# Utilization of Phosphogypsum and Sediment in Subgrade Material for Pavement Construction
## 2 Materials and Methods
### Sample Preparation  
In Figure 1, the block diagram illustrates all the analyses conducted throughout the experimental work. First, the physico-mechanical experiments and thermal analysis was performed.  
To assess the contamination of sediment, analysis of the pseudo-total trace metal contents using nitric acid digestion was done, following the US EPA standard method [23]. Accuracy was ensured by analyzing triplicate samples, with relative standard deviations (RSD) consistently below 10%. Metal concentrations were determined using atomic absorption spectroscopy (AAS) with a Perkin Elmer AAnalyst(tm) 700 (PerkinElmer Inc., Shelton, CT, USA), adhering to standard procedures [24].  
In Serbia, sediment quality is classified based on specific criteria, including 10% organic matter and 25% clay. Sediment quality assessments were adjusted based on Official Gazette of the Republic of Serbia, No. 50/2012 [25] by converting values to reflect the actual sediment conditions, considering both organic material and clay content. Metal concentrations were first corrected to align with standard sediment parameters and then classified according to Serbian national evaluation schemes. The overall sediment quality was determined using the most stringent classification parameters [25].  
For the leaching tests, prepared various sample types, denoted by capital letters (F for phosphogypsum, S for sediment) and numbers indicating the percentage weight of each component. The phosphogypsum and sediment content were expressed as a percentages of the total solid weight. The tested samples included F-3 and F-5. Each sample was prepared as monolithic cubes with dimensions of (3 \(\pm\) 0.1) cm per side. These cubes were compacted at an optimal water content, which maximizes the dry density for the given compactive effort, according to ASTM D1557-00 standards [26], achieving a compactive effort of 2700 kN\(\cdot\)m/m\({}^{3}\)[26]. The density of the specimens ranged from 1.38 to 1.54 g/cm\({}^{3}\). After curing at 20 \({}^{\circ}\)C in sealed bags for 28 days, the samples were subjected to various tests to evaluate their performance.  
For physical analysis, three mixtures were analyzed. The first represents the examination of pure sediment without the addition of 0% binder and then with the addition of 3 and 5% phosphogypsum. Their application is reflected in their use as a road embankment and whether they fulfill the properties for the construction of such a layer. The idolized test is shown in the following block diagram (Figure 1).  
Figure 1: Block diagram for sample analysis.  
We then sampled the material, which was dried, and determined its natural humidity. After that, the preparation of the granulometric composition of the material started, which was tested according to the standard SRPS EN ISO 17892-4:2017 [27]. Then, the plasticity of the material was determined, and Afterberg's consistency limits were defined. The next step was to determine the maximum compaction at the optimal moisture content of the material, i.e., Proctor's test. At the defined optimal moisture content of the material, bodies were made to test the load capacity of the material (California load capacity index) and uniaxial compressive strength after 7 and 28 days. The uniaxial compressive strength test was performed on 3 and 5% binder samples. It is impossible to test on pure sediment because it does not have sufficient strength.