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