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The unusual behavior of a Milos bentonite in cement suspensions
Applied Clay Science 54 (2011) 189–195
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
The unusual behavior of a Milos bentonite in cement suspensions K.-H. Ohrdorf a,⁎, S. Kaufhold b, F. Rüßmann c, K. Ufer d, H. Flachberger e a
Ingenieurbüro für Bentonit-Technologie Dipl.-Ing. Ohrdorf, Wiesbaden, Germany Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany c Dyckerhoff AG, Wiesbaden, Germany d TU Bergakademie Freiberg, Institute of Mineralogy, Freiberg, Germany e Montanuniversität Leoben, Chair of Mineral Processing, Austria b
a r t i c l e
i n f o
Article history: Received 27 October 2010 Received in revised form 5 August 2011 Accepted 5 August 2011 Available online 1 November 2011 Keywords: Bentonite Rheological parameters Soluble silica
a b s t r a c t Bentonites from different deposits worldwide often show unexplainable differences with respect to their performance in different applications. Examples are the water uptake capacity and bleaching ability (Kaufhold, 2001; Kaufhold et al., 2010). The present study focused on the rheological parameters of bentonite-cement slurries used in civil and underground engineering. By measuring the viscosity and yield point of several different Na+-activated calcium bentonites mixed with blast furnace cement, behavior of a Milos bentonite was detected characterized by an unusual increase of the viscosity after addition of the cement. A detailed mineralogical characterization was conducted to identify the reason. The most striking difference of the samples was the content of soluble silica making up 27 mass% of the bentonite (soluble in hot sodium carbonate solution). However, the addition of silica gel to bentonites free of soluble silica proved the effect on the rheological parameters viscosity and yield point. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bentonite-cement suspensions are recently applied in some special fields of civil and underground engineering. In retention walls the bentonite of such a suspension acts as a supporting agent of the trench during excavation and provides low permeability of water after the setting of the cement. The process in which the bentonite-cement suspension remains inside the trench is called the “One-Phase-Method” (Fig. 1) and was inaugurated by the French civil engineering firms Soletanche and Bachy in the 1970s. The bentonite was stirred with water, and a pre-swelling time of up to 24 h was allowed to develop the best achievable viscosity and yield point of the commercially available bentonite at the site. Then, the cement and additives (e.g. mineral filler) were added to the aged bentonite suspension. As an alternative to this method, dry premixes of bentonite, cement, and additives made in special mixing plants of cement factories have become more and more popular. Based on the research of the firms Anneliese Cement AG and Dyckerhoff AG in 1988, a patent was granted to Anneliese (DE 3633736A1) which is exclusively used by Dyckerhoff. In Germany and Austria most of the constructors of retention walls use such premixes. The advantages of this method are: – the premixes are easier to handle, – they require less site equipment, ⁎ Corresponding author. E-mail address: [email protected] (K.-H. Ohrdorf). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.08.001
– there is no pre-swelling required, – there is better recipe and quality control of the mixture during production. However, the disadvantage is that the recipe cannot be easily modified on site in the case of special site requirements (e.g. special soil properties). The one-phase-retention wall method requires increased performance of activated calcium bentonites, particularly with respect to viscosity and thixotropy during the time of trenching and excavation. Therefore, bentonites with high swelling capacity are required. Activated bentonites are known to lose appreciable swelling capacity in the presence of free Ca ++-ions. Therefore, a certain “cement-stability” is required. This change of the rheological properties most probably can be attributed to a cation exchange whereby the Na +/Ca ++ ratio determines the rheological properties. The reasons, however, for the rather different performance of different bentonites with respect to the change of rheological properties cannot be explained yet. Possibly, different selectivity constants for the cation exchange of the different smectites play a role. In addition, minor components such as carbonates are known to affect cation exchange processes (e.g. Kaufhold and Dohrmann, 2009a,b). The carbonate anions react with the Ca++-ions of the cement. Though the term “cement-stability” is not defined exactly, in industrial usage a bentonite is called “cement stable” if it does not significantly change its rheological properties after being mixed with cement and within the time of trenching until the cementation starts. The three economically most important European regions for bentonite production: Milos (GR), Bavaria (D) and Sardinia (I) have been
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selected in order to compare the rheological properties in cement slurries. Each of the samples was composed of at least 50 individual samples of different horizons and stocks of the deposits. Commercially available hydrophobized precipitated silica “Vulkasil S”, hydrophilic
fumed silica “Aerosil 90” and water glass solution were used as additives to bentonites free of soluble silica. The preparation for mineralogical and chemical analysis included crushing to b2 mm, homogenizing, drying at 80 °C to a moisture content of approx. 5 mass% and grinding (b63 μm) in an agate mortar. The mineralogical composition was determined by the XRD-Rietveld technique as described by Ufer et al. (2008). The chemical composition of the powdered samples was determined using a PANalytical Axios and a PW2400 spectrometer. The samples were prepared by mixing with a flux material and melting into glass beads. The beads were analyzed by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). To determine loss on ignition, (LOI) 1 g of the sample material was heated to 1030 °C for 10 min. The CEC was determined by the so-called Cu-triene method (Meier and Kahr, 1999). The amount of soluble silica was determined by using 0.02, 0.04, 0.06, 0.08, 0.1 g (pre-dried at 105 °C for 24 h) samples and a 50.0 mL 2% sodium carbonate solution. The suspension was shaken at 60 °C. The Si content in the supernatant was determined by ICPOES. The reproducibility was approximately ±0.1%. The total carbon, sulfur and organic carbon contents of air-dried samples (170–180 mg) were measured with the LECO CS-444Analysator. The carbonates were removed by reaction with HCl at 80 °C. To determine the rheological properties, the samples were activated at 30 mass% of moisture in laboratory scale with the I.B.O. activation method. This method describes an industrial activation technique to create new surfaces by destroying continuously the sediment structure of a bentonite from a given deposit in such a way that no further mixing time or shear forces can increase the efficiency of the cation exchange under the conditions of a commercial production. The efficiency of the cation exchange can be detected by the yield point of the bentonite suspension. The guidelines to determine the suspension properties for retention walls are a combination derived from the API Spec. 13 for drilling fluids, the DIN 4126 and 4127 for retention walls and the DIN 18136 and 18310 for geotechnics.
Fig. 2. Activation curve of three bentonites of high viscosity from the three European economically most important mining regions.
Fig. 3. Activation curve of three different bentonites of low viscosity of the three European economically most important mining regions.
Fig. 1. Trenching using a bentonite-cement slurry.
K.-H. Ohrdorf et al. / Applied Clay Science 54 (2011) 189–195
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Fig. 4. Marsh viscosity of bentonite- and bentonite-cement slurries (arrow shows the lower limit of the desired value range).
The preparation of the suspension has to follow the specific instructions given by each of the cement manufacturers. In the present paper the suspensions were prepared as follows: 80 g of activated bentonite was added to 2 L of deionized water at 20± 2 °C and stirred for 10 min with a dissolver-type stirrer at 3,000 ± 200 rpm. A 3 L cup of 150 mm inside diameter was used. After swelling for 1 h 400 g of a blast furnace cement type CEM III/C (Dyckerhoff SOLIDUR®) was added. The suspension was then stirred under the same conditions for 10 min. To determine the viscosity, a Marsh-funnel according to the API Spec. 13, Sect. 2 (1982) with an outlet tube diameter of 4.76 mm was filled with 1500 mL of the suspension, and the outflow of 1000 mL was measured after a relaxation time of 30 s. Known as the “Marsh-time” or “Marsh-viscosity” this data allows the assessment of the flow properties and processability of the suspension during
trenching and excavation. The value of water is approx. 28 s, whereas the guidelines (desired value) for bentonite-cement-suspensions require a Marsh-time of 35 to 40 s. The yield point relates particularly well to the stability of the trench and the processability of the suspension. The yield point was measured with the “Kugelharfe” (would be “sphere harp” in English), a device which can be easily handled on site (DIN 4126). Ten small spheres with different diameters and densities were held by strings and numbered according to increasing mass and diameter. The slurry was poured into a cup, and after a relaxation time of 60 s all spheres were slowly dipped into the slurry. The first number in row from the lightest (no. 1) to the heaviest (no. 10) sphere which sinks into the slurry represents the yield point of the slurry and is converted in N/cm2 by a specific scheme. The guidelines (desired value) for a bentonite-cement-suspension require a yield point of 14–50 N/cm 2 corresponding to sphere no. 4–8.
Fig. 5. Yield point of bentonite- and bentonite-cement suspensions.
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montmorillonite. It was assumed that accessory minerals could play an important role.
2. Results and discussion 2.1. Rheological data
2.2. Chemical and mineralogical investigation The rheological properties of industrially activated bentonites directly correlate with the “chemical degree of activation” (CAG), i.e. the amount of sodium carbonate needed for the cation exchange according to the nature of the bentonite and the “technical degree of activation” (TAG), i.e., the feasibility to activate lumpy raw materials of 30 ± 3 mass% of moisture in a production plant. The maximum values of the rheological properties were reached only if both degrees of activation approach 100%. Any under- or over-activation inevitably leads to a loss of rheological properties, independent of the deposit-related bentonite characteristics. Typical activation curves following the CAG of bentonite from different deposits were obtained with a 5 mass%-suspension and a newly developed method using the Rheometer RS 600 made by ThermoFisher GmbH, Karlsruhe. The suspensions were then screened at 0.075 mm to eliminate particles above the measuring distance of the plate-cone combination of the Rheometer (0.105 mm) resulting in differences of the solid concentration. In order to eliminate these small differences and to make the yield points comparable, a “specific yield point” (yield point per gram bentonite) was calculated, and the optimum of the CAG could be clearly defined (Ohrdorf, 2010) (Figs. 2 and 3). In order to compare bentonite deposits of different origins the identification of the optimum activation is necessary. The large content of Ca ++-ions in the suspension derived from the cement, causes an aggregation of the montmorillonite platelets (Lagaly, 1993). Therefore, mostly blast furnace slag cements are used in bentonitecement-slurries. The data in Figs. 4 and 5 proved that – the optimum activation can minimize but not completely avoid the loss of rheological properties upon addition of cement, – the assumption which was made up to now, that only Milos bentonites can be used in bentonite-cement slurries is incorrect. At CAG = 100% and at TAG = 100%, activated Bavarian and Sardinian bentonites of high basic viscosities can be used as well, – a Milos bentonite was identified which had a comparatively low basic viscosity but obtained high viscosity after addition of the cement.
The results of the mineralogical characterization are summarized in Table 2. ASP and V3 were two samples of the Milos bentonite, showing the anomaly described, V2 was a Milos bentonite with good rheological properties, V4, V5, were Bavarian bentonites with good and bad rheological properties, V6B, V6G were Bavarian and Sardinian bentonites also with good and bad rheological properties. As expected, all samples are mainly composed of smectites with partially significant contents of illite/muscovite. However, the large illite/muscovite content of samples V4 and V5 was attributed to a systematic error of the Rietveld refinement which depends on the quality of the structural models and particularly on the disorder models. Hence considering the sum of 2:1 clay minerals is probably closer to the true composition. The contents of the minerals not needing a disorder model in the Rietveld refinement (carbonates, quartz, feldspar) are supposed to be close to the real contents. The contents of the minerals not needing a disorder model in the Rietveld refinement (carbonates, quartz, feldspar) are supposed to be close to the real contents. This was confirmed by comparing the calcite content calculated with the LECO data. Sample V4 contained ankerite, pyrite was found only in sample V3, and dolomite only in sample V7. Furthermore, the kaolinite content of V5 is particularly large, which is properly reflected by the Al2O3-content. Samples ASP and V3, from the same location, contain appreciable amounts of low crystalline silica polymorphs (cristobalite, opal-CT, and probably some opal-A) which results in the large amount of soda-soluble silica (Fig. 7). The exchangeable cation population before activation did not vary significantly. The significant differences of the Ca ++-contents can be attributed to the different content of carbonates which partially dissolve throughout during the CEC determination (Dohrmann and Kaufhold, 2010). The most striking property of the samples with unusual rheology was the high content of soda-soluble silica of 27 mass% of the bentonite. The soda-soluble silica, which is also dissolved in contact with the alkaline cement pore water, may react with the cement. One possible reaction is a formation of CSH phases.
This bentonite fulfilled all the required bentonite-cement slurry properties including Marsh viscosity, “Kugelharfen” yield point, filtration and sedimentation behavior (Table 1). The unusual behavior of this Milos bentonite was only observed after activation, i.e. when Na +-ions were present (Fig. 6). At CAG = 0%, i.e. calcium bentonite, the Marsh viscosity became similar to water. Adding sodium carbonate until CAG = 100%, the Marsh viscosity of the bentonite-cement suspension was significantly increased exceeding the Marsh viscosity of the pure bentonite suspension in spite of the presence of the Ca ++-ions of the cement. Based on viscosity tests with a pure bentonite suspension, this rheological anomaly could not be related to the type of the
Table 1 Properties of the Milos bentonite slurries with unusual rheological behavior. Bentonite suspension
Bentonite-cement suspension
Marsh viscosity
Liquid limit “Kugelharfe”
Marsh viscosity
Liquid limit “Kugelharfe”
Filtrate
24 h sedimentation
s
N/m2
s
N/m2
mL
%
47 38–55
56.8 37–67
38.2 b55
0.5 b3
32.8 8.1 Desired value (Dyckerhoff)
Fig. 6. Marsh viscosity of a bentonite-cement suspension versus CAG; CAG = 0 refers to calcium bentonite (arrow shows the lower limit of the desired value range).
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Table 2 Mineralogical and chemical characterization of the bentonites. Sample
ASP
V2
Mineralogical composition Quartz Smectite Illite/muscovite Feldspar OpalCT Cristobalite Calcite Dolomite Ankerite Gypsum Kaol/halloysite Anatase Pyrite Sum
(XRD-Rietveld according to Ufer et al., 2008) [mass%] 3 3 [mass%] 54 77 [mass%] [mass%] 5 4 [mass%] 13 [mass%] 19 10 [mass%] 2 [mass%] [mass%] [mass%] 2 4 [mass%] 4 [mass%] 1 [mass%] [mass%] 100 100
C-/S-analysis (LECO) Organic C Inorg. C Calcite (modal) Total C Total S
[mass%] [mass%] [mass%] [mass%] [mass%]
V3 3 53 6 13 20
V4
V5
V6 B
V6 G
V7
11 51 24 4
9 46 19 6
4 90
10 78
5
11
9
1
2
1 1
2 2 4 0 100
20
100
100
100
0
100
0.0 0.1 0.6 0.1 0.3
0.0 0.3 2.5 0.3 0.8
0.0 0.1 0.9 0.2 0.3
0.0 1.3 11.0 1.4 0.0
0.0 0.0 0.2 0.1 0.0
0.0 0.2 1.6 0.2 0.2
0.0 0.1 0.6 0.1 0.0
0.0 0.4 3.2 0.4 0.0
27.0 27.1 22.3 23.9 22.2
9.3 8.0 7.6 7.2 7.0
27.3 22.8 18.0 14.5 12.8
4.1 3.0 2.4 2.1 1.8
4.7 3.4 2.8 2.4 2.0
5.9 4.1 3.3 2.8 2.4
5.8 4.0 3.0 2.5 2.1
Cation exchange capacity (Cu-triene-method) Na+ [meq/100 g] K+ [meq/100 g] 2+ Mg [meq/100 g] 2+ Ca [meq/100 g] Sum cations [meq/100 g] CEC [meq/100 g] S-T [meq/100 g]
7 0 28 27 62 53 9.0
10 1 31 85 127 78 49.6
6 0 30 26 63 56 6.7
0 3 18 55 76 62 14.6
0 2 20 41 62 62 0.1
0 3 54 60 117 104 12.8
0 3 50 51 104 100 3.4
1 4 38 37 81 73 7.6
Chemical composition of main elements (XRF) SiO2 [mass%] TiO2 [mass%] Al2O3 [mass%] Fe2O3 [mass%] MnO [mass%] MgO [mass%] CaO [mass%] Na2O [mass%] K2O [mass%] P2O5 [mass%] (SO3) [mass%] LOI [mass%] Sum [mass%]
65.0 0.3 13.9 2.0 0.0 2.1 1.0 0.3 0.5 0.0 0.4 14.4 99.8
52.8 0.6 15.7 4.5 0.0 2.8 3.2 0.4 0.7 0.1 1.6 17.2 99.7
66.3 0.3 14.2 2.1 0.0 2.2 1.0 0.3 0.5 0.0 0.3 12.6 99.8
48.8 0.4 15.2 4.9 0.1 3.2 6.7 0.3 1.7 0.1 0.0 18.3 99.7
52.2 0.4 20.0 5.6 0.1 2.7 1.4 0.4 1.4 0.1 b0.01 15.6 99.7
50.1 0.3 16.2 3.7 0.0 5.6 2.1 0.0 1.1 0.1 0.3 20.0 99.5
51.4 0.3 16.2 4.2 0.0 5.5 1.6 0.1 1.3 0.1 0.0 19.2 99.8
52.2 0.5 14.6 5.5 0.1 5.3 2.1 0.2 2.4 0.1 0.0 16.8 99.8
Soda soluble silica (ICP) 0.02 g/0 mL 0.04 g/50 mL 0.06 g/50 mL 0.08 g/50 mL 0.1 g/50 mL
However, CSH phases were not detected by electron microscopy (NanoSEM). One open question is why the unusual rheology was not observed in the case of calcium bentonites. On the one hand, the Na +-ions could form water glass as an intermediate product. Alternatively, the pH could be important because calcium bentonites are known to effectively buffer the pH at around 6.5, which decreases the dissolution of silica to finally prove the mechanism, additional tests are required.
4.6 3.4 2.6 2.4 1.9
which of course decreases the solubility in aqueous solutions. Hydrophobized silica gels such as “Vulkasil S” from Lanxess AG did not show any increase of the viscosity and yield point. Two silica gels, the hydrophyllic “Aerosil 90” from Evonik AG and sodium water glass solution were tested with positive results (Fig. 9). Although the level of the Milos bentonite could not be reached, the significant increase of the viscosity and yield point of the cement suspensions with Bavarian and Sardinian bentonites after addition of silica was evident (Figs. 8, 9).
2.3. Explanation of the unusual rheological behavior 3. Summary and conclusions The idea that soluble silica could be responsible for the unusual rheological behavior was tested by using a mixture of activated bentonite free of soluble silica and different types of synthetic silica gels. However, silica gels show strong differences in the rheological properties of the mixtures. This can be easily explained by the different structures and solubilities of the gels. Moreover, some silica gels were hydrophobized
By measuring the viscosity and yield point of several bentonites mixed with standard blast furnace cement, an unusual increase of the viscosity of one of these, a Milos bentonite, was observed. The most striking property of this bentonite was the high content (up to 27 mass%) of soda-soluble silica. The presence of sodium ions was a
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precondition but the effect of sodium ions on the unusual increase of the rheological properties could not be evaluated. The influence of soluble silica was verified with the addition of Aerosil 90 and sodium water glass solution. The Milos bentonites contain variable amounts of soluble silica due to the acid post formational alteration (Decher, 1997; Kaufhold, 2001; Kaufhold and Decher, 2003). Therefore, the content of soda-soluble silica of bentonites might be an additional quality-determining parameter of bentonites which are intended to be used in combination with cement. References
Fig. 7. Soluble-silica content of bentonites at different solid liquid ratios.
Decher, A., 1997. Bentonite der Insel Milos/ Griechenland. — Dissertation RWTH Aachen, ISBN 3-86073-602-7, 194 p. Dohrmann, R., Kaufhold, S., 2010. Determination of exchangeable calcium of calcareous and gypsiferous bentonites. Clays and Clay Minerals 58, 513–522. Kaufhold, S., 2001. Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle. — Doctoral thesis of the RWTH Aachen: 188 p., online available: http://134.130.184.8/opus/frontdoor.php? source_opus=256. Kaufhold, S., Decher, A., 2003. Natural acidic bentonites from the island of Milos, Greece. Zeitschrift für Angewandte Geologie 7–13 (ISSN 0044–2259, 2/2003).
Fig. 8. Increase of the viscosity by adding 27 mass% of “Aerosil 90” (arrow shows the lower limit of the desired value range).
Fig. 9. Increase of the yield point by adding 27 mass% of “Aerosil 90” (arrow shows the lower limit of the desired value).
K.-H. Ohrdorf et al. / Applied Clay Science 54 (2011) 189–195 Kaufhold, S., Dohrmann, R., 2009a. Three new, quick CEC methods for determining the amounts of exchangeable calcium cations in calcareous clays. Clays and Clay Minerals 57 (3), 251–265. Kaufhold, S., Dohrmann, R., 2009b. Stability of bentonites in salt solutions I sodium chloride. Applied Clay Science 45 (3), 171–177. Kaufhold, S., Dohrmann, R., Klinkenberg, M., 2010. Water uptake capacity of bentonites. Clays and Clay Minerals 58, 37–43. Lagaly, G., 1993. Tonminerale und Tone. Steinkopff Verlag, Darmstadt.
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Meier, L.P., Kahr, G., 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with triethylenetetramine and tetraethylenepentamine. Clays and Clay Minerals 47, 386–388. Ohrdorf, K.-H., 2010. Zu Fragen der alkalischen Aktivierung von Bentoniten. Dissertation Montanuniversität Leoben. Ufer, K., Stanjek, H., Roth, G., Dohrmann, R., Kleeberg, R., Kaufhold, S., 2008. Quantitative phase analysis of bentonites by the Rietveld method. Clays and Clay Minerals 56 (2), 272–282.
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
The unusual behavior of a Milos bentonite in cement suspensions K.-H. Ohrdorf a,⁎, S. Kaufhold b, F. Rüßmann c, K. Ufer d, H. Flachberger e a
Ingenieurbüro für Bentonit-Technologie Dipl.-Ing. Ohrdorf, Wiesbaden, Germany Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany c Dyckerhoff AG, Wiesbaden, Germany d TU Bergakademie Freiberg, Institute of Mineralogy, Freiberg, Germany e Montanuniversität Leoben, Chair of Mineral Processing, Austria b
a r t i c l e
i n f o
Article history: Received 27 October 2010 Received in revised form 5 August 2011 Accepted 5 August 2011 Available online 1 November 2011 Keywords: Bentonite Rheological parameters Soluble silica
a b s t r a c t Bentonites from different deposits worldwide often show unexplainable differences with respect to their performance in different applications. Examples are the water uptake capacity and bleaching ability (Kaufhold, 2001; Kaufhold et al., 2010). The present study focused on the rheological parameters of bentonite-cement slurries used in civil and underground engineering. By measuring the viscosity and yield point of several different Na+-activated calcium bentonites mixed with blast furnace cement, behavior of a Milos bentonite was detected characterized by an unusual increase of the viscosity after addition of the cement. A detailed mineralogical characterization was conducted to identify the reason. The most striking difference of the samples was the content of soluble silica making up 27 mass% of the bentonite (soluble in hot sodium carbonate solution). However, the addition of silica gel to bentonites free of soluble silica proved the effect on the rheological parameters viscosity and yield point. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bentonite-cement suspensions are recently applied in some special fields of civil and underground engineering. In retention walls the bentonite of such a suspension acts as a supporting agent of the trench during excavation and provides low permeability of water after the setting of the cement. The process in which the bentonite-cement suspension remains inside the trench is called the “One-Phase-Method” (Fig. 1) and was inaugurated by the French civil engineering firms Soletanche and Bachy in the 1970s. The bentonite was stirred with water, and a pre-swelling time of up to 24 h was allowed to develop the best achievable viscosity and yield point of the commercially available bentonite at the site. Then, the cement and additives (e.g. mineral filler) were added to the aged bentonite suspension. As an alternative to this method, dry premixes of bentonite, cement, and additives made in special mixing plants of cement factories have become more and more popular. Based on the research of the firms Anneliese Cement AG and Dyckerhoff AG in 1988, a patent was granted to Anneliese (DE 3633736A1) which is exclusively used by Dyckerhoff. In Germany and Austria most of the constructors of retention walls use such premixes. The advantages of this method are: – the premixes are easier to handle, – they require less site equipment, ⁎ Corresponding author. E-mail address: [email protected] (K.-H. Ohrdorf). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.08.001
– there is no pre-swelling required, – there is better recipe and quality control of the mixture during production. However, the disadvantage is that the recipe cannot be easily modified on site in the case of special site requirements (e.g. special soil properties). The one-phase-retention wall method requires increased performance of activated calcium bentonites, particularly with respect to viscosity and thixotropy during the time of trenching and excavation. Therefore, bentonites with high swelling capacity are required. Activated bentonites are known to lose appreciable swelling capacity in the presence of free Ca ++-ions. Therefore, a certain “cement-stability” is required. This change of the rheological properties most probably can be attributed to a cation exchange whereby the Na +/Ca ++ ratio determines the rheological properties. The reasons, however, for the rather different performance of different bentonites with respect to the change of rheological properties cannot be explained yet. Possibly, different selectivity constants for the cation exchange of the different smectites play a role. In addition, minor components such as carbonates are known to affect cation exchange processes (e.g. Kaufhold and Dohrmann, 2009a,b). The carbonate anions react with the Ca++-ions of the cement. Though the term “cement-stability” is not defined exactly, in industrial usage a bentonite is called “cement stable” if it does not significantly change its rheological properties after being mixed with cement and within the time of trenching until the cementation starts. The three economically most important European regions for bentonite production: Milos (GR), Bavaria (D) and Sardinia (I) have been
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selected in order to compare the rheological properties in cement slurries. Each of the samples was composed of at least 50 individual samples of different horizons and stocks of the deposits. Commercially available hydrophobized precipitated silica “Vulkasil S”, hydrophilic
fumed silica “Aerosil 90” and water glass solution were used as additives to bentonites free of soluble silica. The preparation for mineralogical and chemical analysis included crushing to b2 mm, homogenizing, drying at 80 °C to a moisture content of approx. 5 mass% and grinding (b63 μm) in an agate mortar. The mineralogical composition was determined by the XRD-Rietveld technique as described by Ufer et al. (2008). The chemical composition of the powdered samples was determined using a PANalytical Axios and a PW2400 spectrometer. The samples were prepared by mixing with a flux material and melting into glass beads. The beads were analyzed by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). To determine loss on ignition, (LOI) 1 g of the sample material was heated to 1030 °C for 10 min. The CEC was determined by the so-called Cu-triene method (Meier and Kahr, 1999). The amount of soluble silica was determined by using 0.02, 0.04, 0.06, 0.08, 0.1 g (pre-dried at 105 °C for 24 h) samples and a 50.0 mL 2% sodium carbonate solution. The suspension was shaken at 60 °C. The Si content in the supernatant was determined by ICPOES. The reproducibility was approximately ±0.1%. The total carbon, sulfur and organic carbon contents of air-dried samples (170–180 mg) were measured with the LECO CS-444Analysator. The carbonates were removed by reaction with HCl at 80 °C. To determine the rheological properties, the samples were activated at 30 mass% of moisture in laboratory scale with the I.B.O. activation method. This method describes an industrial activation technique to create new surfaces by destroying continuously the sediment structure of a bentonite from a given deposit in such a way that no further mixing time or shear forces can increase the efficiency of the cation exchange under the conditions of a commercial production. The efficiency of the cation exchange can be detected by the yield point of the bentonite suspension. The guidelines to determine the suspension properties for retention walls are a combination derived from the API Spec. 13 for drilling fluids, the DIN 4126 and 4127 for retention walls and the DIN 18136 and 18310 for geotechnics.
Fig. 2. Activation curve of three bentonites of high viscosity from the three European economically most important mining regions.
Fig. 3. Activation curve of three different bentonites of low viscosity of the three European economically most important mining regions.
Fig. 1. Trenching using a bentonite-cement slurry.
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Fig. 4. Marsh viscosity of bentonite- and bentonite-cement slurries (arrow shows the lower limit of the desired value range).
The preparation of the suspension has to follow the specific instructions given by each of the cement manufacturers. In the present paper the suspensions were prepared as follows: 80 g of activated bentonite was added to 2 L of deionized water at 20± 2 °C and stirred for 10 min with a dissolver-type stirrer at 3,000 ± 200 rpm. A 3 L cup of 150 mm inside diameter was used. After swelling for 1 h 400 g of a blast furnace cement type CEM III/C (Dyckerhoff SOLIDUR®) was added. The suspension was then stirred under the same conditions for 10 min. To determine the viscosity, a Marsh-funnel according to the API Spec. 13, Sect. 2 (1982) with an outlet tube diameter of 4.76 mm was filled with 1500 mL of the suspension, and the outflow of 1000 mL was measured after a relaxation time of 30 s. Known as the “Marsh-time” or “Marsh-viscosity” this data allows the assessment of the flow properties and processability of the suspension during
trenching and excavation. The value of water is approx. 28 s, whereas the guidelines (desired value) for bentonite-cement-suspensions require a Marsh-time of 35 to 40 s. The yield point relates particularly well to the stability of the trench and the processability of the suspension. The yield point was measured with the “Kugelharfe” (would be “sphere harp” in English), a device which can be easily handled on site (DIN 4126). Ten small spheres with different diameters and densities were held by strings and numbered according to increasing mass and diameter. The slurry was poured into a cup, and after a relaxation time of 60 s all spheres were slowly dipped into the slurry. The first number in row from the lightest (no. 1) to the heaviest (no. 10) sphere which sinks into the slurry represents the yield point of the slurry and is converted in N/cm2 by a specific scheme. The guidelines (desired value) for a bentonite-cement-suspension require a yield point of 14–50 N/cm 2 corresponding to sphere no. 4–8.
Fig. 5. Yield point of bentonite- and bentonite-cement suspensions.
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montmorillonite. It was assumed that accessory minerals could play an important role.
2. Results and discussion 2.1. Rheological data
2.2. Chemical and mineralogical investigation The rheological properties of industrially activated bentonites directly correlate with the “chemical degree of activation” (CAG), i.e. the amount of sodium carbonate needed for the cation exchange according to the nature of the bentonite and the “technical degree of activation” (TAG), i.e., the feasibility to activate lumpy raw materials of 30 ± 3 mass% of moisture in a production plant. The maximum values of the rheological properties were reached only if both degrees of activation approach 100%. Any under- or over-activation inevitably leads to a loss of rheological properties, independent of the deposit-related bentonite characteristics. Typical activation curves following the CAG of bentonite from different deposits were obtained with a 5 mass%-suspension and a newly developed method using the Rheometer RS 600 made by ThermoFisher GmbH, Karlsruhe. The suspensions were then screened at 0.075 mm to eliminate particles above the measuring distance of the plate-cone combination of the Rheometer (0.105 mm) resulting in differences of the solid concentration. In order to eliminate these small differences and to make the yield points comparable, a “specific yield point” (yield point per gram bentonite) was calculated, and the optimum of the CAG could be clearly defined (Ohrdorf, 2010) (Figs. 2 and 3). In order to compare bentonite deposits of different origins the identification of the optimum activation is necessary. The large content of Ca ++-ions in the suspension derived from the cement, causes an aggregation of the montmorillonite platelets (Lagaly, 1993). Therefore, mostly blast furnace slag cements are used in bentonitecement-slurries. The data in Figs. 4 and 5 proved that – the optimum activation can minimize but not completely avoid the loss of rheological properties upon addition of cement, – the assumption which was made up to now, that only Milos bentonites can be used in bentonite-cement slurries is incorrect. At CAG = 100% and at TAG = 100%, activated Bavarian and Sardinian bentonites of high basic viscosities can be used as well, – a Milos bentonite was identified which had a comparatively low basic viscosity but obtained high viscosity after addition of the cement.
The results of the mineralogical characterization are summarized in Table 2. ASP and V3 were two samples of the Milos bentonite, showing the anomaly described, V2 was a Milos bentonite with good rheological properties, V4, V5, were Bavarian bentonites with good and bad rheological properties, V6B, V6G were Bavarian and Sardinian bentonites also with good and bad rheological properties. As expected, all samples are mainly composed of smectites with partially significant contents of illite/muscovite. However, the large illite/muscovite content of samples V4 and V5 was attributed to a systematic error of the Rietveld refinement which depends on the quality of the structural models and particularly on the disorder models. Hence considering the sum of 2:1 clay minerals is probably closer to the true composition. The contents of the minerals not needing a disorder model in the Rietveld refinement (carbonates, quartz, feldspar) are supposed to be close to the real contents. The contents of the minerals not needing a disorder model in the Rietveld refinement (carbonates, quartz, feldspar) are supposed to be close to the real contents. This was confirmed by comparing the calcite content calculated with the LECO data. Sample V4 contained ankerite, pyrite was found only in sample V3, and dolomite only in sample V7. Furthermore, the kaolinite content of V5 is particularly large, which is properly reflected by the Al2O3-content. Samples ASP and V3, from the same location, contain appreciable amounts of low crystalline silica polymorphs (cristobalite, opal-CT, and probably some opal-A) which results in the large amount of soda-soluble silica (Fig. 7). The exchangeable cation population before activation did not vary significantly. The significant differences of the Ca ++-contents can be attributed to the different content of carbonates which partially dissolve throughout during the CEC determination (Dohrmann and Kaufhold, 2010). The most striking property of the samples with unusual rheology was the high content of soda-soluble silica of 27 mass% of the bentonite. The soda-soluble silica, which is also dissolved in contact with the alkaline cement pore water, may react with the cement. One possible reaction is a formation of CSH phases.
This bentonite fulfilled all the required bentonite-cement slurry properties including Marsh viscosity, “Kugelharfen” yield point, filtration and sedimentation behavior (Table 1). The unusual behavior of this Milos bentonite was only observed after activation, i.e. when Na +-ions were present (Fig. 6). At CAG = 0%, i.e. calcium bentonite, the Marsh viscosity became similar to water. Adding sodium carbonate until CAG = 100%, the Marsh viscosity of the bentonite-cement suspension was significantly increased exceeding the Marsh viscosity of the pure bentonite suspension in spite of the presence of the Ca ++-ions of the cement. Based on viscosity tests with a pure bentonite suspension, this rheological anomaly could not be related to the type of the
Table 1 Properties of the Milos bentonite slurries with unusual rheological behavior. Bentonite suspension
Bentonite-cement suspension
Marsh viscosity
Liquid limit “Kugelharfe”
Marsh viscosity
Liquid limit “Kugelharfe”
Filtrate
24 h sedimentation
s
N/m2
s
N/m2
mL
%
47 38–55
56.8 37–67
38.2 b55
0.5 b3
32.8 8.1 Desired value (Dyckerhoff)
Fig. 6. Marsh viscosity of a bentonite-cement suspension versus CAG; CAG = 0 refers to calcium bentonite (arrow shows the lower limit of the desired value range).
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Table 2 Mineralogical and chemical characterization of the bentonites. Sample
ASP
V2
Mineralogical composition Quartz Smectite Illite/muscovite Feldspar OpalCT Cristobalite Calcite Dolomite Ankerite Gypsum Kaol/halloysite Anatase Pyrite Sum
(XRD-Rietveld according to Ufer et al., 2008) [mass%] 3 3 [mass%] 54 77 [mass%] [mass%] 5 4 [mass%] 13 [mass%] 19 10 [mass%] 2 [mass%] [mass%] [mass%] 2 4 [mass%] 4 [mass%] 1 [mass%] [mass%] 100 100
C-/S-analysis (LECO) Organic C Inorg. C Calcite (modal) Total C Total S
[mass%] [mass%] [mass%] [mass%] [mass%]
V3 3 53 6 13 20
V4
V5
V6 B
V6 G
V7
11 51 24 4
9 46 19 6
4 90
10 78
5
11
9
1
2
1 1
2 2 4 0 100
20
100
100
100
0
100
0.0 0.1 0.6 0.1 0.3
0.0 0.3 2.5 0.3 0.8
0.0 0.1 0.9 0.2 0.3
0.0 1.3 11.0 1.4 0.0
0.0 0.0 0.2 0.1 0.0
0.0 0.2 1.6 0.2 0.2
0.0 0.1 0.6 0.1 0.0
0.0 0.4 3.2 0.4 0.0
27.0 27.1 22.3 23.9 22.2
9.3 8.0 7.6 7.2 7.0
27.3 22.8 18.0 14.5 12.8
4.1 3.0 2.4 2.1 1.8
4.7 3.4 2.8 2.4 2.0
5.9 4.1 3.3 2.8 2.4
5.8 4.0 3.0 2.5 2.1
Cation exchange capacity (Cu-triene-method) Na+ [meq/100 g] K+ [meq/100 g] 2+ Mg [meq/100 g] 2+ Ca [meq/100 g] Sum cations [meq/100 g] CEC [meq/100 g] S-T [meq/100 g]
7 0 28 27 62 53 9.0
10 1 31 85 127 78 49.6
6 0 30 26 63 56 6.7
0 3 18 55 76 62 14.6
0 2 20 41 62 62 0.1
0 3 54 60 117 104 12.8
0 3 50 51 104 100 3.4
1 4 38 37 81 73 7.6
Chemical composition of main elements (XRF) SiO2 [mass%] TiO2 [mass%] Al2O3 [mass%] Fe2O3 [mass%] MnO [mass%] MgO [mass%] CaO [mass%] Na2O [mass%] K2O [mass%] P2O5 [mass%] (SO3) [mass%] LOI [mass%] Sum [mass%]
65.0 0.3 13.9 2.0 0.0 2.1 1.0 0.3 0.5 0.0 0.4 14.4 99.8
52.8 0.6 15.7 4.5 0.0 2.8 3.2 0.4 0.7 0.1 1.6 17.2 99.7
66.3 0.3 14.2 2.1 0.0 2.2 1.0 0.3 0.5 0.0 0.3 12.6 99.8
48.8 0.4 15.2 4.9 0.1 3.2 6.7 0.3 1.7 0.1 0.0 18.3 99.7
52.2 0.4 20.0 5.6 0.1 2.7 1.4 0.4 1.4 0.1 b0.01 15.6 99.7
50.1 0.3 16.2 3.7 0.0 5.6 2.1 0.0 1.1 0.1 0.3 20.0 99.5
51.4 0.3 16.2 4.2 0.0 5.5 1.6 0.1 1.3 0.1 0.0 19.2 99.8
52.2 0.5 14.6 5.5 0.1 5.3 2.1 0.2 2.4 0.1 0.0 16.8 99.8
Soda soluble silica (ICP) 0.02 g/0 mL 0.04 g/50 mL 0.06 g/50 mL 0.08 g/50 mL 0.1 g/50 mL
However, CSH phases were not detected by electron microscopy (NanoSEM). One open question is why the unusual rheology was not observed in the case of calcium bentonites. On the one hand, the Na +-ions could form water glass as an intermediate product. Alternatively, the pH could be important because calcium bentonites are known to effectively buffer the pH at around 6.5, which decreases the dissolution of silica to finally prove the mechanism, additional tests are required.
4.6 3.4 2.6 2.4 1.9
which of course decreases the solubility in aqueous solutions. Hydrophobized silica gels such as “Vulkasil S” from Lanxess AG did not show any increase of the viscosity and yield point. Two silica gels, the hydrophyllic “Aerosil 90” from Evonik AG and sodium water glass solution were tested with positive results (Fig. 9). Although the level of the Milos bentonite could not be reached, the significant increase of the viscosity and yield point of the cement suspensions with Bavarian and Sardinian bentonites after addition of silica was evident (Figs. 8, 9).
2.3. Explanation of the unusual rheological behavior 3. Summary and conclusions The idea that soluble silica could be responsible for the unusual rheological behavior was tested by using a mixture of activated bentonite free of soluble silica and different types of synthetic silica gels. However, silica gels show strong differences in the rheological properties of the mixtures. This can be easily explained by the different structures and solubilities of the gels. Moreover, some silica gels were hydrophobized
By measuring the viscosity and yield point of several bentonites mixed with standard blast furnace cement, an unusual increase of the viscosity of one of these, a Milos bentonite, was observed. The most striking property of this bentonite was the high content (up to 27 mass%) of soda-soluble silica. The presence of sodium ions was a
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precondition but the effect of sodium ions on the unusual increase of the rheological properties could not be evaluated. The influence of soluble silica was verified with the addition of Aerosil 90 and sodium water glass solution. The Milos bentonites contain variable amounts of soluble silica due to the acid post formational alteration (Decher, 1997; Kaufhold, 2001; Kaufhold and Decher, 2003). Therefore, the content of soda-soluble silica of bentonites might be an additional quality-determining parameter of bentonites which are intended to be used in combination with cement. References
Fig. 7. Soluble-silica content of bentonites at different solid liquid ratios.
Decher, A., 1997. Bentonite der Insel Milos/ Griechenland. — Dissertation RWTH Aachen, ISBN 3-86073-602-7, 194 p. Dohrmann, R., Kaufhold, S., 2010. Determination of exchangeable calcium of calcareous and gypsiferous bentonites. Clays and Clay Minerals 58, 513–522. Kaufhold, S., 2001. Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle. — Doctoral thesis of the RWTH Aachen: 188 p., online available: http://134.130.184.8/opus/frontdoor.php? source_opus=256. Kaufhold, S., Decher, A., 2003. Natural acidic bentonites from the island of Milos, Greece. Zeitschrift für Angewandte Geologie 7–13 (ISSN 0044–2259, 2/2003).
Fig. 8. Increase of the viscosity by adding 27 mass% of “Aerosil 90” (arrow shows the lower limit of the desired value range).
Fig. 9. Increase of the yield point by adding 27 mass% of “Aerosil 90” (arrow shows the lower limit of the desired value).
K.-H. Ohrdorf et al. / Applied Clay Science 54 (2011) 189–195 Kaufhold, S., Dohrmann, R., 2009a. Three new, quick CEC methods for determining the amounts of exchangeable calcium cations in calcareous clays. Clays and Clay Minerals 57 (3), 251–265. Kaufhold, S., Dohrmann, R., 2009b. Stability of bentonites in salt solutions I sodium chloride. Applied Clay Science 45 (3), 171–177. Kaufhold, S., Dohrmann, R., Klinkenberg, M., 2010. Water uptake capacity of bentonites. Clays and Clay Minerals 58, 37–43. Lagaly, G., 1993. Tonminerale und Tone. Steinkopff Verlag, Darmstadt.
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Meier, L.P., Kahr, G., 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with triethylenetetramine and tetraethylenepentamine. Clays and Clay Minerals 47, 386–388. Ohrdorf, K.-H., 2010. Zu Fragen der alkalischen Aktivierung von Bentoniten. Dissertation Montanuniversität Leoben. Ufer, K., Stanjek, H., Roth, G., Dohrmann, R., Kleeberg, R., Kaufhold, S., 2008. Quantitative phase analysis of bentonites by the Rietveld method. Clays and Clay Minerals 56 (2), 272–282.