Synthesis of geopolymers from mechanically and thermally treated carbonate-rich clay sediments

Among the many aluminosilicate raw materials suitable for the synthesis of Alkali Activated Materials (AAMs) including geopolymers, the latest research on geopolymers is dealing with an increasing interest in the sustainable use of widely available raw materials and hazardous wastes recycling. In the same vein, the involvement of Apulian illitic carbonate-rich clay sediments for geopolymer production may promote the knowledge on the use of natural clay deposits to produce AAMs, providing a more ecological intended use of such clays, which are currently involved in the local industry of cements and ceramic tiles. Although illite/smectite clay sediments have been found to be suitable for AAMs, the complex mineralogical composition of Apulian carbonate-rich illitic clays open some challenges about the most effective processing methodology aimed at the increase of the clay reactivity for the synthesis of geopolymers and to produce a positive effect of calcium compounds on the final product strength. Apulian carbonate-rich clay samples were subjected to thermal activation in red-ox atmosphere (temperatures between 400 and 900°C), mechanical activation (milling for 5, 10 and 15 minutes) and to a combination of such treatments. A preliminary geopolymer synthesis has been conducted on differently treated samples, selected on the basis of the highest amorphous content and Si and Al release in alkaline solution, to assess the effects of different clay pre-treatments on clay reactivity. Moreover, two solutions (NaOH 8M and Na2SiO3 with SiO2/Na2O = 2.06) were used as activators, to investigate how the presence of soluble silica influences the final reaction products and properties of geopolymers. The preparation of the pastes started from mixing the treated clays and the two different activators, adopting the lowest liquid/solid ratio to ensure the minimum paste workability condition (L/S ranging from 0.5 to 0.8). The resulting geopolymers have been characterised through XRPD, FT-IR, SEM-EDAX analysis and compressive strength tests. In particular, a positive correlation between the compressive strength and the amorphous content of the raw materials (and, subordinately, the sodium silicate/solid ratio) was observed. The best strength results are observed for the geopolymers obtained after alkali activation with sodium silicate (L/S = 0.8) of the thermally activated clay at 800°C in oxidising atmosphere, probably due to the contribution of the increased content of soluble silica and highly reactive lime and portlandite. Additionally, the activation of treated clays with sodium silicate brought the geopolymers to show a more reacted and homogeneous geopolymeric matrix enriched in calcium, identified as (N,C)-A-S-H gel, compared to geopolymers obtained after clay activation by NaOH. In this latter case, low reacted matrix resulted in the formation of zeolites as secondary reaction products and in very low compressive strength values.