Costa, Letizia Teresa (2024) Alkaline activation of natural and artificial pozzolans as binders for improving waste soils. [Tesi di dottorato]

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Abstract

Over the years, there has been considerable interest in sustainable construction and the production of alternative materials to those normally used. The reason for this interest lies in the fact that although traditional binders, such as lime and cement, offer excellent mechanical performance and are suitable for various applications, their production is undoubtedly responsible for huge rates of CO2 emissions into the atmosphere due to high production temperatures. To overcome this problem, nontraditional binders have been studied and designed in recent decades, such as those produced by alkali activation, which represent a group of innovative cementitious materials produced by combining silica- and alumina-rich solids with a solution of alkali salts, resulting in a mixture of gels and crystalline compounds that eventually harden into a new matrix [1]. Alkali binders are synthesized by the chemical reaction between an amorphous precursor, rich in alumina (Al2O3) and silica (SiO2), and a sodium- or potassium-based activator. During alkaline activation processes, the dissolution of the aluminosilicate oxides in the alkaline activator promotes a polycondensation reaction that leads to the formation of a sodium aluminosilicate gel with cementitious properties [2-3]. The activation process of these materials not only takes place cold, so it is considered Zero Impact, but it uses waste materials, therefore, it represents an environmentally sustainable production that promotes the Circular Economy. The use of nontraditional binders in geotechnical applications is of an innovative nature and is considered a very interesting topic, as it could enable a real shift to materials with a lower environmental impact. However, it should be kept in mind that the activation of aluminosilicate powders is a complex condition to accomplish, as multiple factors can influence the chemical reactions that characterize this process. Therefore, the primary objective of this study is to examine the factors that regulate the alkaline activation process of natural pozzolans and to understand how each of these may influence, qualitatively and quantitatively, the dynamics of the kinetic reactions produced. Although the chemical reactions during experimentation differ from one pozzolan to another, it is still possible to define common or "standard" dynamics through a statistical study of the processes described in the literature and confirmed by observation of the chemical and physical parameters of some natural pozzolans that have been studied.

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