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Effect of Curing Time and Bentonite Content on the Quantitative Evaluation of Engineering Behavior of Cement Treated Clayey Mixtures Under Soaked Conditions
B.Sc., M.Sc., Ph.D., F.G.S., Lecturer, Laboratory of Soil Mechanics and Foundation Engineering,Geotechnical Engineering Division, Department of Civil Engineering, Aristotle University of Thessaloniki, Thesssaloniki, Greece |
ABSTRACT
Cement - stabilization in clayey soils results from two distinct chemical processes, cation exchange and flocculation. These processes are associated with the potential bonds developed between cement and soil grains.
The efficacy of cement - stabilization method is related with the composition of soils and the stabilization parameters such as percentage of cement, compaction and curing time. The curing time (time of reactions provided by cation exchange and flocculation processes) plays a vital role for the development of efficient cement stabilization. Strength and durability loss of cement stabilized clay soils after soaking in water is attributed to water absorbing capacity of dominant clay mineral (e.g. montmorillonite).
The swelling clays such as bentonite soaked in water, due to environmental conditions, result to volume increase causing macro and micro-fracturing in cement stabilized soil mass. These fractures accelerate water penetration and establish the initiation a slaking which causes strength and durability loss. For these reasons, in this research work, the influence of bentonite content was evaluated on strength and slaking of clay mixtures prepared with constant percentage of kaolin and different proportions of bentonite. Also the rate of strength and durability gained during curing and soaking in water, of cement stabilized active bentonite-sand mixtures, was evaluated in terms of parameters Strength Development Index (SDI) and Slaking Development Index (SLDI) in relation to cement content. By relating strength or slaking values developed during curing time with the corresponding strength or slaking values developed during soaking time, the slope parameter k of these linear relationships was evaluated for cement stabilized active bentonite-sand mixtures. Finally it was found that strength and durability values were increased with prolonged increase of curing and soaking time, increase of cement content and decrease portion of active clay mineral.
Keywords: curing, soaking, strength, slaking, bentonite, cement stabilization.
INTRODUCTION
Problematic clayey soils, exhibit high compressibility, high swelling potential, reduced strength, low permeability and durability and consequently are low quality material for construction and present difficulties in construction (Pinto et al, 2003). In such circumstances, engineering properties of problematic soils can be enhanced by the addition of cement, thereby producing an improved construction material. The cement-soil mixing (surface or deep) has been used for many diverse applications including foundations, retaining structures, liquefaction mitigation, temporary support of excavations, water control, pollution control (stabilization / solidification techniques) and structures that protect the natural environment. This method has a basic target, to find the most efficient and economical technique of mixing cement with soil, so that problematic soils obtain properties more like to those of a soft rock.
The soaking in water (due to environmental effect) increases the strength loss and slaking of a cement stabilized soil by time and decreases the bearing capacity in construction works. Few other adverse effects include: 1) increase in settlement of foundation, 2) increase in loosening of the surrounding soil in excavation work, 3) increase of volume change and erosion deterioration, 4) increase in loss of strength and slaking in cement stabilized soil slopes, 5) increase of erosion damage (land subsidence, chemical corrosion of the soil).
The water intrusion during soaking, especially in bentonite creates swelling, disrupts the interparticle contacts and the cement bonds and reduces the strength and durability of the cement stabilized clay. The absorbed water fills in around the old contacts, interacts with clay mineral surfaces and alters their surface properties (Dhakal et al, 2002; Stavridakis, 2005c). Finally the lower strength and durability of clay soils stabilized by cement, after soaking in water (environmental conditions of wetting) are attributed to the higher water absorbing capacity of the active clay fraction (e.g. bentonite). Besides the clay mineral composition is one of the most dominant factor affecting the engineering properties such as strength and durability and determining the chemical and physical properties of a clayey soil (Stavridakis, 2003).
Cement which is mixed with soil, is hydrated turning into the well-known hydrated compounds if water content is enough. The main hydration products are silicates and calcium hydrate aluminates and hydrated lime which deposits forming a separate crystal solid phase. In particular cement stabilization of clayey materials results from the complex combination of mechanical and chemical bonds which depend on reactions between cement and soil particle surface (Chew et al, 2004).
Cement - stabilization of clayey soils, within the time constraints usual in the field, (e.g. in transportation engineering projects - road pavements, embankments, temporary support of excavations, shallow foundations etc) provides that curing time (the time of development reactions by cation exchange and flocculation processes) plays a vital role to the achievement of efficient cement stabilization. In particular curing time influences strongly the engineering properties such as strength and durability of cement stabilized soil mass as well as the construction and usage time of infrastructural soil works (Kamruzzaman, 2002). From this point of view a research was carried out in order to study and characterize the influence of curing and soaking time, cement content and composition of clayey soils stabilized by cement on the quantitative evaluation of their engineering properties. In this effort both slaking and unconfined compressive strength were carried out on two groups of bentonite - kaolin - sand mixtures stabilized by 4% cement, compacted at 100%, 95%, 90% and aged for 7 and 28 days.
Mixtures of the first group were prepared with constant percentage (25%) of kaolin and 12.5, 20, 25, 30% of natural bentonite while mixtures of the second group contained constant percentage (62.5%) of kaolin and 0, 10, 20, 30% of natural bentonite. Also diagrams were presented in this research work to define the influence of curing and soaking time on the slope k (linear relationships) of strength and slaking development (early strength and durability) from 7 days of curing to 7 + 7 and 7 + 14 days of curing and soaking as well as from 28 days of curing to 28 + 7 and 28 + 14 days of curing and soaking of active bentonite - sand mixtures stabilized by cement.
In accordance with the above the increase of strength and decrease of slaking compared with an original strength and slaking value were evaluated in terms of Strength Development Index (SDI) and Slaking Development Index (SLDI). In particular in active bentonite - sand mixtures, contained 20Ma = 20% active bentonite, 80A = 80% sand (20Ma80A), the variation of SDI and SLDI were estimated in relation to the increase of cement content. It was found that the increase of bentonite content influences negatively the development of strength and durability. Also the experimental results showed that longer curing and soaking time and higher cement content influence positively the rate of strength and durability gained during cement stabilization and consequently the variation of early strength and durability. Finally the increase of curing, soaking time and cement content and the decrease of the active clay - mineral portion influence positively the rate of potential bonds (developed between soil grains) gained during cements stabilization.
Description of materials
In all practical cases, the primary ingredient necessary for stabilizing soils is calcium (cement). In addition to plasticity reduction, Portland cement, by its inherent nature of producing strength - developing hydration products, provides improved strength and durability. Therefore the effectiveness of stabilization is based on the time (curing - soaking time) of reactions provided by cation exchange and flocculation processes. These are related with the number of positions of exchangeable ions (type of clay minerals - active or inactive) of a clay and the amount of liberated calcium ions from cement (% of cement) which influence the durability (bonding effect) and unconfined compressive strength (bearing capacity).
For these reasons compressive strength and slaking tests were carried out on clayey admixtures stabilized by cement, consisting of three clayey soils namely natural and active Bentonite and Kaolin (Table 1).
Bell (1976), Croft (1967) and others have characterized the behaviour of bentonite (montmorillonite) as active and that of kaolinite as inert. Kaolinite and well organized (well crystallized) soil minerals appear to have little effect on the hydration of cement and hardening proceeds normally, after short curing periods by using small amounts of cement, (Stavridakis, 2005b). By contrast clay minerals with an expansive lattice (i.e. bentonite), have a profound influence on the hardening of cement and require large amounts of cement, to develop satisfactory strength and durability (Bell, 1976; Croft, 1967).
Table 1. Index properties of clays
In this research a commercially available kaolin (K) with LL = 34%, commercial active bentonite (Ma) with LL = 486% and sand (A) were used while a second type of bentonite (Mn) with LL = 111.5% was from natural source (Table 1). The sand used was uniform, fine to medium grained (74µm/840µm) with a uniformity coefficient of 2.19.
Sample preparation and testing procedure
Bonding (clayey soil grains - cement) determines the ease with which micro-macrofractures during slaking process, can propagate through the specimen. Slaking, under environmental conditions of wetting - drying and potential stresses (i.e. during seismic events or movements due to landslides), (Tatsuoka et al, 1997) is an aspect of cement stabilized soils, behaviour that has been neglected in favour of other properties such as strength. However it is an important feature of many commonly, encountered engineering problems concerning surface or deep soil - cement mixing (stabilization) when problems of durability arise in soil - cement mass because of water table fluctuations, in transportation engineering problems (Owttrim, 1988), in dam construction when the dispersive properties of clays (Na - montmorillonites) are not suitable accounted, (Shah and Ahmad, 2003). For these reasons the following tests were performed:
a) The slaking durability test (Franklin and Chandra, 1972) used to predict the potential deterioration of durability due to climatic wetting and drying.
Also this test contributes to the improvement of engineering properties of soil by relating the cement stabilization parameters (percentage of cement, degree of compaction, curing time) with the potential bonds developed between soil grains during this improvement by cement.
- The slaking (100-Id2) was measured using the device and testing procedure developed by Franklin. The apparatus combines the effects of both soaking and abrasion in order to accelerate the rate of weathering (bonding effect).
- The cylindrical specimens tested in slake durability test had a diameter of 35.5 mm and were 23.7 mm in length.
- Finally the slake - durability index (Id2 second cycle) was calculated as the percentage ratio of final to initial dry sample weight (Table 2).
b) The unconfined compressive strength was measured using a commercially available device named Versa Tester (Soil Test Inc.). The cylindrical specimens tested in unconfined compressive test had a diameter of 35.5 mm and were 71 mm in length. The displacement rate was 0.6604 mm/min.
- The specimens were prepared at the optimum moisture contents and maximum dry densities (Standard Proctor test) according to BSI 1377 d2 test and BSI 1924.
Table 2. Classification and characterization of durability (after Franklin and Chandra, 1972)
- The cylindrical samples were prepared according to ASTM 1632-96.
- The Atterberg limits were estimated according to BSI 1377.
- The percentages of cement (4, 8, 12%) were selected to give noticeable change in strength and durability.
- A compaction range from 90 to 100% of Standard Proctor maximum dry density obtained in the laboratory is often specified in field projects. This compaction range was adopted for this study.
- The clay - sand cement mixtures were cured at approximately 95.5% relative humidity and a temperature of 21oC for 7 and 28 days. After curing time specimens of active bentonite - sand mixtures were soaked in water for 7 and 14 days.
- The test programme included 252 (84x3) specimens of unconfined compressive tests as well as 924 (84x11) specimens of slake durability index tests for the clayey mixtures (Tables 3, 5 and 6).
Presentation and discussion of results
As can be seen in this study, unconfined compressive strength and durability increase as the cement content and curing + soaking time increase. This is due to the formative of cementitious products (reticular CSH and platy CASH - where C = CaO, S = SiO2, A = Al2O3 and H = H2O) in the clay - cement matrix and the change of the treated clay fabric to flocculated type.
The pozzolanic reaction is found to be very significant as curing and soaking time increasing. At prolonged curing and soaking periods significant portion of Ca2+ ions diffuses within the treated clay matrix to permit the pozzolanic reaction. These reactions based on the liberated calcium ions from the cement of soil - cement mixture occupy the positions of exchangeable ions on the clay minerals. The number of these positions depends upon the proportion of clay in soil - cement mixture or the type of clay minerals, (Croft, 1968). Also the calcium ions react with the silicates and aluminates from the soil minerals.
The latter coat the grains to form a skeletal structure of considerable strength. Therefore the amount of liberated calcium ions depends on the curing and soaking time and the amount of cement which in turn are related to slaking and unconfined compressive strength (Za-Chien-Moh, 1965). Thus the strength and durability increase significantly at prolonged curing and soaking periods leading to the formation of more structural clay (skeletal cementation). According to these the increase of strength and decrease of slaking are functions of the cement content used, curing and soaking time and composition of a cement stabilized soil, (Stavridakis, 2005a). Finally, the critical cement content, to achieve the stabilization of strength and durability, decreases with the decrease of active clay mineral (i.e. montmorillonite) portion and the prolonged increase of curing and soaking time (Stavridakis, 2004).
For the above reasons the development of unconfined compressive strength and slaking (100 - Id2) (%) was calculated in relation to cement content and curing + soaking time. The rate of strength increase and slaking decrease related to cement content increase was evaluated in terms of parameter Strength (SDI) and Slaking (SLDI) Development Index. The Strength Development Index (SDI), (Uddin, 1995; Siddique and Hossain, 2003), related to cement content was defined as follows:
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(1) |
where qu of stabilized sample = strength developed with 8% or 12% cement, compaction of 100%, curing time of 7 or 28 days and curing + soaking time 7+7, 7+14 or 28+7, 28+14 days
- qu of 4% cement treated sample = strength developed with 4% cement, compaction of 100%, curing time of 7 or 28 days and curing + soaking time 7+7, 7+14 or 28+7, 28+14 days
The Slaking Development Index (SLDI), related to cement content was defined as follows:
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(2) |
negative values of SLDI correspond to slaking decrease; where
- slaking of stabilized sample = slaking developed with 8% or 12% cement, compaction of 100%, curing time of 7 or 28 days and curing + soaking time 7+7, 7+14 or 28+7, 28+14 days.
- slaking of 4% cement treated sample = slaking developed with 4% cement,
- compaction of 100%, curing time of 7 or 28 days and curing + soaking time 7+7, 7+14 or 28+7, 28+14 days.
Figure 1 shows the influence of bentonite content on strength and slaking values of cement stabilized clayey mixtures prepared with 25% kaolin (Tables 3 and 4). As can be seen the increase of bentonite content decreases strength and increases slaking values. It is obvious that 4% cement is not enough to control slaking and strength in cement stabilized clay mixtures containing 25% kaolin and more than 25% bentonite content. Clay mixtures, prepared with 25% kaolin and less than 20% bentonite content, are stabilized effectively with 4% cement exhibited high strength and low slaking values. Figure 2 shows the influence of bentonite and kaolin content on strength and slaking values of cement stabilized clayey mixtures prepared with 62.5% kaolin (Tables 3 and 4). It is clear that clay mixtures, contained more than 20% bentonite content, appeared low strength and high slaking values while clay mixtures, prepared with bentonite content less than 10%, exhibited high strength and low slaking values. By comparing Figure 1 with Figure 2 it seems that the effective stabilization area moves from
Table 3. Influence of bentonite content of clayey mixtures, prepared with constant percentage (25% and 62,5%) of kaolin and cured for 7 and 28 days, on strength and slaking development index values (Figures 1 and 2).
Table 4. Empirical relationships between unconfined compressive strength and slaking values (Figures 1 and 2).
clay mixtures containing 25% of kaolin and bentonite content less than 20%, to clay mixtures containing 62.5% kaolin and bentonite content less than 10%. This is attributed to the increase of kaolin content from 25% to 62.5%. Therefore the higher the percentage of kaolin the larger the number of contacts of grain - kaolin and the greater the amount of cement needed to develop strong bonds for enough durability.
Figure 1. Influence of bentonite content of clayey mixtures prepared with 25% of kaolin stabilized by cement on their strength and slaking.
Figure 2. Influence of bentonite content of clayey mixtures prepared with 62.5% of kaolin stabilized by cement on their strength and slaking.
It is obvious that 4% cement is not enough to control slaking and strength of cement stabilized clay mixtures containing 62.5% kaolin and more than 10% bentonite content. On the other hand Figures 3 and 4 exhibit the relationship between strength or slaking values of 7 days curing and 7+7 or 7+14 days curing and soaking, of active bentonite - sand (20Ma80A) mixtures (Tables 5, 6 and 7). Strength (Figure 3) and slaking (Figure 4) values are fitted with linear relationships for this clay mixture with good correlation. These figures also show that soaking time influences negatively the development of strength and durability of an active bentonite - sand mixture. In particular at prolonged soaking periods such as 14 days the strength and durability increase in comparison with the corresponding strength and durability of 7 days soaking of active bentonite - sand mixtures.
Figures 5 and 6 show the development of strength and slaking values of 28 days curing, 28+7 and 28+14 days curing and soaking of active bentonite - sand (20Ma80A) mixtures (Tables 5, 6 and 7).
Table 5. Unconfined compressive strength and slaking values of 20Ma80A mixture
compacted at 100%, under cured and soaked conditions.
Table 6. Unconfined compressive strength and slaking values of 20Ma80A mixture
compacted at 90%, under cured and soaked conditions.
Figure 3. Influence of soaking time on strength of clay mixtures (20Ma80A) cured for 7 days.
Figure 4. Influence of soaking time on slaking of clay mixtures (20Ma80A) cured for 7 days.
Figure 5. Influence of soaking time on strength of clay mixtures (20Ma80A) cured for 28 days.
Figure 6. Influence of soaking time on slaking of clay mixtures (20Ma80A) cured for 28 days.
Table 7. Development of strength and slaking values of 20Ma80A mixture during curing
or curing and soaking conditions, (Figures 3, 4, 5 and 6).
Table 8. Strength and slaking development index values of 20Ma80A mixture under cured
or cured and soaked conditions (Figures 8, 9, 10 and 11).
This development is fitted with linear relationships with good correlation. It seems that the slope parameter (k) of these linear relationships is influenced strongly from the soaking time of cement stabilized active bentonite - sand mixtures. In other words the development of early strength and durability of cement treated active bentonite - sand mixtures are connected strongly to the slope parameter k (Figures 3, 4, 5 and 6). In particular Figure 7 reveals the strong connection of curing and soaking time with the slope parameter (early strength and durability) either for strength (kqu 7, kqu 28) or for slaking (sl7, sl28) development.
Figures 8 and 10 show the influence of cement content on SDI of active bentonite - sand mixtures cured for 7 and 28 days respectively (Tables 5 and 8). These figures also show the influence of soaking time on the development of strength (SDI). In particular the higher the percentage of cement added and longer soaking time, the higher the strength development index of clay mixtures (20Ma80A) cured for 7 or 28 days.
Figure 7. Influence of soaking time on the slope parameter k (early strength and durability) of strength (kqu 7, kqu 28) and slaking (ksl 7, ksl 28) development of clay mixtures (20Ma80A) cured for 7 and 28 days (Table 7).
Figure 9 reveals the development of slaking (SLDI) of active bentonite - sand mixtures (20Ma80A), the higher the cement content used the higher the slaking development index, (Tables 5 and 8). This figure also shows that percentages of cement content fluctuating from 4% to 8% are not enough to control slaking in cement stabilized clay mixtures cured for 7 days and soaked for 7 or 14 days (Tables 5 and 8). In clay mixtures (20Ma80A) cured for 7 days and stabilized with more than 8% cement the SLDI of mixtures soaked for 14 days is higher than mixtures soaked for 7 days. Also Figure 11 exhibits the development of slaking (SLDI) of active bentonite - sand mixtures cured for 28 days. It is clear that 4% cement is enough to control the development of slaking, (Tables 5 and 8). In accordance with the aforementioned, the increase of cement content, curing and prolonged soaking time influence positively the development of slaking index (rate of potential bonds gained, between soil grains, during cement stabilization).
CONCLUSIONS
(1) The increase of bentonite content decreases strength and increases slaking values, of cement stabilized clayey mixtures.
(2a) Clay mixtures prepared with 25% or 62.5% kaolin and less than 20% or 10% bentonite content respectively are stabilized effectively with 4% cement exhibited high strength and low slaking values.
Figure 8. Influence of curing, soaking time and cement content on Strength Development Index (SDI) of cement stabilized clay mixtures (20Ma80A) cured for 7 days.
Figure 9. Influence of curing, soaking time and cement content on Slaking Development Index (SLDI) of cement stabilized clay mixtures (20Ma80A) cured for 7 days.
Figure 10. Influence of curing, soaking time and cement content on Strength Development Index (SDI) of cement stabilized clay mixtures (20Ma80A) cured for 28 days.
Figure 11. Influence of curing, soaking time and cement content on Slaking Development Index (SLDI) of cement stabilized clay mixtures (20Ma80A) cured for 28 days.
(2b) Cement content of 4% is not enough to control slaking and strength of cement stabilized clay mixtures containing 25% or 62.5% kaolin and more than 25% or 20% bentonite content respectively.
- The aforementioned are attributed to the increase of kaolin content from 25% to 62.5%.
(3) The experimental results indicate linear relationships, with good correlation, between, a) strength or slaking values of 7 days curing and 7+7 or 7+14 days curing and soaking, b) strength or slaking values of 28 days curing and 28+7 or 28+14 days curing and soaking.
- The slope parameter (k) of these linear relationships either for strength (kqu) or slaking (ksl) development is connected with the early strength and durability respectively. These parameters are related strongly with the soaking time.
(4) Strength (SDI) and slaking (SLDI) development index have been introduced in order to increase the understanding of variation in strength and slaking development related to cement content as well as curing and soaking time of cement stabilized clay mixtures.
(5) The increase of cement content, curing and prolonged soaking time increase the development of strength (SDI) and slaking (SLDI) index (rate of potential bonds gained during cement stabilization).
(6) A combination of strength and slaking development index with curing and soaking time, type and portion of bentonite and cement content of stabilized clayey - sand mixtures reveals useful information for efficient cement - stabilization that could be used for specific applications such as grouting, cement - soil mixing (deep or surface), control of water flow through soil or even improvement of geotechnical properties of problematic soils.
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