Use of fly ash from fluidized bed boilers in clinker-slag-ash based binders
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Use of fly ash from fluidized bed boilers in clinker-slag-ash based binders
Elżbieta Janowska-Renkas 0
Jolanta Kowalska 0
0 Department of Engineering Building Materials, Faculty of Civil Engineering, Opole University of Technology , Katowicka 48, 45-061 Opole , Poland
The study presents the state of knowledge regarding physical and chemical properties, as well as trends for application of fly ashes from combustion in fluidized bed boilers in building materials. Clinker - slag - ash based binders were tested that contained up to 40 mass % of fly ashes from combustion in fluidized bed boilers. It was demonstrated that fluidized bed combustion fly ashes (FBC fly ash), apart from granular blast furnace slag, could be the ingredient of low clinker Portland cements (ca. 20% by mass). These cements, compared to CEM I Portland cement, have higher water demand and durability in the corrosive environment, and a lower compressive strength value. Based on test results of binders with various content of blast furnace slag and fly ash, the clinker - slag - ash based binder was singled out, which demonstrated the higher durability in the corrosive environment. It was found that production of clinker - slag - ash based binders was possible in the strength class 32.5 even with 30% by mass of FBC fly ash content.
1 Introduction
A contemporary cement industry is formed by actions aiming at reduction of CO2 emissions,
saving of natural resources, reduced outlays, on energy used, etc. [1]. One of more significant
trends for changes is application of mineral additives as the ingredient of cement for general
purposes.
Reduction of Portland clinker content in the cement, and thus application of a larger
quantity of non-clinker based ingredients, becomes a part of a sustainable development
strategy [2]. The most commonly used mineral additives to cement are: silica fly ash, granular
blast furnace slag and the limestone [3].
Fly ashes coming from coal combustion in fluidized bed boilers are a continuously
growing group of combustion process by-products [
1-7
]. It mainly results from the increasing
number of this type of furnaces, due to necessity to reduce SO2 and nitrogen oxides emission
to air. Main phases of fly ashes from the fluidized bed combustion are dehydrated minerals
that form a waste rock if a primeval character, unreacted sorbent (CaCO3), free CaO and
anhydrite [1-6]. The low temperature of fuels combustion in fluidized bed boilers (800
900oC) causes that no liquid phase occurs in the bed, as well as no mullite and of glazed,
round forms of ash grains [
1-7
].
Clayey minerals, which are the main ingredient of FBC ashes, in temperature of 850oC
undergo hydratation and form a dehydrated aluminium silicate substance with much higher
pozzolanic activity than conventional fly ashes [
1-7
]. FBC fly ashes, contrary to ashes formed
in traditional furnaces, contain irregular grains of high fineness and constituents which
undergo hydratation, demonstrating binding properties [1-6].
A characteristic feature of FBC fly ashes is their fineness and thus the high specific
surface area. A main factor, which has the impact on diversity of the particle size distribution,
is a type of the plant and its operating parameters. Basic properties of FBC fly ashes are
formed by: a type and a method of fuel combustion, a type of sorbent used and a type of
fluidized bed boiler design [3].
The main threat resulting from FBC fly ashes application in concretes, is the calcium
sulphate CaSO4, present in ashes, which may cause formation of delayed ettringite
and destruction of the concrete [1-6]. Therefore, the content of sulphates in FBC fly ashes
should be monitored when they are used in cement materials. Reactions, which occur during
cement hydratation with FBC fly ashes, are complex and they are still a subject of research,
and possibilities of their application in the cement and concrete materials have been presented
in numerous studies [
1-6, 9, 12
].
Currently the main issue is the management of FBC fly ashes, which do not meet
the standard [
16
], regarding possibility of mineral additives application to the cement [3, 5].
Tests of FBC fly ashes carried out so far demonstrate that they may be used as the mineral
additive to cement, which shows both pozzolanic and hydraulic properties. On the other hand,
due to presence of calcium sulphate in their composition, they may be used as a time
controller for cement binding process [
3, 5, 12
]. In recent years a few studies have been
published, where authors demonstrate that the total replacement of general purpose cement
with the ash – slag binder, allows to get materials with increased durability [
1-6,9,12
].
According to some researchers, the quantity of FBC fly ashes introduced into cement may
reach from 15% to 30% by mass. [1-6, 12].
The objective of the study was to determine composition of the clinker – slag – ash based
binder, which allowed (...truncated)