Characterization of red mud/metakaolin-based geopolymers as modified by Ca(OH)2

Abstract

Geopolymers are an emerging class of materials that offer an alternative to the Portland cement as the binder of structural concrete. One of the advantages is that the primary source of their production is waste alumosilicate materials from different industries. One of the key issues in geopolymer synthesis is the low level of mechanical properties due to porosity as well as the high activity of conductivity carriers. It can often lead to limited application possibilities, so the objective is to obtain an enhanced strength as well as decreased cracking tendency through microstructure modification. The introduction of Ca(OH)2, under certain pH conditions could lead to the filling-the-pores process and improving the mechanical properties. The aim was to understand the role that calcium plays in the geopolymer synthesis, and to define which reaction prevails under the synthesis conditions: formation of geopolymer gel or calcium silicate hydrate that contains aluminum substitution (CASH). The synthesis was performed with different raw materials (with or without red mud) and different alkalinity conditions. Ca(OH)2 was the obligatory supplement to both of the mixtures. Different techniques were performed for the testing of reaction products, as well as to define the microstructural changes as the generator of improved mechanical properties and changed electrical conductivity. The characteristics of the geopolymer's macrostructure were defined by means of an SEM analysis. Compressive strength and electrical conductivity are among the investigated product's properties. X-ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR) were used for the identification of various crystalline phases and an amorphous phase.