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Effect of Cement on Silica Fume and Flyash

Earlier notion of using high amounts of cement for concrete has now changed on favour of increased use of high amounts of mineral ad-mixtures and super plasticizers with reduced amounts of cement and water in the concrete mixtures. Energy plays a crucial role in growth of developing countries, like India. In context of low availability of non-recoverable energy sources coupled with requirements of large quantities of energy to materials like cement, steel etc.

In India about million tones of fly ash has been produced by 68 major thermal power stations and are likely to be doubled within next 10 years. It has been a published fact from research that waste materials like fly ash; silica fume etc, through their use as construction materials can be converted into meaningful wealth. Also, a partial replacement of cement with fly ash is desirable, and indeed essential due to a variety of technical, economical and ecological reasons.

Researchers have reported that silica fume smaller in size and round shape fills the voids between the coarser cement particles which may be otherwise occupied with water. A properly proportional fly ash and silica fume in concrete mix improves properties of the concrete that may not be achievable through the use of Portland cement alone. The resulting concrete mix becomes strong, durable and economical and also eco-friendly as it utilizes an ecological hazardous material.

Fly ash is a by-product of burning pulverized coal to generate electric power. There are periodic variations in the operation of a power station can results in occasionally varying properties of the fly ash. Differences in the fly as produced by different power stations. Even the same power station will produce fly ash with varying properties if the coal used is non-uniform.

A simple picture of the behavior of concrete containing fly ash cannot be presented because fly ash is to a single material of nearly constant composition. Fly ash depends on the chemical properties and the fineness of the Portland cement in the mix.

It is not surprising that there is no simple relation between the proportions of fly ash in the total cementations material and the properties of the resulting concrete of otherwise fixed. Inevitable attempts to relate, by a single equation, the strength of concrete, even of fixed proportions, to the various properties of fly ash such as fineness, residues of particles above certain size, pozzolanic indices, carbon content, glass content, and chemical composition, have been unsuccessful.

Indeed, this situation is to be expected, given that no single equation can predict the strength properties of Portland cements alone from their physical and chemical properties. The fly ash in concrete makes efficient use of product of hydration of cement such as calcium hydroxide C-H which is otherwise a source of weakness in normal cement concrete converts it into denser and stronger C-S-H compounds by pozzolanic reaction.

The heat generated during hydration initiates the pozzolanic reaction of fly ash. Silica fume is a by-product of silicon or Ferro-Silica industry and is times finer than cement. A silica fume is also referred to as micro silica or condensed silica fumes. It consists of amorphous silica and has high reactivity towards lime.

When SF is used in concrete mix, its introduction affects the physical arrangement of the system, particularly near the aggregate surface where porosity exists. Silica fume starts reacting at the early stage of hydration process. All these result in more uniform, stronger transition zone potential of micro cracking. The specific gravity of silica fumes is generally 2. This value can be compared with the specific gravity of Portland cement, which is 3.

A concrete mix containing fly ash is cohesive and has reduced bleeding capacity. The mix can be suitable for pumping and for slip forming; finishing operations of fly ash concrete and made easier. The influence of fly ash on the properties of fresh concrete is linked to the shape of the fly ash particles. Most of these are spherical and solid, but some of the large particles are hollow spheres, know as cenospheres.

High carbon content in fly ash is that it adversely affects workability. Variation in carbon content may also lead to earratic behavior with respect to air entrainment some air entraining agent becoming adsorbed by the porous carbon. The effect of chemical reactions, fly ash has a physical effect of improving the microstructure of the hydrated cement paste.

The main physical action is that of packing of the fly ash particles at the interface of coarse aggregate particles, which are absent in the mortar used in the test. The extend of packing depends both on the fly ash and on the cement used: better packing is achieved with coarser Portland cement and with finer fly ash. One beneficial effect of packing on strength is a reduction in the volume of entrapped air in the concrete.

But the main contribution of packing lies in a reduction in the volume of large capillary pores. It is worth noting that the positive influence of the fineness of fly ash is coupled with it spherical shape. Therefore, grinding of fly ash, although it increases fineness, may result in the destruction of spherical particles with consequent increase in water demand of the mix due to the irregular angular shape of the fly ash particles.

The beneficial influence of fly ash upon water demand does not extend beyond a fly ash content of 20 percent by mass. An excessive content of fly ash is not beneficial from the point of view of strength development either. The limiting content is probably around 30 percent by mass of total cementitious material as can be seen from figure below.

Average values of strength of concrete cylinder moist cured at 23oC are shown in table above. It is worth adding that maximum size of aggregate was 9. Although an early reason for the use of the various cementations materials in concrete was their influence on the rate of development of heat and of strength, even more important is their influence on the resistance of concrete to chemical attack which is the consequence not only of the chemical nature of the hydrated attack, which is the consequence not only of the chemical nature of the hydrated cement paste but also of its microstructure.

It is no exaggeration to say that the cementations materials have a major influence on all aspects of durability related to the transport of attacking agent through concrete. The cementations materials considered are finer than Portland cement and therefore, improve particle packing, so that provided adequate wet curing is applied, their presence reduces permeability. Even though the use of fly ash reduced permeability, it allows faster carbonation.

The increase in the rate of carbonation is greater when fly ash is used with Portland blast furnace cement. The enhanced carbonation need not necessarily be large in practice when mixes with proper mix proportions are used.

Also, carbonation may reduce the permeability, but not when both fly ash and ggb are present in the mix. Likewise, good resistance to sulfate attack was observed with class C fly ash contents up to 50 percent and 10 percent of silica fumes.

Control of the alkali-silica reaction is specialized topic in which a detailed knowledge of the aggregate to be used is necessary. However, the beneficial effects of the incorporation of fly ash about 30to40 percent by mass in the blended cement should be noted. These materials contain only a small amount of water-soluble alkalis so that, at a given content of cementations material which includes Portland cement with high alkali content, the presence of fly ash in the blended cement reduces the total alkali content in the mix.

Thus, the use of these materials may obviate the need for low-alkali cement but the absence of expansive reactions should be verified by test. The beneficial effects of the inclusion of silica fume in steam-cured concrete at 65o C upon its penetrability by chlorides was confirmed by Campbell and Detwiler.

For significant improvement, the minimum silica fume content was 10 percent in Portland-cement-only concrete, but 7. It may be added that curing Portland-cement-only concrete at 50 o C was found to result in increased penetrability by chlorides.

The effectiveness of superplasticizer in enhanced by the presence of silica fume. The same dosage maintained the slump when the silica fume content was 10percent by mass of cementations material. Example of the relation between the 28 day compressive strength of mm cubes and the water cementations material are show in figure above.

The same figure also showed the relation for concrete containing Portland cement only. The presence of silica fume affects significantly the properties of fresh concrete.

The mix is strongly cohesive and, in consequence, there is very little bleeding, or even more. The reduced bleeding can lead to plastic shrinkage cracking under drying conditions, unless preventive measures are taken.

On other hand, voids caused by trapped bleed water are absent. The cohesive character of the mix affects the slump so that, for both mixes equally to be capable of compaction, the mix with silica fume needs a slump 25 to 50mm. The cohesiveness of concrete containing silica fume makes it satisfactory for pumping and for underwater concreting as well as for use as flowing concrete. Entrained air remains stable, but an increase dosage of the air-entraining admixture is required because of the high fineness of silica fume.

In addition, there are problems in obtaining a suitable air-void system when super plasticizers are used. There are no reports of incompatibility of silica fume with admixtures in general. It is useful to observe that the retarding effect of lignosulfonate based admixtures is smaller when silica fume is present in the mix. Consequently larger dosages of these admixtures can be used without causing an excessive retardation. In addition to the pozzolanic reaction between the amorphous silica in silica fume and calcium hydroxide produced by the hydration of Portland cement, silica fume contributes to the progress of hydration of the latter material.

This silica fume contributes to the progress of hydration of the latter material. This contribution arises from the extreme fineness of the silica.

Thus, early strength development takes place. Silica fume dissolves in a saturated solution of calcium hydroxide within a few minutes. Therefore as soon as enough Portland cement has hydrated to result in saturation of the pore water with calcium hydroxide, calcium silicate hydrate is formed on the surface of the silica fume particles.

Their reaction proceeds, initially, at a high rate For example, when the mass of silica fume was 10percent of the total mass of cementations material, one-half of the silica fume was observed to react in 1 day; and two-thirds during the first 3 days.

However, subsequent reaction was very slow, only three-quarters of the silica have hydrated at 90 days. The acceleration of hydration processes by silica fume occurs also when, in addition to Portland cement, ggbs is present in the mix. A consequence of the rapidity of the early reactions in concretes containing silica fume is that the development of heat of hydration in such concretes may be as high as when rapid- hardening Portland cement is used alone.

The behavior of concrete with silica fume beyond the age of about 3 months depends on the moisture conditions under which the concrete is stored. Under dry storage conditions, retrogression of strength typically up to 12 percent below the peak value at about 3 months was observed in test on laboratory specimens. However, the strength of concrete containing silica fume, determined on cores up to 10 years old, clearly show no retrogression of strength.

This finding is of importance because the behavior of test specimens in which moisture gradient exist may be misleading. One consequence of the high early reactivity of silica fume is that the mix water is rapidly used up; in other words, self-desiccation takes place.

At the same time, the dense microstructure of the hydrated cement paste makes it difficult for water from outside, if available, to penetrate toward the unhydrated remnants of Portland cement of silica fume particles. In consequence, strength development ceases much earlier than with Portland cement alone; some experimental date are shown in table below from which it can be seen that there was no increase in strength beyond 56 days. The contribution of silica fume to the early strength development up to about 7 days is probably through improvement in packing, that is, action as a filler and improvement of the interface zone with the aggregate.

The bond of the hydrated cement paste with aggregate, especially the larger particles, is greatly improved, allowing the aggregate better to participate in. Some contrary arguments about the role of silica fume have been advanced, but they are likely to reflect specific test conditions rather than intrinsic behavior. The contribution of a given amount of silica fume to the strength of concrete arising from packing and interface effects should remain constant with time.

This is unlike the effect of pozzolanic activity which continues to take place. Indeed, at a fixed content of silica fume, the increase in strength of concrete between 7 and 28 days was found to be independent of the value of the 7 day strength.

Effect of Thermal-cured Hydraulic Cement Admixtures on the Mechanical Properties of Concrete

The applications of steel slag powder and steel slag aggregate in ultra-high performance concrete UHPC were investigated by determining the fluidity, nonevaporable water content, and pore structure of paste and the compressive strength of concrete and by observing the morphologies of hardened paste and the concrete fracture surface. The results show that the fluidity of the paste containing steel slag is higher. The nonevaporable water content of the hardened paste containing steel slag powder is close to that of the control sample at late ages. Both steel slag powder and steel slag aggregate react and connect tightly to gels and hardened paste, respectively. The UHPC containing steel slag aggregate demonstrates higher compressive strengths. Steel slag is a by-product of steel manufacturing [ 1 ].

Request PDF | The effect of high fly ash content on the compressive It was shown by Kearsley and Wainwright [17] that 67% replacement of cement with FA did not Influence of expanded vermiculite powder and silica fume on The waste toner additive was collected from used printer cartridges.

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Slag properties are important since they: i have a decisive effect on both process control and on the quality of the product and E. Copper slag is one of the materials that is considered as a waste which could have a promising future in construction Industry as partial or full substitute of aggregates. Attractive Colour. The granulated copper slag is made up of regularly shaped, angular particles, mostly between 4. Aggregate acts as reinforcement Table 2.

Keywords: Fad , compressive strength , flexural strength , porthland , DRX. Authors: P. Parthiban , J.

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Muhit1, S. Ahmed2, M. Amin1 and M. Raihan1 1. The utilization of these industrial by products is becoming popular throughout the world because of the minimization of their potential hazardous effects on environment. Seven types of mix proportions were used to cast the test specimens for both groups.

The use of fly ash in portland cement concrete PCC has many benefits and improves concrete performance in both the fresh and hardened state. Fly ash use in concrete improves the workability of plastic concrete, and the strength and durability of hardened concrete. Fly ash use is also cost effective. When fly ash is added to concrete, the amount of portland cement may be reduced. Benefits to Fresh Concrete. Generally, fly ash benefits fresh concrete by reducing the mixing water requirement and improving the paste flow behavior. The resulting benefits are as follows:.

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