Mechanochemical activation of natural kaolins: a sustainable, low carbon, process for the synthesis of geopolymeric cements

According to the roadmap defined for the year 2020 by the European Union (EU) Commission for a resource efficient Europe [1], the reduction of greenhouse gases from industrial processes is a priority task. In the EU countries, most of these emissions come from the ETS sector, where cement industry is one of the most important. The case of Italy is paradigmatic. With 0,35 Mt y -1 of CO 2 [2], Italy is one of the major CO 2 emitting countries in Europe, and is ranked as 19 th in the world. The major role played by the cement industry in Italy comes by the fact that Italy is the second cement producer in Europe [3]. High emission of CO 2 occurs in cement production, because the primary component of cement (clinker) is obtained by heating at 1450°C stoichiometric mixtures of carbonate and silico-aluminate rocks in rotary kilns. For this reason, an increasing number of research programmes in the field of advanced materials is recently devoted to test the suitability of non-hazardous decarbonized materials (e.g. fly ash, volcanic tuffs) as partial substitutive of clinker in cement formulations as well as to develop new, low-carbon, cementitious binders. Among these novel cements, Si-Al geopolymers [4] are the most promising for the years to come, as they could completely replace clinker. While clinker uses the hydraulic reaction to form a nanomaterial with randomly oriented hydrated calcium silicates, several Si-Al materials can rapidly react in alkaline solutions. This approach is more flexible than that thehydraulic one, because different 2D and 3D polymers can be obtained as a function of the Si/Al ratio of the reagent and the type of compensatory cations introduced in the structure.The general formula of geopolymers can be written as:C.[-(SiO 2 )z-AlO 2 -]n .wH 2 OWhere C is a monovalent or divalent cation, z is an integer from 1 to 3, n is the degree of polycondensation, and w the water molecules coordinated by the structure. Depending on theSi/Al ratios and C, nano-materials made of randomly oriented Si-Al crystals displaying different thermal and mechanical properties can be formed. Metakaolins (MKAs) are the most used materials in the synthesis of Si-Al based geopolymers [4]. MKAs are amorphous materials obtained by dehydration of natural kaolins, where kaolinite crystals are, by far, the dominant mineral. Dehydration of kaolinite is usually performed by heating kaolins at temperatures ranging from 650 to 800°C for more than 2 hours. The CO 2 emission produced by burning fuel to heat the kilns for the thermal treatment of kaolins makes the Si-Al based geopolymer production less environmentally sustainable. Although it is known that kaolins can also be converted into MKAs by grinding them in millswhose moving masses rotate at high speeds (800 to 2.000 r.p.m.) [5], the potential of mechanochemical processing of kaolins for the synthesis of Si-Al geopolymers still needs tobe better investigated. With the MECAGEOPOLY CNR-MIUR Flagship Project, coordinated by IMC, we wanted to demonstrate the feasibility of mechano-chemical processing inproducing Si-Al geopolymeric binders at an industrial level, in order to further reduce CO 2 emission. The mechanochemical treatment was investigated using a kaolin whose thermalbehavior was known. The material was grinded under different conditions and times, and the various MKAs obtained were characterized by XRD, FTIR-AR, TGA-DTG-DTA, 27 Al, 29 Siand 1 H MAS NMR, and other techniques. The ability of mechanically activated MKAs to make Si-Al geopolymers was thus investigated, and the chemical, mechanical and thermal properties of the obtained products determined. The results of this study will be presented and critically discussed.