The Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete

Shweta Goyal, Lecturer, Thapar University, Maneek Kumar, Head, Civil Engineering Department, Thapar University, Patiala, Professor Bishwajit Bhattacharjee, Head, Civil Engineering Department IIT Delhi.

This paper deals with the effect of granular characteristics of mineral admixtures like silica fume and flyash added in binary or ternary combinations on the water requirement of resultant concrete. The role of superplasticizers in modifying the rheology has been investigated. Superplasticizers are the admixtures that are added to concrete in very small dosages and modify the water requirement of resultant mix and improve fresh properties of concrete.

Measurement of workability is made by slump test and Vee-bee time test in order to have the correlation between the two and amount of compaction achieved is studied by measuring fresh density of concrete. It is found that superplasticizers become necessary with the reduction of water binder ratio and flyash and silica fume affect the fresh concrete in opposite ways. Also, the relation that exists between slump and Vee–bee time for normal concrete without superplasticizers does not remain valid for concrete having mineral admixtures and superplasticizers.

Introduction

The use of high range water reducers (superplasticizers), condensed silica fume and other fine mineral admixtures have lead to the production of high-strength concrete [1]. Mineral admixtures are used in order to increase strength and improve durability of concrete. Blast furnace slag, flyash and silica fume are some of the mineral admixtures used in varying proportions to achieve the desired results. The mineral admixtures also affect the properties in fresh state, which are directly related to development of strength and durability of hardened concrete. Economics (not always) and environmental considerations have also had a role in the growth of mineral admixture usage.

Much research has been conducted for improving both fresh and hardened properties by using various mineral admixtures. It is reported that fly ash contributes to increase flowability in the fresh state, a dense microstructure and develop higher mechanical properties at the later stage due to the pozzolanic reaction [2,3]. Silica fume, on the other hand, has very fine particles–average particle size is less than 1Fm, which decreases the flowability in fresh state of concrete although, provides a dense microstructure and improved mechanical properties at early stages due to fast pozzolanic reaction [4, 5]. Silica fume is considered to be most efficient in contributing towards both early and later age properties of concrete. However, in India, silica fume comes under the category of costly materials, whereas flyash is abundant in our country and its production is increasing day by day. In the study undertaken, silica fume and flyash are used in combination to see the effect on improvement in fresh properties.

It is widely known that better fluidity is achieved by addition of superplasticizer. The increase of superplasticizer in concrete began in 1960s and has proved to be a milestone in concrete technology and in the field of construction [6]. There is no doubt that the use of admixtures had a profound impact on the concrete practices in India during the last few years [7]. The superplasticizer is adsorbed on the cement particles, which deflocculates and separate, releasing trapped water from cement flocks [8]. Currently available superplasticizers are micro molecular organic agents which are often divided into four groups according to their chemical contents as sulphonate melamine formaldehyde, sulphonate napthalene formaldehyde, modified lignosulphonates and copolymers containing sulphonic and carboxyl groups [9]. The family of superplasticizers based on polycarboxylic products is more recent (1980s). These materials are of higher reactivity; they do not contain the sulphonic group and are totally ionized in alkaline environment. These do not have the side effect of delaying the curing of concrete [10]. In the present study, poly-carboxylic group based superplasticizer is used as a chemical admixture.

It is believed that admixtures mainly affect the flow behavior of cement paste and do not alter the behavior of aggregates. Therefore, in most of the studies on concrete rheology and selection of chemical admixtures, tests on cement pastes have been conducted [1, 3, 8]. The results are then related to concrete workability. Unfortunately, the relation between cement paste rheology and concrete rheology has never been completely established [11]. The main reason behind it is that cement rheology is typically measured under conditions that are never experienced by cement paste in concrete. The values that are usually reported in literature do not take into account the contribution of aggregates [12]. The aggregates act as heat sink and shear the cement paste during mixing process. Therefore, in order to predict concrete rheology accurately, the tests are directly conducted on concrete. For this, one of the most commonly used methods for measuring concrete workability, i.e. slump cone test, is used. Slump cone test is the typically quantified field test for measuring concrete workability. However, in a survey conducted by National Ready Mix Concrete Association (NRMCA) and the National Institute of Standards and Technology (NIST) [13], it is determined that slump cone is not representative of the ease of handling high performance concrete in field, because in slump cone test, concrete does not undergo the same treatment as is met in the field. Therefore, along with the slump cone test, Vee–bee time is also noted, because in this test, concrete experience almost same vibrations as experienced in field.

The objective of the study is to look at the rheological characteristics of concrete which has silica fume and fly ash present either as binary or ternary combination with ordinary Portland cement. Secondly, the validity of existing relation between slump and Vee–bee time is checked for the mineral admixture concrete containing superplasticizers.

Materials

ASTM Type I Portland cement is used in this study. Its chemical composition is given in Table 1. The chemical and physical characteristics of two mineral admixtures silica fume and flyash can be seen in this table.

Aggregates

Crushed granite with a maximum nominal size of 10 mm was used as coarse aggregate and natural riverbed sand confirming to Zone II with a fineness modulus of 2.52 was used as fine aggregate. The properties of aggregates are listed in Table 2.

Superplasticizer

Poly-carboxylic group based superplasticizer, Structro 100 (a product of Fosroc chemicals), is used throughout the investigation. This group maintains the electrostatic charge on the cement particles and prevents flocculation by adsorption on the surface of cement particles [14]. It is a light yellow colored liquid complying with requirements of IS 9103 – 79, BS 5075 Part III and ASTM – C494 Type F. The specific gravity of superplasticizer is 1.2 and solid content is 40 percent by mass.

Mixture Details and Preparations

To explore the effect of superplasticizer, the rheological properties are studied for three water binder ratios: 0.25, 0.35 and 0.45. The three series obtained from three water binder ratios are designated as M1, M2 and M3 respectively for water binder ratios of 0.25, 0.35 and 0.45. The quantity of mineral admixtures is varied from 0 to 30 percent and is used either in a binary or a ternary combination. The mix designs used in the study are shown in Figure 1 and the mix details of specimens are listed in Table 3 and Table 4.

The mix preparation is very important because it influences its rheological behavior. The following procedure was adopted for mixing.

The cementitious materials (Portland cement, silica fume and flyash) were mixed together separately in a container. Coarse aggregates and the fine aggregates were mixed in a mixer rotated at slow speed of about 140 rev./min. for 1 minute. The cementitious material was then put in the mixing drum and the resultant mixture was dry mixed for one minute followed by addition of half of the total water content during the next one-minute mixing. The remaining water along with superplasticizer was then added and mixed at high speed of about 285 revolutions per minute for 1.5 minutes or till the uniform and homogeneous mix is achieved. (Superplasticizer was taken as percentage by mass of binder which included cement, silica fume and flyash if any present. Water content of superplasticizer was taken into account when calculating the total water content of the mix [15].)

The prepared mix is used for obtaining slump and Vee–bee time. In all 24 mixes are prepared and three determinations of slump and Vee–bee time are made for each sample and the mean value is taken. It is worth mentioning at this stage that for the selected dose of superplasticizer, no segregation was observed at any stage.

Results and Discussions

For each of the mix, the superplasticizer dose is given step increments and the corresponding Vee–bee time and slump is noted. The saturation point is obtained from the slump verses superplasticizer dosage curves; and is taken as that value of superplasticizer beyond which it will not increase the slump with any further increase in dosage. (In other words, superplasticizer has no further plasticising effect). The results of these tests are presented in Figure 2 to 4 where slump is plotted against superplasticizer dosage and in Figure 5 where optimum superplasticizer dosage is plotted against water binder ratio. The nomenclature of mixes used is already presented in Table 4. The results are discussed as below.

Effect on Mineral Admixtures on Rheological Properties

The effect of the addition of a mineral admixture is detected by an increase in the slump or a reduction of water content or a reduction of superplasticizer dosage needed to obtain the same slump. The results are represented in Figure 2 to 4 in which the variation of slump is plotted as a function of superplasticizer dosage for three series of water binder ratios studied.

(a) OPC – SF System

For the same water binder ratio, with increase in silica fume content in concrete, the value of lump decreases and hence the optimum superplasticizer dosage increases, which can be attributed to high specific surface of silica fume with an average particle size of 0.1Fm. However, this is not the sole factor affecting the increase in superplasticizer demand for silica fume mixture. Long with the high specific surface area, the particles of silica fume are chemically highly reactive and have affinity for multilayer adsorption of superplasticizer molecules, which is also supported by other researchers [16, 17]. As a result, with increase in silica fume percentage, the quantity of superplasticizers in the concrete system decreases leading to steep increase in the superplasticizer dosage. The same type of behavior is observed for entire water binder range with an exception for water binder ratio of 0.25. At this ratio, with the addition of 5% silica fume, the optimum dosage of superplasticizer decreased by a small amount from 4% (for control mix) to 3.75%. This reverse trend can be explained by considering the dispersion action of flocculated cement particles by silica fume particles in combination with superplasticizer. Actually, the effectiveness of superplasticizer is enhanced in the presence of silica fume [18]. Similar observation is also made in some previous studies also [19, 20].

(b) OPC – FA System

The addition of flyash has just the opposite effect on the mix properties in terms of workability and optimum dosage of superplasticizer as compared to silica fume. With incorporation of flyash, the water demand and hence optimum percentage of superplasticizer required reduce as compared to the control mix without mineral admixtures for all water binder ratios studied. The reduction in water demand of concrete caused by the presence of flyash is ascribed to its spherical shape, which reduces the frictional forces among the angular particles of OPC, called ball – bearing effect [21]. These spherical particles easily roll over one another, reducing inter-particle friction. The spherical shape also minimizes the particle’s surface to volume ratio, resulting in low fluid demands. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand [22]. In other way, the effect of flyash can be considered similar to the action of superplasticizer

(c) PC–SF–FA System

From the above discussion, it can be stated that flyash act improves flowability and silica fume has a reverse effect, when added individually. Thus, it is thought that when used in combination, the beneficial effect of flyash on fluidity is used to compensate the loss of slump with silica fume addition. As expected, when the different combinations of silica fume and flyash are used, the slump values were higher and optimum superplasticizer dosage was lower in comparison with the corresponding mixes having only silica fume. The slump obtained increased with increase in flyash content in the mix and decreased with increase in silica fume content. For all the three water binder ratios, TC2 gave least superplasticizer dosage while MC3 gave maximum superplasticizer dosage. Thus, it can be said that the addition of flyash led to the production of economical mixes with greater workability.

Effect of Water Binder Ratio on Optimum Superplasticizer Dosage

Figure 5 shows the results of optimum superplasticizer dosage obtained for all mixes at various water binder ratios. From the figure, it is observed that as the water binder ratio decreases, the optimum dosage of superplasticizer increases. With the decrease in water binder ratio, more number of superplasticizer molecules are required for adsorption on the surface of cement and mineral admixture particles to increase the fluidity of the mix. The optimum dosage increases sharply as the water binder ratio is decreased from 0.35 to 0.25 as compared to the shift from 0.45 to 0.35. For example, in the control mix, the optimum superplasticizer dosage increased from 1.25% to 4% as the water binder ratio is decreased from 0.45 to 0.35. This is because at very low water – binder ratio, cement particles are very close and to overcome inter particle friction and inter particle forces of attraction, higher optimum dose of superplasticizer is required.

Relation Between Slump and Vee-bee Time

In order to formulate a relation between slump and Vee–bee time for mineral admixture concrete, Vee–bee time test is also conducted simultaneously to slump test. Figure 6 shows the graph for slump with Vee– bee time. In the graph, the doted line shows the approximate relationship between slump and Vee–bee time for the normal ordinary Portland cement concrete without using superplasticizers and the solid line is the best fit obtained for the test results in the present study. The marked shift of the present curve for concrete containing mineral admixtures and superplasticizers from the existing curve for normal concrete can be observed from the graph. For higher values of Vee–bee time, the amount of slump required is almost same from both the curves. However, when the Vee–bee time is lesser than 5 seconds, the difference in the values of slumps obtained from the two curves differ in the range of 20 to 50 mm. Since Vee–bee time is the representative of actual compaction in the field, it can be said that for equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only. This shift in the curve can be due to the effect of cohesive nature of the mix with silica fume, flyash and superplasticizers.

Effect of Mineral Admixtures on Fresh Density

In order to study the effect of mineral admixtures of superplasticizer on the degree of compaction achieved, fresh density of final mixes were also determined and the same is presented in Table 5. The fresh density of all the mixes lies in the similar range, although the mixes with flyash have a density somewhat higher than the other mixes which can again be due to ball bearing effect of flyash.

Conclusion

On the basis of the studies carried out, it can be concluded that in the binary system, silica fume increases the superplasticizer demand at a constant workability due to its high surface area and its strong affinity for multi— layer adsorption of superplasticizer molecules. Flyash addition, on the other hand, decreases the water demand and hence optimum percentage of superplasticiser for constant workability due to its ball – bearing effect that reduces frictional forces among binder particles. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand. Three-component system is much preferred for high performance concrete because in it, silica fume act as a filler and flyash controls rheology.

The existing relationship between slump and Vee –bee time changes with the addition of mineral admixtures and superplasticizer. For equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only.

Acknowledgments

This research is supported by the Department of Science and Technology Grant. The authors would like to acknowledge the authorities concerned for its assistance in carrying out the research.

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