Assessment of two clayey materials from northwest Sardinia (Alghero district, Italy) with a view to their extraction and use in traditional brick production

Two clayey raw materials collected in the Porto Ferro and Lago Baratz areas of Alghero in northwest Sardinia (Italy) were used to prepare handmade brick samples that were fired under oxidizing conditions at temperatures ranging between 750 and 1000 °C to evaluate their possible use in the brickmaking industry. Both raw materials are rich in quartz and phyllosilicates but only the sample from Porto Ferro contains calcite. Granulometrically, the sample from Porto Ferro is a silty sand and shows higher plasticity than the clay from Lago Baratz which is classified as a sand. After firing, the samples acquire a red-orange colour and undergo significant mineralogical and textural changes. Phyllosilicate depletion is accompanied by the crystallisation of mullite and, only in Porto Ferro samples, the breakdown of calcite is followed by the formation of gehlenite, wollastonite and anorthite. Optical and electronic microscopic observations revealed that the temper grains are larger and more abundant in the bricks from Lago Baratz than in those from Porto Ferro. As firing temperature increased, pores became ellipsoidal in shape and the matrix became vitrified. The samples from Lago Baratz also show fissures at grain boundaries, which indicate the formation of an extensive network of pores and make these bricks very brittle when fired at low temperatures (750 and 800 °C), so much so that hydric tests could not be performed on them. Bricks produced with the clayey material from Porto Ferro achieved the best results in terms of hydric behaviour, with a reduction in the water absorption capacity, and in terms of compactness, with an increase in ultrasound wave velocity. However, both Porto Ferro and Lago Baratz bricks behaved in similar ways when submitted to the salt crystallisation test, especially at 1000 °C when a sufficient degree of vitrification was reached to ensure high quality samples.