Technical Questions

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The effect of pozzolans on water demand

Q. Does the use of a pozzolan increase the mixing water requirement?


A. In general, fly ash reduces the water requirement a few percent, whereas most other pozzolans increase it a few percent (ACI 232.R). Silica fume at high dosages will have a large effect in increasing the amount of water needed to achieve the desired workability because of the high surface area, but this is almost always compensated by using a high range water-reducing admixture (ACI 234R). Silica fume at low dosages (<5% by mass of cementitious material) reduces the water demand because it fills the voids between cement grains occupied by water displacing the trapped water. At high dosages, particle packing is complete for the most part and excess silica fume is adding more surface area, and thus increases the water demand.


References: SP-1(02); E3-13 ; SP-221; ACI CT-18; ACI 232.1R-12; ACI 232.2R-03; ACI PRC-232.4-20; ACI 234R-06; ASTM C618; ASTM C1240

Topics in Concrete: Pozzolan; Cementitious Material; Materials in Concrete; Concrete Fundamentals

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