Improving Concrete With Fly Ash

Fly Ash In Concrete

Many electric power plants in the United States are fueled by coal. So... what does this have to with concrete? Well, in addition to producing electricity these plants also create an important ingredient that enhances the durability of concrete products.

Fly ash is that portion of coal that is non-combustible. It is composed of silica, alumina and calcium and solidifies as microscopic, glassy spheres that are collected from the exhaust. Chemically, they are the same as pozzolan, which has very similar properties to volcanic ash.

In fact, the birthplace of pozzolan is a city named Pozzuoli located in Italy which is famed for its centuries old Pantheon and Colosseum having withstood the test of time for durability. In part, the reason for this longevity is ash-based concrete.

Because fly ash is small, it can effectively fill voids which are commonplace with standard Portland cement. These hard round particles act almost like ball-bearings to permit less water in the concrete composition. A main benefit is that it reduces the amount of non-durable calcium hydroxide or lime and, in fact, actually converts it into calcium silicate hydrate helping to improve the strength of the concrete.

Bear in mind that concrete is composed essentially of aggregates and cementitious paste. The best grade of concrete requires a smooth gradation of material from rock all the way down to the finest particles. To put it another way, the largest particle should fill most of the volume and smaller rock and sand should progressively fill in between.

The ideal final product should have as little lime as possible in the paste that ultimately coats every particle in the mixture. Adding fly ash to the cement helps to significantly convert the lime into CSH, thereby filling excess voids with the strength of the aggregates.

The majority of concrete strength is established within the first 28 days of placement. Ironically, fly ash concrete will usually have less strength during that period of time when compared with straight cement concrete. But, interestingly, it will develop a great deal more durability over time. In fact, in one year the concrete made with fly ash will typically have a gain of 1000 PSI.

It has been verified that the addition of fly ash to cement produces more cementitious paste and results in a lower unit weight. This helps improve the concrete flows by reducing the amount of water needed to produce a given slump. This reduction can be anywhere between 2 and 10 percent.

It is also easier to pump concrete that is made with fly ash. Workmen who are assigned to finish the placement frequently notice a creamier texture and see less holes and segregation when stripping the forms. Also, they observe less corrosion as a result of a reduced ingress of water, oxygen and chemicals.

Most people think that the ideal weather for setting concrete is very warm. In fact, too much heat causes hydration of the concrete thereby causing thermal cracking and leading to depletion of its strength and durability. Even very thin slabs can suffer these damaging effects and harm the ability to get the desired finish.