# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 2 Materials and Methods
### Samples Analysis
Each specimen was subjected to vertical confinement within a rigid mold and placed in contact with water from the base for a fixed period of 24 h, conducted in triplicate at a controlled room temperature (20 \(\pm\) 2 \({}^{\circ}\)C) and relative humidity (50-60%). Vertical expansion was measured as percentageincrease in specimen height using a micrometer gauge. Reported expandability values corresponded to the mean of three replicates, ensuring statistical reliability. To assess the degree of chemical weathering of the residual clays, two indices were applied: the chemical index of alteration (CIA) [20] and the chemical index of weathering (CIW) [21]. These indices quantify the extent of depletion of mobile cations, primarily Ca\({}^{2+}\), Na\({}^{+}\), and K\({}^{+}\), relative to Al\({}_{2}\)O\({}_{3}\), which is considered immobile during weathering processes. The CIA was calculated using molar proportions of the major oxides according to the formula CIA = (Al\({}_{2}\)O\({}_{3}\)/(Al\({}_{2}\)O\({}_{3}\) + CaO + Na\({}_{2}\)O + K\({}_{2}\)O)) \(\times\) 100, where CaO represents the Ca oxide content associated with silicate minerals, excluding contributions from carbonates and phosphates. In this study, due to the predominantly silicate nature of the samples and low carbonate content, total CaO was used as a first-order approximation. The CIW modifies the CIA by excluding K\({}_{2}\)O, which may behave conservatively in certain environments or become fixed in secondary minerals like illite. The CIW was calculated as CIW = [(Al\({}_{2}\)O\({}_{3}\)/(Al\({}_{2}\)O\({}_{3}\) + CaO + Na\({}_{2}\)O)] \(\times\) 100. Both indices were determined using major oxide data obtained by XRF of the <0.063 mm fraction. Oxide values were converted to molar proportions prior to calculation. To further explore the consistency and geochemical meaning of both indices, the CIA/CIW ratio was considered as an indicator of post-alteration K mobility, with values close to 1 suggesting minimal K addition or remobilization, while lower values may indicate diagenetic processes or incomplete leaching. These indices were interpreted in combination with bulk chemical composition, mineralogical data (XRD), granulometric distribution, and exchangeable cation content to provide a comprehensive assessment of weathering intensity and its implications for ceramic suitability. Ceramic assays aimed to evaluate the behavior of ceramic bodies throughout the various stages of the ceramic processes. Plasticity is a property that defines the rheological behavior in the presence of water, varying with the crystalline nature of the clay minerals [22]. The plasticity index (PI), as well as the plastic (PL) and liquid (LL) limits, were evaluated following the Atterberg limits [23] by the difference between the maximum water content above which the mass loses consistency to be moldable (LL) and the minimum water content above which the mass becomes moldable (PL), PI = LL \(-\) PL [6]. [PERSON] [24] defined a diagram to identify the plastic properties of materials, while [PERSON] [25] identified the main domains in the diagram for clay materials [26] using four classes: no plasticity (0 < LL\(<\) 20), low plasticity (20 < LL \(<\) 35), medium plasticity (35 < LL \(<\) 50), and high plasticity (LL \(>\) 50). Specific surface area (SSA) was assessed using the BET (Brunauer-Emmett-Teller) method with an Gemini II 2370 equipment (Micromerities(r) Instrument Corporation, Atlanta, GA, USA) [27]. Thermal analyses allowed the assessment of raw material behavior under a uniform heating rate, enabling the design and control of the firing process. Operating conditions for thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were identical, as the thermograms were recorded simultaneously by thermal analysis (STA) [28; 29], using a Netzsch STA 449 C thermobalance (NETZSCH-Geratebau GmbH, Selb, Germany) equipped with a TASC 414/3A temperature control system.