Low temperature steam curing hydration of lime-fly-ash compacts

CEMENT and CONCRETE RESEARCH. Vol. 13, pp. 887-894, 1983. Printed in the USA. 0008-8846/83 $3.00 + 00. Copyright (c) 1983 Pergamon Press, Ltd.

LOW TEMPERATURE STEAM CURING HYDRATION OF LIME-FLY-ASH COMPACTS A. Marcialis, L. Massidda, U. Sanna Institute of Applied, Chemistry & Metallurgy University of Cagliari, Italy

(Communicated by F. Massazza) (Received May 9, 1983)

ABSTRACT Low pressure steam-curing hydration of mixtures with different fly-ash-to-lime ratios compacted at 50 and 150 MPA was studied. The pastes were cured for between 8 and 72 h at 60 ° , 75 ° and 90oc. For comparative purposes duplicate samples were treated for 28 days at 25oc. Compressive strength tests were perform ed on the hydrated samples and the results obtained compared with those pertaining to combined lime and porosity. Higher lime contents of the anhyd r o u s mixtures correspond to greater amounts of reacted p r o d u c t a n d enhanced strengths. Higher temperature accelerates the reaction rate and produces an increment in strength, even if moderate in passing from 75 ° to 90°C. This can be attributed to the corresponding increase in porosity of the specimens which contr a s t s with the positive effect produced by the progression of the pozzolanic reaction.

Introduction Considerable amounts of fly-ash are produced by burning coal in the r m a 1 power stations. As a consequence, in several countries the need to find an appropriate use for this solid waste has provided an impetus to many studies. Various proposals have been advanced for fly-ash disposal; however, their main use remains that of pozzolanic additions in cements and concretes (i, 2, 3). In Italy for a long time fly-ash did not constitute a problem and for this rea son apart from a few studies dating back to the early 6O's (4, 5, 6) practica ! ly nothing was published until a few years ago. The current Italian energy policy, which promotes the use of coal in ther mal power stations will implicate the disposal of substantial amounts of flyash and the need may therefore arise to find an adequate use for them. In this article the authors examine the possibility of obtaining products with good mechanical properties by means of low-pressure steam curing of flyash-lime mixtures compacted at different pressures according to a procedure al ready described elsewhere (7, 8).

887

888

Vol. A. Marcialis,

13, No. 6

et al.

Experimental Materials A fly-ash residue from the combustion of a South African coal burned the Sulcis power station at Portovesme (Cagliari) was used in the tests. composition is shown in Table i. TABLE

i

SiO 2

41.7 %

AI203

28.3 %

CaO

9.1%

MgO

1.5%

Fe203

5.2%

Na20

0.6%

K20

i.O %

SO 3

3.O%

C (unburned)

7.0 %

L.O.I.

6.8%

S.S. Blaine

4000 cm2/g

The lime was composed of Merck calcium Preparation

in Its

hydroxide

and assayed 91% Ca(OH)

2"

of the Specimens

Three fly-ash-lime mixtures were prepared containing 20, 30 and 40% Ca(OH) 2 respectively. After m o i s t e n i n g with 10% water the pastes were compacted into cylindrical moulds with cross section of 3.80 cm 2 and length 2.50 cm at a pressure of 50 and 150 MPa. Finally the specimens were h y d r a t e d with saturated steam for curing times rainging from 8 to 72 h at 60 ° , 75 ° and 90°C. For comparative purposes duplicate specimens were treated for 28 days at 25oc with relative humidity of more than 95%. Tests Compressive strength tests were carried out with a Giazzi press model M58 and p o r o s i t y measured using a Carlo Erba m e r c u r y p o r o s i m e t e r mod. 200. Ca(OH) 2 contents of the samples were dosed by means of t h e r m o g r a v i m e t r i c analysis using a Mettler TA 3000 thermoanalyzer and CAHN thermoscales mod. RG-HV. The dif fractograms of the hydrated pastes were obtained with a General Electric XRD5 diffractometer; an ARL scanning electron m i c r o s c o p e was used for the micrographs. Results

and Discussion

The compressive strengths of the samples are summarized in Fig. i. The letters a and b refer to samples pressed at 50 and 150 MPa respectively. The dashed li ne indicates the strength attained by an homologous specimen treated for 28 days at 25°C. The figure shows that the strengths obtained depend to a varying degree on the different experimental parameters. Their effects can be summarized as follows. Increasing compacting pressure from 50 to 150 MPa enhances compressive strength by between 15 and 40%.

Vol.

13, No. 6

889 STEAM CURING,

bO[

30

HYDRATION,

LIME, FLYASH, REACTION,

la

POZZOLAN

lb

f I

.. 50 -

I

I

I

2b

2a

4o

~30

S I

~

I

I

5O --

I

L

I

I

3a

I 3b

I

i

'

i

i

I

I

8

15

36

72

8

15

36

FIG.

I,

72 Tpme ( hours I

1

Compressive strength as a function of curing time. I. Ca(OH) 2 20%, F.A. 80%; 2. Ca(OH) 2 30%, F~A. 70%; 3. Ca(OH) 2 40%, F.A. 60%. 0: 60%; • : 75°C; A: 9OOC. ......... Specimen treated for 28 days at 25°C. The increment in strength is roughly proportional to the initial Ca(OH)^ content. The effect of temperature differs in that the increment in strength t a i n e d b y increasing temperature from 60 ° to 75°C is much more pronounced than that produced by the increase from 750 to 9OOC. Almost all the samples treated at 75 ° and 9OoC exhibit, even after only 8 hours curing, strengths higher than those attained at 25°C; by comparison, for the specimens treated at 60oc, 15 to 36 hours are required to achieve analogous results. In pastes pressed at 50 MPa only very small increments in strength are exhibited beyond 36 h ageing, whilst for those compacted at 150 MPa strengths continue to increase. With prolonged hydration a gradual combination of the calcium hydro x i d e with fly-ash takes place. This is illustrated in Fig. 2, which shows the lime reacted at 75Oc as a function of curing time, and in Fig. 3, which shows com bined lime after 72 hours as a function of temperature. One can observe th a t all the curves are roughly linear. Moreover, the amount of reacted lime increa ses with increasing lime content of the anhydours mixtures - a feature already observed by others in pozzolanic materials (3).

890

Vol. 13, No. 6 A. Marcialis,

-_x tO E

•

Lime 20%

•

Lime 30%

•

Lime

et al.

40%

£ =

i .r

~

I-

6

...--.B

J I 8

I 15

I 72 Time ~hourst

I 36

FIG.

2

Reacted lime per i O O g of anhydrous mixture as a function of curing time at 75oc (com paction pressure 150 MPa)

ILime 201& • Lime 30% • Litlne 40e/j

K

o '(

OD

8'

FIG. 0

6

3

Reacted lime per I O O g of anhydrous mixture as a function of temperature after 72 h cu~ ing (compaction pressure 150 MPa)

L_____

I

I

I

6O

75

9O Temper a t u r e l ° C )

On the other hand, no correlation exists between reacted lime and strength; whereas the former increases almost linearly with temperature, this is not so in the case of the latter. The diffractometric analysis of the hydrated pastes did not afford any in formation on the nature of the lime-pozzolana reaction products. In fact, Fig. 4 which shows the diffraction patterns relating to the specimens with 40% lime treated for 72 hours at the three different temperatures, illustrates how the reduction in the peaks typical of Ca(OH) 2 does not coincide with the formation of new hydrated crystalline phases.

Vol.

13, No.

6

891 STEAM CURING,

HYDRATION,

LIME,

FLYASH,

REACTION,

POZZOLAN

L

L L

L

L

L

C L

FIG.

4

L = lime D i f f r a c t o g r a m s of: anhydrous sample (a); s p e cimens h y d r a t e d for 72 h at 6OoC (b); 75 o G (c) and 9OOC (d). (Compaction pressure 150MPa) d L

L L

7

L

20

The results of pore size d i s t r i b u t i o n analyses p e r f o r m e d o n s a m p l e s p r e s ~ ed at 150 MPa are shown in Figs. 5 and 6. Figure 5 shows the cumulative volume and pore size d i s t r i b u t i o n of pastes hydrated for different curing tim e s at 75oc, whilst Fig. 6 shows the effect of temperature on specimens tre a t e d for 72 hours at different temperatures. From Fig. 5 one can observe how the increase in both curing time and initial lime content produces a reduction in total p o r o s i t y and a v a r i a t i o n in pore size distribution. In fact, a great e r abundancy of the finer size classes is observed. The increase in temperatu r e from 60o to 75oc results in a r e d u c t i o n in p o r o s i t y (Fig. 6) whilst that from 75 ° to 90°C gives rise to an increase in total p o r o s i t y and a shift of the p ~ re sizes towards the coarser size classes. The SEM m i c r o g r a p h s of Fig. 7 show the aspect of the hydrated specimens. Alongside the balls of fly-ash, some of shich are h o l l o w (as can be o b s e r v e d from Fig. 7a) h y d r a t e d phases are present, composed of irregular agglomerates which in some details appear in the form of coarse fibres, more or less abundant, d e p e n d i n g on the reaction conditions. Conclusions Steam curing h y d r a t i o n of fly-ash lime compacts allowed to obtain products with good strengths even after relatively short ageing.

892

Vol. 13, No. 6 A. Marcialis, et al. ~,r.

,,I~ , ,ill I 4it I liu I I)f i A

to

~.rr M~ ....

~4 I rio i 4 4i

|

i :, o ! u f

A

~o~

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;

so ;

J

,14 ,

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io o~ i i l

i

r A

An ~

60

An ~a~p

80 Cl5

Annvarous samel~

A~h~a,OUS

Anhydrous $aepl~

FIG. 5

g

o eo,

Cumulative curves and pore size distribution of the anhydrous specimens and those hydrated at 75oc for different curing times (compaction pressure 150 MPa)

15 h

40. 20°

e0°

36h

72 h

~0

7B

3Q

167

7~4

~

79

36

T67

~4

~00

Pore rzdlus I~m) Pr essu~e { M P a ) 94

20

44

10

LIME FA

160- " ,

02

94

2U

4A

20% 80%

10

Q2

94

20

4.4

LIME 30% r A 70'r~

1.0

0.2

LIME 40% r A 60%

40.

" ':'~"

FIG. 6 60"C

60°C

Cumulative curves and pore size distribution of the anhydrous specimens and those hydrated for 72 h at different temperatures (compaction pressure 150

~

40.

-

60

60°C

75°C

75°C

MPa) .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

60Oc

75oc

____.90oc

167

764 3500

79

36

167

764 354)0

Pore r a d i u s ( ~ m )

79

36

167

764 3500

Vol.

13, No.

6

893 STEAM CURING,

HYDRATION,

LIME,

FIG.

FLYASH,

REACTION,

POZZOLAN

7

SEM micrographs of specimens cured for 72 hours - 1500 x ~compacti o n pressure 150 MPa) a) Ca(OH) 2 20%, F.A.

80%,

t = 75°C

b) Ca(OH) 2 40%, F.A.

60%,

t = 60°C

c) Ca(OH) 2 40%, F.A.

60%,

t = 75°C

d) Ca(OH) 2 40%,

60%,

t = 90°C

F.A.

Strength increases linearly with increasing lime content and, at a diminishing rate, with increasing curing time and temperature. Combined lime in-creases in an approximately linear fashion with increasing curing time and temperature. For these reasons, no direct relationship therefore exists betwe e n strength and combined lime. Analogous results were found for lime-pozzolana m i x tures cured at 2OoC (9). The absence of such a relationship probably depend s on the fact that the specimens treated at 9OoC have greater porosity than those aged at lower temperatures. In fact, the increment in strength produced by the greater degree of reaction of the product at 90°C, is supposedly reduced by the simultaneous increase in porosity.

894

Vol. 13, No. 6 A. Marclalis, et al.

X-ray analysis of the pahses produced by reaction during hydration reveal ed them to be non-crystalllne and under SEM observation they appear in the form of coarse, sometimes fibrous, agglomerates. Acknowledgements The authors wish to thank the Societ~ Italcementi, and in particular Dr. U. Costa of the Laboratorio Chimico Centrale for carrying out the thermoanalyses.

References 1. M. Kokubu and J. Yamada, Proc. 6th Intern. Congr. Chem. Cem., Moscow, p. 1-5 (1974) 2, R. Sersale, Proc. 7th Intern. Congr. Chem. Cem., Paris, IV, i, P" 1-18 (1980) 3. K. Takemoto and H. Uchikawa, Proc. 7th Intern. Congr. Chem. Cem., Paris, IV, ~, p. 1-29 (1980) 4. F. Massazza and M. Cannas, Ii Calore, 32, p. 11 (1961) 5. F. Massazza and M. Cannas, Ii Calore, 33, p. 9 (1962) 6. F. Massazza and M. Cannas, Ii Calore, 33, p. 58 (1962) 7. M. Collepardi, A. Marcialis, L. Massidda and U. Sanna, Cement and Concre t e Research, 6, p. 497 (1976) 8. L. Massidda and U. Sanna, Silicates Industriels, in press. 9. F. Massazza and U. Costa, Siliconf, Budapest, I, p. 537-552

(1977)