Effect of some industrial wastes, transition metal oxides and firing temperature on the properties of geopolymeric ceramic products

Abstract

In the present work, we evaluated the possibility for using some raw materials and industrial wastes in manufacturing of geopolymers (green or sintered, dense or porous and colored green geopolymer) with high technological and mechanical properties according to the requirements of industrial applications. Kaolin was firstly converted into metakaolin after firing at 850oC for two hours and used successfully as a base material for fabrication of geopolymer in the presence of 31 % alkali activator (mix of sodium hydroxide and sodium silicate). These geopolymer exhibited interested properties as 31% porosity, 1.27g cm−3 bulk density and 31MPa compressive strength. An attempt to study the sintering of prepared geopolymers was conducted with very low sintering rate. The sintering temperature had a substantial effect on the properties of sintered geopolymer. It is found that mechanical strength of the sintered geopolymer increased with increasing the firing temperature up to 1000°C (64MPa) then decreased at 1200°C (about 15MPa). This means sintered ceramic bodies with interesting properties were obtained after sintering of green geopolymer. In another part of this study, silica fume waste material was utilized as additive to produce geopolymer foams which have many applications. The addition of silica fume was for two reasons, firstly, the silica fume was used as pore forming agent to produce porous geopolymer. It includes Si metal which reacts with the alkali activator and produces hydrogen gas. This gas forms porous geopolymer. The second one is an environmental reason, i.e. to get rid of the huge accumulated volumes of volatile silica vapor (silica fume) produced after silicon and ferro-silicon industries as a by-product and recycling them again in a safe way through manufacturing of geopolymers. This might be reflected positively on environmental problems in Egypt. Geopolymer foams were in-situ prepared by alkali-activation of metakaolin with partially replacement by 2.5, 5 and 7.5 mass-% silica fume. Using this waste material in combination with low processing temperature, an economic production process of foamed geopolymer product was introduced. Geopolymer foams with good mechanical properties were obtained. The porosity of prepared geopolymer increased with increasing the amount of added silica fume.