Fly ash and slag in concrete

Hello, I'm wondering how much fly ash or slag is used to replace cement in concrete? If a yard of concrete is roughly 4,000 lbs, my understanding is fly ash might replace 20% of the cement (3,200 lbs cement, 800 lbs fly ash) while slag might replace closer to 50% (2,000 lbs slag, 2,000 lbs cement). Are these figure correct? Thanks!

Answer: No, not quite

Here is a basic mixture of 1 yard of 3000 psi concrtete.

1. 517 pounds of cement
2. 1560 pounds of sand
3. 1600 pounds of stone
4. 32 - 34 gallons of water

The SLAG or FLY ASH only replace some of the CEMENT content of the yard of concrete not the sand of stone.

The redi mix companies I use typically replace about 20% of the cement with slag.

So 20% of 517 is about 103 pounds in a typical yard of concrete.

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