Effect of Fly Ash on fresh and harden concrete Properties

Dnyan Deshmukh

Staff member

Effects on properties of fresh concrete

The absolute volume of cement plus fly ash normally exceeds that of cement in similar concrete mixtures not containing fly ash. This is because the fly ash normally is of lower density and the mass of fly ash used is usually equal to or greater than the reduced mass of cement. While it depends on the proportions used, this increase in paste volume produces a concrete with improved plasticity and better cohesiveness. In addition, the increase in the volume of fines from fly ash can compensate for deficient aggregate fines.

Fly ash changes the flow behavior of the cement paste. The generally spherical shape of fly ash particles normally permits the water in the concrete to be reduced for a given workability. Concrete with fly ash retains slump for more time compared to non-fly ash concrete in hot- weather conditions

Using fly ash in air-entrained and non air-entrained concrete mixtures usually reduces bleeding by providing greater surface area of solid particles and a lower water content for a given workability

Improved pumpability of concrete usually results when fly ash is used. For mixtures deficient in the smaller sizes of fine aggregate or of low cement content, the addition of fly ash will make concrete or mortar more cohesive and less prone to segregation and bleeding. Further, the spherical shape of the fly ash particles serves to increase workability and pumpability by decreasing friction between particles and between the concrete and the pump line.

Time of setting:
The use of fly ash may extend the time of setting of concrete if the portland cement content is reduced. The setting characteristics of concrete are influenced by ambient and concrete temperature; cement type, source, content, and fineness; water content of the paste; water soluble alkalies; use and dosages of other admixtures; the amount of fly ash; and the fineness and chemical composition of the fly ash. When these factors are given proper consideration in the concrete mixture proportioning, an acceptable time of setting can usually be obtained. The actual effect of a given fly ash on time of setting may be determined by testing when a precise determination is needed or by observation when a less precise determination is acceptable.

When fly ash concrete has a longer time of setting than concrete without fly ash, such mixtures should be finished at a later time than mixtures without fly ash. Failure to do so could lead to premature finishing, which can seal the bleed water under the top surface creating a plane of weakness. Longer times of setting may increase the probability of plastic shrinkage cracking or surface crusting under conditions of high evaporation rates. Using very wet mixtures containing fly ashes with significant amounts of very light unburned coal particles or cenospheres can cause these particles to migrate upward and collect at the surface, which may lead to an unacceptable appearance. Some situations are encountered when the addition of fly ash results in stickiness and consequent difficulties in finishing. In such cases the concrete may have too much fine material or too high an air content.

Air entrainment:
Fly ash concrete generally loses air content. The loss of air depends upon a number of factors. Properties and proportions of fly ash, cement, fine aggregate, length of mixing or agitating time.

The use of fly ash in air-entrained concrete will generally require a change in the dosage rate of the air-entraining admixture. This is because ,some fly ashes with LOI values less than 3 percent require no appreciable increase in air entraining admixture dosage. To maintain constant air content, admixture dosages must usually be increased, depending on the carbon content as indicated by LOI, fineness, and amount of organic material in the fly ash. When using a fly ash with a high LOI, more frequent testing of air content at the point of placement is desirable to maintain proper control of air content in the concrete. Required air-entraining admixture dosages may increase with an increase in the coarse fractions of a fly ash. The coarse fraction usually contains a higher proportion of carbon than the fine fraction. The form of the carbon particles in fly ash may be very similar to porous activated carbon, which is a product manufactured from coal and used in filtration and adsorption processes. In concrete, these porous particles can adsorb air-entraining admixtures, thus reducing their effectiveness. In such cases adjustments must be made as necessary in the admixture dosage to provide concrete with the desired air content at the point of placement.

Fly ash concrete with prolonged mixing or agitation prior to placement loses air content in the concrete. Those fly ashes that require a higher admixture dosage tend to suffer more air loss in fresh concrete. When this problem is suspected, air tests should be made as the concrete is

placed to measure the magnitude of the loss in air and to provide information necessary to adjust properly the dosage level for adequate air content at the time of placement. Agitation of the concrete is a prerequisite for loss of air to continue.

Effects on properties of hardened concrete

Compressive strength and rate of strength gain:
Strength at any given age and rate of strength gain of concrete are affected by the characteristics of the fly ash , the cement with which it is used, and the proportions of each used in the concrete. The relationship of tensile strength to compressive strength for concrete with fly ash is not different from that of concrete without fly ash.

After the rate of strength contribution of Portland cement slows, the continued pozzolanic activity of fly ash contributes to increased strength gain at later ages if the concrete is kept moist; therefore, concrete containing fly ash with equivalent or lower strength at early ages may have equivalent or higher strength at later ages than concrete without fly ash. This higher rate of strength gain will continue with time and result in higher later age strengths than can be achieved by using additional cement Compared with concrete without fly ash proportioned for equivalent 28-day compressive strength, concrete containing a typical Class F fly ash may develop lower strength at 7 days of age or before when tested at room temperature. If equivalent 3-day or 7-day strength is desired, it may be possible to provide the desired strength by using accelerators or water-reducers, or by changing the mixture proportions.

In high strength concrete to get early strengths, Silica fume can be used, in combination with fly ash. Simultaneous use of silica fume and fly ash resulted in a continuing increase in 56- and 91- day strengths indicating the presence of sufficient calcium ion for both the silica-fume reaction and the longer term fly-ash reaction to continue . Increased early strengths can be achieved in fly ash concrete by using high-range water reducing admixtures to reduce the water to cementitious material ratio to at least as low as 0.28. The ability of fly ash to aid in achieving high ultimate strengths has made it a very useful ingredient in the production of high-strength concrete

Changes in cement source may change concrete strengths with Class F fly ash as much as 20 percent. For example, cements with alkali contents of 0.60 percent Na2O equivalent or more typically perform better with fly ash for strength measured beyond 28 days. However, when potentially alkali-reactive aggregates are used in concrete, low-alkali cement should be used, even if fly ash is also used.

Bond of concrete:
The bond or adhesion of concrete to steel is dependent on the surface area of the steel in contact with the concrete, the location of reinforcement, and the density of the concrete. Fly ash usually will increase paste volume and may reduce bleeding. Thus, the contact at the lower interface where bleed water typically collects may be increased, resulting in improved bond. Development length of reinforcement in concrete is primarily a function of concrete strength. With proper consolidation and equivalent strength, the development length of reinforcement in concrete with fly ash should be at least equal to that in concrete without fly ash. These conclusions about bond of concrete to steel are based on extrapolation of what is known about concrete without fly ash. The bonding of new concrete to old is little affected by the use of fly ash.

Impact resistance:
The impact resistance of concrete is governed largely by the compressive strength of the mortar and the hardness of the coarse aggregate. Use of fly ash affects the impact resistance only to the extent that it improves ultimate compressive strengths.

Abrasion resistance:
Compressive strength, curing, finishing, and aggregate properties are the major factors controlling the abrasion resistance of concrete. At equal compressive strengths, properly finished and cured concretes with and without fly ash will exhibit essentially equal resistance to abrasion.

Temperature rise:
The chemical reaction of cement with water generates heat, which has an important bearing on the rate of strength development and on early stress development due to differential volume change in concrete. Most of this heat is generated during the early stages of hydration of the alite (substituted C3S) and C3A phases of the cement. The rate of hydration and heat generation depends on the quantity, fineness, and type of the cement, the volume of the structure, the method of placement, the temperature of the concrete at the time of placement, and the curing temperature. The temperature rise can be reduced by using fly ash as a portion of the cementitious material in concrete. As the amount of cement is reduced the heat of hydration of the concrete is generally reduced.

Resistance to high temperatures:
With respect to the exposure of concrete to sustained high temperatures, the use of fly ash in concrete does not change the mechanical properties of concrete in relation to similar concrete containing only Portland cement when exposed to sustained high temperature conditions ranging from 75 to 600 C (170 to 1110 F).

Resistance to freezing and thawing:
The resistance to damage from freezing and thawing of concrete made with or without fly ash depends upon the adequacy of the air-void system, the soundness of the aggregates, age, maturity of the cement paste, and moisture condition of the concrete. Because of the often slower strength gain of concretes with fly ash, more cementitious material (cement plus fly ash) may be used in mixtures to achieve comparable strength at 28 days.

Care should be exercised in proportioning mixtures to insure that the concrete has adequate strength In addition, exposed fly ash concrete to freezing and thawing at very early ages and found no degradation of performance as compared with control concrete.

Permeability and corrosion protection:
Concrete is permeable to water to the extent that it has interconnecting void spaces through which water can move. Permeability of concrete is governed by many factors such as amount of cementitious material, water content, aggregate grading, consolidation, and curing efficiency. The degree of hydration required to eliminate capillary continuity from ordinary cement paste cured at standard laboratory conditions was a function of the water to cementitious materials ratio and time. Required time ranged from 3 days at a water to cement ratio of 0.40 to 1 year at a water to cement ratio of 0.70.

Calcium hydroxide liberated by hydrating cement is water soluble and may leach out of hardened concrete, leaving voids for the ingress of water. Through its pozzolanic properties, fly ash chemically combines with calcium hydroxide and water to produce C-S-H, thus reducing the risk of leaching calcium hydroxide. Additionally, the long-term reaction of fly ash refines the pore structure of concrete to reduce the ingress of chloride ions. As a result of the refined pore structure, permeability is reduced moreover; the reduced permeability of fly ash concrete can decrease the rate of ingress of water, corrosive chemicals, and oxygen.

Reduction of expansion caused by alkali-silica reaction (ASR):
The reaction between the siliceous glass in fly ash and the alkali hydroxides in the portland-cement paste consumes alkalies, which reduces their availability for expansive reactions with reactive aggregates. The use of adequate amounts of some fly ashes can reduce the amount of aggregate reaction and reduce or eliminate harmful expansion of the concrete. Often the amount of fly ash necessary to prevent damage due to alkali-aggregate reaction will be more than the optimum amount necessary for improvement in strength and workability properties of concrete

Sulfate resistance:
As a general rule, fly ash can improve the sulfate resistance of concrete mixtures. The increase in sulfate resistance is believed to be due in part to the continued reaction of fly ash with hydroxides in concrete to continue to form additional calcium silicate hydrate (C-S- H), which fills in capillary pores in the cement paste, reducing permeability and the ingress of sulfate solutions. Fly ashes used in concrete under wetting and drying conditions greatly improve the sulfate resistance of concretes made with all types of cement. The sulfate resistance property of fly ash concrete varied with the ratios of the fly ash to total cementitious material by mass. The sulfate resistance of fly ash concrete is influenced by the same factors which affect concrete without fly ash: curing conditions, exposure, and water-to-cementitious material ratio. The effect of fly ash on sulfate resistance will be dependent upon the class, amounts, and the individual chemical and physical Characteristics of the fly ash and cement used.

Generally, fly ashes with less than 15 percent CaO content will improve the sulfate resistance of concrete. The maximum sulfate resistance will be achieved in a given exposure and situation by employing a low water - cementitious materials ratio, sulfate-resisting portland cement, and fly ash which exhibits good sulfate-resistance qualities. Fly ashes with large amounts of chemically active alumina may adversely affect sulfate resistance.

Drying shrinkage:
Drying shrinkage of concrete is a function of the fractional volume of paste, the water content, cement content and type, and the type of aggregate. In fly ash concrete the addition of fly ash increases the paste volume, drying shrinkage may be increased slightly if the water content remains constant. If there is a water-content reduction, shrinkage should be about the same as concrete without fly ash. Studies indicate with different fly ash cement mixtures no apparent differences in drying shrinkage between concrete with up to 20 percent fly ash content and non-fly ash concrete and when at increased fly ash content resulted in slightly less drying shrinkage.

Efflorescence is caused by leaching of water soluble calcium hydroxide and other salts to external concrete surfaces. The leached calcium hydroxide reacts with carbon dioxide in air to form calcium carbonate, the source of the white discoloration on concrete. The use of fly ash in concrete can be effective in reducing efflorescence by reducing permeability. This reduced permeability helps maintain the high alkaline environment in hardened concrete.

Modulus of elasticity:
The modulus of elasticity of fly ash concrete, as well as its compressive strength, is somewhat lower at early ages and a little higher at later ages than similar concrete without fly ash. The effects of fly ash on modulus of elasticity are not as significant as the effects of fly ash on strength. In general cement and aggregate characteristics will have a greater effect on modulus of elasticity than the use of fly ash.