# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 2 Materials and Methods
### Samples Analysis  
Samples were dried at \(\sim\)40 \({}^{\circ}\)C for a period of 48 h and manually disaggregated. Samples were wet sieved (<0.063 mm), and a portion was pulverized in an agate mill, e.g., for chemical and mineralogical analyses. A <0.002 mm fraction was obtained via sedimentation of non-grounded samples. To determine the relative grain size distribution of the <0.063 mm fraction, samples were analyzed using a Sedigraph\({}^{\circledgtr}\) III Plus V1.01 based on the sedimentation theory (Stokes' law) and the absorption of X-radiation (Beer-Lambert law) [6]. pH was determined in a 1:2.5 soil/water solution using a multiparametric meter Hanna, HI 98494 model (Hanna, Woonsocket, RI, USA). Chemical composition of the <0.063 mm fraction was assessed by X-ray Fluorescence (XRF), with a Panalytical Axios PW4400/40, and with Rh radiation. Raw material mineral phases of <0.063 and <0.002 mm fractions were identified using X-ray diffraction (XRD), with a Philips/Panalytical (Malvern, UK) powder diffractometer, model X' Pert Pro MPD. This equipment uses a Cu X-ray tube operated at 50 kV and 30 mA, with data collected from 2 to 70\({}^{\circ}\) 20 with a step size of 0.01\({}^{\circ}\) and a counting interval of 0.02 s. To identify minerals in diffractograms, we used a combination of procedures proposed by [PERSON] and Brindley [15] and Heigh Score Plus v4.9\({}^{\circledgtr}\) with the International Centre for Diffraction Data (ICDD) database. After diffractogram interpretation, the relative abundance of each mineral phase was semi-quantitatively estimated based on the reflection intensities using the peak area method [16]. Raw materials undergo significant transformations during the firing process, resulting in mineralogical and structural changes. Clay minerals are among the first to decompose, while quartz and hematite tend to remain stable throughout the firing range. The most characteristic neoformed mineral phase is mullite, whose formation is directly associated with the breakdown of phyllosilicates and is therefore dependent on the heating rate. The appearance of mullite implies recrystallization, with grain size being primarily influenced by the maximum temperature reached. For this study, the mineralogical changes that occurred at high temperatures were investigated by XRD on representative portions of each specimen fired at different temperatures, in both the outer and inner parts. Cation exchange capacity (CEC) refers to the maximum number of exchangeable cations that a clay mineral can retain on its surface and within its interlayer spaces [17]. This property plays a key role in defining the physicochemical behavior of clays, particularly in ceramic applications where it can influence plasticity, ion diffusion, and sintering characteristics, with CEC values varying widely depending on mineralogy, particle size, crystallinity, and surface area [18]. In ceramic materials, assessing CEC helps to evaluate the interaction potential of clays with additives and to predict behavior during processing. In addition, the concentrations of the main exchangeable cations, including Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), and K\({}^{+}\), and total Fe, were determined. The CEC was determined on the <0.063 mm fraction using a saturating solution of ammonium acetate (1 M, pH 7), followed by displacement of the adsorbed ammonium ions with KCl. The concentration of the displaced cations was then quantified by atomic absorption spectroscopy (AAS) [14]. Expandability tests evaluate the volumetric variation that a raw material undergoes when it absorbs water by capillarity under well-defined conditions of compaction, moisture content, and confinement, being primarily governed by the nature and proportion of clay minerals present. Samples of the < 0.063 mm fraction were analyzed following standard [19], with tests performed on compacted cylindrical specimens (\(\mathcal{O}\) \(\sim\)50 mm; height \(\sim\)20 mm) prepared from homogenized material.