Fast-setting concrete is a special mix that hardens in about 20 to 40 minutes instead of hours, and it is designed to be poured dry straight into a post hole and then wetted — no mixing required. It is the go-to for setting fence posts, mailbox posts, and deck posts, and for quick small repairs.
The trade-off for that speed is a short working window and less control, so fast-set shines for small, fast jobs and is the wrong choice for slabs or anything you need to finish carefully.
Setting posts with it
This is its best use. Dig the hole (about three times the post width and a third of the post length deep), add a few inches of gravel, set and brace the post plumb, then pour the dry fast-setting mix in around it and add water per the bag — usually about a gallon per bag — letting it soak in. It sets in 20 to 40 minutes and you can hang or load the post within hours. The post hole calculator sizes the bags per hole.
How it is different
Fast-setting mixes use a different cement chemistry that hydrates much faster, so they set hard in well under an hour and reach usable strength quickly. Standard concrete stays workable for 30 to 90 minutes and is finished and cured over days. Fast-set trades that working time for speed.
Where not to use it
Do not use fast-setting mix for slabs, large pours, or anything that needs screeding and finishing — it will harden before you can work it. It is also not the economical choice in volume. For floors, patios, and driveways, use standard ready-mix or a general-purpose bagged mix and finish it properly.
Strength and curing
Quality fast-setting concrete reaches strengths comparable to standard mixes (commonly around 4,000 psi at 28 days), it just gets to early usable strength much sooner. It still benefits from staying damp as it cures — the fast set is about the first hour, not the final strength.
Common questions
Do you have to mix fast-setting concrete for posts?
No — that's its advantage. Pour the dry mix into the hole around the braced post and add water per the bag. It sets in 20 to 40 minutes.
How long does fast-setting concrete take to set?
About 20 to 40 minutes to set hard, with usable strength in a few hours. Full strength still develops over about 28 days.
Can you use fast-setting concrete for a slab?
No. It hardens before you can screed and finish a slab. Use standard ready-mix or a general-purpose bagged mix for flatwork.