Soil, a material in civil engineering, is used as a load-carrying or supporting material, construction material in road bases, embankments, earth dams, clay liners etc., as a result of which there is a constant depletion of this natural resource. Contrarily, due to global warming, soil as a geotechnical material experiences different short- and long-term threats like strength reduction, drying, soil desiccation cracking, shrinkage, microbial oxidation of soil organic matter, fluctuation of the groundwater table, land and exterior erosion, and highly dynamic pore pressure changes (1, 2). Hence, geo-engineering researchers are working hard to overcome this adverse situation. The simultaneously rapid growth of industry produces a large volume of industrial wastes like fly ash (FA), red mud (RM), copper slag (CS), paper pulp, ground granulated blast furnace slag (GGBS), phosphogypsum (PG), rice husk (RH) and so on, which are generally either stored as stockpile or directly deposited into oceans globally. The accumulation of these wastes demands a large land area and causes environmental hazards such as soil and groundwater (GW) contamination and air pollution. Therefore, alternative uses of these wastes for different engineering purposes can solve the aforementioned concerns as well taking another step forward towards sustainable development. The productivity of the food industry is increased by the use of fertiliser to help meet the increasing demand from global population growth. Consequently, there have been hikes in the scale of the fertiliser industry and hence the amount of PG. PG, chemically identified as hydrated calcium sulphate (CaSO₄ 2H₂O), is a by-product waste, 90% of which is produced by the fertiliser industry worldwide from phosphate rock digested by sulphuric acid by wet process (3-5). The amount of PG generated is estimated to be 280 MT per year globally, while India produces 6 million tonnes. However, only 15% of these were utilised due to the presence of phosphorus (P₂O₅), fluoride (F) and other organic substances and the rest was subsequently either landfilled or deposited into the ocean (6-10). Despite the economic benefit of the wet process, it has a demerit in generating as much as 4 to 5 tonnes of PG per tonne production of phosphate fertiliser and other phosphorus compounds (6, 11, 12). PG contains heavy metals and radioactive elements, namely uranium (²³⁸U), radium (²²⁶Ra), and thorium. Of these, ²²⁶Ra produces radon gas which has a short half-life of 3.8 days and its intense radiation capacity causes significant damage to internal organs, so it is classified as TENORM (Technologically Enhanced Naturally Occurring Radioactive Material) (11, 13). The effluent of phosphorus, fluoride, cadmium, total suspended solids, other toxic elements and radionuclides from PG affect human health, for example causing liver dysfunction and lung disease, and also affect both soil and groundwater (14, 15). The presence of phosphoric acid, sulphuric acid and hydrofluoric acid within the porous structure in residual form prevents its application without proper guidelines on a large scale (16, 17). The first utilisation of PG was started in the USA in road construction and since that time it has been used as a binder, filler and other construction material. It has a good filling or binding property as its basic component is CaSO₄ (18), which helps to form ettringite. The general chemical reaction for the production of PG obtained at a temperature of 70‒80 ºC as proved by (19, 20) is shown in Equation [1]:

The strength modification, utilization of optimum percentage of PG for different purposes along with proper justification is summarised in this review paper. The summarised results are presented in tabular and graphical form so as to help the new researchers and user to use Phoshogypsum along with other additives as alternative construction materials. This paper also emphasises the use of optimum percentage of PG for different geotechnical fields to start with so as to save time for the experimental work.

The application of PG can be made easier in a large part of the civil engineering field by analysing its characteristics. Therefore its physical, chemical and other characteristics are compiled for further use. The properties of PG are not only limited to the phosphorus ore but also determined by the method adopted for the extraction of phosphoric acid, the industrial operation efficiency, disposal method, age, location, the height of the landfill storage and the number of water molecules in PG crystals, namely calcium sulphate generated either in anhydrite (CaSO₄), di-hydrate (CaSO₄·2H₂O) or hemi-hydrate (CaSO₄·1/2H₂O) (13, 21‒23). PG feels like a slightly moist powdery material, having silt size particles with very low or no plasticity as an amalgam of gypsum (>90%) and fluorosilicate (24, 25). The existence of orthophosphoric acid, in which the heavy metals and fluorine are in a soluble state, promotes leaching (26). Leaching of these hazardous elements from a PG stack is one of the prime concerns for groundwater contamination (27‒29). Ultimately these dissolved elements are transferred to living bodies via the groundwater (30). The orthophosphoric acid and calcium sulphate (CaSO₄) cause the pH value to lie between 2 and 3, which indicates the acidic behaviour of PG (24, 26). The lower pH value of PG makes it resistive in reaction with soil grains and Ca ions, whereas the addition of cement decreases the acidity, hence encapsulating the impurities present in the PG (24). The pH value determines the solubility of the material and means that it is highly soluble in salt water (31). Distinguishing between gypsum and phosphogypsum is done by microscopic analysis, as both have the same chemical formula. Scanning electron microscopy (SEM) reveals that PG has a more crystalline and distinct structure than gypsum, the crystals of which are hexagonal and rhombic in shape (32). The performance of radioactive elements such as ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, ²³⁸U and ²³⁴U depends upon the parent phosphate rocks (33, 34). Different researchers have referred to different radioactive nuclides as most hazardous; for example, Rutherford (1994) reported ²²⁶Ra, whereas Pérez-López (2007) indicated the ²³⁴U of parent rock is associated with non-mobile fraction whereas PG contains high Uranium concentration bounded to the bio-available fraction (34, 35). The application of PG was banned in the USA in 1990 and the EU discontinued its use in 1992 (36). Due to all these problems, the Environmental Protection Agency (EPA) restricted the use of PG to up to 370 Bq per kg concentration of ²²⁶Ra in agricultural soil and the European Atomic Energy Community (EURATOM) set the limit of 500 Bq/kg (37-40). Despite these contaminates, it cannot be classified as toxic waste because it is not corrosive and the average total elemental concentrations of elements categorised as toxic (Ba, As, Cr, Cd, Hg, Pb, Se and Ag) by the Environmental Protection Agency (EPA) are lower than the allowable toxic elemental criteria for toxic hazardous waste (35). The main constituents of PG and the impurities need to be known for application and in view of environmental concerns. Therefore, the chemical compositions obtained through X-ray fluorescence (XRF) testing are summarised in Table 1, based on different sources. The experimental results proved that the electrical conductivity of PG is greater than soil, with values from 20 to 5 dSm⁻¹ (very saline) and <5 dSm⁻¹ (slightly saline) respectively, which confirms rich nutrients (41). The surface area obtained from Moroccan PG is 0.64 m²/g (42). The mineralogical and morphological characteristics can be obtained by X-ray diffraction and scanning electron microscopy (SEM) spectrophotometry tests. The crystalline morphology characteristics found in SEM analysis of unprocessed PG appear as platelets oriented in a parallelepiped shape, with lengths ranging from a few to 400 micrometres (μm) and a thickness of fewer than 10 μm. The higher value of the aspect ratio becomes a favourable criterion for quick crushing under different stresses (42).

In the presence of a hydraulic binder, the chemical composition aids in understanding the response between PG and other constituents. It can be predicted from the table that the abundant calcium ion in PG can form a gel of calcium aluminate hydrate or calcium-silicate when mixed with other material that is rich in silica and alumina, to reduce the water sensitivity and subsequently increase the mechanical properties. The chemical composition of PG includes radioactive elements, traces of heavy metals such as As, Cr, Cd, Hg, Pb, fluoride, zinc, antimony, and copper (49), along with some other chemical elements that can be used for the production of bricks, blocks, tiles, and artificial stone.

Landfilling of PG waste releases dust as well as leachate of hazardous elements, fluoride and heavy metals, which causes air, soil and groundwater pollution (50, 51). The presence of natural radioactive nuclides and impurities, namely phosphate and fluoride, reduces the strength and extends the setting time, causing a high level of pentafluoroaluminate (ALF^-5) to disturb the gypsum development, thus limiting its use as a construction material to as low as 15% globally (52, 53, 12). Researchers all over the world have been treating the waste to make it safe for disposal and increase the potential for its massive application without environmental concerns. Manjit et al. (1993) washed PG using aqueous hydroxide solution (5‒20%) then dried it at 42 ºC to minimise the amount of P₂O₅, F, and all the contaminants were abridged (54). Aly and Mohammed (1999) recovered fluorine and lanthanides by using nitric acid (HNO₃) (55). A wet sieving and hydro-cyclone method followed by heat drying was adopted by Manjit (1996), who identified the reduction of impurities and increment of pH and SO₃ values (56). The extraction of radionuclides of more than 60% and rare earth elements by organic extractors in kerosene diluent with a liquid to solid ratio of 1:1 at 55 ºC was examined by El-Didamony et al. (57). The exchange or membrane technique was adopted, combined with recrystallisation by Koopman and Witkamp, who showed that more than 90% of the heavy metals and lanthanides were removed (58). Mashifana et al. (2018) used 0.5M citric acid with a 40% solid load concentration based on the studies of (59‒62) and observed remarkable results in removing radionuclides, P₂O₅ and F by up to 92.8%, 34.7% and 18.87% respectively (22, 63). Al-Jabbari et al. washed PG with water and passed it through a 100 mm sieve, then calcined it by hydrated lime at various temperatures, and reported this as the lowest cost method of treatment. The modified PG was free of pollutants, with both setting and strength properties improved (64).

Chemical tests have confirmed that PG is highly acidic and contains radionuclide elements. However, the Atomic Energy Regulation Board of India in 2009 decreed in its directive (number 01/09) that the sale of PG for construction purposes does not require its approval, provided the activity concentration of ²²⁶Ra is less than 1.0 Bq/g (65). Campos et al. examined the radon exhalation of PG-manufactured bricks and plates and found it to be the same as ordinary construction materials, and therefore suggested a safe practice without any prior handling (66).

Mineralogical changes of raw materials after treatment help to understand the structure and strength of the new composite. These changes depend decisively upon the raw materials, their proportions, curing period and their ambience. The mineralogy of PG is crystalline in form and hence has strength potentiality. When PG is mixed with cementitious materials, the formation of portlandite and ettringite indicates good strength (78). When clay-rich soil is modified with either treated or raw PG together with other additives, the new form of minerals is different in both cases. Among the three strength compounds of C-S-H, C-A-H and ettringite, the formation intensity of ettringite was greater, which contributed to the early strength development with the addition of PG. Meanwhile, the C-A-H compound was reduced, whereas the C-S-H gel remained unchanged. The ettringite formed bridged the solid particles due to its needle-like structure, and hence improved the strength as well as the density by compressing the pore space of the soil with high water content (86, 91, 103). Mashifana et al. (2019) reported the XRD analysis of raw PG, RPG20 (lime, FA, 10% BOF and 20% raw PG), treated PG and TPG20 (lime, FA, 10% BOF and 20% treated PG), as shown in Figure 4 to Figure 6 respectively. In both of these composites, kieserite is the common predominant mineral that has a significant role in high strength (63). Mashifana et al. (2018) tested expansive soil, rich in montmorillonite 8(KAl₄ (SiAl)O₁₀ (OH)₄), bentonite (Ca_0.06 Na_0.21 K_0.27) (Al_11.64), kaolinite (Al₂ (Si₂ O₅) (OD)₄), and quartz (SiO₂). When this soil was modified with PG, new hydration products formed; namely calcium magnesium silicide (CaMgSi), sillimanite (Al₂ (SiO₄)O), kaolinite (Al₂ (Si₂O₅) (OD)₄), feldspar (Al₂ Si₂O₈), and trikalsilite ((KNa)AlSiO₄). Treated PG helped to dissolve siliceous and aluminous compounds from the soil lattice. These ions reacted with the calcium ions in the pore water to form calcium silicate hydrate and calcium aluminate hydrate, which coated the soil particles. Subsequently, crystallisation bonded them, hence the strength was significantly improved (45, 104).

The reuse of locally available waste materials on a large scale in various practical fields is a sign of progress towards achieving sustainable development. The fertiliser industry by-product waste phosphogypsum is acidic and contains trace materials and radioactive nuclides like ²²⁶Ra. Therefore, it needs to be either purified or used in different engineering fields with proper guidelines to avoid effects on human health and the environment. Many research studies have been conducted on the use of PG in different fields of civil engineering, like construction material, road base material, and binder and soil stabilisation. All these studies showed that the success of the application of PG largely depends upon the grain sizes, relative consistency, compressibility, and the chemical and mineralogical composition of the material to be treated. The leachate problem and radioactivity problems can be solved to a reasonable extent by treating them with a mild acid. But it can also be concluded that it cannot be used alone due to its acidic behaviour, so it needs to be stabilised with other materials like FA, GGBS, rubber tyre and lime, cement etc. There is another by-product waste, RM obtained from the aluminium industry as an alkaline bauxite residue, and these can neutralise each other because of their opposite characteristics. Future investigations could usefully be attempted by researchers to stabilise the PG with RM and make the mixture suitable for different geotechnical purposes like backfilling of mechanically stabilised earth (MSE) walls, paver blocks, MSE walls, clay liners etc. However, the leaching property of the combined material should still be tested before using it as a construction material due to continuing environmental concerns.