This paper presents the effect of fly ash grain-size
fractions on Portland-fly ash cement hydration and its properties. Siliceous
fly ashes of size fraction of 0-16 and 16 - 32μm,
separated from initial fly ash samples from 1st, 2nd and
3rd hopper in ESP system, were analysed. Cement hydration was
investigated by determination of hydration heat and content of Ca(OH)2 and C3S in cement samples. Water to cement ratio and initial setting
time of cement pastes as well as compressive strength and microstructure of
cement mortars were also analyzed. Results showed that the same amount of the
same size ash fraction can give cement of lower or higher early strength and
its lower or higher increase with time. Incorporation of 20wt% of
ash fraction of 0 - 16μm can produce Portland-fly ash cement CEM II/A-V of strength class
42.5R (from 2nd hopper) or 52.5N (from 3rd hopper).
Cement containing 40wt% of ash fraction of 0 - 16μm from
2nd and 3rd hopper can be classified as pozzolanic
cements CEM IV/A-V of strength class 42.5 and normal or rapid early strength,
respectively. Different development of strength of cement with addition of the
same size ash fraction separated from the initial ash sample from the next hopper in
ESP system is connected with higher depolymerization degree of SiO4 units in ash glass, resulting from the greater amount of AlO4 units
replacing SiO4 units. Ash fraction of 16 - 32μm shows
lower depolymerisation of glass network and as a consequence lower hydration
degree of C3S to portlandite and calcium silicate hydrates (C-S-H).
Cite this paper
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