08-12-2012, 01:23 PM
THE EFFECT OF FLY ASH ON THE FLUIDITY OF CEMENT PASTE, MORTAR, AND CONCRETE
1THE EFFECT OF FLY ASH.pdf (Size: 261.01 KB / Downloads: 170)
Abstract
The addition of ultra-fine fly ash (UFA) to cement paste, mortar and concrete can
improve their fluidity, but some coarse fly ash can’t reduce water. This paper
investigates the effect of fineness and replacement levels of fly ash on the fluidity of
cement paste, mortar, and concrete. The fly ash is collected by electro-static
precipitators and airflow classing technology. Three different finenesses were chosen,
and their replacement levels were 20%, 30%, and 40%, respectively. The experiment
results show that particle size distribution, Zeta potential, density and particle
morphologies of fly ash are the major factors affecting their fluidity.
Introduction
Currently, high-performance concrete (HPC) is widely used due to their technical
and economical advantages. Such materials, so called the 21st century concrete, are
characterized by improved mechanical and durability properties resulting from use of
chemical and mineral admixtures as well as specialized production processes [1-4].
In the modern concrete, active mineral additives, such as fly ash, silica fume and slag,
etc., have been the essential component that provides concrete with higher
compressive strength, great fluidity, and higher durability. The use of fly ash is
accepted in recent years primarily due to saving cement, consuming industrial waste
and making durable materials, especially due to the improvement in the quality
stabilization of fly ash. Because of these, more and more investigations have been
made about their effects on concrete. A popular hypothesis put forward to explain the
workability enhancement due to the use of certain fine mineral admixtures,
especially fly ash or SF, is that the spherical particles easily roll over one another,
reducing interparticle friction [5]. Sakai et al. [6] reported that a higher packing
density was obtained with spherical particles as compared to crushed particles in a
wet state. This resulted in lower water retention in the spherical case and
subsequently lower water demand for a specific workability.
Experiments methods
The tests of fluidity of cement paste fluidity of mortar and workability of concrete
generally conform to Chinese National Standard GB/T 8077-1987, GB/T 2419-1994,
and GBJ80-85 test methods, respectively. In this study, the mortars were prepared at
a water-to-binder ratio of 0.30 and sand-to-binder ratio of 1.5, with 20%, 30%, and
40% equivalent replacement of cement by low-calcium fly ash A, B, and C,
respectively. The concretes are prepared with three different finenesses fly ash A, B,
and C and the equivalent replacement level is 20%, 30%, and 40%. The effect of the
addition of FA is expressed by the changes in concrete slump, water demand, or
water reducing rate in the concrete.
Results and Discussion
Effect of UFA on cement setting time
The initial and final setting time of cement paste containing C fly ash is shown in Fig.
2. When the UFA replacement level is 20%, the initial and final set time of
UFA-cement paste was about 9.56 and 11.25 hours, respectively. When the UFA
replacement level increased to 40%, the initial and final setting tines of UFA-cement
paste are prolonged to about 11.85 and 13.58 hours, respectively. In general, the set
time of UFA-cement paste is prolonged with the increase of UFA. The outer surface
of UFA particle increase with the increase of UFA, the amount of absorbed calcium
ions increased. That inhibits calcium ions concentration build-up in fresh paste
during early hydration, resulting in the setting time is prolonged, thus, the heat of
hydration decreases.
Effect of UFA on mortar fluidity
Fig. 3 shows the water demand of mortar containing different finenesses and
replacement levels of FA for a given flow, where flow table tests were used. All
water demand ratios of FA mortar are lower than 100%, that is, A, B, and C fly ashes
all have water reducing function. When the replacement is equal, the water demand
ratio of C fly ash is the lowest, B fly ash is the second, and A fly ash is the highest. In
general, the finer the cementitious material, the larger surface area the material.
Thus, the more water will be absorbed by the material, that is the negative effect of
UFA water reducing. At the same time, the finer fly ash, the more spherical-like fly
ash particles morphology, the internal friction in fresh mortar reduce, the better
“lubricant effect” fly ash has, on the other hand, the finer fly ash, the better close
packing effect fly ash has when it partially replace cement, those are positive effect
of UFA water reducing. The increase of positive effect is bigger than that of negative
effect when the fineness of fly ash is increased in this study, Consequently the water
demand ratio decrease with the increase of fly ash fineness. A fly ash is more porous
than B and C fly ash, more water is adsorbed into FA pores. With the replacement
level of A fly ash increasing, the water demand ratio decreases a little. When the
replacement levels of B and C fly ash are increased from 0% to 30%, the water
demand ratio reduce, but when the replacement is increased to 40%, the increase of
negative effect of UFA water reducing surpass that of positive effect, the water
demand ratio increase slightly. In order to obtain better fluidity, B and C fly ash
cannot replace cement too much.
Effect of FA on concrete workability
High slump and low slump loss of fresh concrete were considered as an assurance
for good concrete casting, vibrating and finishing. Generally, superplasticizer
increases concrete slump, but causes high slump loss when compared with the plain
concrete having the same initial slump. Because of a low water-to-binder ratio in
HPC, when the same amount of water is lost through evaporation or by cement
hydration, the slump loss was more significant.
Fig. 4 shows the water reducing rate of concrete containing different finenesses and
replacement levels FA when the slump is equal. All fly ash have water reducing
effects. When the replacement is 30%, the water rate of A, B, and C fly ash are 2.1%,
7.9%, and 9.7%, respectively. The water reducing rate increases with the level of fly
ash replacement. The fly ashes actually function as a kind of mineral water reducers.
The test results of HPC containing C fly ash were presented in Table 3. As to plain
concrete, the slump loss after 1 hour is 8.5%, 2 hours is 19.1%, and 3 hours is 36.2%,
but the slump loss of concrete containing 30% UFA after 1 hour is 2%, 2 hours is
8.2%, and 3 hours is 10.2%. UFA could not only increase slump and spread, but also
reduce the slump loss. First, because UFA can prolong the setting time, which result
in decreasing slump loss of concrete, secondly UFA have huge specific surface area
which can adsorb some superplasticizer, thirdly the Zeta potential of UFA is negative
in de-ionized water.
Conclusions
The fly ashes studied were collected by electro-static precipitators and airflow
classing technology. Due to their spherical shape and smooth surface features, the fly
ashes demonstrated improved water reduction effect with increased fineness. Based
on the test results, the following conclusions can be drawn:
• The incorporation of ultra-fine C fly ash may increase the setting time of
cement paste.
• The water demand ratio of UFA decrease with the increasing of fineness.
• The water reducing rate of 30% ultra-fine C fly ash reach 10%, ultra-fine C
fly ash is a kind of good mineral water reducer.
• Ultra-fine C fly ash has significantly increased the slump and reduced the
slump loss of concrete.