Concrete + Fly ash
Properties of the material
Shapes
Fly ash consists mainly of hollow spherical shapes which creates a ball bearing effect in the mix, improving workability without increasing water requirements. Fly ash also improves the pump-ability of concrete by making it more cohesive and less prone to segregation. The spherical shape improves the pump-ability by decreasing the friction between the concrete and the pump line. In addition, some fly ashes have been shown to significantly decrease heat generation as the concrete hardens and strengthens. Fly ash, as do all pozzolanic materials, generally provide increased concrete strength gain for much longer periods than mixes with Portland cement only.
Durability
The biggest reason to use …show more content…
The spherical shape of fly ash particles and the glassy nature of slag particles reduce the amount of water necessary to achieve equivalent workability. They can also improve the concrete’s pump-ability. Performance and suitability
Fly ash will increase the concrete performance. In its hardened state, fly ash creates additional benefits for concrete, including:
• Higher Strength. Fly ash continues to combine with free lime, increasing compressive strength over time.
• Decreased Permeability. Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability.
• Increased Durability. Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater.
• Reduced Efflorescence. Fly ash chemically binds free lime and salts that can create efflorescence, and dense concrete holds efflorescence producing compounds on the inside.
• Reduced Shrinkage. The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying …show more content…
However, concrete containing fly ash generally offers superior resistance to sulfate attacks as they lower permeability, thereby restricting the ingress and movement of sulfate-bearing ions. They also reduce the compounds that can react with sulfates. For example, class F fly ashes are very effective in improving sulfate resistance. The effectiveness of class C fly ashes is depends on the ash chemistry and the replacement level.
Advantages and Disadvantages of the material
Economy
• The cost of fly ash is less than cement, however fly ash does not substitute cement and gain strength when mixed with water; it compliments cement in that it uses by-products of the cement hydration process to build strength. As a result, fly ash can be blended with cement in order to achieve more cost effective cement solution.
Long term strength development
• Fly ash is a synthetic pozzolan material. In the presence of Calcium Hydroxide and water it will form C-S-H (Calcium Silicate Hydrate) which gives fly ash based concrete the ability to develop strength over prolonged periods of