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Microstructure, permeability and rheology of bentonite — cement slurries
CEMENT and CONCRETE RESEARCH. Vol. 20, pp. 45-61, 1990. Printed in the USA. 0008-8846/90. $3.00+00. Copyright (c) 1990 Pergamon Press plc.
MICROSTRUCTURE, OF
PERMEABILITY AND RHEOLOGY - CEMENT SLURRIES
BENTONITE
P L E E * , F. L E B E D E N K O * F. OBRECHT** M. LETELLIER** and H. VAN DAMME ** Centre de Recherches CECA, 19, rue David d'Angers, 75019 PARIS, FRANCE **C.N.R.S.-C.R.S.O.C.I., IB, rue de la F4rollerie, 45071 ORLEANS CEDEX 02, FRANCE D.
(Communicated by D.M. Roy) (Received Dec. 30, 1988) Abstract The microtexture, the water loss, the permeability, and the flow behaviour of bentonite-cement slurries has been interpreted in terms of two major effects : a large calcium ions concentration, on one hand, and a high pH on the other hand.
I - INTRODUCTION
Clay-water systems, and more specifically bentonite-water systems, are among the most extensively investigated colloidal media [1-4] The industrial applications of these materials are essentially in foundry moulds, drilling muds and civil engineering perforations [5]. Cohesion and viscosity are the basic properties which are looked for. In addition, a low water loss and a good impermeabilisation power are generally needed in the two latter applications. In civil engineering, the interest is currently devoted to mixed bentonite-cement aqueous slurries, in view of simplifying some operation procedures which previously involved the successive use of bentonite slurries on one hand, and cement pastes on the other hand. Though simpler, the use of mixed slurries involves a larger viscosity and a larger water loss than with pure bentonite slurries. Crude european bentonites are mostly Ca 2+- and Mg 2+ _ exchanged minerals which do not meet the technical specifications needed for such applications and require an Na2CO 3 activation step, which displaces part of the divalent ions by Na ÷ . It is the purpose of this paper to study a mixed bentonite-cement slurry based on such an activated bentonite, and to understand its behaviour (water loss, permeability, rheology) in microstructural terms. Before reporting and discussing our results, we shall briefly summarize the main structural and physico-chemical properties of bentonite pastes.
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Vol. 20, No. l O. Plee, et al.
BENTONITE
PASTES
i General behaviour It is now well established [6-11] that the microstructure of concentrated aqueous bentonite pastes, as revealed by electron microscopic techniques (TEM and SEM) and small-angle X-ray scattering , is a connected and porous network of anisometric particles. Contrary to the classical "card-house" and "book-house" models proposed by VAN OLPHEN [12], in which interparticular connectivity is achieved through edge-to-face associations, the more recent results show that face-to-face associations are preferred. Connectivity and isotropy are achieved thanks to the flexibility of the clay sheets, and the resulting network generates a lenticular porosity in which the pore walls are (imperfectly) oriented stacks of individual sheets. The stacking order within the pore walls is far from being perfect. Most interlamellar distances correspond to the thickness of three to four layers of water molecules, but larger distances are frequently encountered. Thus, the system is characterized by a broad hierarchy of pore sizes. Ca 2+- or, more generally, multivalent ions - tend to favour the stacking of the clay sheets. One consequence is to stiffen the pore walls and to hinder the connectivity of the solid network. High ionic strengths induce similar modifications, up to the critical flocculation concentration, at which the connectivity is totally lost due to the formation of large and dense spheroidal particles, pH variations modify the edge-charge of the clay sheets thanks to the acid-base equilibria of the terminal OH groups and, as a matter of consequence, modify the interparticular forces. This leads, close to the zcp at high pH, to a gelation phenomenon. A bentonite paste submitted to a pressure gradient, in filtering, undergoes and ordering process in which the particles get thicker, by the addition of new sheets to existing particles, and tend to orient themselves parallel to the filtration plane. In other terms, the average pore size and the angular distribution of pore walls decrease. The size and the number of residual pores determines the amount of water retained by the material. Na +- bentonite pastes contain a large number of small interparticular pores surrounded by flexible walls, whereas Ca 2÷ -bentonite pastes contain a smaller number of larger pores surrounded by thicker and stiffer walls. Upon filtration under a given pressure, P, the Na-clay yields highly oriented and connected cakes, whereas the Ca-clay yields more disordered and less connected deposits. The water retention capacity is larger in the former case, due to the larger number of small pores. The largest residual pore size, rK, under a given pressure, P, is given by [13] : r K = 2 ~/P where ~ is the surface tension of water (0.072 N m l ) . As far as the rheological behaviour is concerned, it is clear that the development of elastic properties is directly connected to the degree of connectivity and the long range forces in the system. Thus, at constant interaction potential (constant pH, constant ionic strength), Na + bentonites pastes are characterized by a larger storage modulus (G') and a larger yields stress (To) than the Ca 2+ - bentonite pastes. Like most dispersed media, bentonite pastes also exhibit a shear-thinning behaviour, due to the rupture of the network at high shear rate.
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47 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
2 Cement-containing pastes Although the properties of bentonite-cement slurries are of practical interest, very few results have been published on the subject. Some data are available on the compression strength of hardened slurries [14]. Nevertheless, from the known chemistry of cement hydration [15], one can pick up three major factors which are likely to modify the properties of the bentonite slurries : (i) a dramatic pH rise ; (ii) a supersaturation of Ca 2+ ions ; and (iii) less important, a non negligible K + ion concentration. According to section II.i, the expected consequences are as follows : (a) The high Ca 2+ concentration is expected to yield a Ca 2÷- exchanged bentonite with thick pore walls and a low water retention capacity ;(b) This effect should be further enhanced by the large ionic strength, which is a well-known flocculation factor, due to the compression of the electrical double layers ; (c) The pH of Portland cement pastes (12-13) is precisely in the range where bentonites are known to gelate. Thus, one is led to predict a strong increase of the water loss and also an increase of the viscosity. As will be shown hereafter, both conclusions are confirmed by the experiments. EXPERIMENTAL i Materials The cement was ordinary Portland cement (CPJ 45). The bentonite was provided by CECA S.A. (FTP3). The bentonite slurries were prepared by dispersing the clay in distilled water (0.03 by clay/iL water) with a deflocculating turbine at 2700 rpm for 5 min. The slurry was then allowed to stand for one hour. The bentonite-cement slurries were prepared by mixing, in the same conditions (2700 rpm ; 5 min) O.I kg of cement with the bentonite suspension prepared as described above. In order to simulate the physico-chemical effects of cement on the bentonite, two sets of bentonite slurry samples were prepared by adding either CaCI 2 at constant pH (8.1), or by increasing the pH by varying the CaO/CaCI 2 ratio, while keeping the total calcium concentration constant at 10 .2 M. Addition the cement.
and dispersion
of those chemicals was performed as with
2 Methods 2.1 SEM. Small samples (N 0.5 ml) of the slurries were frozen in liquid nitrogen and immediately freeze-dried in order to avoid mierotextural modifications. The freeze-dried samples were examined in a Cambridge instrument. 2.2 NMR. The spin-lattice relaxation time, TI , of the 2H nuclei of water in bentonite-cement slurries prepared with heavy water was measured using a Brucker MSL spectrometer at 90 MHz. The purpose of this measurement was to monitor the increase of the interface area due to the hydration of the cement particles. The method is based on the existence of a fast exchange between the water molecules close to the solid surface (and which relax faster than the molecules in "bulk" water) and those far from the surface (which are undistinguishable from "bulk" water). Thanks to this fast exchange, only one average relaxation rate, Ti I , is measured which is the
Vol.
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20, No. 1
D. Plee, et a l .
weighted molecules
average :
of
the
relaxation
rates
of
the
two
populations of
(2)
Til = Xa TI"1a + XbTi
where x a and x b are the molar fractions of each population. Ti$ and TiaI are the r e l a x a t i o n rate of the p e r t u r b e d and u n p e r t u r b e d water molecules, respectively. A c c o r d i n g to FRIPIAT et al.[16], the thickness of the surface film (population b) is constant, and of the order of I nm. Thus, x b is directly proportional to the area of the solid/liquid interface. This m e t h o d has b e e n successfully applied to clay suspensions [16] and to cement pastes [17]. 2.3 Specific surface area. The specific surface areas of the freeze-dried samples were d e t e r m i n e d by N 2 BET a d s o r ption using a Carlo Erba automatic sorptiometer. 2.4 Water loss and permeability. Both properties were determined by filtering 0.i I of slurry under a differential pressure, P, of 7 x 10 5 Pa, and following the amount of filtrate as a function of time. The model used to calculate the permeability, k, assumes that the m i c r o s t r u c t u r e of the cake in f o r m a t i o n does not undergo m o d i fications or, in other terms, that Darcy's law can be applied, with a constant k :
(3)
d Vf/dt = Q - (k/w). S. (P/e) Q
is the filtrate flow (m3s'1); Vf is the filtrate volume at time t ; S is
the f i l t r a t i o n surface (m2); ~ is the filtrate v i s c o s i t y (Pa.s); e is the cake thickness (m). Defining v (< i) as the volume ratio of cake over slurry, and taking into account the volume c o n s e r v a t i o n equation (which applies as long as no air enters the cake) : d V s + d V c + d Vf = 0
(4)
where V s and V c are the volume of slurry and cake, d Vc = - v d Vs
respectively,
one has
:
(5)
and (6)
d Vf - - (i -v) d V s I n t e g r a t i o n of eq. 3 leads to V~
- S 2. k.
P.
(i - v).
t/~.v
(7)
w h i c h is used to calculate k from the experimental Vf - f(t) curves. From k, one can also define an h y d r a ulic radius, rN, assuming that the porous m e d i u m can be d e s c r i b e d by a t h r e e-dimensional array of cylindrical pores [18] : r~ - 24 k / ~ (8) w h e r e • is the p o r o s i t y of the cake. The c a l c u l a t e d by immersion into petroleum.
cake volume and porosity were
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49
BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
2.5 Rheology. The shear stress vs shear rate curves ("flow" curves) were determined with a FANN (couette geometry) viscosimeter. The measurements were performed starting from high shear rates (1020 s'1), for six values of . At each value of 9, a stabilisation time was necessary, due to the thixotropy of the medium. The shear stress, 7, was recorded only after this stabilisation period. The rheological functions 7 = f(@) were fitted using a least square procedure.
RESULTS AND DISCUSSION
i Microtexture Plate I contains a SEM micrograph of a freeze-dried suspension of the bentonite used in this work and Plate II shows the microtextural evolution of the standard bentonite - cement slurry freeze-dried after increasing ageing times. The ageing time was counted starting from the end of mixing. With respect to the bentonite suspension which shows a rough but connected surface, the slurry is, from the very beginning, characterized by
PLATE I BENTONITE + cacl2
[ C a ] - OM
10 -2M
5x10-2M
10-I M
10#m i
SEM
t
micrographs of f r e e z e - d r i e d bentonite suspensions at increasing CaCI 2 concentration
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D. P l e e , e t a l .
a disconnected and very disordered microstructure, showing cavities of the order of 2 to 5 microns. This trend amplifies as time goes on, and the presence of the clay is less and less visible as the hydrated calcium silicate and aluminate phases grow. The very characteristic fibrous shape of type I C-S-H [19] is clearly apparent after a few hours. In order to understand the influence of the cement hydration reactions on the clay microtexture, a set of bentonite suspension samples at increasing [Ca2÷] concentration was prepared (see III-i). Plate I shows the SEM micrograph of these samples in the Ca concentration range from 10 .3 to 10 "I M, i.e. in a range covering the Ca concentration generated during the hydration of Portland cement [15]. Up to 10 .2 M, no significant microtextural modification can be detected, in the micrometric range. Beyond 10 .2 M, a dramatic aggregation process takes place, leading, at 10 I M, to the formation of large and well individualized spheroidal particles, leaving huge holes of the order of 5 to iO microns. A second set of freeze-dried bentonite suspensions prepared at constant Ca concentration and increasing pH was examined, as shown in Plate II!. Although the surface of the samples looks rougher than that of the reference suspension (Plate I, first micrograph) no evidence for a significant modification of the permeation pathways can be detected on the micrographs. Comparison of Plates I, II and III shows that the microstructure of the clay in the bentonite-cement slurry is not identical to that of the clay in pure bentonite suspensions at high Ca concentration and high pH. This is likely to stem from the physical presence of the cement hydration products, intimately mixed with the clay and preventing the latter from developping large, well individualized and pure clay aggregates. Nevertheless, it should be pointed out that both high Ca concentrations and the presence of cement lead to very similar spatial distribution of matter, with large aggregates, separated by large holes, in the micronic range. On the contrary, a high pH leads to a drastically different - rough but connected - microstructure.
2 Solid-liquid interface area Figure i shows the increase of surface area of freeze-dried bentonite-cement slurries as a function of ageing time. After a short induction period, the increase is quasi-linear, from ~ 4 m2/g at ta = 0 to 25 m2/g after 25 hours, i.e., roughly, an increment of ~ I m2/g/hour. The NMR measurements on the wet slurries, performed over longer periods of time, are reported in Figure 2. They also evidence an induction period of a few hours, followed by a quasi-linear increase of Til , and finally, after ~ 50 hours, a saturation tendency. The relative increase of Til in the first 25 hours (~ a factor of 5) is in good agreement with the increase of surface area measured by N 2 adsorption on freeze-dried samples. Also shown on Figure 2 are the Ti I values measured for a bentonite-cement slurry to which a classical superplasticizer (polynaphtalene sulfonate) was added (0.8% with respect to cement weight). The main result is that this additive blocks the development of the solid-liquid interface area, as already shown by LETELLIER et al. for pure cement pastes [17].
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51 BENTONITE-CEMENT,
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PLATE II
BENTONITE/CEMENT
t=O 8
t=lh 8
t~2h
t~4h
ta=6h
ta=24h lO#m o
4
SEM micrographs of freeze-dried standard bentonite-cement s l u r r i e s increasing ageing time ( t a ) .
after
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Vol. 20, No. l D. Plee, et al.
PLATE I I I BENTONITE + C a O / C a C I 2
ph:9.89
ph:11.85
; [ca]tot=lO-2M
R=O.25
R=0.75
1OF m
ph:11.50
R=0.50
ph:12.07
R:I.0
SEM micrographs of freeze-dried bentonite suspensions at increasing pH and constant calcium concentration (i0- M). R is the CaO/CaCl2 r a t i o . 3 Water loss The measurement of the water loss (WL) is a first macroscopic parameter allowing for a quantitative evaluation of the microtextural modifications evidenced in Plates I to III. As shown in Figure 3, in the pressure range from 105 to 6 x 105 Pa, the WL of a fresh bentonite-cement slurry is always 80%. This figure drops considerably (by c.a. iO to 20%) after an ageing time of 6 hours, due to the hydration of cement. It should be noted that the WL does not increase linearly with the applied differential pressure. A sudden increase is observed when one goes from 2 to 3 x 105 Pa. This behaviour is probably associated with the existence of a stress threshold for the rupture of the brittle interparticular bonds generated by the hydration of the calcium silicate and (or) aluminate phases [20] . This is supported by the considerable amplitude increase of the effect (the WL "jump") when the slurry is aged. The WL of the bentonite-cement slurries is always much larger than that of a bentonite suspension (3%, w/w)) in distilled water (pH 8.5), which is close to 28% at P = 6 x 105 Pa . However, as shown in Figure 4, addition of calcium ions dramatically increases the WL, up to ~ 98% at
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53
BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
30
BENT/CEM
O
lo
~
o_o I
I
$
10
I
ta [h I
I
15
20
25
Figure 1 - N 2 BET surface area of bentonite-cement v_~s ageing time [Ca] > IO "2 M. At this point, the water loss of the bentonite suspension is even larger than that of the bentonite-cement slurry. This considerable rise takes place between 10 .3 and 10 .2 M, and should be related to some critical flocculation concentration. However, one should notice that the clay losses most of its water retention capacity while the microstructural changes at micronic scale are not yet detectable (Plate I). This suggests that the water retention capacity in this pressure range is due to pores smaller than those which would be visible at the scale of the micrographs in Plates I to III, in agreement with eq. 1 which predicts that r ~ 2500 A for P - 6 x 105 Pa.
10~______~.~~_~ oi
o
I
o
PN$ I
10o
!
t.[h]
15o
I
2o0
Figure 2 - 2H spin-lattice relaxation rate in bentonite-cement slurries vs ageing time. The upper curve is for the standard slurry. The lower curve has been obtained with a slurry containing & PNS superplasticizer.
54
Vol. 20, No.
l
D. Plee, et a l .
0.9
0
©~O-BENT/c=cI 2
0.8
A p = s = I O S P=
©
o.5
0.?
/
0.6
I
[~
1
2
I
?
T
r
3
4
5
6
O.
I
I
1~lrr
I
10-2
A p [,,, ,0]
Figure 3 - Water loss of bentonitecement slurries vs filtration differential pressure, for different ageing times.
I
I I I II11
10-I
(Col (M)
Figure 4 - Water loss (WL) of bentonite suspensions under a pressure of 5 x lO 5 Pa, vs calcium concentration in the suspension. The dotted line represents the WL in calcium-free water.
4 P ermeability Eq. 7, which predicts that Vf should increase as t I/2, was verified for all the samples. This means that Darcy's law can be applied to those systems, as already concluded by PROST for hectorite and kaolinite paste [21,22] Figure 5 shows a few Vf = f(t I !2) plots for the standard
•
I 2001
ta=O h =I h =2h =4h =6h
• 0 • •
15(]
10(
5(
i
i
h
i
/
i
1
2
3
4
5
6
~/t'(mn o.s)
Figure 5 - Typical filtration curves for the standard b e n t o n i t e - c e m e n t slurries after increasing ageing time.
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55 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
bentonite-cement sluries. In these plots, the plateau values correspond to the WL equilibrium. Table I and Figure 6 summarize the numerical values and the evolution of k as a function of the ageing time. A thirteenfold increase of k is observed in 6 hours. This corresponds, at constant porosity, to a N 4 fold increase of the hydraulic radius r H (eq. 8), from N 2 ~ to N 7,5 ~, which is in good qualitative agreement with the cavities observed by SEM (Plate I). Table I - Numerical parameters characterizing the filtration of the standard bentonite-cement slurry under a differential pressure of 6 x 105 Pa, after increasing ageing time (ta).
ta
(hour)
v is the WL is the is the k is the r H is the
v
WL
~
k (m 2 )
rH (rim)
0
0.20 0.92 0.36 5.6 x 10 "14 1930
i
0.24 0.90 0.38 2.4 x 10 "13 3890
2
0.29 0.89 0.34 3.9 x I0"13 5250
4
0.48 0.82 0.34 7.8 x 10 "13 7420
6
0.48 0.77 0.43 7.8 x I0"13 6600
cake-slurry volume reduction factor. fractional water loss of the slurry upon filtration. porosity of the cake. permeability of the cake. hydraulic radius of the cake.
The permeability of the fresh bentonite-cement slurry (~ 5.6 x 10 "14 or N 56 m Darcy) is more than three orders of magnitude larger than that the pure bentonite suspension (2.9 x I 0 1 7 m 2 ; Table II). Upon addition CaCI 2 to the bentonite suspension, its permeability drops, down
m2 of of to
0.6 x I 0 1 7 m 2 at [Ca] = 10 "I M . However, one should realize that this slight drop of k is over compensated by the dramatic drop of the volume reduction factor of the cake (or, equivalently, by the larger water loss). In other words, inspite of a lower permeability, a bentonite suspension at [Ca] = IO'IM filtrates much faster than a bentonite suspension in distilled water, because the filtering cake is much thinner in the former case. As far as the "apparent" filtration velocity (volume of filtrate passing through the cake per unit time) is concerned, the bentonite suspension at [Ca] > I O 2 M is equivalent to the bentonite-cement slurry. The very small volume fraction occupied by the clay cake at high Ca concentration (v = 1.6%) suggests that the apparent volume of the bentonite cement cake (v = 20%) is almost entirely due to the cement paste, the clay being totally collapsed around or between the cement hydration products. Increasing the pH of the bentonite suspension by addition of CaO (see section III-I) at constant calcium concentration (IO'2M) does not modify significantly the permeability of the cake, up to pH = 12. Several semi-empirical expressions have been proposed to relate the permeability of porous media to their porosity and specific surface area per unit volume, S o . The most widely known is the KOSENY-CARMAN expression [23] : k = 0.2 ~ / ( i - ~)2S~T (9)
56
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D. Plee, et al.
where T is a tortuosity factor. The agreement b e t w e e n the k's measured experimentally for the b e n t o n i t e - c e m e n t slurries (Table i) and the values c a l c u l a t e d from • and S o (Figure 2), using eq. (9), assuming T = i, is bad. The c a l c u l a t e d values are too low by about two order of magnitude. One may reasonably consider that this discrepancy is due to a large tortuosity factor in the cake. Table II - Numerical parameters c h a r a c t e r i z i n g the f i l t r a t i o n of a 3% (w/w) aqueous slurry of b e n t o n i t e as a function of pH and calcium concentration, under a differential pressure of 6 x 105 Pa.
[CaCI 2 ] pH 8.1 8.1
v
(M)
WL
k (m 2 )
~
0.68
0.28 0.98
2.9 x 10 "17
5 x 10 .3 0.32
0.67 0.96
1.3 x 10 "17
0
8.1
10 .2
0.025 0.98
0.4 x 10 1 7
8.1
10 I
0.016 0.98
0.6 x 10 "17
Ii. i0
i0 2
0.038 0.94
0.7 x 10 1 7
Ii. 80
i0 2
0.032
0.93
0.6 x 10 1 7
12. O0
I0" 2
0.046
0.94
0.7 x i0"17
12.66
i0 2
0.206
0.77
2.0 x 10 1 7
v is the c a k e / s l u r r y volume ratio. WL is the fractional water loss of the slurry upon filtration. is the porosity of the cake. k is the p e r m e a b i l i t y of the cake. The porosity, ~, is not tabulated w h e n v is very small and the error considerable.
BENT/CEM
<>
I
I
I
2
I
I
3 4 ta[h]
! S
! 6
Figure 6 - P e r m e a b i l i t y of the standard b e n t o n i t e - c e m e n t v s ageing time of the slurry.
filtration
cake
1
57
Vol. 20, No. I BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
5 Rheolog¥ A priori, the rheological behaviour of the slurries is more difficult to understand in microstructural terms than the water loss or the permeability. Indeed, no simple model is expected to fit the complexity of the interactions in a concentrated mixture of flexible sheets, needles, and porous spheroidal aggregates. The reference bentonite suspension behaves almost as an ideal Bingham fluid (Figure 7). its behaviour is satisfyingly described by the Herschell-Bulckley law : T = ~o + a ~ (i0) with n very close to i. The yield stress, To, is of the order of 70 Pa. Table III - Rheological parameters obtained from a least-square fit of the data by eq.lO, • - T o + a ~ , for bentonite suspensions (3%, w/w) at increasing calcium concentration. Data recorded after two hours of settling.
[Ca](M) To(Pa ) a n pH
0 68 0.05 i 10.2
5 x 10 .3 10 .2 5 x 10 .2 10 I 37
12
13
9
0.06
0.15
0.25
0.25
0.88
0.75
0.72
0.72
9.2
8.66
8.05
7.85
Addition of CaCI2, up to 10 "I M, drastically decreases the yield stress down to ~ I0 Pa, with n remaining relatively large (> 0.7). In other terms, the rheological behaviour tends towards a simple shear-thinning model (Figure 8) (remind that the solid concentration is low : 0.03, w/w). This
150
o?
~
[co]°o
BENT
100
I,-
50
[¢o1-1o-'- . _ _ . _ ~
!
SO0
!
~, [,.,]
1000
Figure 7 - Flow curves for the standard bentonite suspension in water and in a IO'IM calcium chloride solution.
58
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D. Plee, e t a l .
300
BENT
[Ca] = 10-2 .
200
= I,-
100
PH : 8.86
I
f !
!
soo Figure 8 - Flow curves and pH = 12.06.
~ [..,]
for the standard b e n t o n i t e
,ooo suspensions
at pH = 8.86
is in a g r e e m e n t with the C a - i n d u c e d a g g r e g a t i o n of the clay sheets, which diminishes the total excluded volume of the solid fraction in the system. With isolated clay sheets, which are ~ i0 A thick, or with stacks of sheets, the e x c l u d e d volume, V e , (which is close to the gyration volume) may be c o n s i d e r a b l y larger than the real volume of the solid phase. For disks of diameter d and thickness h (h << d), the e x c l u d e d volume is given by [24] : v e = ~d3/8
(ii)
BENT/CEM 300
-
t a = 3h
~
200 ta=O
100
I
500
I
~/ [s-~]
100o
Figure 9 - Flow curves for a fresh standard b e n t o n i t e - c e m e n t for a slurry aged for 3 hours.
slurry
and
Vol. 20, No. l
59 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
whereas the real volume is only Vr m w d2h/4
(12)
A simple calculation shows that, if all the clay sheets were isolated in a 3% (w/w) suspension, their total excluded volume would be 13 times larger than the total suspension volume ! This is an indication that the clay already forms aggregates ( " t a c t o i d s " ) of more than I0 sheets in this concentration range, in agreement with previous studies [16]. Table IV - Rheological parameters obtained from a least-square fit of the data by eq.lO, T = ~o + a ~ ,
for bentonite suspensions (3%, w/w) at
increasing pH and constant calcium concentration ([Ca] = 10 .2 M). Data recorded after two hours of settling.
pH
8.86 ii. I0 11.80 12.00 12.06
To(Pa ) a n
14
28
0.3
1.3
38
0
8
0.67 0.56
0.35
0
61
70
0.2
0.2
Increasing the pH (Table IV) leads first (8.90 ~ pH < 11.80) to an increase of the yield stress, from i0 to 40 Pa, while, in the same time, n decreases from 0.7 to 0.5. In other words, the gel strength and the shear-thinning behaviour are more typical of a pseudo-plastic fluid. Interestingly, beyond pH = 11.80, the rheological model switches from pseudoplastic behaviour to a simple shear-thinning power law (Figure 8) : T -- a "~
(13
with low n values (N 0.2). Table V - Rheological parameters obtained from a least-square fit of the data by eq.lO, T = ro + a ~ , for the standard bentonite-cement slurry, after increasing ageing time, t a .
t a (hour) To a n
0
2
3
4
5
87
0
0
O
0
ii
68
73
71
66
0.33 0.22 0.22 0.22 !0.22
Remarkably, the bentonite-cement slurries exhibit also a crossover from pseudo-plastic to simple shear-thinning behaviour when they are aged over two hours (Table V and Figure 9). This has to be paralleled with the increase of pH which happens also over a period of about two hours during the hydration of Portland cement [15]. Considering the evolution of the pure bentonite suspension beyond pH : 11.80 , this suggest that the clay controls to a large extent the rheological behaviour of the slurry.
60
Vol. 20, No. I D. Plee, et al.
CONCLUSIONS
Qualitatively, the filtering properties and the microstructure of bentonite-cement slurries, as revealed by SEM, are in good agreement. The observations and measurements performed on bentonite suspensions at large Ca concentrations show that the filtering behaviour of the slurries can be reasonably well understood in terms of a calcium-induced aggregation process. This is the major factor which controls the increase of the permeability and the water loss. On the other hand, the rheological properties of the slurries are not amenable to the same rationale. They seem to be controlled more by the pH rise associated with the hydration reactions of cement than by the Ca or ionic strength effects. High pH values (but nevertheless < 13) are known to viscosify bentonite suspensions, as observed when cement is added to a bentonite suspension, whilst Ca addition, at medium pH, has the opposite effect. Acknowledgments CECA S.A. is gratefully acknowledged for authorizing publication.
for supporting this research and
REFERENCES
[i]
S.D. LUBETKIN, S.R. MIDDLETON and R.H. OTTEWILL, Lond., A 311, 353 (1984).
Phil. Trans. R. Soc.
[2]
P.F. LOW, Proc. Int. Clay Conf. Denver ; L.G. SCHULTZ, H. VAN OLPHEN and F.A. MUMPTON Eds., The Clay Minerals Society, pp.247 (1987).
[3]
J.J. FRIPIAT, M. LETELLIER and P. LEVITZ, Phil. Trans. R. Soc. Lond., A 311, 287 (1984). [4] I.T. ROSENQUIST, Phil. Trans. R. Soc. Lond., A 311, 369 (1984). [51 I.E. ODOM, Phil. Trans. R. Soc. Lond., A 311, 391 (1984). [6] H. BEN RHAIEM, D. TESSIER and C.H. PONS, Clay Miner., 21, 9 (1986).
[7]
D. TESSIER and G. PEDRO, Proc. Int. Clay SCHULTZ, H. VAN OLPHEN and F.A. MUMPTON Society, pp.78-84 (1987).
[8]
H. BEN RHAIEM, C.H. PONS and D. TESSIER, Proc. Int. Clay Conf., Denver ; L.G. SCHULTZ, H. VAN OLPHEN and F.A. MUMPTON Eds., The Clay Minerals Society, 292-297 (1987).
[9]
C.H. PONS, D. TESSIER, H. BEN RHAIEM and D. TCHOUBAR, Proc. Int. Clay Conf., Bologna, Pavia ; H. VAN OLPHEN and F. VENIALE Eds., Elsevier, Amsterdam, pp. 165-186 (1982).
[iO] C.H. PONS, F. ROUSSEAUX and D. TCHOUBAR,
Conf., Denver ; L.G. Eds., The Clay Minerals
Clay Miner., 17, 327 (1982).
[ii] D. TESSIER, Ann. Agron., 29, 319 (1978). [12] H. VAN OLPHEN, "An Introduction to Wiley-lnterscience, New York (1978).
Clay
Colloid
Chemistry",
Vol. 20, No. I
61 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
[13] G. SPOSITO and J.V. GIRALDEZ,
Soil Sci. Soc. Am. J., 40, 352 (1976).
[14] J.L. PORTIER,
Annales Inst. Techn.
[15] for instance (1983).
: D.D.
DOUBLE,
Bat., N°423,
Phil.
Trans. R.
[16] J.J. FRIPIAT, J. CASES, M. FRANCOIS Interf. Sci., 89, 378 (1982).
and
19 (1984). Soc. Lond., A 310, 53
M. LETELLIER,
[17] M. LETELLIER, H. VAN DAMME, B. MORTUREUX and M. REGOURD, Conf. Cement, Rio, Sept. 1986. [18] C. FERNANDEZ-PINEDA
and J.I. MENGUAL,
J. Coll.
Interf.
J. Coll.
Proc.
Int.
Sci., 61, 102
(1977). [19] S. DIAMOND, 7th Int. Vol. IV, pp. 113-128. [20] J. SKALNY, Sept. 1978. [21] R. PROST,
I.
JAWED
Clay Miner.,
Cong. Cement Chemistry,
and
H.F.W.
TAYLOR,
Septima,
Paris
(1980),
World Cement Techn.,
183,
14, 173 (1979).
[22] R. PROST, Agronomie, ~, 29 (1984) [23] P.C. CARMAN, "L'4coulement de gr~s ~ travers les milieux poreux", Bib. Sciences et Techniques Nucl4aires, Presses Univ. France, Paris (1961). [24] P.G. de GENNES, Press (1976).
"The Physics of liquid crystals",
p. 38, Oxford Univ.
MICROSTRUCTURE, OF
PERMEABILITY AND RHEOLOGY - CEMENT SLURRIES
BENTONITE
P L E E * , F. L E B E D E N K O * F. OBRECHT** M. LETELLIER** and H. VAN DAMME ** Centre de Recherches CECA, 19, rue David d'Angers, 75019 PARIS, FRANCE **C.N.R.S.-C.R.S.O.C.I., IB, rue de la F4rollerie, 45071 ORLEANS CEDEX 02, FRANCE D.
(Communicated by D.M. Roy) (Received Dec. 30, 1988) Abstract The microtexture, the water loss, the permeability, and the flow behaviour of bentonite-cement slurries has been interpreted in terms of two major effects : a large calcium ions concentration, on one hand, and a high pH on the other hand.
I - INTRODUCTION
Clay-water systems, and more specifically bentonite-water systems, are among the most extensively investigated colloidal media [1-4] The industrial applications of these materials are essentially in foundry moulds, drilling muds and civil engineering perforations [5]. Cohesion and viscosity are the basic properties which are looked for. In addition, a low water loss and a good impermeabilisation power are generally needed in the two latter applications. In civil engineering, the interest is currently devoted to mixed bentonite-cement aqueous slurries, in view of simplifying some operation procedures which previously involved the successive use of bentonite slurries on one hand, and cement pastes on the other hand. Though simpler, the use of mixed slurries involves a larger viscosity and a larger water loss than with pure bentonite slurries. Crude european bentonites are mostly Ca 2+- and Mg 2+ _ exchanged minerals which do not meet the technical specifications needed for such applications and require an Na2CO 3 activation step, which displaces part of the divalent ions by Na ÷ . It is the purpose of this paper to study a mixed bentonite-cement slurry based on such an activated bentonite, and to understand its behaviour (water loss, permeability, rheology) in microstructural terms. Before reporting and discussing our results, we shall briefly summarize the main structural and physico-chemical properties of bentonite pastes.
45
46
Vol. 20, No. l O. Plee, et al.
BENTONITE
PASTES
i General behaviour It is now well established [6-11] that the microstructure of concentrated aqueous bentonite pastes, as revealed by electron microscopic techniques (TEM and SEM) and small-angle X-ray scattering , is a connected and porous network of anisometric particles. Contrary to the classical "card-house" and "book-house" models proposed by VAN OLPHEN [12], in which interparticular connectivity is achieved through edge-to-face associations, the more recent results show that face-to-face associations are preferred. Connectivity and isotropy are achieved thanks to the flexibility of the clay sheets, and the resulting network generates a lenticular porosity in which the pore walls are (imperfectly) oriented stacks of individual sheets. The stacking order within the pore walls is far from being perfect. Most interlamellar distances correspond to the thickness of three to four layers of water molecules, but larger distances are frequently encountered. Thus, the system is characterized by a broad hierarchy of pore sizes. Ca 2+- or, more generally, multivalent ions - tend to favour the stacking of the clay sheets. One consequence is to stiffen the pore walls and to hinder the connectivity of the solid network. High ionic strengths induce similar modifications, up to the critical flocculation concentration, at which the connectivity is totally lost due to the formation of large and dense spheroidal particles, pH variations modify the edge-charge of the clay sheets thanks to the acid-base equilibria of the terminal OH groups and, as a matter of consequence, modify the interparticular forces. This leads, close to the zcp at high pH, to a gelation phenomenon. A bentonite paste submitted to a pressure gradient, in filtering, undergoes and ordering process in which the particles get thicker, by the addition of new sheets to existing particles, and tend to orient themselves parallel to the filtration plane. In other terms, the average pore size and the angular distribution of pore walls decrease. The size and the number of residual pores determines the amount of water retained by the material. Na +- bentonite pastes contain a large number of small interparticular pores surrounded by flexible walls, whereas Ca 2÷ -bentonite pastes contain a smaller number of larger pores surrounded by thicker and stiffer walls. Upon filtration under a given pressure, P, the Na-clay yields highly oriented and connected cakes, whereas the Ca-clay yields more disordered and less connected deposits. The water retention capacity is larger in the former case, due to the larger number of small pores. The largest residual pore size, rK, under a given pressure, P, is given by [13] : r K = 2 ~/P where ~ is the surface tension of water (0.072 N m l ) . As far as the rheological behaviour is concerned, it is clear that the development of elastic properties is directly connected to the degree of connectivity and the long range forces in the system. Thus, at constant interaction potential (constant pH, constant ionic strength), Na + bentonites pastes are characterized by a larger storage modulus (G') and a larger yields stress (To) than the Ca 2+ - bentonite pastes. Like most dispersed media, bentonite pastes also exhibit a shear-thinning behaviour, due to the rupture of the network at high shear rate.
Vol. 20, No. I
47 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
2 Cement-containing pastes Although the properties of bentonite-cement slurries are of practical interest, very few results have been published on the subject. Some data are available on the compression strength of hardened slurries [14]. Nevertheless, from the known chemistry of cement hydration [15], one can pick up three major factors which are likely to modify the properties of the bentonite slurries : (i) a dramatic pH rise ; (ii) a supersaturation of Ca 2+ ions ; and (iii) less important, a non negligible K + ion concentration. According to section II.i, the expected consequences are as follows : (a) The high Ca 2+ concentration is expected to yield a Ca 2÷- exchanged bentonite with thick pore walls and a low water retention capacity ;(b) This effect should be further enhanced by the large ionic strength, which is a well-known flocculation factor, due to the compression of the electrical double layers ; (c) The pH of Portland cement pastes (12-13) is precisely in the range where bentonites are known to gelate. Thus, one is led to predict a strong increase of the water loss and also an increase of the viscosity. As will be shown hereafter, both conclusions are confirmed by the experiments. EXPERIMENTAL i Materials The cement was ordinary Portland cement (CPJ 45). The bentonite was provided by CECA S.A. (FTP3). The bentonite slurries were prepared by dispersing the clay in distilled water (0.03 by clay/iL water) with a deflocculating turbine at 2700 rpm for 5 min. The slurry was then allowed to stand for one hour. The bentonite-cement slurries were prepared by mixing, in the same conditions (2700 rpm ; 5 min) O.I kg of cement with the bentonite suspension prepared as described above. In order to simulate the physico-chemical effects of cement on the bentonite, two sets of bentonite slurry samples were prepared by adding either CaCI 2 at constant pH (8.1), or by increasing the pH by varying the CaO/CaCI 2 ratio, while keeping the total calcium concentration constant at 10 .2 M. Addition the cement.
and dispersion
of those chemicals was performed as with
2 Methods 2.1 SEM. Small samples (N 0.5 ml) of the slurries were frozen in liquid nitrogen and immediately freeze-dried in order to avoid mierotextural modifications. The freeze-dried samples were examined in a Cambridge instrument. 2.2 NMR. The spin-lattice relaxation time, TI , of the 2H nuclei of water in bentonite-cement slurries prepared with heavy water was measured using a Brucker MSL spectrometer at 90 MHz. The purpose of this measurement was to monitor the increase of the interface area due to the hydration of the cement particles. The method is based on the existence of a fast exchange between the water molecules close to the solid surface (and which relax faster than the molecules in "bulk" water) and those far from the surface (which are undistinguishable from "bulk" water). Thanks to this fast exchange, only one average relaxation rate, Ti I , is measured which is the
Vol.
48
20, No. 1
D. Plee, et a l .
weighted molecules
average :
of
the
relaxation
rates
of
the
two
populations of
(2)
Til = Xa TI"1a + XbTi
where x a and x b are the molar fractions of each population. Ti$ and TiaI are the r e l a x a t i o n rate of the p e r t u r b e d and u n p e r t u r b e d water molecules, respectively. A c c o r d i n g to FRIPIAT et al.[16], the thickness of the surface film (population b) is constant, and of the order of I nm. Thus, x b is directly proportional to the area of the solid/liquid interface. This m e t h o d has b e e n successfully applied to clay suspensions [16] and to cement pastes [17]. 2.3 Specific surface area. The specific surface areas of the freeze-dried samples were d e t e r m i n e d by N 2 BET a d s o r ption using a Carlo Erba automatic sorptiometer. 2.4 Water loss and permeability. Both properties were determined by filtering 0.i I of slurry under a differential pressure, P, of 7 x 10 5 Pa, and following the amount of filtrate as a function of time. The model used to calculate the permeability, k, assumes that the m i c r o s t r u c t u r e of the cake in f o r m a t i o n does not undergo m o d i fications or, in other terms, that Darcy's law can be applied, with a constant k :
(3)
d Vf/dt = Q - (k/w). S. (P/e) Q
is the filtrate flow (m3s'1); Vf is the filtrate volume at time t ; S is
the f i l t r a t i o n surface (m2); ~ is the filtrate v i s c o s i t y (Pa.s); e is the cake thickness (m). Defining v (< i) as the volume ratio of cake over slurry, and taking into account the volume c o n s e r v a t i o n equation (which applies as long as no air enters the cake) : d V s + d V c + d Vf = 0
(4)
where V s and V c are the volume of slurry and cake, d Vc = - v d Vs
respectively,
one has
:
(5)
and (6)
d Vf - - (i -v) d V s I n t e g r a t i o n of eq. 3 leads to V~
- S 2. k.
P.
(i - v).
t/~.v
(7)
w h i c h is used to calculate k from the experimental Vf - f(t) curves. From k, one can also define an h y d r a ulic radius, rN, assuming that the porous m e d i u m can be d e s c r i b e d by a t h r e e-dimensional array of cylindrical pores [18] : r~ - 24 k / ~ (8) w h e r e • is the p o r o s i t y of the cake. The c a l c u l a t e d by immersion into petroleum.
cake volume and porosity were
Vol. 20, No. 1
49
BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
2.5 Rheology. The shear stress vs shear rate curves ("flow" curves) were determined with a FANN (couette geometry) viscosimeter. The measurements were performed starting from high shear rates (1020 s'1), for six values of . At each value of 9, a stabilisation time was necessary, due to the thixotropy of the medium. The shear stress, 7, was recorded only after this stabilisation period. The rheological functions 7 = f(@) were fitted using a least square procedure.
RESULTS AND DISCUSSION
i Microtexture Plate I contains a SEM micrograph of a freeze-dried suspension of the bentonite used in this work and Plate II shows the microtextural evolution of the standard bentonite - cement slurry freeze-dried after increasing ageing times. The ageing time was counted starting from the end of mixing. With respect to the bentonite suspension which shows a rough but connected surface, the slurry is, from the very beginning, characterized by
PLATE I BENTONITE + cacl2
[ C a ] - OM
10 -2M
5x10-2M
10-I M
10#m i
SEM
t
micrographs of f r e e z e - d r i e d bentonite suspensions at increasing CaCI 2 concentration
50
Vol. 20, No. l
D. P l e e , e t a l .
a disconnected and very disordered microstructure, showing cavities of the order of 2 to 5 microns. This trend amplifies as time goes on, and the presence of the clay is less and less visible as the hydrated calcium silicate and aluminate phases grow. The very characteristic fibrous shape of type I C-S-H [19] is clearly apparent after a few hours. In order to understand the influence of the cement hydration reactions on the clay microtexture, a set of bentonite suspension samples at increasing [Ca2÷] concentration was prepared (see III-i). Plate I shows the SEM micrograph of these samples in the Ca concentration range from 10 .3 to 10 "I M, i.e. in a range covering the Ca concentration generated during the hydration of Portland cement [15]. Up to 10 .2 M, no significant microtextural modification can be detected, in the micrometric range. Beyond 10 .2 M, a dramatic aggregation process takes place, leading, at 10 I M, to the formation of large and well individualized spheroidal particles, leaving huge holes of the order of 5 to iO microns. A second set of freeze-dried bentonite suspensions prepared at constant Ca concentration and increasing pH was examined, as shown in Plate II!. Although the surface of the samples looks rougher than that of the reference suspension (Plate I, first micrograph) no evidence for a significant modification of the permeation pathways can be detected on the micrographs. Comparison of Plates I, II and III shows that the microstructure of the clay in the bentonite-cement slurry is not identical to that of the clay in pure bentonite suspensions at high Ca concentration and high pH. This is likely to stem from the physical presence of the cement hydration products, intimately mixed with the clay and preventing the latter from developping large, well individualized and pure clay aggregates. Nevertheless, it should be pointed out that both high Ca concentrations and the presence of cement lead to very similar spatial distribution of matter, with large aggregates, separated by large holes, in the micronic range. On the contrary, a high pH leads to a drastically different - rough but connected - microstructure.
2 Solid-liquid interface area Figure i shows the increase of surface area of freeze-dried bentonite-cement slurries as a function of ageing time. After a short induction period, the increase is quasi-linear, from ~ 4 m2/g at ta = 0 to 25 m2/g after 25 hours, i.e., roughly, an increment of ~ I m2/g/hour. The NMR measurements on the wet slurries, performed over longer periods of time, are reported in Figure 2. They also evidence an induction period of a few hours, followed by a quasi-linear increase of Til , and finally, after ~ 50 hours, a saturation tendency. The relative increase of Til in the first 25 hours (~ a factor of 5) is in good agreement with the increase of surface area measured by N 2 adsorption on freeze-dried samples. Also shown on Figure 2 are the Ti I values measured for a bentonite-cement slurry to which a classical superplasticizer (polynaphtalene sulfonate) was added (0.8% with respect to cement weight). The main result is that this additive blocks the development of the solid-liquid interface area, as already shown by LETELLIER et al. for pure cement pastes [17].
VoI. 20, No. I
51 BENTONITE-CEMENT,
SLURRIES, MICROSTRUCTURE,
RHEOLOGY
PLATE II
BENTONITE/CEMENT
t=O 8
t=lh 8
t~2h
t~4h
ta=6h
ta=24h lO#m o
4
SEM micrographs of freeze-dried standard bentonite-cement s l u r r i e s increasing ageing time ( t a ) .
after
52
Vol. 20, No. l D. Plee, et al.
PLATE I I I BENTONITE + C a O / C a C I 2
ph:9.89
ph:11.85
; [ca]tot=lO-2M
R=O.25
R=0.75
1OF m
ph:11.50
R=0.50
ph:12.07
R:I.0
SEM micrographs of freeze-dried bentonite suspensions at increasing pH and constant calcium concentration (i0- M). R is the CaO/CaCl2 r a t i o . 3 Water loss The measurement of the water loss (WL) is a first macroscopic parameter allowing for a quantitative evaluation of the microtextural modifications evidenced in Plates I to III. As shown in Figure 3, in the pressure range from 105 to 6 x 105 Pa, the WL of a fresh bentonite-cement slurry is always 80%. This figure drops considerably (by c.a. iO to 20%) after an ageing time of 6 hours, due to the hydration of cement. It should be noted that the WL does not increase linearly with the applied differential pressure. A sudden increase is observed when one goes from 2 to 3 x 105 Pa. This behaviour is probably associated with the existence of a stress threshold for the rupture of the brittle interparticular bonds generated by the hydration of the calcium silicate and (or) aluminate phases [20] . This is supported by the considerable amplitude increase of the effect (the WL "jump") when the slurry is aged. The WL of the bentonite-cement slurries is always much larger than that of a bentonite suspension (3%, w/w)) in distilled water (pH 8.5), which is close to 28% at P = 6 x 105 Pa . However, as shown in Figure 4, addition of calcium ions dramatically increases the WL, up to ~ 98% at
Vol.
20, No. I
53
BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
30
BENT/CEM
O
lo
~
o_o I
I
$
10
I
ta [h I
I
15
20
25
Figure 1 - N 2 BET surface area of bentonite-cement v_~s ageing time [Ca] > IO "2 M. At this point, the water loss of the bentonite suspension is even larger than that of the bentonite-cement slurry. This considerable rise takes place between 10 .3 and 10 .2 M, and should be related to some critical flocculation concentration. However, one should notice that the clay losses most of its water retention capacity while the microstructural changes at micronic scale are not yet detectable (Plate I). This suggests that the water retention capacity in this pressure range is due to pores smaller than those which would be visible at the scale of the micrographs in Plates I to III, in agreement with eq. 1 which predicts that r ~ 2500 A for P - 6 x 105 Pa.
10~______~.~~_~ oi
o
I
o
PN$ I
10o
!
t.[h]
15o
I
2o0
Figure 2 - 2H spin-lattice relaxation rate in bentonite-cement slurries vs ageing time. The upper curve is for the standard slurry. The lower curve has been obtained with a slurry containing & PNS superplasticizer.
54
Vol. 20, No.
l
D. Plee, et a l .
0.9
0
©~O-BENT/c=cI 2
0.8
A p = s = I O S P=
©
o.5
0.?
/
0.6
I
[~
1
2
I
?
T
r
3
4
5
6
O.
I
I
1~lrr
I
10-2
A p [,,, ,0]
Figure 3 - Water loss of bentonitecement slurries vs filtration differential pressure, for different ageing times.
I
I I I II11
10-I
(Col (M)
Figure 4 - Water loss (WL) of bentonite suspensions under a pressure of 5 x lO 5 Pa, vs calcium concentration in the suspension. The dotted line represents the WL in calcium-free water.
4 P ermeability Eq. 7, which predicts that Vf should increase as t I/2, was verified for all the samples. This means that Darcy's law can be applied to those systems, as already concluded by PROST for hectorite and kaolinite paste [21,22] Figure 5 shows a few Vf = f(t I !2) plots for the standard
•
I 2001
ta=O h =I h =2h =4h =6h
• 0 • •
15(]
10(
5(
i
i
h
i
/
i
1
2
3
4
5
6
~/t'(mn o.s)
Figure 5 - Typical filtration curves for the standard b e n t o n i t e - c e m e n t slurries after increasing ageing time.
Vol. 20, No. l
55 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
bentonite-cement sluries. In these plots, the plateau values correspond to the WL equilibrium. Table I and Figure 6 summarize the numerical values and the evolution of k as a function of the ageing time. A thirteenfold increase of k is observed in 6 hours. This corresponds, at constant porosity, to a N 4 fold increase of the hydraulic radius r H (eq. 8), from N 2 ~ to N 7,5 ~, which is in good qualitative agreement with the cavities observed by SEM (Plate I). Table I - Numerical parameters characterizing the filtration of the standard bentonite-cement slurry under a differential pressure of 6 x 105 Pa, after increasing ageing time (ta).
ta
(hour)
v is the WL is the is the k is the r H is the
v
WL
~
k (m 2 )
rH (rim)
0
0.20 0.92 0.36 5.6 x 10 "14 1930
i
0.24 0.90 0.38 2.4 x 10 "13 3890
2
0.29 0.89 0.34 3.9 x I0"13 5250
4
0.48 0.82 0.34 7.8 x 10 "13 7420
6
0.48 0.77 0.43 7.8 x I0"13 6600
cake-slurry volume reduction factor. fractional water loss of the slurry upon filtration. porosity of the cake. permeability of the cake. hydraulic radius of the cake.
The permeability of the fresh bentonite-cement slurry (~ 5.6 x 10 "14 or N 56 m Darcy) is more than three orders of magnitude larger than that the pure bentonite suspension (2.9 x I 0 1 7 m 2 ; Table II). Upon addition CaCI 2 to the bentonite suspension, its permeability drops, down
m2 of of to
0.6 x I 0 1 7 m 2 at [Ca] = 10 "I M . However, one should realize that this slight drop of k is over compensated by the dramatic drop of the volume reduction factor of the cake (or, equivalently, by the larger water loss). In other words, inspite of a lower permeability, a bentonite suspension at [Ca] = IO'IM filtrates much faster than a bentonite suspension in distilled water, because the filtering cake is much thinner in the former case. As far as the "apparent" filtration velocity (volume of filtrate passing through the cake per unit time) is concerned, the bentonite suspension at [Ca] > I O 2 M is equivalent to the bentonite-cement slurry. The very small volume fraction occupied by the clay cake at high Ca concentration (v = 1.6%) suggests that the apparent volume of the bentonite cement cake (v = 20%) is almost entirely due to the cement paste, the clay being totally collapsed around or between the cement hydration products. Increasing the pH of the bentonite suspension by addition of CaO (see section III-I) at constant calcium concentration (IO'2M) does not modify significantly the permeability of the cake, up to pH = 12. Several semi-empirical expressions have been proposed to relate the permeability of porous media to their porosity and specific surface area per unit volume, S o . The most widely known is the KOSENY-CARMAN expression [23] : k = 0.2 ~ / ( i - ~)2S~T (9)
56
Vol.
20,
No.
D. Plee, et al.
where T is a tortuosity factor. The agreement b e t w e e n the k's measured experimentally for the b e n t o n i t e - c e m e n t slurries (Table i) and the values c a l c u l a t e d from • and S o (Figure 2), using eq. (9), assuming T = i, is bad. The c a l c u l a t e d values are too low by about two order of magnitude. One may reasonably consider that this discrepancy is due to a large tortuosity factor in the cake. Table II - Numerical parameters c h a r a c t e r i z i n g the f i l t r a t i o n of a 3% (w/w) aqueous slurry of b e n t o n i t e as a function of pH and calcium concentration, under a differential pressure of 6 x 105 Pa.
[CaCI 2 ] pH 8.1 8.1
v
(M)
WL
k (m 2 )
~
0.68
0.28 0.98
2.9 x 10 "17
5 x 10 .3 0.32
0.67 0.96
1.3 x 10 "17
0
8.1
10 .2
0.025 0.98
0.4 x 10 1 7
8.1
10 I
0.016 0.98
0.6 x 10 "17
Ii. i0
i0 2
0.038 0.94
0.7 x 10 1 7
Ii. 80
i0 2
0.032
0.93
0.6 x 10 1 7
12. O0
I0" 2
0.046
0.94
0.7 x i0"17
12.66
i0 2
0.206
0.77
2.0 x 10 1 7
v is the c a k e / s l u r r y volume ratio. WL is the fractional water loss of the slurry upon filtration. is the porosity of the cake. k is the p e r m e a b i l i t y of the cake. The porosity, ~, is not tabulated w h e n v is very small and the error considerable.
BENT/CEM
<>
I
I
I
2
I
I
3 4 ta[h]
! S
! 6
Figure 6 - P e r m e a b i l i t y of the standard b e n t o n i t e - c e m e n t v s ageing time of the slurry.
filtration
cake
1
57
Vol. 20, No. I BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
5 Rheolog¥ A priori, the rheological behaviour of the slurries is more difficult to understand in microstructural terms than the water loss or the permeability. Indeed, no simple model is expected to fit the complexity of the interactions in a concentrated mixture of flexible sheets, needles, and porous spheroidal aggregates. The reference bentonite suspension behaves almost as an ideal Bingham fluid (Figure 7). its behaviour is satisfyingly described by the Herschell-Bulckley law : T = ~o + a ~ (i0) with n very close to i. The yield stress, To, is of the order of 70 Pa. Table III - Rheological parameters obtained from a least-square fit of the data by eq.lO, • - T o + a ~ , for bentonite suspensions (3%, w/w) at increasing calcium concentration. Data recorded after two hours of settling.
[Ca](M) To(Pa ) a n pH
0 68 0.05 i 10.2
5 x 10 .3 10 .2 5 x 10 .2 10 I 37
12
13
9
0.06
0.15
0.25
0.25
0.88
0.75
0.72
0.72
9.2
8.66
8.05
7.85
Addition of CaCI2, up to 10 "I M, drastically decreases the yield stress down to ~ I0 Pa, with n remaining relatively large (> 0.7). In other terms, the rheological behaviour tends towards a simple shear-thinning model (Figure 8) (remind that the solid concentration is low : 0.03, w/w). This
150
o?
~
[co]°o
BENT
100
I,-
50
[¢o1-1o-'- . _ _ . _ ~
!
SO0
!
~, [,.,]
1000
Figure 7 - Flow curves for the standard bentonite suspension in water and in a IO'IM calcium chloride solution.
58
Vol.
20, No. I
D. Plee, e t a l .
300
BENT
[Ca] = 10-2 .
200
= I,-
100
PH : 8.86
I
f !
!
soo Figure 8 - Flow curves and pH = 12.06.
~ [..,]
for the standard b e n t o n i t e
,ooo suspensions
at pH = 8.86
is in a g r e e m e n t with the C a - i n d u c e d a g g r e g a t i o n of the clay sheets, which diminishes the total excluded volume of the solid fraction in the system. With isolated clay sheets, which are ~ i0 A thick, or with stacks of sheets, the e x c l u d e d volume, V e , (which is close to the gyration volume) may be c o n s i d e r a b l y larger than the real volume of the solid phase. For disks of diameter d and thickness h (h << d), the e x c l u d e d volume is given by [24] : v e = ~d3/8
(ii)
BENT/CEM 300
-
t a = 3h
~
200 ta=O
100
I
500
I
~/ [s-~]
100o
Figure 9 - Flow curves for a fresh standard b e n t o n i t e - c e m e n t for a slurry aged for 3 hours.
slurry
and
Vol. 20, No. l
59 BENTONITE-CEMENT, SLURRIES, MICROSTRUCTURE, RHEOLOGY
whereas the real volume is only Vr m w d2h/4
(12)
A simple calculation shows that, if all the clay sheets were isolated in a 3% (w/w) suspension, their total excluded volume would be 13 times larger than the total suspension volume ! This is an indication that the clay already forms aggregates ( " t a c t o i d s " ) of more than I0 sheets in this concentration range, in agreement with previous studies [16]. Table IV - Rheological parameters obtained from a least-square fit of the data by eq.lO, T = ~o + a ~ ,
for bentonite suspensions (3%, w/w) at
increasing pH and constant calcium concentration ([Ca] = 10 .2 M). Data recorded after two hours of settling.
pH
8.86 ii. I0 11.80 12.00 12.06
To(Pa ) a n
14
28
0.3
1.3
38
0
8
0.67 0.56
0.35
0
61
70
0.2
0.2
Increasing the pH (Table IV) leads first (8.90 ~ pH < 11.80) to an increase of the yield stress, from i0 to 40 Pa, while, in the same time, n decreases from 0.7 to 0.5. In other words, the gel strength and the shear-thinning behaviour are more typical of a pseudo-plastic fluid. Interestingly, beyond pH = 11.80, the rheological model switches from pseudoplastic behaviour to a simple shear-thinning power law (Figure 8) : T -- a "~
(13
with low n values (N 0.2). Table V - Rheological parameters obtained from a least-square fit of the data by eq.lO, T = ro + a ~ , for the standard bentonite-cement slurry, after increasing ageing time, t a .
t a (hour) To a n
0
2
3
4
5
87
0
0
O
0
ii
68
73
71
66
0.33 0.22 0.22 0.22 !0.22
Remarkably, the bentonite-cement slurries exhibit also a crossover from pseudo-plastic to simple shear-thinning behaviour when they are aged over two hours (Table V and Figure 9). This has to be paralleled with the increase of pH which happens also over a period of about two hours during the hydration of Portland cement [15]. Considering the evolution of the pure bentonite suspension beyond pH : 11.80 , this suggest that the clay controls to a large extent the rheological behaviour of the slurry.
60
Vol. 20, No. I D. Plee, et al.
CONCLUSIONS
Qualitatively, the filtering properties and the microstructure of bentonite-cement slurries, as revealed by SEM, are in good agreement. The observations and measurements performed on bentonite suspensions at large Ca concentrations show that the filtering behaviour of the slurries can be reasonably well understood in terms of a calcium-induced aggregation process. This is the major factor which controls the increase of the permeability and the water loss. On the other hand, the rheological properties of the slurries are not amenable to the same rationale. They seem to be controlled more by the pH rise associated with the hydration reactions of cement than by the Ca or ionic strength effects. High pH values (but nevertheless < 13) are known to viscosify bentonite suspensions, as observed when cement is added to a bentonite suspension, whilst Ca addition, at medium pH, has the opposite effect. Acknowledgments CECA S.A. is gratefully acknowledged for authorizing publication.
for supporting this research and
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[i]
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[2]
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[3]
J.J. FRIPIAT, M. LETELLIER and P. LEVITZ, Phil. Trans. R. Soc. Lond., A 311, 287 (1984). [4] I.T. ROSENQUIST, Phil. Trans. R. Soc. Lond., A 311, 369 (1984). [51 I.E. ODOM, Phil. Trans. R. Soc. Lond., A 311, 391 (1984). [6] H. BEN RHAIEM, D. TESSIER and C.H. PONS, Clay Miner., 21, 9 (1986).
[7]
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[9]
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Vol. 20, No. I
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[13] G. SPOSITO and J.V. GIRALDEZ,
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[15] for instance (1983).
: D.D.
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J. Coll.
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Int.
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[22] R. PROST, Agronomie, ~, 29 (1984) [23] P.C. CARMAN, "L'4coulement de gr~s ~ travers les milieux poreux", Bib. Sciences et Techniques Nucl4aires, Presses Univ. France, Paris (1961). [24] P.G. de GENNES, Press (1976).
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