Improving Shear Strength and Other Mechanical Properties of Clayey SoilAsh Mixtures Omer Nawaf Maaitah Associate Professor, Civil and Environmental Engineering Department, Faculty of Engineering, Mu’tah University, P.O Box 7, Karak – Jordan e-mail:
[email protected]
Nafeth A. Abdel Hadi Assistant Professor, Dept. of Civil Engineering, Al-Balqa' Applied University, Amman, Jordan
Monther A. Abdel Hadi Assistant Professor, Dept. of Civil Engineering, Al-Ahliyya Amman University, Amman, Jordan
ABSTRACT With development of infrastructure and increasing transportation demands, construction of better pavements with longer lives is required. Low quality of unsaturated compacted borrowed material that used in sub-base and base course can be mixed with high calcium ash to produce cement treated base and roller compacted concrete (RCC) without ordinary Portland cement. Durable pavements, embankments and dams can be constructed with very long life and low cost by utilizing the high calcium ash. Limy ash was obtained by the direct combustion of the bituminous lime stone of Al-Karak at a temperature of 950 °C . The laboratory tests have been selected with respect to construction needs and possible post construction conditions. The objective of this study is to utilize ash in stabilizing brown clayey soil. The shear strength of the clay has been raised from 220 kPa to 1080 kPa. The CBR value has been raised from 4.5 % to 120% for clay. Various mortars and construction elements can be produced at normal room curing temperature without using ordinary Portland cement. The swelling potential has been illuminated when significant ash amount used. The initial consumption of ash value is around 100% to give higher strength and CBR and lower swell potential.
KEYWORDS:
Bituminous limestone, Brown Clay, Pavement, Ash and shear
strength.
INTRODUCTION It is not uncommon for low quality sub-base and base course to remain stable for many years without proper treatment. In Jordan country-side roads receive some insufficient treatment to stabilize the surface layer which may fail during or soon after several heavy rainstorms. Rainfallinduced surface layer of road to fail which is the most common type of damage in Jordan local - 1261 -
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roads. Jordan climate is arid in summer with huge rainfall during winter. Each year millions of Jordanians Dinars are spent to road maintenance. Small and large cracks occur during the winter in Jordan roads. Despite the efforts that have been made to improve gentle treated roads, many issues are still not fully understood. In Jordan, considerable amount of construction activity is carried out at relatively shallow depths where soil is likely to be unsaturated and subjected to low stress levels. One particular example is the pavement. The occurrence of damage to road founded on unsaturated sub-grade is due to the collapse of the soil structure upon wetting-drying process, has come to be well recognized. It has recently become apparent that these unsaturated soils can cause damage to roads because of the change in the shear strength of unsaturated soil as the degree of saturation changes (Maaitah et al 2004). For a rainfall-induced shallow collapse, the failure mechanism could be as follows: (i) water infiltration to the sub-grade results in an increase in saturation and then a decrease in the matric suction of the soils, (ii) the decrease in the latter reduces the soil shear strength, and (iii) the reduction in soil shear strength subsequently causes the pavement layers to become unstable and then fail (Reinson, 2001). Moreover, once the pavement layer is completely saturated, the unsaturated component of the soils disappears completely. Poor subgrade soil conditions can result in inadequate pavement support and reduce pavement life. Soils may be improved through the addition of chemical or cementitious additives. Soil stabilization is a technique introduced many years ago with the main purpose to render the soils capable of meeting the requirements of the specific engineering projects (Kolias et al., 2005). Several additives, which may be utilized for ground modification such as cement, lime and mineral additives such as fly ash, silica fume, rice husk ash..., have been used under various contexts. On the other hand, extensive studies have been carried out on the stabilization of soft soils using various additives mentioned above (Al-Rawas and Goosen, 2006). One of the great benefits of using fly ash is to stabilized recycle pavement material which is reported by (Li et al 2008). There is many other benefits of fly ash may be divided into three categories (adapted from Ferguson, 1993 and Parsons et. al 2005): Firstly: as drying agent. Fly ash hydrates when exposed to water. As a water consumer, it can be used as a drying agent for wet soils when acceleration of the drying process is desired. Secondly: as control of volume change. Fly ash reduces shrink/swell behavior because it does not experience significant volume change itself, so its addition acts to dilute the effects of the swelling clays that are present. It also contains some lime, which acts through ion exchange to reduce the clay particles’ affinity for water (Ferguson and Levorson, 1999). Thirdly: increase strength. Fly ash acts as a weak cementing agent that increases the strength of the treated soil. The aim of the current work is to stabilize unsaturated brown sandy clay soil by high calcium ash. The possibilities to utilize the solid waste ash material in various geotechnical engineering aspects will be investigated such as the improvement and stabilization of weak materials like brown clay as a sub-grade in roads and highways. Large feasible deposits of bituminous limestone are existed in Al-Karak (a city 120 km south of Amman, Jordan). Using Ash in stabilizing soil can be of great benefit to the environment of the site preparation.
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Many tests have been performed to investigate the physical, chemical, and mechanical properties of ash. Also, many tests such as Proctor, California bearing ratio, direct shear, and free swelling have been carried out on stabilized soil. The testing has been conducted at 1, 7, and 28 days for ash-soil mixtures at different percentages of ash content. METHOD OF SOLUTION
Problem Definition It is not uncommon for low quality sub-base and base course to remain stable for many years without proper treatment. In Jordan country-side roads receive some insufficient treatment to stabilize the surface layer which may fail during or soon after several heavy rainstorms. Rainfallinduced surface layer of road to fail which is the most common type of damage in Jordan local roads. Jordan climate is arid in summer with huge rainfall during winter. Each year millions of Jordanians Dinars are spent to road maintenance. Small and large cracks occur during the winter in Jordan roads. Despite the efforts that have been made to improve gentle treated roads, many issues are still not fully understood. In Jordan, considerable amount of construction activity is carried out at relatively shallow depths where soil is likely to be unsaturated and subjected to low stress levels. One particular example is the pavement. The occurrence of damage to road founded on unsaturated sub-grade is due to the collapse of the soil structure upon wetting-drying process, has come to be well recognized. It has recently become apparent that these unsaturated soils can cause damage to roads because of the change in the shear strength of unsaturated soil as the degree of saturation changes (Maaitah et al., 2004). For a rainfall-induced shallow collapse, the failure mechanism could be as follows: (i) water infiltration to the sub-grade results in an increase in saturation and then a decrease in the matric suction of the soils, (ii) the decrease in the latter reduces the soil shear strength, and (iii) the reduction in soil shear strength subsequently causes the pavement layers to become unstable and then fail (Reinson, 2001). Moreover, once the pavement layer is completely saturated, the unsaturated component of the soils disappears completely. Poor subgrade soil conditions can result in inadequate pavement support and reduce pavement life. Soils may be improved through the addition of chemical or cementitious additives. Soil stabilization is a technique introduced many years ago with the main purpose to render the soils capable of meeting the requirements of the specific engineering projects (Kolias et al., 2005). Several additives, which may be utilized for ground modification such as cement, lime and mineral additives such as fly ash, silica fume, rice husk ash..., have been used under various contexts. On the other hand, extensive studies have been carried out on the stabilization of soft soils using various additives mentioned above (Al-Rawas and Goosen, 2006). One of the great benefits of using fly ash is to stabilize recycled pavement material which is reported by Li et al. (2008). There is many other benefits of fly ash may be divided into three categories (adapted from Ferguson, 1993 and Parsons et al, 2005): Firstly: as drying agent. Fly ash hydrates when exposed to water. As a water consumer, it can be used as a drying agent for wet soils when acceleration of the drying process is desired. Secondly: as control of volume change. Fly ash reduces shrink/swell behavior because it does not experience significant volume change itself, so its addition acts to dilute the effects of the
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swelling clays that are present. It also contains some lime, which acts through ion exchange to reduce the clay particles’ affinity for water (Ferguson and Levorson, 1999). Thirdly: increase strength. Fly ash acts as a weak cementing agent that increases the strength of the treated soil. The aim of the current work is to stabilize unsaturated brown sandy clay soil by high calcium ash. The possibilities to utilize the solid waste ash material in various geotechnical engineering aspects will be investigated such as the improvement and stabilization of weak materials like brown clay as a sub-grade in roads and highways. Large feasible deposits of bituminous limestone are existed in Al-Karak ( a city 120 km south of Amman, Jordan). Using Ash in stabilizing soil can be of great benefit to the environment of the site preparation. Many tests have been performed to investigate the physical, chemical, and mechanical properties of ash. Also, many tests such as Proctor, California bearing ratio, direct shear, and free swelling have been carried out on stabilized soil. The testing has been conducted at 1, 7, and 28 days for ash-soil mixtures at different percentages of ash content.
Characterization of the bituminous limestone and ash Huge deposits of bituminous limestone or what is called bituminous oil shale is an indigenous source of energy in Jordan. Ash could be the solid waste product of possible utilization of the AlKarak bituminous limestone. The following investigations shall concentrate on determining the physical, chemical, and mechanical properties of ash. The bulk density of Al-Karak bituminous limestone is 19.5 kN/m3 and the specific gravity is of 2.39. X-ray fluorescence results indicate that the bituminous rocks are dominated by calcite 20%-80%. Quartz 10%-40%, kaolinite 5-10%, apatite 4-14%, and dolomite 2-3.6% are the other constituents. Minor minerals (around 5%), as feldspar, pyrite, goethite, gypsum, opal and muscovite are also present (Abu Ajammieh, 1980, Hamarneh, 1995 and Nafeth et al, 2008). The rock is essentially composed of CaO, SiO2, Al2O3, and Fe2O3. Trace elements are considered relatively high. The properties of ash are summarized in table 1. The ash sample was obtained by direct combustion at a temperature of 950 °C. The sample was allowed to cool down to the ambient temperature (30 0C) and then was ground under dry conditions to obtain the possible minimum grain size. Small ball mills and Los Angles machine were used.
Table 1: Ferguson (1999) classification of class C&F compared with Al-Karak ash (Analysis was done using XRF technique, Ferguson et al. (1999) Oxide% SiO2 % Al2O3 % Fe2O3 % CaO % MgO % SO3
Class C ash 54.9 25.8 6.9 8.7 1.8 0.6
Class F ash 39.9 16.7 5.8 24.3 4.6 3.3
Karak ash 33.82 3.25 1.44 45.84 2.47 5.71
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Experimental Results The representative sample was collected from brown sandy clay at the Northern part of alKarak city. The soil samples are considered as problematic soils due to their poor strength characteristics and high susceptibility to successive swelling-consolidation behavior during the rainy winter and arid summer. The XRD analyses of the brown soil have shown that quartz (SiO2) and smectites are the major components. The chemical composition of the parent soil samples is given in Table 2. Following ASTM standards, the physical properties of the brown soil are summarized in Table 3. The high liquid limit indicates a high exposure to moisture sensitivity due to the presence of high percentage of the clay fraction. This result reveals the cause of problems exhibited by this soil as a subgrade material. Grain size distribution is carried out as per ASTM D-422 which is illustrated in Figure 1. The soil classification is presented in Table 4.
100
Passing %
80 60 40 20 0 0.0001
0.001
0.01
D (mm)
0.1
1
10
Figure 1: Particle size distribution In this paper two kinds of tests have been conducted using direct shear testing device. The first set of tests was at 7 days curing and the second set at 28 days of curing. The ash content and degree of saturation were used as two variables in each set. The normal stress was used as a variable also. The major works were carried out with different percentage of ash at 28 days curing. The samples were mixed with water and then were sieved to make sure the water is well distributed. The results are presented in the following figures. Tests on natural soil were performed for purpose of comparison.
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Table 2: Chemical composition of the clay soil (L.O.I = 7.5%,* ICP technique) Oxide clay soil (BC0) SiO2 Fe2O3 CaO MgO MnO TiO2 K2O P2O5
% % % % % % % %
67.1 7.1 1.9 0.8 0.16 1.7 0.8 0.11
Table 3: Physical and mechanical properties of clay soil Parameter Liquid limit (LL) Plastic limit (PL) Shrinkage limit (SL) Plasticity index Passing #200% Clay fraction % Specific gravity Unconfined compressive strength Maximum dry density California Bearing Ratio
Representative soil 53 26 17 27 94 56.6 2.72 650 kPa 16.5 kN/m3 4
2. Stabilization of clay with ash All standard testing for strength determination of ash as a self-cementing material have shown a high stability (Ferguson, 1993 and Khoury 1993). No minor features of disintegration or disturbance of the prepared samples during the curing stage in water at normal ambient temperature of 28 0C are observed (Boarfman et al 2001). This reflects the opposite behavior of normal soils that could not sustain under saturation conditions. On the other hand, ash behaved as cementaceous material that has the possibility to gain strength under normal curing time and conditions as the other different types of cements. The addition of water under surface normal conditions would lead to the following hydration reactions: CaO + H2O → Ca (OH) 2 (Portlandite) SiO2 +Ca (OH) 2 +H2O → Ca.SiO2.2H2O (CSH) Al2O3 +Ca (OH) 2 +H2O → Ca O.Al2 O3.2H2O SiO2 + Al2 O3 + Ca (OH) 2 + H2 O → Ca. SiO2. Al2 O3.H2O (CAS)
(1) (2) (3) (4)
Atterberg tests are conducted on clay by adding 10, 20, 30, and 50 percent of ash. The results have revealed that the consistency limits have changed when compared with the untreated
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sample. The results have shown that the liquid limit has decreased and the plastic limit has increased when the soil is mixed with ash samples as shown in table (4). The variation of the consistency limits is very close at the same ash content regardless of the ash type since all of the ash samples are not plastic. The group index for the same soil has also changed with increasing the ash content. The liquid limit variation with ash content is illustrated in table (4). This indicates that the LL decreases with the increase of the ash content regardless of the ash type. The soil plasticity has decreased until the non-plastic behavior has appeared when it is mixed with the proper percentage of ash. This is referred to the proportional increase of the non plastic ash portion in the tested samples. The plasticity index (PI) is decreasing with increasing the ash content as shown in Figure 2 and Table 4. This result means that the soil has improved by mixing with a proper amount of the non-plastic ash.
30 25 PI %
20 15 10 5 0 0
10
20
30
40
Ash content %
Figure 2: Variation of PI with ash content
Table 4: Summary of consistency limits for ash soil mixtures
Ash content % 0.0 10 20 30 50 Liquid limit 46 42 42 40 N.P Plastic limit 23 25 26 28 Shrinkage limit ---- ---- 26 20 Plasticity index 23
50
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17 16 12 Group index (AASHTO) A-7-5 A-7-6 A-7-6 A-6 Group index (UC) MH CH CL CL
Compaction characteristics
Dry Density kN/m3
The results of compaction behavior following ASTM D 698 for the ash-brown soil mixtures have revealed that the soil mixture has a wider range of moisture content at which the soil is compacted with lower effort. The results of compaction tests that are carried out for ash-brown soil mixtures are shown in Figure 3. The maximum dry density of ash-brown soil mixtures has decreased at small values when the mixtures are mixed with up to 30% of ash. The limited decrease of dry density is related to the low ash content in all the compaction trials due to the high plasticity of the treated samples. The ash-soil mixtures with 50% of ash and more have shown a non-plastic mixture with a considerable decrease of dry density (14.5 kN/m3) and a wider range of stability at higher moisture content when it is compared with the parent untreated sample. The MDD of the ash-brown soil with 100% of ash has decreased to 13.9 kN/m3 when it is compared with 16.5 kN/m3 of the original untreated brown soil. However, the sensitivity to water has decreased due to increase the ash content.
17 16.5 16 15.5 15 14.5 14 13.5 13 12.5 12
Natural soil
Ash =30%
Ash =20%
Ash =50%
Ash=100%
10
15
20 25 Moisture Content %
Figure 3: Compaction behavior of ash brown soil mixtures
30
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350 Shear strength kPa
300 250 200 150 100 Normal stress = 10 kPa
ash = 0 %, Natural soil
50
Normal stress =50 kPa
0 0
20
40
60
80
Sr % Figure 4: Effect of degree of saturation on the unsaturated shear strength for natural soil
Shear strength for stabilized brown clay Strength is considered as one of the most important parameters that reflect the efficiency of introducing the Al-Karak ash for stabilizing representative clays. The improved compressive strength results will be reflected on the other mechanical and engineering properties as shear strength and swelling in addition to California Bearing Ratio (CBR) of the ash treated soil. Figure (4) shows the effect of degree of saturation on the shear strength to natural soil. The strength drops dramatically as the saturation exceed 40% for untreated soil. This means that the compacted unsaturated soil is sensitive to any change in water content. The buildup of strength is not instantaneous and increases with the increase of time and ash content as shown in Figures 5, 6, 7, 8, and 9. This is due to the exchange of calcium cations supplied from lime, (CaO). Hydrated lime reacts with clay mineral surfaces in the high pH environment creating cementaceous products (Qubain et al., 2000). Carbonation of portlandite Ca(OH)2 is responsible for the precipitation of calcite which acts as a cementing material. Figure 5 shows as the curing time increases the shear strength increases for the ash-brown soil mixtures. In addition, as the ash percent increases the shear strength increases as illustrated in the Figure 5. The strength increases as the normal stress increases as stated in Figure 6.
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curing 1 day
1270
curing 7 days
Normal stress=10 k
curing 28 days
Shear strength kPa
1200 1000 800 600 400 200 0 0
20
40
60
80
100
Ash %
Figure 5: Effect of curing time on the shear strength of ash brown clay mixture
Shear strength kPa
1000 800 600 400
Curing time = 7 days Ash =30 %
200 0 0
20
40
60
80
100
Normal stress kPa
Figure 6: Effect of confining stress on shear strength of stablized soil Ash Modification Optimum or Initial consumption of ash value is greater than 50% as shown in Figure 5. The percent of ash can be defined as the Initial consumption of ash gives an indication of the minimum quantity of ash that must be added to a soil to achieve a significant change in properties. The expected investment or utilization of bituminous limestone rocks in the future as a source of crude oil extraction by retention processes, or for use as an energy source for electricity generation by direct combustion, will lead to the production of 52% by weight of the original rock as solid waste (ash). The huge quantities of ash are considered as one of the main
1
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obstacles when considering the environment, cost of dumping and site preparation. The possibilities to utilize the solid waste ash material in various geotechnical engineering aspects will be investigated such as the improvement and stabilization of weak materials like soil and marl as a subgrade in roads and highways. Therefore, it is of great importance to use 1 ash to 1 soil in mixing to increase the strength and to remove the waste.
Shear strength kPa
Ash = 0%
Ash = 50%
Ash = 30%
1200 1000 800 600 400 200 0 0
20
40
60
80
Normal stress =10 k curing 28 days
100
Sr %
Figure 7: Effect of ash content and degree of saturation on shear strength of stablized soil Figure 7 shows that the sensitivity toward variation in degree of saturation. The strength increases slightly as the saturation increases up to 40 % and then becomes nearly constant for untreated soil (Figure 4). The result in Figure 7 shows the effect of ash on soil which removes the drop in strength as the saturation increases beyond 40 % as illustrated in figure 4. The same conclusion can be drawn from Figures 8 and 9 which show that the amount of ash plays an important role in increasing the shear strength. The sensitivity toward water decreases as the amount of ash increases as it clear from comparison between figures 8 and 9. In this situation a significant level of strength gain is developed through a long term pozzolanic reactions. Calcium from the ash reacts with alumina and silica released from the clay mineral surfaces, leading to the formation of calcium silicate hydrates and calcium aluminates hydrates as it is confirmed by XRD analysis. The pozzolanic reaction can continue for a long period of time, as long as the pH remains high (above 10). As a result of this reaction, the ash treated soil exhibits substantial strength gain, improvements in shear strength and long term durability. Field demonstrations have indicated that the improved soil properties are maintained over 20 to 40 years (Little, 1999 and Qubain et al., 2000.).
1
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Shear strength kPa
1000 980 960 940 Ash=50%
920
Curing 28 day, Normal stress =10 kPa
900 0
20
40
60
80
100
Sr % Figure 8: Effect of degree of saturation on stablized soil The time factor is important for the ash alkalis to react with the pozzolanic part of the brown clay to change the poorly cemented soil into a stable cemented material. The compressive strength results after 7 days are variable due to the variation of ash content and are distinguished by poor gain of strength. The reaction will continue until the mixture pH is deceased by the alkali–silica reaction.
680 Shear strength kPa
670 660 650 640
Normal stress =10 kPa, curing 28day
630
Ash = 30%
620 0
20
40
60 Sr %
80
100
120
Figure 9: Effect of degree of saturation on stablized soil The long term strength buildup continues through the continuous ash- alkalis reaction with soil-pozzolanic constituent and or the addition of CO2 to the ash brown clay mixtures. The CO2 source is expected to be derived from the dissolved CO2 gas in water or from the organisms that live in the media as bacteria or from the atmosphere. The strength buildup of the S3 ash mixtures in spite of the law CaO content is related to the variable content of ettringite as revealed by the XRD results. The long term reaction will continue as a result of the uptake of atmospheric CO2 according to the following reaction. CaO +CO2 → CaCO3
(5)
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California Bearing Ratio The California bearing ratio of the compacted ash-brown soil mixtures are cured for 28 days in tight plastic bags to maintain the optimum moisture content. Curing was at normal ambient temperature. The samples are soaked for 96 hours directly before testing; the results are shown in Figure 10. The CBR results show increase trend as ash content increases. It is clear that the Initial consumption of ash value is around 100% as shown in Figure 10). 160 140 120 CBR
100 80 60 40 Curing Time 28 days
20 0 0
10
20
30
40
50
60
70
80
90
Ash percent %
Figure 10: Effect of ash on CBR
Swelling behavior The addition of ash to soil revealed an efficient action in improving the swelling properties. The reduction of free swelling potential is considered as an important factor to prevent excessive water absorption of sandy silty clay soils subgrade and hence reducing the pumping phenomenon under rigid pavements and reducing the subgrade stress by increasing the stiffness of the base course material instead of increasing its thickness. However, the addition of extra quantities of ash above the optimum quantity has no effect on swelling behavior as shown in Figure 11. The optimum quantity of ash is around 100% of ash.
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Ash content =0 % Ash content =10 % Ash content =20 % Ash content =30 % Ash content =50 %
12 10 Free swell %
1274
8 6 4 2 0 1
10
100 Time (min)
1000
10000
Figure 11: Effect of ash on swelling
Discussion The ash and Portland cement are essentially composed of lime (CaO). Silica (SiO2) and alumina (Al2O3) are present at higher concentrations in the Portland cement and react with CaO at about 1425 0C to form alite (C3S). Heat treatment of the bituminous rocks and Portland cement raw material involves dehydration, thermal decomposition of clay minerals (300-650 0C), decomposition of calcite (›800 0C), the formation of belite (C2S), tricalcium aluminates (C3A), and tetracalcium alumina ferrite (C4AF). The liquid phase and sintering at about 1425 0C form alite which is responsible for the strength of concrete. Al-Karak bituminous limestone ash has revealed a self cementaceous behavior for the various prepared samples. The development of the self cementaceous properties and strength are controlled by the temperature of combustion of the bituminous limestone, the lime content and the curing period. The results are obtained through the standard mixtures of each ash sample. The mixtures are prepared and cured under the same standard procedures and conditions. Each ash type has its own chemical composition as a result of the variation of the combustion temperature. All of the ash samples are essentially composed of CaO. The variable CaO values are the result of L.O.I variation. The three types of ash samples have a relatively similar chemical composition and are analogous to the raw materials used to produce the Ordinary Portland Cement (OPC). The alkali content which is presented by CaO, the pozzolanic content which is presented by (SiO2 +Al2O3+ Fe2O3), and the variable content of SO3 are found in both the ash samples and OPC raw material. The strength buildup in all the ash samples is related to the setting reactions of lime (CaO) with the pozzolanic constituents to produce calcium silicate hydrate (CSH) and
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calcium aluminate hydrate (CAH). High pH solution is highly reactive with amorphous Al-Si rich phases at normal room temperature. Sulfate minerals as ettringite are expected to form because of the availability of SO3 in the Karak bituminous limestone and its combusted products. The hydration products of the ash as identified by the XRD technique are portlandite, ettringite, calcium silicate hydrate and calcium aluminum hydrate. The reactions are not spontaneous and are time dependent. Curing period of 28 days and more has influenced the strength results. High strength values are obtained with intact samples indicating no disintegration features under fully saturated conditions. All hydrated samples have shown a similar behavior to the hydrated OPC products but with lower compressive strength. To stabilize the soil with ash (ash-brown soil mixtures) is prepared with equivalent weights (ash/soil =1:1, and 1:2). The mixtures are compacted to maximum dry density and optimum moisture content. The strength gain in the ash soil mixtures has increased with the increase of silica content in the stabilized soil samples. The CBR values for mixtures of the stabilized brown soil with ash has shown that the CBR has increased from 4% for the unstabilized parent soil samples to more than 120%. Complex reactions are expected in the various ash mixtures. Lime (CaO), plays an important role in the strength buildup. The increase of CSH and CAH phases in ash samples and its related mixtures is responsible for the increase of compressive strength. A non-combusted background bituminous limestone mortar is used. The background sample has indicated a low strength as it is submerged in water for 28 days as a result of the absence of binding phases. This is due to the absence of lime and amorphous Al-silicates. This result confirms the role of CaO content in the strength buildup. Cement manufacturing involves grinding, blending, pre-calcination of the raw materials, clinker burning and cement grinding. Limestone (CaCO3) and other materials containing calcium, silicon, aluminum and iron are crushed and milled into a raw meal. This raw meal is blended and is then heated in the pre-heating system to initiate the dissociation of carbonate to calcium oxide and carbon dioxide. The meal then proceeds to the kiln for heating and reaction between calcium oxide and other elements to form calcium silicates and calcium aluminates at a temperature up to 1450 0C. The reaction products leave the kiln as a clinker. The clinker leaves the kiln here to be cooled, mixed with gypsum, ground into a fine powder (cement). The setting of cement involves a number of stages at different rates. A complex series of reactions take place as the cement reacts with water. Setting of C2S involves slow hydration reactions and the formation of Portlandite Ca(OH2) and calcium silicate hydrate. These products are expected to be the essential components of ash hydrated products. In the ash samples, the excess lime is present and the formation of tri-calcium aluminate C3S is favored. In Portland cement, the addition of gypsum could lead to the formation of ettringite Ca6 Al2O6(SO4) 3.31-32 H2O. As the mixture ages, the fibrous crystalline network continues its growth, and the crystals become coarser and form interlocking network. The critical aspects in setting include the formation of CSH and CASH phases to provide the observed strength of the ash. Portlandite Ca (OH)2 plays an important role in the setting reaction. Portlandite reacts with silicates and aluminum rich phases to form insoluble compounds which contribute to the strength
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formation (pozzolanic reactions). Excess portlandite reacts with atmospheric CO2 to precipitate calcium carbonate that helps in strengthening the product after aging. The combusted bituminous rocks in central Jordan have indicated the presence of two groups of minerals; high temperature which is equivalent to clinker cement (Khoury, 1993) and low temperature which is similar to the hydrated cement products (Khoury and Nasser, 1982). The low temperature mineral group has a similar composition to the hydrated cement products and has been precipitated from high alkaline circulating water (pH > 12.5). This naturally occurring alkaline water is analogous to the cement percolating water (Khoury, 1993).
CONCLUSIONS 1. Subgrade or embankments of clay soil as representative soils can be stabilized efficiently by using ash. 2. The representative brown clay soil (i.e. subgrade) under highways can be easily treated and modified to fit the construction purposes. 3. The initial consumption of ash value is 100% to give higher strength and CBR. Also this percent will minimize the swelling potential in soil. 4. Utilization of Al-Karak ash for various engineering purposes will contribute in minimizing the environmental impact of the expected huge output of solid waste resulting from the retort of the bituminous rocks. 5. The strength buildup is related to ash content, ash type and curing period. 6. Shear strength and the CBR increase as curing time increases……. 7. Shear strength the CBR increase as ash percent increases 8. The strength increases as the normal stress increases.
REFERENCES Abu Ajamieh, M. (1980). An Assessment of the Karak oil shale deposit. Internal report, NRA. Al-Rawas, A.A., Hago, A.W. and Al-Sarmi, H. (2005). Effect of lime, cement and sarooj (artificial pozzolan) on the swelling potential of an expansive soil from Oman. Building and Environment, Vol. 40, pp 681-687. ASTM D 2435-96. Standard Test Method for One-Dimensional Consolidation Properties of Soils. ASTM International. ASTM D 4318, 2003. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils (D4318-00), American Society for Testing and Materials. ASTM D 698. Standard Test Method for Laboratory Compaction Characteristics of Soil Using
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