How Fly Ash Protects Concrete


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An extremely important aspect of the durability of concrete is its permeability. Fly ash concrete is less permeable because fly ash reduces the amount of water needed to produce a given slump, and through pozzolanic activity, creates more durable CSH as it fills capillaries and bleed water channels occupied by water-soluble lime (calcium hydroxide).

 

Fly ash improves corrosion protection. By decreasing concrete permeability, fly ash can reduce the rate of ingress of water, corrosive chemicals and oxygen — thus protecting steel reinforcement from corrosion and its subsequent expansive result.

 

Fly ash also increases sulfate resistance and reduces alkali-silica reactivity. At this point a distinction between Class C and Class F fly ashes needs to be made. While both improve the permeability and general durability of concrete, the chemistry of Class F ashes has proven to be more effective in mitigating sulfate and alkali-silica expansion and deterioration in concrete. Some Class C fly ashes have been used to mitigate these reactions, but must be used at higher rates of cement replacement.

 

Fly Ash in concrete can reduce sulfate attack in two additional ways:

 

– Fly ash reduces calcium hydroxide, which combines with sulfates to produce gypsum. Gypsum is a material that has greater volume than the calcium hydroxide and sulfates that combine to form it, causing damaging expansion.

– Aluminates in the cement also combine with sulfates to form expansive compounds. By replacing cement, the amount of available aluminates is reduced, thereby lowering the potential for this type of expansive reaction.

 

In reducing alkali-silica reactivity, fly ash has the ability to react with the alkali hydroxides in portland cement paste, making them unavailable for reaction with reactive silica in certain aggregates. Certain studies suggest that greater than 30% replacement with fly ash for cement has a dramatic effect in combating this expansive reaction.

 

How Fly Ash Reduces Heat of Hydration in Concrete

 

The hydration of cement is an exothermic reaction. Heat is generated very quickly, causing the concrete temperature to rise and accelerating the setting time and strength gain of the concrete. For most concrete installations, the heat generation is not detrimental to its long-term strength and durability. However, many applications exist where the rapid heat gain of cement increases the chances of thermal cracking, leading to reduced concrete strength and durability. In these applications, replacing large percentages of cement with fly ash (fly ash generates only 15 to 35 percent as much heat as compared to cement at early ages) can reduce the damaging effects of thermal cracking.

 

While the first structures to apply this concept in earnest were hydroelectric dams built in the 1930s and 1940s with 40% to 50% cement replacement, warm weather concreting and the risk of thermal cracking is a problem that exists today for all concrete. Warm weather will naturally raise the temperature of concrete aggregates, which make up the majority of the mass in concrete.

 

This natural heating of the aggregates, coupled with solar heating at the construction site, can cause even thin concrete slabs to suffer the damaging effects of thermal cracking, along with finishing difficulties caused by rapid uncontrolled setting. Replacing 20% to 35% of the cement for “everyday” concrete in warm conditions will help reduce thermal cracking and provide the time needed to obtain the desired finish.

 

 

FLY ASH CHECKLIST: Enhancing Concrete Workability

 

The “ball-bearing” effect of fly ash particles creates a lubricating action when concrete is in its plastic state. This creates benefits in:

 

Workability

Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.

Ease of Pumping

Pumping requires less energy and longer pumping distances are possible.

Improved Finishing

Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.

Reduced Bleeding

Fewer bleed channels decrease permeability and chemical attack. Bleed streaking is reduced for architectural finishes.

Reduced Segregation

Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes.