Synthesis of Special Cements from Mixtures Containing Fluidized Bed Combustion Waste, Calcium Carbonate and Various Sources of Alumina

Special cements are obtained from non-portland clinkers. In particular applications, when rapid hardening and high chemical resistance are required, they behave better than ordinary cements [1-4]. The peculiar composition of special cements can also be exploited for giving a more pronounced environment-friendly character to their manufacturing process. In this regard important features are: a) low synthesis temperatures; b) reduced thermal input and CO2 generation; c) easy grindability; d) possibility of using hardly recoverable wastes and by-products. All the above mentioned features are present in calcium sulphoaluminate cements [5-14] based on 4CaOּ 3Al2O3ּ SO3 (C4A3S, according to the cement chemistry notation under which C=CaO; A=Al2O3; S =SO3; S=SiO2, H=H2O). Calcium sulphoaluminate is able to generate, upon hydration, ettringite (C6AS3H32 ), a compound which regulates all the technical properties of calcium sulphoaluminate cements. When C4A3S reacts with calcium sulphate and water, in the absence of lime, a non-expansive ettringite is rapidly formed together with aluminium hydroxide, thus giving a high mechanical strength at early ages [4, 15-17]. Among the other phases present in C4A3S-based cements, dicalcium silicate (C2S) can play an important role because it is able to add strength and durability at later ages. Secondary constituents, such as gehlenite (C2AS), calcium sulphosilicate (C S S 5 2 ) and various calcium aluminates, have generally a poor hydraulic behaviour and provide a small contribution to the technical properties.Fluidized bed combustion (FBC) waste, mainly composed by exhausted sulphur sorbent and coal ash, contains CaO, SiO2, Al2O3 and SO3 as major oxides, thus representing a potential raw material for the manufacture of calcium sulphoaluminate cements [18]. Both disposal in landfill and re-use of FBC waste is generally made difficult by its chemical and mineralogical composition. Upon hydration, exothermal and expansive phenomena occur due to the relatively high content of lime and calcium sulphate [13]. Moreover the utilization of FBC ash in the ordinary cement and concrete industry is hindered by its poor pozzolanic activity due to the reduced glass content related to combustion temperatures which are significantly lower than those of traditional pulverized coal combustors [14]. The identification of suitable application fields for such waste is therefore of critical importance. This paper is focused on the use of a CFBC (circulating fluidized bed combustion) fly ash as raw mix component for the synthesis of calcium sulphoaluminate cements. The raw mix composition was designed in order to combine all available sulphate and silica into calcium sulphoaluminate and dicalcium silicate, respectively. To this end additional sources of calcium and aluminium oxides were required: the former was given by pure calcium carbonate; the latter by pure alumina, red mud and/or bauxite.