**1. Introduction**

Soil stabilization is a technique used to improve the geotechnical properties of soil, either physically or chemically. Different types of stabilization methods exist, and each process varies with the type of additive used. Additives include lime, cement, bitumen geosynthetics, and some industrial by-products like flyash, slag, coal, and stone dust, chemicals, reagents, and recycled materials like rubber tire chips, waste plastics, and crushed glass that follows recent advanced bio-stabilization techniques like microbialinduced calcite precipitation and enzyme-induced calcite precipitation [1]. Among these stabilization methods, the most commonly adopted process is the addition of calciumbased materials like lime, cement, and flyash [2,3]. However, these additives have their own limitations in terms of carbon emissions [4,5]. Stabilization with lime and cement also causes a problematic expansion in the presence of sulphate [6]. In silica-rich soils, adding lime decreases the soil performance beyond its optimum level because the soil develops silica gel that withholds water and retains the soil plasticity [7]. Expansive semiarid soils have been treated with lime and tested for their lime leachability. At 4% lime content, the lime leachability was minimized with increase in the curing period due to pozzolanic reactions [8]. The laterite soil when stabilized with lime caused a decrease in the unconfined compressive strength (UCS) and California bearing ratio (CBR) with the increase in the delay of compaction in hours [9,10]. High-plastic clays stabilize with lime, and the UCS and the coefficient of permeability increase with the increase in the delay of

**Citation:** Amulya, G.; Moghal, A.A.B.; Almajed, A. A State-of-the-Art Review on Suitability of Granite Dust as a Sustainable Additive for Geotechnical Applications. *Crystals* **2021**, *11*, 1526. https://doi.org/10.3390/cryst11121526

Academic Editors: Antonella Sola, Cesare Signorini, Sumit Chakraborty and Valentina Volpini

Received: 8 November 2021 Accepted: 28 November 2021 Published: 7 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

compaction due to the formation of pozzolanic reactions [11]. The stabilization of sand with cement induces cohesion, but is not effective in developing interfacial friction [12]. Previous studies have shown that brittleness is associated with cement-stabilized sands [13]. Alkaline materials, such as cement, lime, and gypsum, make the treated soil brittle with concentration increase and alter the soil pH [14]. Polypropylene fibers and flyash–cement mixtures are used to improve the unconfined compressive strength of clay. The presence of composite cement turns the treated soil to brittle; however, due to the presence of fibers, the treated soil exhibits a plastic behavior upon load application [15]. In order curb the usage of cement and respective alkaline materials, researchers are working on disposal wastes that act as a sustainable supplementary cementitious material. Recently, biopolymer stabilization of plastic and non-plastic fines is gaining attention owing to its sustainable approach and associated low carbon footprint emissions [16]. Coal gangue is another sustainable generated from the coal production process. Coal gangue utilization can reduce ecological issues, but due care should be given to associated leaching of trace elements [4,5,17]. Enzyme-mediated calcite precipitation is a technique used to improve the compressive strength of sand. The substitution of magnesium sulphate in enzymes yields a better improvement compared to conventional calcite precipitation [18,19]. Many other additives like polypropylene fibers, cement kiln dust, ground-granulated blastfurnace slag, and slag play a major role in stabilizing specific soils [20]. In the previous works, every stabilizer has been limited to some aspects (e.g., carbon emission, production cost, groundwater chemistry, change in soil pH, UV radiation, reactivity, etc.) that drive sustainable stabilizers. This review explores the potential applications of granite dust, a waste by-product, as an efficient stabilizer for improving the geotechnical properties of problematic soils, including Atterberg's limits, compaction properties, unconfined compressive strength, permeability, and California bearing ratio, among others. The mechanisms responsible are described based on the physicochemical characteristics of granite dust. The optimum dosage of granite dust in various soils for different applications is proposed to facilitate practical utilization.

Granite dust is an industrial by-product with an ever-increasing demand in the construction industry. It is deposited in huge amounts at quarry sites and crushing industries [21]. Granite dust is a non-plastic material that exhibits high shear strength with zero carbon emissions. The fine state of stone dust results in a large specific surface area. The physical properties, chemical composition, and mineralogy of stone dust vary with the type of parent rock, but is consistent with the quarry at site [22]. Granite dust is an industrial by-product originating from the primary crushing stage of aggregates [23]. These are fine aggregates produced with particle diameters less than 4 mm [24]. The quality of a stone dust depends on the rock type, origin, and processing method. The global production of stone dusts from different plants is approximately 1.48 billion tons and produced by 1430 companies. On an average, a typical rock produces roughly approximately 400–500,000 tons of aggregate every year [25]. Approximately, 20–25% of this goes as unused material [26]. In India, approximately 200 million tons of quarry by-products is produced annually [27]. Mined boulders and blasted rocks from quarry sites are hauled into a crusher bin and fed to crushers [25]. Crushing can be done in three to four stages (i.e., primary, secondary, tertiary, and quaternary). In the primary and secondary stages, two major crusher units are fed with quarried rock to produce aggregates of different sizes determined by demand [25]. An overview of production of quarry fines is described in the Figure 1. Screening is done in each crushing stage to obtain a usable end product.

Table 1 shows that granite dust is an industrial by-product that has high density and zero carbon emission and is abundant and chemically inert with water. The specific gravity of granite dust is greater than the specific gravity of soils [21], which ranges from 2.6 to 2.8.

**Figure 1.** An overview of stages involved in the production of quarry fines.



## **2. Physical and Chemical Properties of Granite Dust**

*2.1. Morphology and Mineralogy of Granite Dust*

The physical appearance of granite dust changes with topography. The X-ray diffraction and scanning electron microscopy (SEM) of granite dust show mineralogy and morphological variations. Granite is an intrusive igneous rock formed from magma. It is predominantly white, pink, or gray in color. These rocks mainly comprise feldspar, quartz, mica, and amphibole minerals. Dust formed out of granite quarry and aggregate crushing plants varies in the physical appearance of granite dust relevant to location. Table 2 presents petrological details of different granite dusts sourced from different parts of the world.


**Table 2.** Petrological details of granite dust(s) from different regions of the world.

Quartz, granite, limestone, dolomite, and sandstone are the major rock types used by the crushed stone industry. Granite dust is produced from aggregate crushing plants. Most parts of fines are passing the No. 200 sieve and defined as fine aggregate with a particle size less than 4 mm in diameter. The chemical composition of granite dust is an important material characteristic which plays a key role in stabilization. It differs with location, formation and the type of rock available.

### *2.2. Granite Dust and Composition*

Table 3 provides the composition of a granite dust which give a rough estimate of various chemical elements in support of the content provided in Table 2.

**Table 3.** Chemical composition of granite dust (Sourced from [36–39]).


#### **3. Granite Dust as a Sustainable Material**

Sand mining is the process of removing sand from the foreshore. Approximately 47 to 59 billion tons of material is mined globally. Sand utilization in construction leads to unjustifiable sand mining caused by the increment in development activities, which are unacceptable. The available sources of characteristic sand are draining. High-class sand can be moved from a significant distance, causing an economical constraint. Therefore, the structure quality relies on a partial or complete material replacement. Granite dust discarded in a huge amount creates a financial and ecological expense to the industry [40]. Granite dust can avoid detrimental effects on the environment, which are caused by the excessive mining of river sand [41]. Some granite dust applications are in geotechnical aspects like embankment, backfills, road-paving materials, underground cavity fillers, barrier wall materials and sub-base.

#### **4. Effect of Granite Dust Addition on the Geotechnical Properties**

#### *4.1. Atterberg Limits*

Granite dust is a non-plastic material that cannot be influenced by water. Hence, adding granite dust to plastic soils reduces the plasticity index by breaking the particle– water–particle bond and the liquid and plastic limits. Works have been performed on red earth, kaolinite, and sun-dried marine clay, where the Atterberg limits decreased with the dosage increase [21]. The sun-dried marine clay comparatively gave a better response with granite dust addition compared to the other two because the marine clay is a high-plastic soil with a poor gradation curve [21] (Table 4).


**Table 4.** Response of Atterberg limits with an increase in the dosage of granite dust (Modified after [21]).

The high plasticity of soil decreased with the increase in the amount of added granite dust. The liquid limit decreased to 52%, with 60% granite dust addition. Similar works [42,43] have investigated the low-strength/weak soil and concluded that adding granite dust to plastic and high-plastic soils decreases the liquid and plastic limits. Work has also been performed on the granite dust–black cotton soil mixtures and observed the decreasing behavior of Atterberg limits with an increase in granite dust addition [44]. Table 5 lists the summary of the attempts made to improve the Atterberg's limits of different soils with the addition of granite dust.

**Table 5.** Summary of the attempts to improve the Atterberg's limits of different soils with the addition of granite dust.


Decrease in Atterberg's limits of the soil is due to the decrease in finer fraction of the heterogeneous mix. Change in the finer fraction affects the water absorbing capacity of the soil.

#### *4.2. Compaction Attributes*

Maximum dry density (MDD) and optimum moisture content (OMC) are two significant parameters used to assess the field capacity of soil. Adding granite dust to soil increases the MDD and reduces the OMC due to the increase in coarser fraction and the specific gravity of soil–granite dust mixes [43]. Moreover, the increase in the MDD was due to the shift in the gradation curve from a poor to a well-graded mix. In a work on quarry reclamation, granite dust was mixed with silty soil. A decrease in the OMC and an increase in the MDD were observed with the increments in the presence of granite dust (Figure 2) [45]. Generally high-plastic silts show an improved MDD and a decreased OMC with the gradual increase in granite dust substitution [42]. Nwaiwu [43] observed an increase in the MDD of black cotton soil–granite dust mixes and a decrease in the OMC at higher granite dust contents. Irrespective of compaction energy adopted, the addition of granite dust improved OMC and MDD relatively for several soils. Similar observations [22,46] were identified in the case of clays and red earth soils, where an increase in the MDD of mixes were observed with a simultaneous reduction in the OMC values at higher percentages of granite dust dosages was noted. The compaction characteristics of residual soils improved with the addition of granite dust, which consequently led to an increase in the compaction energy [37].

**Figure 2.** Influence of the granite dust dosage on the compaction characteristics S1: 75% soil + 25% granite dust; S2: 50% soil + 50% granite dust; and S3: 25% soil + 75% granite dust (Modified after [45]).

The maximum dry density of mixed soils improves because of the substitution of dust particles in the clay voids and, to some extent, in silts. This will ensure that macro and micro-voids are minimized at higher compactive efforts. The MDD of marine clay increased by approximately 88%, which is higher compared to other soils as seen in Figure 3 (Adding granite dust to soils allows less water to absorb due to the increase in the coarser fraction compared to fines, which is particularly observed in clays, silts and clayey soils). These changes in the soil–granite mix help to improve the engineering properties of the soil.

**Figure 3.** Variation of MDD of different soils with granite dust (Modified after [42,44]).

The change in compaction characteristics is due to the change in particle size distribution of the granite dust mixed soil. Formation of a well graded mix offers greater density and presence of granite dust breaks the water film around the clay particles.
