Effect of Adding Cement Dust Waste on the Geotechnical Properties Behavior of Selected Gypseous Soil in Al-Najaf City

Abstract


Introduction
Gypseous soil is defined as the soil containing gypsum, so it is usually hard especially, when it is unsaturated due to the interconnection of soil particles with gypsum.Whenever the soil is soaked, the dissolving of the cement gypsum between the soil particles causes a collapse in soil strength and a dramatic increase in compressibility (Nashat, 1990).This type of soil is mainly, found in many countries such as Spain, Russia, Argentina, and Australia; while, Iraq accounts for about 9% of the world's gypsum soil.Generally, gypsum soils are found in arid and semi-arid regions of the world (Abbas and Al-Luhaibi, 2020).Several gypseous soils classifications have been applied in French, Australian, American, Iraqi and Russian studies.Iraqi studies use the classification of Al-Baraznji (1973).This study depends on the concentration of gypsum in the soil (Table 1).The classification suggested by (Nashat, 1990) (Table 2), which is used for construction purposes by the National Center for Building Laboratories (NCBL), takes into account the gypsum content in the soil.Gypseous soils have multiple and complex problems.The presence of gypsum, due to its harmful behavior, is one of the most difficult engineering problems, especially when it is accompanied by a change in moisture content in the environment (Nashat, 1990).Another problem is gypsum seeping from the soil, which alters the soil's physical, chemical, and mechanical properties, resulting in significant settlements (Mikheev et al., 1973).
Table 1.Gypsum soil classification in Iraq only (Barazanji, 1973) Gypsum content percent Category 0 -0.30Non gypsiferous 0.30 -3.0 Very low gypsiferous 3.0 -10.0Low gypsiferous 10.0 -25.0 Medium gypsiferous 25.0 -50.0 Highly gypsiferous More than 50.0Extremely gypsiferous  (Nashat, 1990) Gypsum content percent Category 0.0 -10.0Low gypsiferous 10.0 -25.0 Medium gypsiferous 25.0 -50.0 Highly gypsiferous >50.0%Gypcrete Many methods can be utilized to improve the behavior of gypseous soil; one of them is to introduce other materials to the gypseous soil to improve soil properties.In order to improve or stabilize gypseous soil, many additives, including chemical additives, can be utilized to treat soil problems, when stabilizing substances such as acrylate liquid, cement, lime, silicon oil, sodium silicate, and sulfur are used.These materials have good results in improving the behavior of problem soil properties, but they are not environmentally friendly.They could be toxic, pollute groundwater and soil, and change soil pH (Chang and Cho, 2016;Ayeldeen et al., 2017).Several attempts have been mentioned recently, to address the soil problem using biological or excretory methods.Biological additives are environmentally, favorable because of their reduced releases and strong potential to prevent soil denudation.Cement, the widely, utilized binder in building, is known to contribute significantly, to emissions of CO2.Biopolymers are employed as bindings or mixed additives to strengthen and improve soils, which have shown considered an extension in inter particle (Chang and Cho, 2016).Biopolymers are organic polymers that are synthesized by living organisms (Fuwei et al., 2009).Many researchers have examined Xanthan gum as a biopolymer to lower the hydraulic conductivity of clay soils (Bouazza et al., 2009).
Many previous studies had been carried out about this object, such as; Hussain (2005) described gypcrete as soil containing mainly secondary gypsum at or near the surface of the earth.Secondary gypsum is formed either by the upward movement of water rich in sulfate and calcium by capillary action, and evaporation at the surface, or by direct precipitation from groundwater or surface water.Fattah et al. (2013) used grouting in the treatment of collapse of gypseous soils.The study used four different varieties of gypseous soils, each with its own set of characteristics and gypsum content.The testing was done on reshaped samples to see how compressible gypseous soil in different situations.
The acrylate liquid reduced the compressibility of the gypseous soil by more than 60%-70% in the treated samples.This is due to an acrylate liquid film coating the gypsum particles, effectively isolating them from the effects of water.Hayal et al. (2020) investigated the application of nanomaterials in the treatment of gypsum soil collapse, and the study found a considerable change in the geotechnical parameters of the soil sample.When nano-silica is added to the soil, the breakdown potential (collapse potential (CP) decreases as the amount of nano-silica increases up to 1% of the total added nanomaterials.
The main goal of the research is to study the effect of adding cement dust waste on the behavior of some geotechnical properties of gypseous soil.Also, the important aims are to preserve the soil and environment of the city of Al-Najaf from pollution caused by this harmful waste; and to contribute to the disposal of these wastes that cause damage and soil deterioration through their use in improving the specifications of gypseous soil and transforming it into an environment friendly.

Geological Setting
The stratigraphic setting can be demonstrated from the older to the younger (Fig. 2): • Dammam Formation (Middle-Late Eocene) is recognized by neritic shoal limestone; either recrystallized or dolomitized.Nummulite is in the lower part, and miliolids in the upper part (Bellen et al.,1959).• Euphrates Formation (Early Miocene) consists of five unites comprising cream to orange dolomite, and white, soft , oolitic, macrofosiliferous and chalky limestone (Jassim and Goff, 2006) .• Nfayil Formation (Middle Miocene) consists of white marl, red clay, chalky fossiliferous limestone, green sandy limestone, and brownish gray calcareous sandstone. .• Injana Formation (Late Miocene) was known as the Upper Faris Formation.It comprises fining upward cyclic sediments of fine grained clasts in a fluvial to lacustrine environment.• Dibdibba Formation (Pliocene-Plistocene) consists of pebbly sandstone and gravel of granitic source.White quartz cemented into calcite and marl are considered the main minerals.It was deposited in fluviatile to deltaic environment .• Zahra Formation (Pliocene-Plistocene) is composed of reddish brown claystone, yellowish-whitish grey sandstone, and brownish grey limestone.Sometimes it intersects with Dibdibba Formation (Jassim and Goff, 2006).• Quaternary Deposits consist of many of the geological units with different origins, such as fluvial sediments, flood plain deposits, sediment filling valleys and depressions, slope deposits, the gypsum crust, and wind sand deposits.

Field Work
The study area is located in the center of Al-Najaf city, Iraq, southwest of Baghdad.It was represented by two sites; the first (S1) is near Al-Salam residential complex on the Hawly Road of the city, specified by coordinates 44o 21' 42.17" E and 32o 1' 0.904" N. The second site (S2), with the coordinates 44o 17' 48.587" E and 32o 4' 3.788" N is in Al-Adala district, near a modern residential building as explained in Fig. 1.
The soil employed in this study is a usual chaotic gypsum soil.All samples represent disturbed soil samples collected from a depth of 1 to 2 meters below the ground surface from the two selected sites in Al-Najaf city with their coordinates as mentioned above (Fig. 1).
The sample of the cement dust waste was brought from Al-Kufa Cement Plant, during visiting this industrial factory.Cement dust waste is the volatile dust particles that produce from the combustion of cement raw materials (soil and limestone) in rotary kilns.It is precipitated by electrostatic precipitators, instead of releasing them to the air from the chimneys of furnaces and polluting the working environment and surrounding areas.The chemical analysis of cement dust waste, which used in this study, was taken from the quality control department / laboratories in Al-Kufa Cement Plant (Table 3).

Laboratory Work
The National Center for Building Labs and Research/ Babylon Branch (NCCLR) in the Hilla city, conducted laboratory tests, in addition to the laboratories of the Department of Applied Earth Sciences, College of Science, University of Babylon.The laboratory tests with Standard Specifications are listed in Table 4.The first site (S1) is recognized by high gypsum content of 32%.The gypsum content at the second site (S2) is medium, with 21%.According to the Uniform Soil Classification System (USCS), these soils are classified as poorly graded sand (SP).The physical and chemical parameters of gypsum soil are shown in Table 5 along with the test standard.

Cement dust waste -processed soil samples
To prepare soil samples for analysis, the oven-dried soil was sieved at 4.75 mm (ASTM -D422-2004).Cement dust waste-treated soil samples were obtained with cement dust contents of 5%, 10%, and 15% of the soil mass.Cement dust waste was mixed with dry soil.The samples were made by mixing a mixture of cement dust with the soil by adding the three previously mentioned ratios and preparing them to test Atterberg limits, specific gravity, grain size distribution and compaction tests.While the cement dust waste -soil samples mixture equipped with molds and percentage 15% from cement dust to suit direct shear test.Specimens were removed and held at room temperature to replicate the dry condition, and 50% of the harsh samples were placed in distilled water at room temperature before being subjected to direct shear examinations (saturated case).

Atterberg limits tests
Atterberg limits (liquid limit, plastic limit, and plasticity index) were determined according to the methods approved by the American Society for Testing and Materials (ASTM -D4318 -06).The tests for the Atterberg limits were carried out, by taking a sample of the transient soil from sieve No.40 (475μ), and treated with water.It is placed in the Casacrandi device, several attempts may be repeated, and the moisture content is calculated for several attempts.The number of blows is dropped 25 blows on the flow curve to extract the value of the liquid limit of the soil.Fig. 3 and Table 3 show results of the liquid limit for untreated soils 1 and 2. The plastic limit is calculated by rotating a thread of clay with a diameter of 3.25 mm until hair cracks appear and the water content is calculated for them.As for the plasticity index, it is calculated from the difference between the liquid limit and the plastic limit from the following equation.

Specific gravity
ASTM D-854 2010 standard is used to estimate the specific gravity of the soil.To keep the gypsum from dissolving, kerosene was used instead of water.The specific gravity of untreated gypseous soil is 2.38 and 2.45, respectively, according to the results.

Analysis of grain size
American Society for Tests and Materials (ASTM -D422 -04) method was used to conduct this investigation for natural soil.In sieve No. 200, about twenty grams of soil is washed, and the rest is left to dry.A variety of sieves are used to determine the amount of sand and gravel, with the finer at the bottom (Hussein et al., 2021).The particle size distribution of the untreated samples in the research area is demonstrated in Fig. 4.

Compaction test
American Society for Tests and Materials method was used to determine the maximum dry density, optimum moisture content, and their percentages in the current study.ASTM-D698-78 utilized modified compaction energy to obtain the compaction curve for untreated soils 1 and 2 (Fig. 5).

Direct shear tests
To evaluate the strength properties of collected samples, many direct shear experiments were done.The tests were carried out in accordance with ASTM D-3080-2011 for both dry and moisture specimens, whereas the sample was 6 x 6 x 3 cm in size.Both soils 1 and 2 were subjected to a direct shear test, with field densities of 14.45 kN/m 3 and 13.83 kN/m 3 , as well as field moisture content of 8% and 7%, respectively.Table 3 and Figs

Atterberg Limits of Cement Dust -Treated Soils
The results of liquid limit tests for soil 1 and soil 2 show that increasing the cement dust waste additive causes a relatively gradual increase, with the explanation being an increase in fine particles (Figs.8,9 and Table 6).

Specific Gravity of Cement Dust Waste -Treated Soils
The specific gravity (Gs) value of soil 1 and soil 2 decreases as the amount of cement dust added increases.This can be attributed to the small Gs values of cement dust compared to soil.As a result, the more cement dust is added, the lower specific gravity value becomes (Fig. 10 and Table 6).6).

Compaction Test of Cement Dust -Treated Soils
As the cement dust mixture is increased, the maximum dry unit weight decreases, while the optimum water content increases (Figs. 13,14,and Table 6).Similar results have been obtained by other researchers such as Jassam and Younes (2021).The low density of the cement dust additive relative to the density of the soil utilized, is the reason for this result.

Direct Shear Test of Cement DustTreated
Direct shear tests of soils 1 and 2 were carried out using cement dust waste additive at a rate of 15% because it is effective.The tests were taken with the field density of the soil and the natural moisture , in a curing time of one day and seven days.

Dry soil state
The direct shear test was performed on soil samples from S1 and S2 that were dry, and had the same field density of natural moisture content.The treated soil with cement dust waste has a percentage of 15% after curing 1 and 7 days of sample preparation.The results of the C and  values for the treated soil are shown in Table 6 and Figs . 15, 16, 17, and 18. Due to the effect of cement dust waste particles that create bonding bonds with gypsum soil grains, the cohesion value (C) increased as the percentage of additives increased.The slight decrease in the internal friction angle (Ø) can be attributed to the internal friction angle of cement dust, which was less than the internal friction angle of gypsum soil.

Soaked soil state
For the soaked condition, Table 6 as well as Figs.17 and 18 for soil 1 and soil 2 show the values of shear strength coefficients after curing for 1 and 7 days.The cohesion values and internal friction angles can be increased slightly, with the increase of the additive of the mixture due to the good properties of cement dust waste in the case of soaking.

Conclusions
The following conclusions were made as a consequence of the testing approach utilized in this investigation.An increasing, percentage of cement dust waste additive causes an increase in the liquid limit value for both soils.Also, increase in the amount of mixture additive causes a decrease in the specific gravity of both soils.With an increasing mixture additive, an increase in the diameter of the improved gypseous soil granules is noticed through sieve analysis.From the results it is concluded that the maximum dry unit weight decreases and the optimum moisture content increases with an increase of the percentage of cement dust waste additives.The cement dust waste additive influences the shear strength coefficients of gypseous soils by raising a cohesion, and a minor decrease in the internal friction angle for treated soils, especially, at 15% ,and for 7 days curing time.Cement dust is very inexpensive and available as manufacturing waste.As a result, its usage in improving gypseous soil qualities can result in economic benefits through waste recycling, moreover preserving the nature and the environment.

Fig. 1 .
Fig.1.Location map represents the study area

Fig. 5 .
Fig. 5. Relationship between maximum dry unit weight and optimum moisture content for soil 1 and 2.
. 6 and 7 illustrate the results of a direct shear test on untreated soil in dry and moist circumstances, which indicate poor values for both cohesion and internal friction angle.It is concluded from this that additives are important for improving the characteristics of gypseous soils.

Fig. 10 .
Fig.10.Specific gravity changes with cement dust waste mixture for soil 1 and soil 2

Fig. 13 .Fig. 14 .
Fig. 13.Effect of cement dust waste on dry unit weight and optimum moisture content of soil 1

Fig. 15 .Fig. 16 .
Fig.15.Results of direct shear tests on dry soil treated with 15% cement dust waste for 1 and 7 days for soil 1

Fig. 17 .Fig. 18 .
Fig.17.Results of direct shear tests on saturated soil treated with 15% cement dust waste for 1 and 7 days for soil 1

Table 2 .
Classification of gypsum soils for construction purposes in Iraq only

Table 3 .
Chemical analysis of the cement dust waste used in the study

Table 4 .
Laboratory examines performed on soil samples

Table 5 .
Physical and Chemical properties of Untreated soils

Table 6 .
Geotechnical properties of treated soils in cement dus