The effect of initial curing temperature on the performance of oilwell cements made with different types of silica

CEMENT and CONCRETE RESEARCH. Vol. 19, pp. 703-714, 1989. Printed in the USA. 0008-8846/89 S3.00+00 Copyright (c) 1989 Pergamon Press pIc

THE EFFECT OF INITIAL CURING TEMPERATURE ON THE PERFORMANCE OF OILWELL CEMENTS MADE WITH DIFFERENT TYPES OF SILICA

Eliza Grabowski and J.E. Gillott Department of Civil Engineering The University of Calgary

(Received Feb. l,

(Refereed) 1988; in final form March 2], 1989)

ABSTRACT Oilwell cement blended with silica subjected to hydrothermal conditions (2.75 MPa, 230@C) shows changes in engineering properties which depend on the type and proportions of silica. The most significant changes occur after shorter periods of pre-curing. Thus the phase composition and fabric at the onset of hydrothermal reactions has a very significant effect on post-hydrothermal strength and permeability. Variation of the initial curing period and temperature, which influences the rate of both hydration and pozzolanlc reactions, may be used to change the characteristics of hardened cement. Cements cured at lower temperatures showed decreased strength and increased permeability due to a much slower rate of the pozzolanic reaction between silica fume and calcium hydroxide and a reduced rate of cement hydration. After hydrothermal treatment, however, samples pre-cured at lower temperatures for less than 3 to 4 weeks, showed increased compressive strength and slightly decreased permeability. Introduction Compressive strength and water permeability of oilwell cement blends depend on such factors as time and conditions of curing (I, 2), slurry design and use of additives (3, 4, 5, 6), environmental conditions (7, 8), and any additional treatments to which the hardened cement blends are subjected (9, I0, Ii). The ability of oilwell cements to maintain adequate strength and low water permeability is particularly important when the cements are exposed to hydrothermal conditions. Silica in the amount of 30 to 50 percent by weight of cement has been commercially used to combat the strength retrogression and permeability increase observed for ordinary Portland cements (3, 8, II). The extent of improvement in engineering characteristics of hydrothermally treated cement: silica blends depends on the type and proportions of silica used (12) and on the time period of pre-curlng, particularly at shorter ages (5, 12). Evidently the degree of hydration, namely the relative amounts of unhydrated and hydrated calcium silicate hydrates (C-S-H), type of C-S-H and the availability of free silica and

703

704

Vol. E. Grabowski

19, No. 5

and J.E. Gillott

calcium hydroxide at the onset of hydrothermal reaction influence the characteristics (phase content, mlcrostructure and fabric) of hydrothermally formed products. The major product is xonotlite, C6S6H. That type of xonotllte-rlch cement is characterized by different strengths and permeabillties (I0, II) which depend on the pre-curlng period prior to hydrothermal treatment (HT). The assumption was made, therefore, that different temperatures of pre-curing would further affect the engineering characteristics of hydrothermally formed products. That assumption appeared reasonable since the rate of both hydration and pozzolanic reaction will be influenced by temperature (13, 14) and probably to different degrees. Mixes studied contained silica flour, silica fume and a combination of fume with flour or sand in different proportions. The main objective of this work was to investigate the effect of curing temperatures between 3 and 21°C on basic engineering properties of thermal cement blends before and after hydrothermal treatment. Materials and Methods Oilwell cement Class G has been used as a basic cement to prepare blends with silica fume and flour or silica fume and sand. Silica fume, a byproduct of the silicon or ferrosillcon industry (3, 15, 16) is a very reactive pozzolanlc material. Due to its high water demand a superplasticizer was used to achieve the required consistency and workability of the slurries. Silica flour is pulverized quartz with a uniform minus 200 mesh particle size (< 75~m); Ottawa sand, > 150 um size fraction, was used as a coarse silica. Physical and chemical characteristics of the materials used in this work and a detailed description of methods are given in other papers (5, 12). Proportions for laboratory batches of 600 mls of slurry are given in Table I. Content of silica (equivalent SiO2) was kept at 40% by weight of cement.

Weight proportions Mix

W/(C+S)

W/C

TABLE i. (g) used to prepare 600 ml of Cement Slurry Cement

Fume

Flour

Sand

Water

Super plast.

515 515 515 515 515

173 216 113 173

205 50 -

103 50

360 369 365 365 369

8-9 12 3.5 6

.

SF SFFB F FS FSB

0.5 0.5 0.5 0.5 0.5

0.70 0.71 0.70 0.70 0.71

(C + S) is used for cement + silica Control samples for flexural and compressive strength and for permeability tests were stored for 24 hours in the fog-room, demoulded and pre-cured for specified time periods in the fog-room (100% RH., 21°±I°C) prior to HT. Samples for low temperature curing were stored for 2 to 3 hours in the fog-room, sealed in plastlc bags containing water and immersed for about 45 hours in cold water baths at temperatures of 3 ° and 10°C respectively. Then samples were demoulded and kept for specified periods in the cold water baths. Companion samples were cured under fog-room conditions or subjected to HT. Strength and water permeability tests were performed (5), and post-test specimens were investigated by infrared spectroscopy (IR), X-ray diffraction and scanning electron microscopy (SEM).

Vo]. 19, No. 5

705 CURING TEMPERATURE, PERFORMANCE, OILWELL CEMENTS

FOG

(MPo)

ROOM

8

LOW

TEMPERATURE

CURING

~lPO,

........ 0 . . . . . . . . . . . . .

............ o

FR

,o....°°°'" 60.

"1" F-

MIX

//

/

z

? ,,

bJ nI(/14o

/

o IO° .....---° °o°°"" o

tkl >

KEY, 4 0 % SiO z

o0 ,oO"

=..""

so

(/) (/I w I¢ ~,-~

"

/ o""

SF

......... M I X F FR • FOG ROOM (21"C)

. ............... 8 3 0 FR I0 •

. ............

'1~ - - ' ' -

--~-

~ 5°

0 U

0/ I

;

n4

z'e

s6 TOTAL

~o

( DAYS )

AGE

FIG. 1. Compressive Strength of Mixes SF and F Cured at Different Temperatures !

(MPe)

FOG

Z

81 LOW

TEMPERATURE

~

60-

•r SO. I-

ROOM

o

CURING

FR

KEY, 40%

$10 z MIX

f f ' -_ -

•

FIR

........

I0"

FR • FOG

tLi

MIX

FS8 FS ROOM

(2t°C)

n, 40"

I,(n tmJ > 30. ul bJ

n, 20. o. =E

0 o

I0.

/

0

m

~

=O

~

~ TOTAL

(DAYS)

AGE

FIG.2. Compressive Strength of Mixes FS and FSB Cured at Different Temperatures

706

Vol. E. Grabowski

19, No. 5

and J.E. Gillott

Results a)

Fog room and Low Temperature Samples Curing temperature has a major effect on strength development as the rate of hydration changes with temperature. As expected, compressive strength decreased with decreasing temperature; the magnitude of this effect, however, was different for cement mixes containing the different types of silica in different proportions (Figs. I, 2). The largest reduction of ultimate strength was observed for the cement:fume mix (F); 90 days strength values of mixes cured at 3°C and IO°C were only 45% and 60% of fog-room cured mixes respectively. Mixes containing fume, together with either flour (SFFB) or sand (FSB) at the 3:1 ratio, showed slightly smaller strength decreases; the corresponding values were 52% and 68% of those found for fog-room cured samples. Values for mixes containing fume and sand at the i:I ratio cured at 3°C and 10°C were 54% and 74% of the fog-room values. Corresponding 90 day values for the mix with flour (SF) were 70% and 80% of the fog-room values. The trend of all curves was similar and a plateau was reached at 56 days of hydration regardless of curing temperature. Changes in flexural strength with different curing temperatures followed a generally similar pattern, i.e. strength decreased with decreasing temperature. Mixes with fume and sand at the 3:1 (FSB) and I:I ratio (FS) peaked at 28 days and showed very large decreases of flexural strength after longer periods• For the same mixes cured in the fog-room strength increased steadily with time with a plateau at about 56 days. Mixes containing fume and flour in different proportions (SF, SFFB and F) also showed steady increases in strength with time regardless of curing temperature. The types and proportions of silica had the greatest influence on changes in water permeability with curing temperature (Table 2). The mix

Permeability

TABLE 2. [mD] as a Function of Curing Period

Mix Temp Age ms7 56

SF fogroom 21 ° S 2x10-~ • 2 9x10 -4

Mix Temp Age 7 28 56

[days] and Temperature F

i0 °

3°

5.7xlO-~ 2.7x10-~ 5.0x10 -~

fogroom 21 °

3.7x10-~ 3.5x10-~ 5.8xi0 -)

l.Oxl0-~ 1.0xlO -~

I0 °

0 4.0xl0 -6

10 o

1.6x10 -4 6.0x10-.~ i. IxlO ->

3°

0 6.6xI0-9 2.5xI0 -~

SFFB f ogroom 21 °

[°C].

9. Ox10-9 8 6x10-~ 3 5xlO -~

FSB 3o

9.7xi0 -3 1.8xlO-.4 I. 3x10 -)

f ogroom 21 ° 6.1x10 -4 2.3xI0-~ 8.0xlO -9

3=

7.8xi0 -3 1.3x10 -43. Ixl0 -~

containing fume and flour (SFFB) showed the greatest changes; permeability values at ages of 7 and 28 days for samples cured at lower temperatures were significantly higher, than for samples cured in the fog-ro_o~. By 56 days, however, values for all SFFB samples were close to I0 ~ mD. The mix containing fume and sand (FSB) showed slightly higher permeability at fog-

Vol. 19, No. 5

707 CURING TEMPERATURE, PERFORMANCE, OILWELL CEMENTS

room curing than at 3°C after 28 and 56 days. The mix with flour (SF) showed lower permeabilities at lower temperatures after 56 days; at 7 and .28 days the values were higher. The mix with fume (F) was impermeable (< I0-~ mD) up to 28 day~ for all curing conditions and showed slightly higher values (still about i0-~ mD) at 56 days. Due to equipment limitations, the experimental error greatly increases for very low permeabilities and the observed "permeability increase" with curing period may be attributed to that factor. Self-dessication and microcracking, resulting from increased shrinkage of fume-containing mixes, may also contribute to this (5). The major products of hydration of the cement:silica blends cured under fog-room conditions were described previously (5, 12). Lower temperatures of curing resulted in smaller amounts of hydrated phases relative to unhydrated phases and quartz (for flour or sand containing mixes). Cement mixes made with fume and sand contained more quartz than mixes made with fume and flour suggesting that some flour was consumed in a pozzolanic reaction; in mixes cured at lower temperatures the differences in amount of quartz were less evident so further support is provided for that suggestion.

/ C°~ ,

.

"~,~,

.er,

;'~/,

4~--'M

A. F L O U R

C.

t~.kd.

20
:

07

(SF),

5 °C

,

"~

"J'~.

.~

.~, ~ '

" i - . _ ~ _- ~

~ : - ~ 1 1 ~

224

5°C

FUME

B.

" FLOUR

FUME

D.

(F) , 5 °

I0 ° C

FIG. 3. SEM's of Cement:Sillca Mixes Cured 28 days at Low Temperatures

708

Vol. E. Grabowski

19, No. 5

and J,E. Gillott

Lower curing temperatures for mixes containing fume decreased the rate of the pozzolanlc reaction more than the rate of hydration. More CH and silica fume was present relative to the amount of C-S-H. The amount of CH decreased with time of curing for all curing temperatures; for samples cured in the fog-room CH was detectable up to 7 days (5); for 10°C samples - up to 28 days and for 3°C samples - up to 56 days. The above changes in composition of cement:silica blends cured at lower temperatures were confirmed by observation on the scanning electron microscope. All cement: silica blends cured at i0 ° and 3°C showed more grains of unhydrated cement and silica than were present in mixes cured in the fog-room (Fig. 3). Mixes containing fume showed exfollated CH at longer periods of hydration (Fig. 3c); the surface of samples cured at lower temperatures was much more diversified and porous than in samples cured in the fog-room (5, 12). Some rosette-like C-S-H and CH and unreacted fume (Fig. 3C) were visible at early ages of hydration (Fig. 4B, D).

C.

IO°C SEM's of Cement:Sillca

FUME FIG.4. Mixes Cured

D.

3°C

1 day at Low Temperatures

Vol. 19, No. S

709 CURING TEMPERATURE, PERFORMANCE, OILWELL CEMENTS

b)

HTdrothermally treated samples The effect of decreased curing temperature prior to HT on compressive strength depended largely on the type and proportions of silica (Fig. 5). The cement mix with flour (SF) showed slightly higher strengths at 7 days when pre-cured at IO°C than at 3°C or in the fog-room. At 14 days strengths went through a deep minimum for fog-room and 10°C pre-curing. The minimum was not detected however for 3°C pre-curing. At greater ages (14 to 210 days) hydrothermal strengths steadily increased for all pre-curlng conditions. The mix containing fume (F) also showed strength minima; the trough was at about 28 days for samples pre-cured in the fog-room and at about 56 days for IO°C and 3°C samples. Between 56 and 210 days hydrothermal strengths were generally lower for lower pre-curing temperatures than for fog-room conditions.

AFTER

HYDROTHERMAL TREATMENT

~O" 3 O

40(MPa).

30. "~MIX FR,

e'- \o~K" "% .IIr~

\ %%. \ •

z

"-\ . . . . . .

•.

.

.

.

.

.

Y" ...~o'° "%.

~ .......... ..........

.....................

~,1 ,y, Ir~

SF F FOG ROOM (21"C)

"'""-.

FR

..........

3" ,o.

. ............

~, X DAYS) J

d

2~0

(MP=)"

KEY= W >

4 0 % $10 t MIX ,FS8 ...... MIX FS FR • FOG ROOM (21"C)

w E G. 0 U 20

|

(DAYS) TOTAL

AGE

FIG.5. Compressive Strength of HT Mixes, Pre-cured at Different Temperatures

710

Vol. 19, No. 5 E. Grabowski and J.E. Gillott

The mix containing fume and flour (SFFB), precured at 30C, showed higher strength at 7 days than the 10°C and fog-room mixes. From 14 to 90 days the values for all pre-curing conditions were within the limits of experimental error; no strength minima were detected. The corresponding mix with fume and sand (FSB) showed similar 7 day values for all conditions. 14 day values were slightly higher, followed by the trough (minimum) at 28 days for all pre-curing conditions. At 56 and 90 days strengths were arranged in an order of decreasing value with decrease in temperature. The mix with a I:i fume:sand ratio (FS), showed hydrothermal strength values which were higher for 10°C and 3°C than for normal temperature samples for all ages. No minima were detected. Changes in flexural strength values generally followed a similar pattern to changes in compressive strength but results were more scattered and relative errors greater than for compressive strength. Water permeability was less affected than strength (Table 3). Generally for earlier ages it decreased with decreasing pre-curing temperatures with the extent of improvement depending ~ the t y ~ and proportions of silica. Permeability values were between 5x10 -~ to 4x10 mD. TABLE 3. Permeability [mD] of HT Samples as a Function of Age [days] and Initial Temp. [=C]. Mix: Age 7 14 28 49 270

SF

f.r.,21 ° 2.6XI0-~ 4.9Xi0-~ 2.3Xi0-~ 1.7X10 -Z -

i0 ° 1.5X10-~ 5.1x10-~ 2.7Xi0-~ 1.5X10 -z -

Mix: Age

F f.r.,21 °

2.4X10-~ 2.4xI0-~ 4.4XI0-~ 1.2X10 -Z -

6.4Xi0-~ 1.2xlO-~ 7.7XI0-~ 7.1X10-~ 7.1X10 -j

I0 ° 4.8XI0-~ 7.9XI0-~ 3.0X10-~ 5.2XI0-~ 5.7XI0 -5

I 7xlO -2 2.2xlO -z 9 2x10 -j

7. Oxl0 -3

3° 6.2X10-~ 7.2XI0-~ 5.0xlO-~ 9.1xlO-~ 9.8Xi0 -3

FSB

SFFB 3°

f.r. ,21 °

7 14 28 49 90 270

3°

f.r.,21 °

3°

58xi0 -3

29xi0 -2

4 ixlO-

7.9x10--j 7.3x101~ 6.3x10 5. ixl0 -3

8 3xlO--j 8 2xlO -J

6.7x10-~ 7. ixl0 -j

8.0x10 -3 -

7.0x10 -3 -

The main phase formed hydrothermally from all cement:silica mixes was xonotllte, CAS6H* (5,12). Samples of HT mixes precured at lower temperatures showed some -cEanges in the amounts of minor phases detected by analytical methods. The mix with silica flour (SF) contained larger amounts of scawtlte (C7S6CH p) * and smaller amounts of gyrollte (CpS~H) and calcite with decreaslng temperature of pre-curing. The mix wilh- silica fume (F) was characterized by a total absence of gyrolite (detected for fog-room conditions) and by the presence of scawtite at earlier ages of pre-curlng. Trace amounts of truscottlte (C7S12H 2) present at all ages were not affected * C=CaO

S-SiO 2

H=H20

C=CO 2

Vol.

19, No. 5

711 CURING TEMPERATURE,

PERFORMANCE,

OILWELL CEMENTS

by temperature of pre-curing. The mix with fume and flour (SFFB) retained some gyrolite at IO=C, but not at 3°C; more calcite was present in the 10°C mix than in the 3°C or fog-room mixes. Some scawtite appeared at early ages for 3°C and 10°C mixes. Mixes of cement with fume to sand at the two different ratios behaved differently with decreased pre-curing temperatures. The 3:1 mix (FSB), showed a slight increase of scawtite at 14 and 28 days of total age, while the i:I mix, (FS), showed a marked decrease in the relatively large amounts of scawtite and gy=olite which were detected in fog-room cured samples. The fabric of mixes with flour and/or fume was also affected to some degree by decreased temperature (Fig. 6). Needle-like xonotlite, with parallel orientation, was visible in larger amounts in low temperature pre-cured mix SF than in one pre-cured in fog-room. Granular and needle-like fabric was present but needle-like features appeared most frequently in the mix SFFB (Fig. 6 C,D). Needle-like crystals were also present on fracture surfaces of all cement blends containing silica fume. The mix with fume (F) showed some needles at a total age of 7 days. After longer periods it became granular, resembling fog-room cured samples (Fig.7).

A.

C.

5°C

5°C

FLOUR

FUME:

SEM's of Cement:Silica

FLOUR FIG. 6. Mixes (Pre-cured

B.

D.

IO°C

IO°C

i day and HT 6 days)

712

Vol. E. Grabowski

A.

I DAY, 3°C

19, No. 5

and J.E. Gillott

B. 21 DAYS FUME (F)

, 3°C

FIG.7. SEM's of Cement:Fume Mix After 7 days HT

Discussion a)

Control samples - fog-room and low temperature curing The decrease in compressive and flexural strengths due to decreased precuring temperatures was much more pronounced for mixes containing higher proportions of fume. The corresponding increase in water permeability was observed up to 28 days; after longer periods the permeability was slightly lower for cements cured at lower temperatures and blended with flour (SF) and fume-sand (FSB). Water permeability depends not only on degree of hydration, density (porosity) and pore continuity, but also on the concentration and orientation of microcracks. These are most evident in fume-containing mixes and are attributed to self-dessication (2). Changes in microstructure of the interracial zone between flour or sand and the cement-fume matrix may also influence the pore size distribution (17). Therefore these complex effects may lead to different trends in permeability depending on the characteristics of a given mix. Compressive strength on the other hand depends mainly on the porosity of the system (18) being affected by the degree of hydration i.e. by the amount of C-S-H and, if fume is present, by the extent of pozzolanic reaction between fume and CH. It seems that decreases in curing temperature had more effect on the rate of the pozzolanic reaction than on the rate of hydration. Strength decrease with decreasing temperature in the mix with fume (F) was several times larger than that of the mix with flour (SF). When the same amount of cement and fume was used in conjunction with flour (SFFB) or sand (FSB), the strength values remained almost unchanged supporting the above conclusions. CH which reacted completely with fume within 7 days at ambient conditions (5) was still present after 56 days in samples cured at low temperatures; thus the rate of the pozzolanic reaction was much slower. SEM pictures also confirmed the presence of CH, unhydrated cement and silica particles after long periods of hydration at low temperatures.

Vol. 19, No. 5

713 CURING TEMPERATURE, PERFORMANCE, OILWELL CEMENTS

b)

Hydrothermally treated samples

The effect of lower pre-curing temperature on the engineering characteristics of hydrothermally treated samples depended on the type of silica used. When silica flour (SF) was added to the cement the greatest improvement in compressive strength was observed for samples pre-cured for up to 21 days (28 days total age). A deep "trough" present on the strength vs. total age curve of fog-room pre-cured SF samples at 14 days was reduced for IO"C and eliminated for 3°C samples (Fig.5). After longer periods of pre-curlng (56 to 210 days total age) fog-room samples showed increasing hydrothermal strength values with time, while 3 ° and IO°C samples seemed to reach a strength plateau at 56 days. Xonotlite was the high temperature product after all ages i.e. very short and longer periods of pre-curing when silica flour was available. Therefore xonotlite formed in very immature pastes, containing large amounts of unhydrated phases (C~S and CpS), and in mature pastes with an abundance of C-S-H and CH. DifEerences -in initial fabric and/or in the reaction path followed in hydrothermal formation of xonotlite led to significant differences in the strength of the product. Pre-curing at lower temperatures generally resulted in higher strength at corresponding ages, probably due to differences in degree of maturity of the pastes which were similar to those caused by pre-curing for shorter times. It is also pertinent to recall that when fume is added to cement two different kinds of C-S-H are formed at ambient (and low temperature) conditions (5, 12). C-S-H II, with a needle-llke fabric, is the major product formed during cement hydration, while C-S-H I, with granular appearance, is the product of pozzolanic reaction (16). After HT similar differences result probably from inheritance of fabric by epitaxial growth of xonotlite from the two forms of C-S-H. The fabric differences in turn appear to be responsible for the post-hydrothermal differences in strength and water permeability of cement blends made with different forms of silica. For the mix with fume decreased pre-curing temperature affected the pozzolanlc reaction between fume and CH even more than the hydration reaction. Thus more fume remained available together with more unhydrated cement at a given period of curing for hydrothermal transformation into xonotlite. The trough (strength minimum) on the 3 ° and 10°C curves (Fig. 5) was shifted towards later ages as compared to 28 days for fog-room precured samples. Lower temperatures therefore delayed both the hydration and pozzolanic reaction. Differences in mechanism of these reactions and in pre-curlng conditions resulted in a slightly different content of minor hydrothermal phases; the type and fabric of the major hydrothermal phase, xonotlite, was, however, of the greatest importance. When fume and flour (mix SFFB) or fume and sand in different proportions (mixes FSB and FS) were added to cement, increases in hydrothermal compressive strengths with decreasing pre-curing temperatures lay between those found for mixes containing fume or flour only. This is most probably due to a "competitive" effect between the rates of hydration and pozzolanic reaction resulting from differences in availability of free silica and CH related to equilibrium between unhydrated cement and C-S-H. More experiments, particularly measurements of porosity and phase analyses by DTA are needed in order to place tentative explanations on a firmer basis. Temperature of pre-curlng is one of many factors influencing the engineering properties of hydrated cement blends. It has been shown however, that engineering characteristics, mainly compressive strength and to a lesser degree water permeability, can be controlled in practice by varying the period and temperature of curing before the cement is subjected to hydrothermal conditions.

714

Vol. 19, No. 5 E. Grabowski and J.E. Gillott Conclusions

i. 2. 3. 4. 5.

6.

7.

Initial curing temperature affects both the rate of cement hydration and the rate of the pozzolanic reaction between fume and CH. Decrease in curing temperature has a greater effect on the pozzolanic reaction than on the hydration reaction. The content of C-S-H changes with curing period and temperature, more significantly in mixes with fume than flour. Very low permeability of cement-fume blends (for all curing temp.) is attributed to physical effects of pore clogging by fume particles. The effect of initial curing temperature on the characteristics of hydrothermally formed products depends on the type and proportions of silica used to prepare the slurry, and on the precuring period. Changes in water permeability after HT were more random and not very significant with a general tendency to decreased permeability with lower temperature of precuring at early ages of hydration. The mechanism of hydrothermal changes depends on many variables and is not a simple one. Nevertheless decrease of time or temperature of curing may be used to improve hydrothermal strength. References

i. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13. 14. 15. 16. 17. 18.

F.M. Lea, The Chemistry of Cement and Concrete, 3rd Edition, Chapter 9, p. 177, Chem. Publ. Comp. N.Y. (1971). G.G. Carette and V.M. Malhotra, CANMET Report No. MRP/MSL 82-102, CANMET, Energy, Mines and Resources Canada, Ottawa, 25 pp. (1982). D.K. Smith, Cementing, Monograph No. 4, H.L. Doherty Series SPE of AIME Dallas, p. 28, (1976). V.M. Malhotra, Conc. Int., p. 19, April 1984. E. Grabowski and J.E. Gillott, - accepted for publ. in Cem. &Conc. Res. H. Cheng-yi and R.F. Feldman, Cem. &Conc. Res. 15, p. 585, (1985). J.P. Gallus and C.E. Johnson, Presented at the 1983 API Standardization Meeting, pp. 28, (1983). G. Radenti and L. Ghiringhelli, Geothermics, 3, p. 119, (1972). R.A. Kennerley, New Zealand J. of Sci. 4, p. ~53, (1961). J.P. Gallus and D.E. Pyle, SPE 7591, (1978). L.H. Eilers and E.B. Nelson, SPE 9286, (1980). E. Grabowski and J.E. Gillott, - sub. for publ. in Cem. &Conc. Res.. A. Buck and J.P. Burkes, Proceedings, 3rd International Conference on Cement Microscopy, Houston, ICMA, Duncanville, p. 279, (1980). G.G. Carette and V.M. Malhotra, CANMET Report No. 82-IE, CANMET, Energy, Mines and Resources Canada, Ottawa, 15 pp., February 1982. V.M. Malhotra and G.G. Carette, Conc. Constr., 27, No. 5, p. 443, (1982) P.K. Mehta, ist Int. Conf. on the Use of Fly Ash, Fume, Slag and Other Mineral By-Products of Concrete, Montebello, Canada, i, p. I, (1983). R.F. Feldman, Durability of Build. Mat., ~, No. 2, p. 137, (1986). D.M. Roy, G.R. Gouda and A. Bobrowsky, Cem. &Conc. Res. ~, p. 399, (1972). Acknowledgements

The authors wish to express sincere thanks to the Alberta Oil Sands Technology and Research Authority (AOSTRA) for financial support. The conscientious technical assistance of Ken Velcic and secretarial help of Rene Wollin is acknowledged.