## 1 Introduction  
Gypseous soils are soils containing significant amounts of calcium sulfates (CaSO\({}_{4}\times 2\)H\({}_{2}\)O). Such soils are widely distributed in many countries, such as the US, UK, India, Africa, and Australia [1, 2]. Gypseous soils are common in highway pavement construction, but they often pose many problems, such as cracking, collapsing, and pavement layer settlement, thereby making pavement and highway construction on such soil a challenge to geotechnical engineers [1, 3, 4]. One of the proven economical ways to improve the engineering properties of soil samples is via chemical soil stabilization [5, 6, 7]. The most common soil stabilization method is the addition of cement or lime to increase the soil's durability and strength [1, 3, 4, 5, 8, 9, 10, 11]. However, adverse effects of ground heave have been reported when using cement and lime to stabilize sulfate-rich soil. Sulfate-induced heave is well known to have serious negative impacts on the general performance of runways, highways, and other structures that are constructed on lime- or cement-stabilized sulfate-rich soil layers [1, 4, 11]. The application of lime or cement on sulfate-rich soils frequently elicits a series of reactions that result in the formation of ettringite [Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\)\(\times\) 26H\({}_{2}\)O], which is an expansive sulfate-mineral-based compound [2, 4, 12]. Ettringite contains 26 water molecules and can resist expansive increases in volume when exposed to hydration, thereby leading to a significant heave effect, swelling, and decline in compressive strength [13]. Many types of equipment and tests have been utilized to determine the negative impact of sulfate on soil stabilization. The most common equipment and tests are unconfined compressive strength tests used to reveal the influence of ettringite on the strength of soil samples containing sulfate, and linear expansion to observe the magnitude of swelling induced by ettringite formation. [PERSON] et al. [4] demonstrated understanding of the mechanisms of sulfate-bearing soil when stabilized with a calcium-based stabilizer. Unconfined compressive strength and linear expansion tests were conducted on gypseous soil containing a high level of calcium sulfate (10%, 15%, and 20%) treated with 7%, 8%, 9%, and 10% calcium-based stabilizer (cement). The outcomes illustrated that the strength reduced by a factor range of 6-47% at 28 days of curing, while the swelling magnitude depended mainly on the sulfate concentration, with the highest rate of swelling observed in the presence of sulfate at 10%. Treatment of sulfate-containing soils (SO4) with calcium-based stabilizers encourages a reaction between the calcium content of the stabilizer (lime or cement) and the sulfate/aluminum content of the soil. With the presence of water, this generates large quantities of ettringite (a crystalline mineral), causing further changes in volume and damage to the structures built on such soil (see Equation (1)) [2,14,15]. \[6\text{Ca}^{2+}+12\text{OH}^{-}+2\text{Al}^{3+}+3\text{SO}_{4}^{2-}+26\text{H }_{2}\text{O}\rightarrow\text{Ca}_{6}\text{Al}_{2}(\text{SO}_{4})_{3}(\text{ OH})_{12}\times 26\text{H}_{2}\text{O} \tag{1}\]  
Ettringite can transform into another expanding mineral: \(\text{thumasite}[\text{Ca}_{6}\text{Si}_{2}(\text{CO}_{3})_{2}(\text{SO}_{4})_{ 2}(\text{OH})_{12}\times 2\text{H}_{2}\text{O}]\). The transformation occurs due to the replacement of aluminum and sulfate with silica and carbonate at temperatures \(<15\)\({}^{\circ}\)C (see Equation (2)) [2,12]. \[ \begin{array}{ll}\text{Ca}_{6}[\text{Al}(\text{OH})_{6}]_{2}\times&(\text{SO }_{4})_{3}26\text{H}_{2}\text{O}+2\text{H}_{2}\text{SiO}_{4}^{2-}+2\text{CO} _{3}^{2-}+\text{O}_{2}\\ &\rightarrow\text{Ca}_{6}[\text{Si}(\text{OH})_{6}]_{2}\times(\text{SO}_{4})_{ 23}\times(\text{CO}_{3})_{2}\times 24\text{H}_{2}\text{O}+2\text{Al}(\text{OH})_{4}^{-}+ \text{SO}_{4}^{2-}+4\text{OH}^{-}+2\text{HO}_{2}\end{array}