admixtures were obtained and a n a l y s e d as part of an overall physical testing programme. (15) The a d m i x t u r e s studied can be broadly separated into four main classes b a s e d on the main chemical constituents. Class Class Class Class

i. 2. 3. 4.

Admixtures Admixtures Synthetic Admixtures systems.

b a s e d on lignosulphonates. b a s e d on h y d r o x y c a r b o x y l i c acids. detergents. b a s e d on synthetic condensed sulphonate

Partial chemical analyses for these samples, which were carried out using a variety of m e t h o d s developed for the programme, have been r e p o r t e d previously. (15) The chemical analyses from that report are r e p r o d u c e d in Table I. To enable r e p r o d u c i b l e results to be obtained for thermal analysis, the liquid admixtures were freeze-dried and stored over P~O 5 before analysis. The admixtures from Class 3 analysed in the testing p r o g r a m m e were still liquid w h e n dried, so that only admixtures from classes i, 2 and 4 were subjected to thermal analysis. Twenty m i l l i g r a m s of admixture were interground with 200 milligrams of calcined alumina in a boron carbide m o r t a r and pestle. A p p r o x i m a t e l y 150 m i l l i g r a m s of the ground mixture was w e i g h e d into a p l a t i n u m m i c r o c r u c i b l e which was p l a c e d in a ring sample holder (BRII) in a Stone thermoanalyser 202. The thermocouple used was Platinel 2. The furnace was placed over the sample followed by a pressure dome which was used to control the sample atmosphere and pressure up to 250 kPa. Oxygen was slowly admitted to the chamber and the whole system p r e s s u r i z e d with oxygen to 250 kPa. Heatin@ was then commenced at 10°C/min from room temperature to 950uC.

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209

DTA, OXYGEN, CHEMICAL ADMIXTURES, IDENTIFICATION TABLE I CHEHICAL ANALYSIS OF ADHIXTURES Class

Admixturo

px

Solid content % w/v

Inorg. ash % w/v

Llgnosulphonate mg/ml

(from

ref

15)

I

Sugars (as glucose) mg/ml

Sugar acids (as sodium gluconate) mg/ml

Trlethanolamine mg/ml

Na20

CaO mg/ml

~/~

I

6.0

54

10

316

46

nd

7

56

17.3

2

4.5

53

6.5

352

122

nd

-

65

0.8

3

4.5

16

2.9

72

9

nd

6

9.1

5.8

4

6.5

36

8.1

260

19

nd

4

41

5

7.5

37

7.8

196

12

nd

57

35

3.1

6

6.0

41

10.7

288

19

nd

39

4.8

7

6.0

56

9.0

416

43

nd

78

3.4

8

6.5

48

9.4

314

21

nd

9

6.0

62

13.7

416

51

nd

10

5.5

solid

(of 1% solution)

(li~ moisture)

11

6.5

65

6.2

320

109

64

27

1.8

12

4.5

62

5.6

312

115

80

29

2.1

13

8.5

49

14.9

92

14

4.5

51

11.8

15

6.0

57

expanded

16

5.5

58

17

5.0

40

18

6.0

27"

nd

nd

nd

nd

nd

nd

nd

19

4.0

93*

nd

nd

nd

nd

nd

nd

nd

nd

10

3.5

nd

-

44mg/g

590mg/g

Class

266mg/g

nd

263

228

76

110

210

45

50

296

28

13.4

exlpa.nded

Class

-

3.2

37

0.42

42

nd

0.2mg/g

2

12.4

0.2

15

14 5.6

49 48

-

1.6 25

355

3

*final product was liquid or low meltlng wax

Class 4 20

6.0

21

8.0

52

16.3

404*

2

solid 26.71w/w 769"mg/q(9.8~ moisture) *synthetlc sulphonated material

nd

133,~J/g

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Results and D i s c u s s i o n DTA curves for the various samples are given in Figs 1 and 2. Some of the admixtures were e x t r e m e l y h y g r o s c o p i c and the broad e n d o t h e r m o c c u r r i n g b e t w e e n i00 and 200°C for these admixtures were due to evolution of the m o i s t u r e absorbed during handling. Sample 17 was e s s e n t i a l l y a solution of s o d i u m g l u c o n a t e and the solid foamed so m u c h on heating that it proved i m p o s s i b l e to obtain a r e p r o d u c i b l e DTA curve. The DTA curves for the various concrete admixtures are complex and no effort was made to assign the various exothermic peaks. When the samples were analysed under vacuum or in an inert atmosphere, the e x o t h e r m i c peaks were absent and only very small e n d o t h e r m i c peaks were p r e s e n t so that the exothermic peaks are, as expected, due to a series of oxidation steps. I n c r e a s i n g the oxygen pressure to 400 kPa gave no appreciable difference in the DTA curves, while samples run in ambient air showed only small broad e x o t h e r m i c peaks, typical of slow o x i d a t i o n steps. A l t h o u g h the overall pattern for several of the admixutres is similar, no two samples gave exactly the same pattern. This pattern of peaks then provides a suitable identification m e t h o d for admixtures. This is similar to the way in which the peaks in the "fingerprint" region of the IR spectrum are used for the i d e n t i f i c a t i o n of organic compounds. For many of these admixtures the IR spectra were very similar(15) and so are not suitable for identifying each admixture, although the different classes could usually be separated and the source of lignosulphonate identified. The temperature of the final e x o t h e r m for many samples is higher than expected. Organic compounds have usually d e c o m p o s e d by 500°C in oxygen but temperatures up to 720oc were recorded for the final exotherm in some admixtures indicating unusual stability. For samples i0, ii, 12, 16 and 21 there is no m a j o r e x o t h e r m above 500oc. Samples i0, ii and 12 contain only small amounts of organic acid salts other than lignosulphonic acid salts. Sample 21 is a melamine based admixture and sample 16 is essentially a sugar solution. Thus it appears that this high temperature e x o t h e r m is associated with salts of organic acids. When samples i, 2, 8, 14 and 15 were converted to the acid form by passing the diluted admixture through an ion exchange column in the hydrogen form b e f o r e freeze drying, there were no exotherms above 500oc. The profile of the peaks b e l o w 500°C was also altered but as many of the free caboxylic acids are unstable, converting to cyclic lactones on drying, this change was not unexpected. Clearly the presence of sodium or calcium ions confers some exceptional stability to salts of and organic acid, the final b r e a k d o w n being rapid giving the sharp peak. The expected inorganic products, sodium or calcium carbonate give endotherms on d e c o m p o s i t i o n so the reaction must be oxidation of an organic species. Further thermal analysis studies on a series of organic acids and their sodium and calcium salts(16) has shown that the

Vol. 14, No. 2

DTA, OXYGEN,CHEMICALADMIXTURES, IDENTIFICATION

3o,uv

/ ~0 J

®

®

~

A

A

/ ®

II \® ! I I

I I

(Z) ®

® @ I

f

I

I

I

I

100

200

300

400

500

600

700°C I

Temperafure Figure

i.

DTA Curves for Class 1 Admixtures. (30 ~v ~ 0.67°C at 500°C)

211

212

Vol. 14, No. 2 N.B. Milestone

®

@

•

®

le

J

!

I

!

|

i

100

200

300

~00

500

600

I

700°C

Temperature Figure

2.

DTA Curves for Class 2 and Class (30 ~v --- 0.67°C at 500°C)

4 Admixtures.

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DTA, OXYGEN,CHEMICALADMIXTURES, IDENTIFICATION

213

p r e s e n c e o f a high temperature e x o t h e r m is common for m a n y h y d r o x y c a r b o x y l i c acid salts, but is a b s e n t in the pure acids. F u r t h e r w o r k is n e e d e d to e l u c i d a t e this stable entity and its m e c h a n i s m of breakdown. This m e t h o d has been used to c o n f i r m that two samples of s u p p o s e d l y i d e n t i c a l admixtures w e r e in fact different. On a m a j o r c o n s t r u c t i o n site, a d d i t i o n to a b a t c h of c o n c r e t e of an a d m i x t u r e f r o m a drum r e c e i v e d in a s e c o n d shipment to the site, c a u s e d delays in setting time. The s u p p l i e r was unable to d i s t i n g u i s h readily b e t w e e n a d m i x t u r e s from the first and s e c o n d shipments. The DTA method, u s i n g s e p a r a t e samples of k n o w n a d m i x t u r e s from the s u p p l i e r as standards, was able to show that the drums in the s e c o n d s h i p m e n t had been i n c o r r e c t l y labelled a n d were actually a d i f f e r e n t a d m i x t u r e to the first shipment. The chemical f o r m u l a t i o n of the two a d m i x t u r e s was very s i m i l a r and they could only be d i s t i n g u i s h e d by a d e t a i l e d chemical analysis. Conclusion DTA o f c o n c r e t e admixtures in an a t m o s p h e r e of o x y g e n p r o v i d e s a m e t h o d whereby the e i g h t e e n a d m i x t u r e s tested can be u n i q u e l y identified. Changes in raw m a t e r i a l s or c h e m i c a l f o r m u l a t i o n cause changes in peak p r o f i l e s that can be readily d e t e c t e d u s i n g this method. These changes are not r e a d i l y o b s e r v e d by o t h e r rapid s c r e e n i n g m e t h o d s such as IR or UV spectroscopy. Because p r o p e r t i e s such as setting times are a f f e c t e d by small changes to the h y d r o x y c a r b o x y l i c acid fraction, close control of f o r m u l a t i o n is needed. E x o t h e r m s in the range 5 0 0 - 7 0 0 o c in the DTA curves are due to the p r e s e n c e of s o d i u m or c a l c i u m salts of h y d r o x y c a r b o x y l i c acids but do not o c c u r above 500°C for the free acid form. References i. 2. 3. 4. 6. 6. 7. 8. 9. I0. ii. 12.

Y. Mouton, Bull. Liason Lab. Ponds. C h a u x 1958 No. 58 pp 117-34. N.B. Milestone, Cement Concr. Res., 7(1) 45-52 (1977). N.B. Milestone, J. Amer. Ceram. Soc. 62 (7-8) 321-324 (1979). J.F. Young, Proc 5th Int Symp Chem C e m e n t Tokyo 1968 II 256-67. W.C. Hansen, J. Materials, 5(4) 842-855 1970. G.M. Bruere, Nature 212,502-503 (1962). N.Z. Standard 3113. C - ~ e m i c a l A d m i x t u r e s for C o n c r e t e 1979. K.E. F l e t c h e r and M.H. Roberts, C o n c r e t e 5 , 1 4 2 - 1 4 8 and 175-179 (1971). W.J. H a l s t e a d and B. Clarke, Nat. Acad. Sci. Nat Res. Council Pub 940. Highway Research B o a r d Bull No. 310 33-35 (1962). P. Diem and K. Krehl, B e t o n w e r k and F e r t i g t e i l Technik 41 299 and 341 (1975). ~ . Kroone, Mag. Conc. Res. 23 132-134 1971. W.L. Dolch, "Analysis of H a r d e n e d C o n c r e t e for A d m i x t u r e Content." Final Report Joint Highway Research Project. F HWA/IN/JHRP-80/5. NTIS PB81-132433.

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13. 14. 15. 16.

M.H. Simatupang, Z. Kalk Gips. (i0), 427-431 (1975). F.W. Broker and M.H. Simatupang, Z. Kalk Gips. (8), 245-247, (1973). N.B. Milestone, Report No. CD2262, Chemistry Division, DSIR, New Zealand, Dec. 1977. D.E. Rogers, unpublished work.

Appendix List of Admixtures

Investigated

Name

Supplier

Darex WRDA Daratard Discon Eucon WR Eucon WR Retarding Febflow Std Febflow Retarding Leadair R Lissapol Z-N Melment Mighty 150 Plastiment DSE Plastiment N Plastiment PHC Plastiment VZ Plastocrete CLK Pozzolith 80 Pozzolith 100 XR Pozzolith 300N Pozzolith 300R Teric GN-9

W R Grace I!

Stipplecote Products Ltd Euclid Chemical Products H

f!

Jos Nathan Ltd t!

Leadair Industries ICI Hoechst Chemiplas Agencies Sika J! IS t! I!

Embecon II u

ICI

a!