http://www.revistadechimie.ro REV.CHIM.(Bucharest)♦68♦No. 6 ♦2017 1182 The Influence of the Properties of the Material Used for Obtaining Geopolymers on Their Structure and Compressive Strength ILENUTA SEVERIN*, MARIA VLAD Dunarea de Jos University of Galati, 47 Domneasca Str., 800008, Galati, Romania This article presents the influence of the properties of the materials in the geopolymeric mixture, ground granulated blast furnace slag (GGBFS) + wheat straw ash (WSA) + uncalcined red mud (RMu), and ground granulated blast furnace slag + wheat straw ash + calcined red mud (RMc), over the microstructure and mechanical properties of the synthesised geopolymers. The activation solutions used were a NaOH solution with 8M concentration, and a solution realised from 50%wt NaOH and 50%wt Na 2 SiO 3 . The samples were analysed: from the microstructural point of view through SEM microscopy; the chemical composition was determined through EDX analysis; and the compressive strength tests was done for samples tested at 7 and 28 days, respectively. The SEM micrographies of the geopolymers have highlighted a complex structure and an variable compressive strength. Compressive strength varied from 24 MPa in the case of the same recipe obtained from 70% of GGBFS + 25% WSA + 5% RMu, alkaline activated with NaOH 8M (7 days testing) to 85 MPa in the case of the recipe but replacing RMu with RMc with calcined red mud, alkaline activated with the 50%wt NaOH and 50%wt Na 2 SiO 3 solution (28 days testing). This variation in the sense of the rise in compressive strength can be attributed to the difference in reactivity of the materials used in the recipes, the curing period, the geopolymers structure, and the presence of a lower or higher rate of pores, as well as the alkalinity and the nature of the activation solutions used. Keywords: geopolymers, ground granulated blast furnace slag, microstructure, compressive strength In recent years, the rise in waste production has been associated to a number of environmental issues and so the necessity of sustainable development regarding the management of waste has become a main priority. Recycling has always been a positive approach of solving the issue of waste, thus reducind pollution and protecting natural resources. In this context, geopolymeric types binder materials represent a new class of binders materials which can be synthesised from a variety of industrial by- products such as ground granulated blast furnace slag [1- 5], ash [6-10], bauxite red mud [11,12] or certain recycled waste [13-15]. The term „geopolymer” has been associated, by French researcher, J. Davidovits, in 1972 [16], to a new class of binding materials formed with industrial by- products alkaline activated. The process through which geopolymerisation is achieved has been divided into three phases by H. Wang [17]. These phases are: the dissolution of silico-aluminates from different materials which promote the formation of silica and aluminum hydrates, followed by the rearrangement of the chemichal formed species and then the polycondensation/ gelling, thus forming the tridimensional network of the silica-aluminous gel under the action of the alkaline activation solution used. Geopolymers are silico-aluminous mineral materials [18] generally amorphous in structure, which can be obtained at atmospheric pressure and a temperature of about 100 °C [19-21]. This type of material presents low microporosity because in the geopolymeric type binder system there is a very smoll amount of water absorbed due to the amorphous or semicristaline structure resulting from synthesing, which causes high values of compression strength. Through recent studies [22, 23], it was revealed that the raw materials of the synthesized geopolymers plays a significant role in the geopolymerisation reactions and largely influences the mechanical properties and * email: ilenuta.severin@ugal.ro; Tel. + 40753518936 structure of the final product. The alkaline activators generally used in the synthesing of a binder geopolymer type are caustic alkalis or mineral alkalis. Their classification according to their chemical composition was designed by Glukhovsky [24]. Among all known alkali activators, NaOH and Na 2 SiO 3 were selected for this study because they are available on the market at a low cost and it has been demonstrated throughout previous studies [25, 26] that by alkaline activating of some industrial by-products have been obtained very good results. In this study it was investigated the possibility of using ground granulated blast-furnace slag, wheat straw ash, uncalcined red mud, and calcined red mud, resulted from Bayer process, in order to obtain construction material through the geopolymerisation. Also, it was determined the optimal combination of materials, which would allow the use of uncalcined or calcined red mud at 600°C, in mixture with ground granulated blast-furnace slag and wheat straw ash and an alkaline activator, that would lead to a geopolymeric binder with specific physical, chemical, and mechanical properties. Experimental part Solid materials preparation The development of a geopolymer implies a series of operations that leads to obtaining them. To prepare the raw materials some operation are need to reduce the volume through grinding, sieving, and in particular for red mud water removal through calcination. Grinding of ground granulated blast furnace slag was achieved using ball mill PM 400 type, this grinding mill has zirconium oxide containers, with 5 mm diameter balls from the same material. The ratio between the quantity of slag and the quantity of balls was 8:1, and the grinding speed was 200 rpm for 4h. To avoid the powder adhering to the balls surface a quantity of 1 mL of ethanol was added. The