Effect of Blended Fly ash and Superplasticizer on Pozzolanic Activity and Compressive Strength of Cement Paste

Preeti Sharma, Research Scholar, Devendra Tyagi, Associate Professor, Department of Chemistry, D.A.V College Dehradun, H.N.B Garhwal University, Srinagar. S.K Agarwal, Scientist, C.B.R.I, Roorkee

It is a very well known fact that the use of fly ash in masonry and concrete enhances the durability of structure hence use of fly ash is gaining momentum in the cement/ concrete Industry. But due to lack of awareness, utilization of flyash is still too low in India.

Present study covers the factors affecting the pozzolanic activity of flyash

1. Effect of blending of flyash from different sources such as Silo1 (Hopper 1 & Hopper 2 mixture), Silo 2 (Hopper 3, Hopper 4 & Hopper 5 mixture).
2. Effect of superplasticizer.

The main properties that influence the pozzolanic activity of Flyash are Loss on ignition and Fineness.

In this paper, the effect of fly ash percentage of different fields on the compressive strength of cement paste with and without superplasticizer has been studied

Fly ash of Silo-1 is very coarse in nature where as flyash of Silo-2 is fine. Hence flyash from Silo1 and Silo 2 were blended in varying proportion to study the effect on pozzolanic activity. Since Silo 1 flyash account for 80 % of total flyash generated hence efforts have been made to maximize the utilization of Silo 1 flyash with addition of superplasticizer.

The pozzolanic activity increases from field I to V as expected, the noticeable observation is that with the use of superplasticizer, the pozzolanic activity of Silo-1 is comparable to control value of 358 kg/cm².

Introduction

Fly ash is a byproduct of the combustion of the pulverized coal in thermal power plants. Fly ash collected from each hopper in the ESP system are transported and stored in the silo. It is known that the properties of fly ashes collected from each hopper in an ESP system varies as we move from the boiler (Hemming et al., 1994; Itskos et al., 2009; Monzo et al., 1994; Erdogdu et al., 1998; Lee et al., 1999). Fly ash consists of inorganic matter present in the coal that has been fused during coal combustion. The particle diameter of fly ash ranges from <1 to 150 µm. Specific surface area is extremely variable ranging from <200 m²/kg to 800 m²/kg.

It is a most common artificial pozzolana, which is defined as a chemically inert silicious and aluminous material that possesse little or no cementitious value. But when it is in finely divided form and in the presence of water, it reacts with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.

Pozzolanic activity is indicative of lime pozzolana reaction, it is mostly related to the reaction between reactive silica and alumina in fly ash with Ca(OH)₂ librated during the hydration of portland cement to form CSH and calcium aluminate hydrate. Fly ash is suitable for massive concrete structures because its addition as a partial substitute for cement reduces the heat of hydration, thereby improving the overall durability of the concrete.

Researchers have given various factors for measuring or assessment of pozzolanic activity:

1. Quantity of reactive silica
2. Composition of SiO₂ + Al₂O₃ + Fe₂O₃
3. Fineness / surface area of the fly ash particles
4. Measurement of compressive strength/ strength activity index
5. Lime reactivity test specified in Indian Standard IS 3812

Fineness of fly ash is very important property affecting pozzolanic reactivity. Different techniques used for measurement of fineness have shown different pozzolanic activity. Fineness of fly ashes plays an important role in strength development. Some researchers give more emphasis on % retained on sieve while some have shown correlation between blain’s fineness and compressive strength.

The sum of Silica + Alumina + Iron oxide is stipulated in most of the standards as a major requirement. It is also observed that silica and alumina in amorphous form contribute to pozzolanic reactivity. Small quantity of iron present in glass phase is reported to have deleterious effect on pozzolanic activity.

Measurement of compressive strength is rated as the best technique for measurement of pozzolanic reactivity. There are incidences where low lime reactivity fly ash has shown more compressive strength while fly ash having higher lime reactivity shown less compressive strength.

It is generally accepted that the fly ash collected at various Sillo/ESP exhibit no greater difference in their chemical composition, but the glassy content of the higher fields is greater [5-7]. Due to this reason cement/ concrete industry is hesitant to use it in concrete. First and second field fly ashes is coarser than other fields and the quantity generated by these fields are nearly 70-80% of the total generation, so it is very important to enhance its reactivity.

There are different ways to enhance the reactivity like

1. Grinding of fly ash
2. Use of chemical admixtures
3. Blending of fly ash of different fields

In the present study blending of Silo one fly ash with subsequent fields has been evaluated for pozzolanic activity. Effect of superplasticizer on the pozzolanic activity has also been studied. This paper reports the effect of fly ash collected from different fields with different percentages on the compressive strength of cement paste.

Material Used

Fly Ash: Fly ash of different fields from thermal power stations (Near Delhi) was collected. The flyash was analyzed and their physical and chemical properties are given in Table 1 & 2.


Cement: Ordinary Portland cement 43 grade confirming to BIS 8112/1989 had been used in present study. The cement was analyzed and its physical and chemical properties are given in Table 4.


Superplasticizer: Sulphonated Naphthalene Formaldehyde Condensate (SNF) conforming to BIS 9103 (2004) was used in the present study Binder had been used as 1% dose by weight in all the test mixture.

Sand: Standard sand (annore) had been used in the present study.

Experimental Procedure

Control Mixture:- The control was prepared with 250 gm of portland cement, 687.5 gm ( 229 fraction I + 229 fraction II + 229 fraction III) of graded sand and 121 ml of water.

Test Mixture: The test mixture was prepared with 225 gm of cement and 25 gm of pozzolana. The flyash was blended in the following proportions :

• SET 1: Silo 1 (90%) + Silo 2 (10%)
• SET 2: Silo 1 (70%) + Silo 2 (30%)
• SET 3: Silo 1 (50%) + Silo 2 (50%)

The above set of experiments was repeated with 1% superplasticizer addition.

Mixing Procedure: The mortar mixture was prepared using ELE (UK) Automatic mixture. 50 mm cubes were casted for the present study.

Storage of Specimens: After 24 hours of initial curing in a moist room (25 ± 2ºC) with relative humidity not less show 95%. The cubes were placed in air tight glass containers and stored at (65 ± 2ºC) for 6 days.

Determination of Compressive Strength: The compressive strength of mortar cubes was determined after 7 days of demoulding of control and test mixture and average of the three samples has been reported in Table 5

Paste Studies

Cement cubes of 25mm were cast with various percentages (10, 30 and 50%) of fly ash of different Silo with and without superplasticizer at the same consistency level. The compressive strength of these cubes was determined at different time interval of 1,3,7,28,90 and 360 days.

Results and Discussion: In the present study compressive strength method had been used to evaluate pozzolanic activity.

Table 1 clearly indicates that the surface area of fly ash increases as we move from Silo 1 to 2.

The pozzolanic activity of fly ash of Silo 1 & 2 with and without superplasticizer has been reported in Table-5. It was observed in both the cases (Case 1 & Case 2) that the pozzolanic activity increases as we move from Silo-1 to Silo-2.

It can be depicted for Table 5 that with the use of superplasticizer it is possible to enhance the pozzolanic activity of flyash.

Table 5 indicates the effect of blending of flyash from different fields in varying proportions. SET 1 pozzolanic activity was comparable to control value i.e. 340 kg/cm², whereas in case of SET 3 the pozzolanic activity was approximately 20 % more than control, thus indicating that the pozzolanic activity was enhanced from SET 1 to SET 3. Hence it was observed that the activity increased with the increasing proportion of Silo 2 flyash thus indicating that fly ash particles with larger medium size particle are more reactive. Enhancement of pozzolanic activity of fly ash through blending helps reduce the cost of superplasticizer.


The results of the compressive strength of cement paste with different percentages (10, 30 and 50%) of fly ash of Silo-1&2 with and without superplasticizer up to 360 days are given in tables 6-11.

It is clear from the Table 6-11 that the compressive strength of cement paste increases from S-1 to S-2. Since the fineness of fly ash increases from S-1 to S-2, this indicates that the fine fly ash is very reactive and has larger influence on the strength.


Table 6 and 7 gives strength data of cement paste with 10% fly ash of different fields with and without superplasticizer up to 360 days. The high pozzolanic activity of fly ash of Silo-2 the strength is 10-15% more at one day. The trend is similar up to one year. However, for Silo-1 the strength is slightly less compare to control up-to 7days but beyond that it is comparable to control.

With the use of superplasticizer the 1&3 day strength is more than the control and the gain in strength is observed up to one year.

The results of 30% replacement of fly ash of different fields are given in table 8 and 9. Compressive strength of cement paste without superplasticizer exhibits lower values for Silo-1 upto 360 days. However in case of field Silo-2 strength at 90 days are comparable to control. With the use of superplasticizer Silo-1 show comparable strength at 360 days and Silo-2 show comparable strength at 28 days and at 90 and 360 days the strength is approx. 7 % higher than control. This gain in strength is due to reduction in w/b as we move from field Silo 1-2.


The strength data of 50% replacement of fly ash is given in table 10 and 11. The strength is less compare to control upto 28 days with and without superplasticizer for fly ash of different fields. At 90 days compressive strength for this is comparable to control when superplasticizer has been used. Beyond 90 days the compressive strength is either equivalent or more compare to control for Silo-2. The use of fine fly ash also has a packing effect and the filling of the small voids and this helps in the strength development (Chindaprasirt et al., 2004).

Conclusions

• Pozzolanic activity of fly ash of Silo-1 is less than control.
• Pozzolanic activity of Silo 2 is 5-8 % more than control.
• Pozzolanic activity of Silo-1 fly ash with 1% superplasticizer is comparable to control value.
• Blending of fly ash Silo-1 (90%, 70% as 50%) with Silo-2 has comparable pozzolanic activity to control. However, use of superplasticizer with blended fly ash shows enhanced pozzolanic activity
• This study may help in identifying the optimized blend of fly ash from different fields with/without addition of superplasticizer, to be used in cement / concrete industry.
• With 30% replacement of cement with fly ash the compressive strength of cement paste from Silo-1 to Silo-2 is comparable to control in case of Silo-1, where as for Silo-2; the strength is more at 360 days in the presence of superplasticizer. However compressive strength of cement paste with 50% replacement for Silo-2 in the presence of superplasticizer beyond 90 days is either comparable or more than control.
• Test results indicate that fly ash of different fields have noticeable effect on the compressive strength due to different fineness. The fine fly ash (Silo-2) with high surface area is more reactive and thus results in increase in strength.

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