Admixtures

Dear Marianna,

The ACI 234 committee which provides guidelines for the use of silica fume in concrete has the following to say about the effect of silica fume on the color of concrete. “Fresh and hardened concretes containing silica fume are generally darker than conventional concrete. This is particularly apparent for concretes containing higher percentages of silica fume as well as those silica fumes that have a high percentage of carbon. The color difference may lesson and virtually disappear after some time”.

Regular Portland cement concrete can also be darker in early ages and become lighter after a few days. Since silica fume hydrates as at a slower rate than cement the dark color lasts longer. However, as the silica fume hydrates the color of the concrete will become lighter. A damp or moist environment may take a little longer. If color is a major consideration you can use white silica fume. It is a little more expensive but does not darken the concrete.

Hope that answers your question.

Dear Prajot,

ASTM C 494 classifies water reducing and retarding admixtures into the below types:

Type A – Water reducing admixture
Type B – Retarding admixture
Type D – Water reducing and retarding admixture
Type F – High range water reducing admixture
Type G- High range water reducing and retarding admixture

A water reducer when added to a concrete mix increases the slump of the concrete, a retarding admixture delays the hydration process of the cement for some time allowing the concrete to be workable for more time whereas a water reducing and retarding admixture will increase the slump and maintain the slump life for sometime.

For lower grades of concrete where the water to cement ratio is slightly high, to achieve a high slump a Type A or Type D admixture would work for you. If you are mixing and placing the concrete within a short time (say less than 15 minutes) you can use a Type A admixture, if placing of concrete is going to take more time after mixing then you might want to use a Type D admixture.

Similarly for higher grades of concrete where the water to cement ratio is low, to achieve a high slump you might have to use a Type F or Type G admixture. Again, if you are going to place the concrete after mixing within a short time you can use a Type F admixture and if placing the concrete is going to take time then you might want to use a Type G admixture.

If you are a concrete producer the best thing for you to do would be to contact an admixture supplier, provide him your concrete mix design and explain how much slump you need at the placing point after how much time and he should be in a position to suggest you the right admixture for your need. Selecting the right admixture will allow you to place concrete without the need to redoes. In case there is a unforeseen delay during placement and the slump drops below than required, then it is safe to re-dose the concrete using a Type F admixture.

Thanks