# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion  
The granulometric distribution of ceramic materials is essential to assess their technological behavior during shaping, drying, and firing processes. A balance between clay (<0.002 mm), silt (0.002-0.063 mm), and coarser fractions (>0.063 mm) is essential to ensure a desirable plasticity, mechanical strength, and dimensional stability. The studied samples (Figure 2) revealed that their clay fraction varied from 28.6% (E1) to 57.7% (B1). According to [PERSON] et al. [34], clayey materials with >40% clay-sized particles are generally considered highly plastic, being suitable for forming processes like extrusion and pressing [35]. Samples A1-2, B1-2, and C1-2 were classified as highly plastic, favorable for shaping but may require careful drying to avoid cracking due to high shrinkage [2]. Samples D2 and E1 presented lower clay content (<30%), with higher sand fraction content (>0.063 mm:58.0% and 59.0%, respectively). These samples are likely to exhibit lower plasticity and greater dimensional stability and may require blending with more plastic clays to enhance workability [36]. Their coarser fraction content also suggests good drying behavior and reduced risk of deformation during firing, making them potentially suitable for grog or filler materials in ceramic bodies. The silt fraction contributes to the packing density of the ceramic body, with samples showing a 12-23% content, contributing to a balanced particle size distribution that can promote good green strength and sintering behavior [37]. The Winkler diagram (Figure 3), based on the relative proportions of <0.002, 0.002-0.020, and >0.020 mm fractions, further refines the suitability of the clays. Samples A1-2, C1-2, and D1 exhibited a well-balanced distribution, with 48-51% of the clay fraction and >33% sand-sized material classified within [PERSON]'s zone 3, well suited for extruded red ceramic products, such as roofing tiles, with adequate plasticity and dimensional stability [38]. Samples B1-2, due to their thinner particle content (>54% < 0.002 mm), were projected out of Winkler's zones, and would benefit from blending with other materials. Samples D2 and E1, both with <30% clay and >60% coarser particles, were classified in Winkler's zone 1, classified as suitable for solid bricks. To improve the suitability of these raw materials for ceramic applications, an increase in the 0.002-0.020 mm fraction would enhance the projection of the samples within domains defined in the Winkler diagram. Mineral phases identified in the <0.063 mm fraction showed that phyllosilicates are dominant, ranging from 38% (C1) to 81% (B2), with variable amounts of quartz (SiO\({}_{2}\)), plagioclase ((Na,Ca)[(Si,Al)AlSi\({}_{2}\)]O\({}_{8}\)), and K-feldspar (K(AlSi\({}_{3}\)O\({}_{8}\))) (Table 1; Figures S1 and S2). In ceramics, quartz acts as a skeletal filler, enhancing mechanical strength and reducing drying shrinkage [42]. Feldspars contribute to alkali fluxes, promoting vitrification during firing, particularly in an intermediate temperature range (900-1100 \({}^{\circ}\)C). Carbonate phases, such as calcite (CaCO\({}_{3}\)) and dolomite (CaMg(CO\({}_{3}\))\({}_{2}\)), release CO\({}_{2}\) upon decomposition (600-800 \({}^{\circ}\)C), influencing porosity and acting as fluxing agents, and were found in low amounts, except in sample D1. The samples' pH ranged 5.5 to 7.9, reflecting differences in mineralogical composition, particularly in the presence or absence of carbonates. A near-neutral pH (6.3-7.0) is typical of kaolinite and illicit clays with low carbonate content. Clay sample color varied from light beige (C1) to bright orange (B1), reflecting differences in mineralogical composition, grain size, and possibly weathering processes.