X-Ray Diffraction Assessment of Expanding Minerals in a Semi-Arid Environment

Abstract

In semi-arid areas, light buildings, highways, and pavements are frequently damaged by the subsurface swelling or shrinkage of expansive soils during both wetting and drying cycles. The goal of this research is to explore the X-ray diffraction of natural clay with bentonite additives in order to determine the amount of expanding minerals in the clay based on changes in the diffractometer profile and diffraction intensity. Mineralogical studies are crucial for determining the geotechnical behavior of these soils. Five semi-arid areas were chosen to explore the key minerals that influence geotechnical behavior. The various geological backgrounds were reflected in differing expansivities, and X-ray diffraction revealed considerable mineralogy differences between the five zones under consideration. Non-sharp peaks rose above background intensities in zones containing smectite clay minerals. Significant expanding minerals produced distinct peaks in the clays. Adding 10, 20, 30, and 40% commercial bentonite changed the peak size and area beneath the peak. Overlapping intensities in clay minerals can affect the intensity of peaks in lower 2θ ranges. This was discovered to influence the method of quantification and can be improved by the usage of heating or glycolation processes. The diffraction profile for each examined area is supplied, along with an identification of expansion minerals. The methodology is provided for estimating clay minerals in areas with similar geological origins. Qatif clays were discovered to be the most expansive with estimated expanded mineral concentrations ranging from 23.9 to 34.7%. The remaining four clays had mineral concentrations ranging from 4.4 to 20%. Two proposed semi-quantitative methods are investigated. The peak intensity method produced better results than the area under the peak method.

1. Introduction

The X-ray powder diffraction patterns indicate that clays are made up of crystalline particles. The clay minerals are layer-lattice silicates made up of a combination of two structural units known as tetrahedrons and octahedrons [1]. The tetrahedrons are silicon-oxygen units and octahedrons are aluminum–oxygen–hydroxyl units. Smectite or Palygorskite are chain silicates consisting of an octahedron and two tetrahedron units. The discoveries of Max von Laue, Bragg, and others in the years 1912 to 1915 are considered the backbones for all XRD research conducted thereafter [2]. X-ray diffraction is now a routine test used to identify the presence of different types of clay minerals. The most common smectite is montmorillonite, with a general chemical formula: (Na, Ca)_0.33(Al, Mg)₂Si₄O₁₀(OH)₂(H₂O)_n [3]. The quantitative analysis or estimation of mineral content is still a challenge to geotechnical engineers. The smectite group minerals, which are responsible for expansion in clays, need to be identified and quantified in order to assess the associated expansion risk. Globally, expansive soil is seen as a possible risk to light structures and facilities constructed in semi-arid areas susceptible to seasonal variations in moisture levels. These soils have notable volume fluctuations as the moisture content varies.

For establishing both qualitative and quantitative mineralogical identification, the X-ray diffraction technique is frequently employed. When a target element is inserted into the anode of an evacuated tube, high-energy cathode rays rip the electrons out of it, producing X-radiation. Short wavelengths are generated, and they vary depending on the target substance. The radiation consists of two parts, kα and kβ. To make things simpler, kβ was filtered through screens. According to Dafalla and Ali [4], X-rays with a single wavelength can be used to measure crystal spacing and distances that identify certain minerals. Cu, Co, Fe, and Cr tubes generate the common radiation utilized in X-ray crystallography.

Saudi Arabia is one of the countries that are hardly hit by the expansive soil problems. Al-Qatif area is the most affected due to the highly plastic nature of its clay. Ahmed [5] used X-ray diffraction to study the mineralogical content of many clay samples from Al-Qatif. Along with the attapulgite, he also identified quartz, montmorillonite, palygorskite, and dolomite. The kaolinite and mica minerals found in Tabuk, as well as the kaolinite and illite minerals found in Al-Ghatt, cannot explain the shale’s very expansive character [5]. Dhowian et al. [6] were unable to detect smectite minerals in these regions. Strong intensity peaks from neighboring minerals in diffractometer records can have an impact on smectite minerals. According to Mahmoud [7], the increased expansivity and shrinkage of Al-Qatif clay in comparison to Al-Ghatt clay shale is due to the presence of smectite minerals.

According to previous studies, there have not been many studies relating the mineralogical composition of expansive soils in the area to their geotechnical properties. The goal of this study is to provide a complete evaluation of the country’s expanding soil distribution for a broad audience. Furthermore, it supplies the fundamental engineering and geotechnical qualities needed for planning and construction. The mineralogy of the expansive soils in the tested areas revealed prospective hazard zones in Saudi Arabia. Earlier research on the geotechnical characteristics of some expansive soils in the eastern and northern regions of Saudi Arabia included the studies in the references: [6,8,9,10,11,12,13,14,15,16]. Additional studies in [17,18,19,20,21,22] are all focused on the characterization and description of the expanding clay and its interaction with soil structure. Recent research on the characteristics of unsaturated soil, hydromechanical properties, and hydraulic conductivity was presented by [23,24,25,26,27]. Previous research has not thoroughly explored how these soils’ mineralogical composition relates to their geotechnical qualities. Dafalla [27] studied the impact of bentonite additives on the final clay void ratio for varying vertical effective stresses in kaolinite bentonite mixtures.

It Is believed that Saudi Arabia’s infrastructure, particularly its light structures, sustains damage worth hundreds of millions of US dollars per year [6,8]. Roads, earth embankments, boundary walls, and one-story buildings are examples of light constructions. A vast network of roadways, including expressways, highways, ring roads, intercity arteries, and major and secondary roads, spans the entirety of Saudi Arabia. Secondary or agricultural roads are types of pavements that are highly susceptible to the expansion characteristics of the subsurface soil. This kind of road is frequently affected by the uplift forces due to the expansiveness of the soil. The yearly maintenance budget for a city like Tabuk, in the northwest of Saudi Arabia, approaches hundreds of millions of US dollars.

It was discovered that the peak area intensities correlate in a linear order with the addition of various standard proportions. The minerals found in the clay of Al-khod were assessed based on the intensities [28]. Al-Rawas et al. [28] followed the approach of Dafalla and Ali [4], in which one known addition of internal standard is considered.

Information provided by Moore and Reynolds [29] and Brown and Brindley [30] was consulted to identify clay minerals. At least five clays from various regions of Saudi Arabia were found to contain the clay mineral groups montmorillonite/palygorskite, illite, and kaolinite.

Methods used in investigating the XRD results included the Reference Intensity Ratio (RIR), Visser and Wolff [31], Mineral Intensity Factor (MIF), Moore and Reynolds [29], and full pattern summation methods [32]. These are the most used in the quantification approaches. Rietveld refinement [33] is also a popular method.

The RIR is an instrument-independent constant internal standard method. This is likely to give different outcomes, but when using the same instrument, comparing the intensity of reflections can provide reasonable results. Every mineral is said to have a distinct mineral intensity factor (MIF value), which is derived from the distinct XRD reflection measured under particular experimental parameters. In the full pattern summation method, X-ray data are initially entered into the complete pattern summing program using the full pattern summation approach. Subsequently, conventional clay mineral characteristics are chosen. By altering the percentage of each standard pattern mineral, the program automatically fits the total recorded XRD patterns (the computed patterns) of pure standard clay minerals to measured patterns.

X-ray diffraction analytical techniques and methods for quantitative analysis can be very complex, but they can be simplified for determining the smectite group minerals responsible for expansion by adopting a single machine, a known internal standard, or a reference expansive mineral. This approach might be seen as crude, but it was found sufficient to predict the concentration of highly expansive clay minerals.

Intensities measured as ordinates or areas under the curve were found to serve the purpose of geotechnical applications.

2. Materials and Methods

2.1. Natural Clays

Five expansive soils from Saudi Arabia were considered in this study. These soils represent the common types of clay that show a tendency to swell. They belong to variable geological origins and with different mineralogy. Al-Qatif clay, Hufof clay, Al-Ghatt, Tabuk, and Al-Hudaiba clay were sampled from shallow test pits and transported to Riyadh, King Saud University laboratories. The collected materials were pulverized after air drying and sieved through sieve No. 40 according to ASTM standards. Figure 1 shows areas reporting expansive soils in Saudi Arabia. Al-Qatif and Hofuf clays are of high to medium plasticity derived from calcareous parent sedimentary rocks. Al-Ghatt, Tabuk, and Al-Hudaiba are shale formations that are fine-grained weak rocks composed of sedimentary deposits, mainly clay or silt, oriented and laminated, flaky in nature, and often referred to as silty shale or clayey shale [21].

Table 1. Basic properties of soils selected for this study.

Soil/Property	Specific Gravity, Gs	Liquid Limit, LL (%)	The Plastic Limit, PL (%)	Shrinkage Limit, Sh. L (%)	USGS * Classification
Hafuf (Hf)	2.7	37.0	27.0	23.0	ML-OL
Hudaibah (Hb)	2.7	33.0	23.0	16.0	CL
Ghatt (Gt)	2.9	59.3	33.0	14.0	MH-OH
Qatif (Qf)	2.7	160.0	60.0	15.0	CH-OH
Tabuk (Tk)	2.8	43.0	27.0	21.0	MH-OH

* USGS stands for United States Geological Society.

presents the index properties of the five selected soils.

2.2. Bentonite Clay

The bentonite clay is selected as a highly expansive reference material for use as an internal standard to predict the quantities of expanding minerals in natural clays. HY OCMA bentonite was obtained from a local supplier.

Table 2. Properties of HY OCMA bentonite.

Property	Value
Specific gravity, Gs	2.76
Liquid limit, LL (%)	223
Plastic limit, PL (%)	72
Plasticity Index, PI (%)	151

presents the index properties of the five selected soils.

Table 3. Typical chemical composition of OCMA grade bentonite.

Fe2O3 (%)	K2O (%)	Na2O (%)	Al2O3 (%)	MgO (%)	SiO2 (%)	TiO2 (%)	CaO (%)
2.9	0.1	1.9	17.0	4.6	55.2	<0.1	0.9

Source: Ore-Arabian Gulf Region—OCMA Grade.

presents the chemical composition of the bentonite.

3. Experimental Program

3.1. Characterization and Compaction

Five expansive soils from locations where expansion issues are recognized were subjected to characterization experiments. The tests were conducted in compliance with the American Society of Testing and Materials (ASTM) guidelines. The specific gravity [34] and Atterberg limits [35] were determined along with soil classification. The essential results of characterization are summarized in Table 1.

The five soils underwent grain size distribution testing in the lab, including sieve analysis [36] and hydrometer analysis [37]. To achieve reliable test results, materials that passed through sieve No. 200 were used for the hydrometer analysis tests. Before testing, the materials were soaked in sodium hexametaphosphate, a deflocculating agent. The ASTM D698 [38] standard proctor compaction tests were conducted.

3.2. The XRD Analysis

The equipment used is the Rigaku MiniFlex 600 (Rigaku Corporation, Tokyo, Japan). A 40 kV Cu Kα radiation source was used in the test program to achieve the X-ray diffraction profile at a scanning speed of two degrees per minute for five locations of expansive soils from Saudi Arabia. Testing was performed on powdered samples collected from Al-Qatif, Hafuf, Al-Ghatt, Tabuk, and Hudaiba. A bentonite material rich in montmorillonite is used as an additive to the natural clay in ratios of 10, 20, 30, and 40 percent by weight of dry mass.

Smectite minerals are present in the region of d = 12 Å to 15 Å, where the basal spacing at peaks is found. The d spacing of Na⁺ montmorillonite is 12 Å, whereas that of Ca⁺⁺ montmorillonite is 14 to 15 Å. At d = 14 Å, other minerals such as vermiculite and chlorite may also be present. Illite can be observed at d = 10 Å. Based on these guides and the X-ray diffraction data, expanding minerals of the smectite group can be verified. There are methods to distinguish between overlapping peaks that make use of heating, glycolation, and other procedures. Certain peaks change or vanish when certain treatments are applied. When a calibrated curve is created with varying concentrations, peak intensities can be utilized to estimate the mineral proportions [4].

The equipment used to analyze the samples is set up to carry out a methodical process that includes measuring peak intensities, flattening the graph, eliminating background intensity, and applying Ka2 elimination. One of the most popular open-source software is Profex, which is an efficient program for the refinement of powder X-ray diffraction (XRD) data. Figure 2 shows a typical example of the procedure. After this procedure, peaks are sought. The Bragg equation is used to calculate the d-spacing based on the peaks displayed at 2θ. It is then possible to identify the matching mineral that results from these simulations. To identify one mineral from another, testing involving chemical or physical elements may occasionally be necessary. Procedures like heating and glycolysis could be used. New peaks may form when heated to 300 °C and 600 °C. The crystal structure has six independent cell parameters, including three lattice lengths and three angles. All these parameters are susceptible to change with extended heating; the change in crystal structure and symmetry can lead to disintegration, resulting in the formation of amorphous mineral matter. Certain intense peaks may shift as a result of glycolation. Glycolation and heating were not considered within the scope of this study and were suggested as future work and a further detailed study.

The International Center for Diffraction Data (ICDD) is an organization dedicated to collecting, editing, publishing, and distributing powder diffraction data for the identification of materials. It can be used to confirm minerals in X-ray patterns.

Table 4. Selected clay minerals d-spacings as typically obtained by X-ray diffraction [30].

Mineral	d (Å)
Mica biotite	24
Mica–smectite	22
Mica–smectite	19.4
Smectite–glycerol	17.8
Smectite	16.8–17
Smectite	15–15.5
Chlorite–Smectite	14.5
Chlorite	14–14.3
Smectite, Na	12.4
Palygorskite	10.3–10.5
Smectite, Glycerol	8.9
Chlorite–Smectite	8.0
Sepiolite	7.4–7.6
Nacrite	7.18
kaolinite	7.15
Smectite, Glycerol	5.9
Palygorskite	3.25
Biotite	2.45
Kaolinite	2.29

presents selected clay minerals d-spacings as typically obtained by X-ray diffraction

4. Results and Discussion

4.1. Gradations

The most useful test for characterizing expansion and swelling in clays is the particle size distribution. The expansiveness of the clay cannot be explained by the particle size distribution alone; it must be considered in conjunction with the soil’s mineralogy, confinement, and placement circumstances [20]. It can be seen that for the five tested soils, a well-graded pattern is observed (Figure 3). For materials that exhibit expansion and contraction, the range of pore sizes and particle sizes is important. When there are more voids in the mixture, the clay particles can spread into more areas without increasing the volume overall. In clayey soils where a certain particle size is absent, the gap grade may be applicable. This only happens in the presence of cementation and flocculation.

It is evident from looking at the graphs in Figure 3 that clays with higher proportions of large-sized particles expand less. When the granular particles are small and confinement is created by outside forces, the mineralogical composition of the clay may play a role. Because of their large proportion of coarse particles, the clays from Zulfi and Al-Hafof are examples of low-swelling clays.

4.2. Moisture Density Relationship

There is a significant variance in the maximum dry density and optimal moisture content for the soil samples collected from five different regions of Saudi Arabia, as indicated by the moisture–dry density relationship as shown in

Table 5. Compaction parameters for the five investigated soils.

Soil	Optimum Moisture Content, %	Maximum Dry Density, kN/m3
Hudaiba	14.2	17.6
Hafuf	17.1	17.3
Qatif	38	11.8
Ghatt	25	16
Tabuk	22	15

.

Al-Qatif clay is the clay with the highest degree of flexibility. For this clay, a moisture content of between 35 and 40 is ideal. The optimal moisture content ranged from 15 to 25% for the other four clays tested, and the maximum dry density was found to be between 15 and 18.5 kN/m³. It seems that the shale formation has a low to medium potential for expansion.

4.3. Swelling Properties

The free swell test, which mostly depends on how quickly water percolates into the soil, aims to determine the swell percentage under no surcharge load. When the rate of swelling starts to decline significantly, it is considered to have finished the primary swelling stage. When graphing the proportion of swell against time, it is not an exact point but can be approximated. Figure 4 illustrates that, with the exception of the Tabuk clay, the main swelling takes 800–2000 min. Owing to the coarse texture or perhaps the shale’s composition, Tabuk has a high rate of water penetration. While extremely imprecise, the free swell is a helpful metric for comparing various clays.

The Qatif and Hazm clays showed exceptionally high free swell reported In the range of 10 to 22%. Similarly to how ASTM D4829 [39] classifies soils,

Table 6. Expansion index and expansion potential according to ASTM D4829 (2011) [39].

Expansion Index, EI	Potential Expansion	Soils
0–20	Very Low	-
21–50	Low	Hafuf, Tabuk
51–90	Medium	Ghatt
91–130	High	-
>130	Very High	Qatif

displays the soils’ expansion index (EI). Al Mahbashi and Dafalla [40] presented the swelling pressure and yield stress for 10 soils from Saudi Arabia, including the five soils of this study. The swelling pressure varied from 110 to 450 kPa. The yield stress, defined as the stress at which excessive deformation takes place for the five tested soils, is observed between 50 and 100 kPa.

4.4. Minerals Detected by X-Rays

The angles at which intense reflections occur and their strength are the two common parameters of significance in the diffractometer recordings. The d-spacing, which is presented in the Bragg equation, is computed using these data. With the exception of low angles of diffraction (2 thetas < 10), the d-spacings are dependable to within approximately 2% as long as the peaks are symmetrical and sharp (Brindly and Brown, [30]).

Strong peaks have the potential to affect and cover other nearby mineral peaks, and many clays do not match these requirements. Overlaps also happen. The accuracy of the results could also be impacted by the preparation process. In terms of reflection intensity, samples prepared in an aluminum frame mount may differ slightly from those prepared in a glass smear mount or double-sided sello-tape mount.

Five samples were used in this investigation. Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 illustrate the X-ray diffraction graphs for the chosen soil areas in Saudi Arabia.

It is evident that there are numerous overlaps in the reflections between 2θ = 6 and 2θ = 10. The following minerals can be observed.

• Montmorillonite group (smectite) with various d-spacings (d—14.4 to 15.6 Å).
• Vermiculite (d—14 Å).
• Chlorite (d—14 Å)
• Illite (d—10 Å)
• Muscovite and Biotite (d—10 Å).
• Other minerals.

The presented X-ray diffraction graphs are marked with letters that stand for variable minerals. A stands for quartz, B for kaolinite, C for Na-Al silicate, D for calcium sulfate hydrate, G for illite, H for palygorskite, K for montmorillonite, J for muscovite, E for endellite, and F for dickite.

It can be seen that all five analyzed samples contained kaolinite, quartz, and calcite. Most of the shale clays probably contain minerals like feldspar. The gypsiferous clay found close to the surface in the Al-Ghatt region is explained by the gypsum mineral noted.

4.5. X-Ray Peak Intensity Method

The measured X-ray peak intensities are found to vary with the mineral concentration, as highlighted in the literature survey. In order to make use of this property, it is necessary to keep the experimental setup, sample preparation, energy source, filters, etc. Achieving this in reality is hard, and minor differences can be the result of variable intensity reflections. X-ray short wavelengths vary depending on the target substance. Running a background base scan can be useful to estimate the size of a peak caused at a particular 2θ location. Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 present diffraction patterns for the clay-only material and for the clay with additives of 10%, 20%, 30%, and 40% bentonite. Peak intensities were measured as ordinates compensated for the background intensities.

Table 7. Intensity size above baseline.

Clay Composition	Intensity Above Baseline Qatif
Natural Clay	30
Natural Clay Plus 10% bentonite	40
Natural Clay Plus 20% bentonite	60
Natural Clay Plus 30% bentonite	65
Natural Clay Plus 40% bentonite	65

,

Table 8. Intensity size above baseline (Al-Ghatt).

Clay Composition	Intensity Above Baseline Ghatt
Natural Clay	4
Natural Clay Plus 10% bentonite	5
Natural Clay Plus 20% bentonite	6
Natural Clay Plus 30% bentonite	5
Natural Clay Plus 40% bentonite	15

,

Table 9. Intensity size above baseline (Hafuf).

Clay Composition	Intensity Above Baseline Hafuf
Natural Clay	4
Natural Clay Plus 10% bentonite	5
Natural Clay Plus 20% bentonite	5
Natural Clay Plus 30% bentonite	5
Natural Clay Plus 40% bentonite	10

,

Table 10. Intensity size above baseline (Tabuk).

Clay Composition	Intensity Above Baseline Tabuk
Natural Clay	5
Natural Clay Plus 10% bentonite	5
Natural Clay Plus 20% bentonite	5
Natural Clay Plus 30% bentonite	10
Natural Clay Plus 40% bentonite	20

and

Table 11. Intensity size above baseline (Hudaiba).

Clay Composition	Intensity Above Baseline Hudaiba
Natural Clay	4
Natural Clay Plus 10% bentonite	5
Natural Clay Plus 20% bentonite	7
Natural Clay Plus 30% bentonite	12
Natural Clay Plus 40% bentonite	10

present a tabular list of measured intensity reflections. Plots were established for each clay, and a best linear fit line was drawn as shown in Figure 10. The expansive mineral content in the clay can be obtained by substituting y = 0 in the best-fit equations provided for the tested clays. The best-fit linear lines for the five tested soils are given as:

Qatif clay: y = 0.95 x + 33 R² = 0.88

(1)

Hafuf clay: y = 0.12 x + 3.4 R² = 0.63

(2)

Ghatt clay: y = 0.29 x + 2.8 R² = 0.94

(3)

Tabuk clay: y = 0.35 x + 2 R² = 0.72

(4)

Hudaiba clay: y = 0.19 x + 3.8 R² = 0.80

(5)

Table 12. Area under the peak measured from the baseline reflection.

Bentonite %	Area Method
	Qatif	Ghatt	Hafuf	Tabuk	Hudaiba
40	271.755	344.3	573.18	339	264.33
30	279.24	120.015	215.25	301	325.5
20	197.79	195.65	392.6	141.12	100.8
10	207.5	111.23	178.08	191.805	152.8
0	73.44	23.125	34.435	46	22.05

presents a summary of the measured expansive clay minerals content for the five tested soils. Al-Qatif reported a value of 34.7% expansive minerals within the natural clay. This is found in agreements of the measurements using other methods reported by (Azam). The clay minerals, including illite, smectite, and attapulgite, are found to be responsible for the volume change and the swelling potential (Azam).

Al-Qatif clay is a highly plastic material that is rich in montmorillonite and palygorskite, which are typical swelling minerals. The percentages of montmorillonite and palygorskite in this clay are between 3% and 23% and 5% to 33%, respectively [5]. Hufuf clay reported expanding minerals of 20% as measured using this procedure. This amount is far less than that of Al-Qatif and can be explained by the lower swelling properties despite both soils belonging to the calcareous origin.

The soils of clayey and silty shale origin measured low concentrations of swelling minerals, reported as 5.7%, 9.7%, and 16% for Tabuk, Al-Ghatt, and Alhudaiba clays, respectively. Expanding clay minerals as low as 5% can be a serious concern. These clays reported damage and distress to many structures and pavements within their geographical zones. Other factors contribute to the intensity of damage more than the content of expanding minerals. These include the placement conditions: initial moisture content, dry density, overburden pressure, and confinement. A clay rich in montmorillonite that is kept wet all the time poses less risk than a clay with low-expanding minerals that is subjected to significant drying.

4.6. X-Ray Peak Integrated Area Method

The general assessment approach is based on the fact that peak intensities of a specific mineral, expressed as integrated areas, are proportionate to the concentration of that mineral in the clay. From the presented linear relationship shown in Figure 11, one can infer the natural content of montmorillonite in the clay. Because of unpredictable factors that could significantly affect the resultant intensities, this method should only be used for low accuracy and semi-quantitative analysis. Al-Rawas et al. [41] compared this approach with a constant mineral standard, in which he added clay increments to a standard mineral and observed the impact on the peak size and area. This confirmed the same trend for Al-Khod clays in Oman and presented better accuracy. The investigated soils yielded the following equations for the integrated area method:

Qatif: y = 4.6837x + 112.27,   R² = 0.80

(6)

Ghatt: y = 6.5114x + 28.637,   R² = 0.73

(7)

Hafuf: y = 11.147x + 55.777,   R² = 0.72

(8)

Tabuk: y = 6.952x + 64.746,   R² = 0.85

(9)

Hudaiba: y = 6.5726x + 41.644,   R² = 0.72

(10)

The quantities quoted using the area method were found to give lower estimates. This is likely due to the influence of peaks closer to the peaks of concern. Al-Qatif clay yielded an expansive mineral content of 23.9% compared to 34.7% in the peak intensity method. Other clays were reported in the range of 4.4% to 9.31% compared to a range of 5.7% to 20% in the peak intensity method.

Due to uncontrollable factors and the variable chemistry of smectite minerals, the authors insist on considering it semi-quantitative but state it can serve the purpose of assessing the risk of expansion.

The results using the area under the peak method tend to underestimate the expansive mineral content as reported in Table 12.

Table 13. Percentage of expansive minerals in the Saudi Expansive soils.

Area	% of Expansive Minerals Peak Intensity Method	% of Expansive Minerals Integrated Area Method
Qatif	34.7	23.9
Ghatt	9.7	4.4
Hufuf	16.2	5.0
Tabuk	5.7	9.3
Hudaiba	20.0	6.33

gives the percentage of expansive minerals in the Saudi Expansive soils.

The X-ray diffraction patterns revealed the presence of sodium–aluminum silicate minerals in low- to medium-plasticity shale clays from Tabuk, Hudaiba, and Ghatt. A total of 20% of the Hudaiba clay’s mineral content was expanding. These contain both non-expanding and expanding clay minerals, and they encompass a broad variety of minerals. For Hudaiba, the montmorillonite group mark has been reported. Every sample that was analyzed showed distinct quartz peaks. It is shown that kaolinite is more prevalent in clayey and silty shale.

5. Conclusions

This study presented the geotechnical characterization of clays obtained from the regions known to be expansive in the Kingdom of Saudi Arabia. The X-ray diffraction revealed the mineralogy of five clays from Al-Qatif, Al-Gatt, Al-Hufuf, Tabuk, and Al-Hudaiba.

The knowledge of expansive mineral quantities can raise the alarm of possible risks for construction in the area. The five soils studied were all of concern, and it can be stated that even 5% expanding minerals can pose a serious risk. The Tabuk area, which reported only 6% expansive mineral content, experienced pavement damage and light structural movements resulting in huge annual maintenance costs. This semi-quantitative analysis is proposed for all areas reporting clays in order to exclude the minerals causing volume change.

The highly plastic clay of Al-Qatif, which has the highest content of expansive minerals requires special attention and treatment using chemicals or other additives. Soil replacement or the use of carefully designed rigid foundations may be an alternative.

Though their expanding mineral concentrations are lower, Hafuf clays are nonetheless of the calcareous kind. This clay exhibited high peaks in illite. Quartz is widely used, just like other clays. As seen in Hafuf, there is calcium sulfate hydrate. Illite and kaolinite are the two main minerals found in Hafuf clay, according to Dhowian et al. (1990) [6].

There were non-sharp peaks seen within the zones of smectite clay minerals, above the background intensities. Two methods were utilized to assess the mineral content in these clays: the peak intensity method and the area under the peak of the XRD profile. It was discovered that adding 10, 20, 30, and 40% commercial bentonite altered the peak size and had an impact on the shape of the diffraction profile. For many clay minerals, the intensity of peaks in the lower ranges of 2θ is influenced by overlaps created by other strong intensities. This condition makes clay minerals indistinguishable without treatments using heating or glycolation. Each examined area’s diffraction report is provided, along with an identification of possible expansion minerals.

Al-Qatif clay yielded an expansive mineral content of 23.9% compared to 34.7% in the peak intensity method. Other clays were reported in the range of 4.4% to 9.31% compared to a range of 5.7% to 20% in the peak intensity method.

The results using the area under the peak method tend to underestimate the expansive mineral content.