# Utilization of Phosphogypsum and Sediment in Subgrade Material for Pavement Construction
## 2 Materials and Methods
### Physico-Chemical Analysis and Thermal Analysis  
2.3.1. pH, Dry Matter and Organic Matter  
The content of dry matter and organic matter was determined by drying a 5 g sample at 105 \({}^{\circ}\)C to a constant mass and then annealing at 550 \({}^{\circ}\)C in order to determine the loss of mass by annealing in accordance with the so-called NEN procedure of the Netherlands Normalization Institute (NNI): NEN 5754 [28]. The pH value was determined according to the standard method SRPS ISO 10390:2007 [29], using a Sentix(r)21 electrode (pH-meter 340i, WTW, Washington, DC, USA). The SRPS ISO 11265:2007 [30] method was used to determine the specific electrical conductivity (TetraCon electrode and Cond 3210 conductometer).  
2.3.2. Thermal Analysis  
The TGA (thermogravimetric analysis) method was used to measure the changes in weight and to obtain phase changes of the material (contaminated sediment and sediment with phosphogypsum-PG) as a function of temperature and time under controlled conditions using Labsys Evo (Setaram, Austin TX, USA). Along with TGA, DTA (differential thermal analysis) was also applied, which allows us better identification of weight loss. The sample (ap-proximately 30 mg) was placed in an alumina crucible and TGA/DTA measurements were performed in the temperature range from 25 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C with a heat change of 10 K/min in an argon atmosphere.  
2.3.3. XRF  
The chemical composition of samples was determined using SPECTRO XEPOS XRF, Kleve, Germany, using TurboQuant (TQ) method (range of elements: Na-U, air-cooled low power X-ray tube with a Pb/Co anode). The samples for XRF analysis were dried, and then 5 g of sample and 1 g of micropowder C (CEREOX BM-0002-1, Fluxana, Bedburg-Hau, Germany) were mixed together and homogenized, and then a pellet with a diameter of 32 mm was formed.  
2.3.4. Leaching Test  
To assess the long-term leachability of heavy metals from the stabilization and solidification (S/S)-treated materials, the ANS method 16.1 was utilized [26]. This method allows us to track how the cumulative fraction of these metals leached from the treated materials changes over time. An applied mathematical diffusion model was based on [PERSON]'s second law to analyze the leaching rate as a function of time, providing insights into how metals migrate from the material. The leachability index (LX), derived from the ANS 16.1 test results [26], is a crucial performance metric used by Environment Canada to evaluate the effectiveness of waste treatment for disposal or utilization [31]. This index measures the relative mobility of contaminants, with values ranging from 5, indicating highly mobile contaminants (De = 10\({}^{5}\) cm\({}^{2}\)/s), to 15, suggesting very low mobility (De = 10\({}^{15}\) cm\({}^{2}\)/s) [32]. To understand the leaching mechanisms controlling the release of metals, the relation ship between the logarithm of the cumulative fraction of metals released (log(Bt)) and the logarithm of time (log(t)) was analyzed. According to [PERSON] and [PERSON] (1992) [33], this relationship helps identify the dominant leaching mechanism. A slope close to 1 (ranging from 0.60 to 1.00) indicates that dissolution is the primary mechanism, where the material dissolves from the surface more quickly than it diffuses through the pore spaces of the soil matrix. Conversely, a slope of approximately 0.5 (ranging from 0.40 to 0.60) suggests that diffusion controls the leaching process, leading to a slower release of heavy metals. A slope less than 0.40 typically points to surface wash-off as the main release mechanism. For our study, samples were meticulously prepared, and leachate was collected and replaced following the procedures outlined in the ANS 16.1 test [26]. This approach provided a comprehensive understanding of how effectively the treated materials contain or release heavy metals over time.