Manufacturing and technological characterization of porous aggregates from lutites from Buenos Aires province (Argentina)

Abstract

According to their physicochemical characteristics, density and resistance, porous aggregates can be used for thermal and acoustic insulation, as building blocks, lightweight aggregate concrete, structural concrete and for geotechnical, gardening and hydroponics applications. In and around Olavarría in central Buenos Aires province, a large stock of lutites is extracted from the upper layers of limestone quarries used to produce Portland clinker. Lutites are a potentially viable raw material for manufacturing porous aggregates, while generating a value-added alternative for regional lutites. The aim of this work is to evaluate the mineralogical composition, main physical properties, pore structure and mechanical resistance of porous aggregates obtained from two lutites (L1 and L2) with different chemical and mineralogical compositions. Different thermal treatments were used: gradual heating (G) and direct heating (D) up to maximum temperatures of 1150 °C and 1200 °C. The mineralogical phases in the lutites contribute to the formation of crystalline and vitreous phases, necessary to trap gases and generate porosity when at viscous liquid state. Considering the crystalline phases, formation of hematite was identified in all sintered aggregates; formation of anorthite (feldspar) in the lutite with higher calcite content (L1) and formation of a spinel in the lutite with higher Fe and Mg content (L2). The type of heat treatment influenced the porosity of the aggregate: with increasing porosity, particle density decreased, as also did its compressive strength. Selection of the heat treatment will depend on the properties required according to the technological application intended for the aggregate. The absorption values in all sintered aggregates were less than 0.1%. The sintering and vitreous phase formation on the outer surface explain the low absorption of the porous aggregates obtained. Thus, for construction applications, the aggregates do not present significant variation in moisture content that should be considered in the water/cement ratio and workability. It was possible to sinter porous aggregates in the laboratory with technological properties comparable to quality lightweight aggregates available on the market: the diameter, particle density and compressive strength of porous aggregates obtained at 1200 °C and direct treatment (L1 D 1200 and L2 D 1200) were similar to those of Liapor® 6.5 2/10.