Technological characterization and ceramic application of gravel pit by-products from middle-course Jarama river deposits (central Spain)

This work, unlike previous others, deals with ceramic products obtained exclusively from gravel pit by-products. The residues are originated from the sand and gravel washing process of middle-course Jarama river Quaternary sediments, located in Madrid region, central Spain. These natural aggregates allow an intense exploitation, because of qquality (well-rounded quartzite) and reserves (up to 700 x 106 m3), generating considerable waste volumes. Thirty silty-clay by-products, collected from seven gravel pits, have been tested at a laboratory scale. The mineralogical composition of these materials is mainly represented by phyllosilicates (muscovite-illite, smectite, kaolinite anc chlorite), quartz and feldpars. From the technological characterization, six representative mixtures (M1 to M6) were designed combining suitable mineralogy and grain size distribution of the 30 samples. A wide range of values were measured on mixture powders, determining specific surface (13 -76 m2/g, BET method), methylene blue index (5-20) and Atterberg limits. Ceramic characterization was performed on extruded bodies, testing drying sensitivity (Bigot curves and Ratzemberger test). Smectitic content influenced ditectly the parameters measured on dried bodies. This influence can be controlled by increasing the silty component, and the ceramic properties measured on these mixtures (M3, M4 and M5) get better: hygroscopicity (less than 2%), drying shrinkage (4-7 cm m-1), keeping the beding strength above 5Mpa. Three maximum firing temeprature were studied: 850, 950 and 1050°C. On mixtures fired at 950°C, firing shrinkage is less than 4 cm m-1, bending strength may reach 69 Mpa and water absorption range from 23% to 0.5%. Efflorescence susceptibility is faint, and colour is red for all samples, getting darker for increasing temperature.The positive results obtained in this set of preliminarly tests drive to envisage new research programs, focused on testing these by-products on a semi-industrial scale, assessing the effective possibilities of using them as ceramic mraw materials.