Properties and structure of oil shale ash pastes. II: Mechanical properties and structure

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 391-400, 1985. Printed in the U S A 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

PROPERTIES AND STRUCTURE OF OIL SHALE ASH PASTES. II: MECHANICAL PROPERTIES AND STRUCTURE H. Baum, A. Bentur and I. Soroka Building Research Station - Faculty of Civil Engineering Technion, Israel Institute of Technology Haifa, Israel (Communicated by J.P. Skalny) (Received Aug. 6, 1984)

ABSTRACT Compressive strength, drying-shrinkage and expansion in water of oil shale ash pastes were studied and compared to the corresponding properties of portland cement paste. X-ray diffraction and some scanning electron microscopy runs were also included in the study. It was concluded that the structure and properties of the ash pastes can be described and explained by the same models which have been suggested for portland cement paste. The only exception was the total porosity of the ash paste which remained unchanged with time. A suitable modification in the structural model of the portland cement paste was suggested to allow for this specific behaviour.

Introduction The first part of the paper (I) presented the tests from which the composition and physical features of oil-shale ash pastes were evaluated and compared to those of portland cement paste. The present part further discusses the mechanical properties, namely compressive strength, dryingshrinkage and expansion in water, of the very same pastes, together with the factors which determine these properties. The ash studied possessed cementitious properties whereas, in most of the reported cases, it has been considered only as a pozzolanic filler (2-5). Hence, only limited data have been reported on the physical structure and the mechanical properties of oll shale ash pastes (6-9), and no detailed quantitative explanation has been offered, as yet, to explain and relate their mechanical properties to their physical structure. As stated previously (i), the purpose of the present study was to suggest an appropriate model by which the mechanical properties of the set ash can be explained. The physical features of the set ash were dealt with in the first paper and the present one is limited, therefore, to its mechanical properties. The data of both papers, however, are considered here in formulating the suggested model of the set ash.

391

392

Vol. 15, No. 3

H. Baum, et al. Experimental In order to avoid repetition, the data with respect to materials, preparation of tests specimens etc., which were presented in the first paper (I), are not included here.

(a)

Compressive strength was determined at the ages of 1,3,7,28,60 and 180 days on six 25mm cubes. Cubes were continuously cured in water to the age of testing, and tested in a saturated surface-dry condition.

(b)

Drying-shrinkage was determined on three 25.4x25.4x160mm (ASTM C490-83a) prisms. The prisms were cured in water to the age of seven days and then allowed to dry at 21±2°C/50±5% RH. Length-change and weight measurements were taken at the ages of 7,8,9,14,21,28,45,60,90 and 180 days. The tests included a portland cement paste with a water/cement ratio of 0.4 and an oil-shale ash paste with a water/ash ratio of 0.8, i.e. drying-shrlnkage was not determined on the ash pastes with the water/ash ratios of 0.9 and 1.0.

(c)

Expansion in water was determined, again, on three 25.4x25.4x160 mm prisms which were stored continuously in water. Length-change and weight measurements were taken at the ages of 1,2,3,7,14,21,28,60,90 and 180 days.

(d)

S c a n n i n ~ e l e c t r o n microscopy. Limited runs were made at the ages of one and 28 days on the set portland cement (w/c = 0.4), and at one and 180 days, on the set oil shale ash paste with a water/ash ratio of 0.8.

Results Compressive Strength Strength data of the cement and the oll shale ash pastes are presented i n T a b l e I. It is clearly evident that the ash pastes were much w~aker than their cement couterparts. Noting that the ash pastes were of a much greater total porosity than the cement paste, i.e. 70 to 85% and 23 to 30% respectively (Table 2, ref. I), such a difference in strength was to be expected. On the other hand, it can be noted that the strength of both pastes are affected in essentially the same way by such strength-determining factors as age and water/binding material ratio. (a) Age. As hydration proceeds with time, the strength of the portland cement paste increases and a similar increase in strength characterizes the ash paste (Table I). Moreoever, it is apparent (Fig. I) that the relative rate of strength development in both pastes is essentially the same. An exception in this respect is the relatively higher strength of the portland cement paste at the early ages up to about 7 days. This higher rate of strength development may be attributed to the presence of C3S in the cement which hydrates much faster than C2S. As no C3S is present in the ash, the relative rate of strength development in the cement is expected to be faster at early ages. Nevertheless, the relative rate of strength development in both pastes may be considered essentially the same. (b) Degree of Hydration. The effect of age stems, of course, from the increase in the degree of hydration with time, which in turn resuts, in the portland cement paste, in lower total porosity. Taking the chemically combined water as a measure of the extent of hydration, it can be

Vol. 15, No. 3

393 OIL SHALE ASH, PASTES,

STRENGTH,

SHRINKAGE,

XRD

Table I Compressive

strength of the oil shale ash and the portland cement pastes Compressive Age, Days

Strength,

Water/Ash Ratio

N/mm 2

w/c Ratio

0.8

0.9

i .0

0.4

I

0.6

0.2

0 .I

4.8

3

1.8

1.0

I .2

20.2

7

4.8

2.7

2 .I

28.5

28

9.0

5.0

4.2

54.3

60

ii .8

7.8

6.0

66.0

180

12.7

8.2

7.0

79.0

~eloo

~" 8O

¢,.5 Z bJ

~

__ CO

6o

W/A ~ - "

_

.t

/

W/C =~ 4

Fig. I

.

Development of relative compressive strength with time in the oil shale ash (full lines) and the portland cement (dotted line) pastes.

t''

~a . z0 o o

,

0 3

7

28

60

180

AGE , DAYS

seen that, indeed, the strength of both the cement and ash pastes increased with the increase in the degree of hydration (Fig. 2). 14

'

I

r

I

I

'

/¢.o. I T '

E E

/

z

60 =w

~,o

so ~-~

z Q.

Y!

4o~ tOW

!

W/A =0.8

j;

IE

82 0

,

l

~ I0 ~

Fig.

2

~ / / ~

I

,

I

0 5 I0 15 20 25 CHEMICALLY COMBINED WATER , %

o

Chemically combined water vs. compressive strength in the oil shale ash (full lines) and the portland cement (dotted line) pastes.

394

Vol. 15, No. 3 H. Baum, et al.

(c)

Porosity. Porosity is generally considered the most important strengthdetermining factor. As mentioned earlier, the increase in the strength of the portland cement paste with the increase in the degree of hydration is attributable to the decrease in the total porosity of the set paste. In the ash pastes, however, the total porosity of the paste remains virtually unchanged with time (Table 2, ref. i), and this particular behaviour constitutes the main difference between the portland cement and the ash pastes. Strength-wise, however, capillary porosity can be substituted in the portland cement paste for total porosity because both simultaneously decrease with the increase in the amount of hydration (I0), and the strength of the portland cement paste similarly increases as the capillary porosity decreases. It may be noted that the same trend of increase in strength with decrease in capillary porosity characterized also the ash pastes (Table 2, ref. I; Table I in this paper). (d) Threshold Diameter. The threshold diameter, which is determined from the turning point of the cumulative pore-size distribution curve of the mercury porosimetry, constitutes a measure of the size of the bigger pores in the sample. The presence of pores in a solid give rise to stress concentration and, adopting the principles of fracture mechanics such as Griffith's theory (Ii), the presence of greater pores will result in a weaker material. Hence, it is expected that a paste of a greater threshold diameter will exhibit a lower strength and vice-versa. It was shown earlier (Fig. 7, ref. i) that the threshold diameter of both the portland cement and the ash pastes decreased with time as the hydration proceed. The very same data are presented in Fig. 3 where the strength is plotted versus the threshold diameter. It can be seen that, as it was to be expected, strength was related to threshold diameter and increased with the decrease in the latter. This effect may provide additional explanation to the increase in the strength of the ash pastes without a corresponding decrease in total porosity. Apparently the effect of the decrease in the capillary porosity and the threshold diameter was great enough to cause increase in the strength of the paste without a corresponding decrease in total porosity. (e) Water/Ash Ratio. The relation between the compressive strength, ~, of the ash paste and its corresponding water/ash ratio, ~, is of the same exponential nature which is observed in the portland cement paste, namely, ~=Ae -b~ (Fig. 4). Accordingly, under conditions in h a n d , _ ~ h ~ 28 days strength of the ash paste can be calculated from o28=1791e Solving for ~=0.4 gives o28=39.2 N/~m~2 which, with due reservation, constitutes a rough estimate of the strength of the ash paste where it was possible to produce such a paste with a W/A ratio of 0.4. The 28 days strength of the corresponding portland cement paste was much higher, i.e. 54.3 N/mm 2 (Table i). It was pointed out earlier that the lower strength of the ash paste was to be expected because, for the same water/binder ratio, the estimated total porosity of the ash paste was greater than the porosity of the corresponding cement paste. Nevertheless, the effect of the water/binder ratio on strength is essentially the same in both pastes. Drying-Shrinkage The shrinkage of the cement paste was much greater than the shrinkage of the ash paste (Fig. 5). The nature of the shrinkage curves, however, is similar and for both pastes the curves can be divided into three parts of roughly of a linear nature. At the early age up to 14 days the shrinkage rate is maximum; at the age interval 14 to 60 days in the portland cement paste, and 14 to 90 days in the ash paste, the rate of shrinkage becomes moderate; and at later ages shrinkage seems to cease altogether with the curves tending to flatten.

Vol. 15, No. 3

!o8 ;~ 18 ~) DAYS ' '

12

395 OIL SHALE ASH, PASTES, STRENGTH,

I

'

I

I

'

I

~e60

-

I

~

-

o - , , ~

I

I

/

DAYS 1 %

6

-

-

i,~

O" = 1 3 . 8 0 6 =D " e ° e

~

6

I0

~28

--

( r -. - 0 9 8 2 )

\\

o

'

SHRINKAGE, XRD

~ ~/4~

"4

-

\\\

-

2

0

J

0

I

,

I

0.2

,

I

0.4-

,

0.6

THRESHOLD

I

I

0.8

1.0

DIAMETER

0.8 0.9 I .0 WATER/ASH RATIO = W/A

, 1.2

, p.m

Fig. 3

Fig. 4

Compressive strength vs. threshold diameter in the oll shale ash pastes.

Relations between compressive strength and water/ash ratio at 7, 28 and 180 days.

O.O

0.2 0.4 ~" 0.6 o N A

_j 0.8 _J

1.0 z -" 1.2 I-

Z

E

Fig. 5

1.6 I--

Drying shrinkage time in the oil (full line) and portland cement pastes.

W/C =0.4

if)

1.8

with shale ash the (dotted line)

-

2.0 7

9

14 AGE

21 (log

;)8

~ ~ 46 60 90

scale)

180

, DAYS

Shrinkage processes can he attributed to several mechanisms (12). Nevertheless, regardless of the mechanism adopted, less shrinkage is to be expected, under otherwise the same conditions, in stronger and less porous pastes (13). Accordingly, contrary to the test results, the shrinkage of the ash paste was expected to be greater than the corresonding shrinkage of the portand cement paste.

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Vol. 15, No. 3 H. Baum, et al.

Under conditions i n hand, however, this was not the case because the volume concentration of the gel in the ash paste, due to Its much greater porosity, was much lower than the volume concentration of the gel in the portland cement paste. As drying-shrlnkage is mostly determined by the water loss from the smaller gel pores rather than from the bigger capillary pores, greater shrinkage is to be expected the higher the gel concentration, explaining, in turn, the greater shrinkage of the portland cement paste. Accordingly, in the portland cement paste (Fig. 6), apparently most of the water lost was gel water and, therefore, such a loss was associated with substantial shrinkage. On the other hand, in the ash paste, the water lost in the first stage up to, say, 20%, was mostly capillary water and as such, was not associated with an appreciable shrinkage. Later, however, most of the water lost was gel water and similarly also the ash paste exhibited a substantial shrinkage, but not as great as the portland cement paste. 0.0

0.2

0.4. N o

0.6

J

-.. 0 . 8 -

Fig. 6

J I.O--

Drying-shrinkage vs. water loss in the oil shale ash (full llne) and the portland cement (dotted line) pastes.

, ,W / - -/ C - =0.4

z_

w 0

1.4--

~

1.6--

I I t t

E

I co 1.8--

I

I

2"00

5

I~ I0 WEIGHT

I

I

15 20 25 L O S S , ~o

I

I

30

35

Fig. 7 Z

z~,~°'0'5F'

I

I

I

~ X

[

~

w Z ~< l

I W/C = 0 . 4

~o... -- -- - O ~ )

I .- .'~ _- " J "~O"

0.1

13

7

18

28

60

90

Expansion in water with time in the oil shale ash (full line) and the portland cement (dotted line) pastes.

180

AGE , DAYS Expansion in Water The oil-shale ash paste exhibited a much lower expansion than the portland cement paste (Fig. 7), i.e. the linear expansion of the portland cement paste at the age of 180 days was 0.5xi0-2% as compared with 0.14xi0-2% in the ash paste with a W/A ratio of 0.8.

Vol. 15, No. 3

397 OIL SHALE ASH, PASTES, STRENGTH,

SHRINKAGE, XRD

The expansion in the portland cement paste is brought about by the increase in volume of the solids on hydration, and the topochemical nature of the hydration process which allows only partial accommodation of the volume increase by the capillary pores of the structure. The lower expansion of the ash paste implies better accommodation of the hydration products by the caplllary pores and this better accommodation can be attributed either to the much greater porosity of the ash paste or to a smaller increase in the volume of the solids on hydration, or to both.

It will be seen later than an appreciable amount of ettrlnglte is formed on the hydration of the ash (Fig. 8). Again, the small expansion of the ash pastes may be attributed to the high content of capillary pores which accommodates the growth of the ettringite crystals without causing an appreciable expansion. The growth of ettrlnglte without an appreciable expansion was observed also by other (14). Structure X-ray diffraction of the set ash (Fig. 8) indicated the formation of ettrlnglte on the first day accompanied by a diminution of the intensity of free CaO and CaSO 4 peaks. No additional crystalline products could be identified. This observation is not totally unexpected once the CSH gel constitutes the main hydration product. The observations made by the scanning electron microscopy runs are grouped in Fig. 9. Generally speaking, it may be concluded that, at the same age, the structure of the portland cement paste is more heterogenous than that of the ash paste (Fig. 9a and b vs. c and d). There are large pores both in the portland cement and the ash pastes, but the pore filling in the portland cement paste is more effective. The structure of the ash paste is characterized by small particles that tend to be round, and covered with fibrous materlals. At 28 days the structure of the portland cement paste Is very congested, and It is hard to recognize structural details (Fig. 9e and f). At 180 days (Fig. 9g and h) the structure of oll shale ash pastes is uniform and it is difficult to distinguish between the individual particles. In general, it seems that most of the area is composed of many fibrous particles which form a quasi three- dimensional net.

c~,so, BEFORE HYDRAT ION

~ ~g COCO,

V~P AFTER

DAY

HYDRATION

E~

E~ 25S

Et~ El~ 276

coco, ~ ~n

e~t ss6

9

E~

Fig. 8 7 DAYS

28

DAYS

J 60

2 @°

X-ray diffraction patterns of the oll shale ash before and after hydration.

398

Vol. 15, No. 3 H. Baum, et al.

CEMENT PASTE

OIL SHALE ASH PASTE •

J

r

.

J

'~''r

r ~" ~ ! "~ ~

'

¢

Fig. 9 Scanning electron microscopy observations of the set oil shale ash and the portland cement pastes.

Vol. 15, No. 3

399 OIL SHALE ASH, PASTES, STRENGTH, SHRINKAGE, XRD

Needle shaped particles are conspicuous in the oil shale ash paste at i day and 180 days. In the oil shale ash pastes, the ettringite needles are very small and not much bigger than C-S-H particles. The structure is composed of small rounded particles, l-5~m in diameter, which is approximately the diameter of the ash particles after grinding.

Discussion It was concluded in the first part of the paper (i) that the structural features, and the factors which control and determine these features, are essentially the same for both the portland cement and the ash pastes. It may be further concluded from the present paper that the mechanical properties of the ash paste, such as strength and volume changes, the factors which affect these properties, and the nature of this effect are, again, essentially the same for both the portland cement and the ash pastes. In a more general way it may be stated that the models which have been suggested to describe the structure of the set portland cement paste and to explain its properties and behaviour, may be adopted for the ash paste. Considering that both 8C2S and C3S hydrate to give a CSH gel, this latter conclusion is not unexpected. Still the portland cement paste models have to be modified to allow for the unchanged total porosity of the ash paste with time. This is not the case in the portland cement paste where total porosity decreases with time as the hydration proceeds. This decrease in porosity is attributed to the increase in the volume of the solids on hydration. It is suggested that, in the ash paste, no such increase takes place, and the volume of the solids remains, therefore, virtually the same. Hence, the total porosity remains also the same. The produced gel, however, subdivides the greater capillary pores into smaller pores, bringing about a reduction in their size whch is reflected in the observed decrease in the threshold diameter (Fig. i0). The deposition of the CSH gel in the pores reduces the volume of the capillary pores, but the volume of the gel pores is increased. Again, total porosity remains virtually unchanged because no change in the volume of the solids is involved. The latter suggestion, that no volume increase of the solids takes place on hydration of the ash, is indirectly supported by the expansion data. The expansion of the set portland cement in water is attributable to the increase in the volume of the solids which is not accommodated by the pores.

Fig. i0 Schematic description of the redution in pore-size in the ash paste without a change in total porosity.

,ORIGINAL BOUNDARY OF PORE

DRATION PRODUCTS

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Vol. 15, No. 3 H. Baum, et al.

Pressure is generated and dilitatlon takes place. Hence, when no volume increase occurs, no dilltatlon is expected explaining, in turn, the much lower expansion of the ash pastes (Fig. 7).

Conclusions This study demonstrated the cementitlous properties of the local oil shale ash which are attributable to the presence of reactive 8C2S. Generally speaking, the structure and properties of the set ash can be described and explained by the very same structural models which have been suggested for the portland cement paste. An exception in this respect, is the total porosity of the ash pastes which remains unchanged with time. This particular behavlour, however, can be explained by introducing some modification in the structural model of the portland cement paste. The ash studied was of a considerably higher specific surface area than that of portland cement. Consequently, the water demand in the ash pastes was rather high resulting, in turn, in a low strength. The great fineness of the ash constitutes one of its inherent properties and, at present at least, the use of such an ash will be feasible only when high strength is not required. Indeed, the use of the oll shale ash in rendering mortars, manufacturing hollow ash-concrete blocks, etc., has been examined with some degree of success (15,16).

References I.

2. 3. 4. 5. 6. 7. 8. 9. i0. Ii. 12. 13. 14. 15.

16.

H. Baum, I. Soroka, and A. Bentur, "Properties and Struture of Oil-Shale Ash Pastes - I: Composition and Physical Features", submitted for publication, Cem. Contr. Res. R. Rohrbach, Zement-Kalk-Gips, 2, 293 (1969). E.M. Lawsow and P.J. Nixon, Bui~dlng Research Establishment Current Paper, CP50/78, pp. 11-19 (1978). P.K. Metha, Cem. Concr. Res. iO, 545 (1980). G.J. Gromko, Trans. Res. Rec. 549, 47 (1975). M. Ish-Shalom, A. Bentur and T. Grlnberg, Cem. Concr. Res. iO, 799 (1980). A. Bentur, M. Ish-Shalom and T. Grlnberg, Cem. Concr. Res. ii, 175 (1981). A. Bentur and T. Grlnberg, Amer. Ceram. Soc. Bull. 61, 1296 (1982). A. Bentur and T. Grlnberg, Amer. Ceram. Soc. Bull. 65, 290 (1984). I. Soroka, "Portland Cement Paste and Concrete", pp. 81-85, The MacMillan Press, London (1979). A.A. Grlfflth, Phil. Trans. Roy. Soc. A. 222, 163 (1920). Ref. I0, pp. 114-123. R.A. Helmuth and D. Turk, J. Res. Dev. Labs, Portland Cem. Assoc. 9, 8 (1967). H.G. Midgley and K. Pettlfer, Cem. Concr. Res. l, i01 (1971). A. Bentur and T. Grlnberg, "Utilization of Ash From Direct Burning of Oil Shales as a Building Material", Israel Ceramics and Silicate Institute, Report to the Ministry of Energy and Infrastructure, The Government of Israel, Haifa, (1980). A. Bentur, Silicate Industrials, XLVII, 163 (1982).