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Coal Combustion By-Products (CCBs) How To ...

Evaluating Fly Ash and Bottom Ash for Concrete Block


Concrete masonry block is a very common building material, and is often loosely (and incorrectly) referred to as cinder block. This latter term is only accurate if cinders, which are akin to bottom ash but mainly produced in boilers called "stokers", are used as the aggregate (cinders are much less common than they used to be). Masonry block contains cement, water, sand, and a coarse aggregate that is usually less than 3/8 inch. Masonry block producers thus use coal combustion byproducts in two ways: fly ash as a cement replacement, and bottom ash as a partial replacement for the sand and/or coarse aggregate. Because the bottom ash is porous, it is imperative that it be thoroughly saturated with water prior to use. This is necessary because, if used dry, the bottom ash will absorb cement in addition to water, which will weaken the block.


The block mix is prepared by combining the solid ingredients (i.e. cement, fly ash, sand, pre-soaked bottom ash) into a forced action mixer. Unlike concrete, the amount of water added is not prescribed. Rather, the water is added until a desired consistency is achieved. If the block is too dry, it will crumble. If it is too wet, it will sag or slump before it hardens.

Preparation of Block

The wet block mix is conveyed to a hopper where it is fed to a block machine. The machine uses vibration and a small amount of pressure to form a block. The block is extracted from the machine and transferred to the curing chamber. If the mix is prepared correctly, the fresh block can be (carefully) moved around before it has set and hardened. The block is then steam-cured (between 150 - 180oF) before being stored outdoors and shipped to its final destination.


Compressive strength of the block is measured in a manner similar to that for concrete, except that capping masonry block requires a much larger mold. The compressive strength tester also must be modified to accommodate the block.

Masonry Mixer or Forced Action Mixer Combines Solid Ingredients to Prepare a Block Mix Block Machine Uses Vibration and a Small Amount of Pressure to Form a Block The Block is then Steam-cured (between 150 - 180 degrees F) Before Being Stored Outdoors and Shipped to its Final Destination

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