###### Abstract  
The study involved the utilization of an industrial waste product, Phosphogypsum (PG) as an additive to lime for the stabilization of soil. Three lime dosages, viz. initial consumption of lime (ICL), optimum lime content (OLC) and less than ICL (LICL) were adopted for stabilizing the soil. The study investigated the stress-strain characteristics of soil composites stabilized with these three lime contents modified with optimum dosages of PG. Mineralogical studies were performed on the spent samples used for a series of determinations of unconfined compression strength tests with various combinations of lime and optimum PG content. The addition of an optimum dosage of PG resulted in an early strength gain of 8.8%, 14.1% and 13.9% and a delayed strength gain of 9.9%, 19% and 19.7% for 3%, 5.5% and 7% for the lime-stabilized soil, respectively. It was found that the addition of PG to the lime resulted in enhanced stiffness, residual strength and reduced brittleness due to the PG amendment of the stabilization reactions. However, in terms of the overall improvement of soil properties, the most favorable benefit was obtained by optimal PG modification of ICL rather than OLC. Microanalysis of the X-ray diffraction scatter also supported the results revealed through stress-strain characteristics. ICL with its optimal PG dosage showed a better progression of pozzolanic reactions when compared to the other two in terms of reduction of peaks of soil minerals and increase in peaks of CSH. little stabilization; phosphogypsum; expansive soil; initial consumption of lime; optimum lime content +
Footnote †: journal: Applied Sciences  
**Citation:** [PERSON]. worldwide per annum [7]. India generates around 12 million tons per annum with re-utilization rates of 40-45% in building materials [8]. PG has been extensively researched for use in building materials and soil modification. Focus on the utilization of PG in soil engineering applications has increased in the recent past [9; 10; 11; 12; 13; 14]. It has also been used as a binary additive to cement as well as for lime-stabilization of soils [6; 15; 16; 17; 18; 19; 20]. In all the investigations adopting the combinations of lime and PG, very few researchers have focused on understanding the effect of the addition of PG to lime in lime-PG systems. In the case of the combination of lime-PG in soil stabilization, such investigations are very scarce. Most previous studies explored the improvement in the strength and bearing of the stabilized soil due to the addition of PG. Only a few investigators have made attempts at analyzing the effects of lime-PG combinations on the soil by performing some detailed studies at the micro level. For instance, [PERSON] et. al. [21] investigated the performance of a fly ash-time-PG binder on stabilized soil, while [PERSON] et al. [22], evaluated the activation of fly ash-time systems using PG for soil stabilization. On the other hand, [PERSON] and [PERSON] [23] focused on the plasticity and swell-shrink characteristics of lime-PG stabilized soil. Both [PERSON] et al. [21] and [PERSON] et al. [22] concluded that the addition of PG to lime/lime-fly ash systems accelerated the pozzolanic reactions. [PERSON] et al. [22] also stated that the development of late strength depended on the ratio of the PG to the lime, resulting in a decrease in the late strength with an increase in the ratio. [PERSON] and [PERSON] [23] concluded that the addition of PG to lime improved its plasticity and swell-shrink behavior. They also found that there is an increase in the utilization of PG with an increase in lime content. At the microstructural level, they found that the addition of PG to lime resulted in a dense compact microstructure. [PERSON] and [PERSON] [24] studied the microstructural development of fly ash modified with lime and gypsum. They concluded that the addition of small quantities of gypsum to lime was sufficient for the enhancement of strength gain at the early curing stage and that the addition of gypsum reduced strength loss due to soaking.