I would like to know the way to stiffen concrete (not too waterly)
while still allow me enough time to work on it.

What I am trying to do is to level the basement floor. My floor is not
level - the difference from one side to another side is around 3
inches. In order to level the floor, I am supposed to use concrete to
build up a dam around the edge of the basement wall, then I can use a
long wood block (2x4) that rides on the dam to level the floor. My
question is about whether the concrete will be too waterly to hold
itself up, especially in the area that I need to build the dam to 3
inches tall.

I guess I should add less water into the concrete mix to make sure it
is stiff enough. But I am afraid that the concrete will dry too fast
that doesn't give me enough time to build the dam very nicely. I guess
making small batches will help in this situation. I am just wondering
whether there is other alternatives.

I am planning to use concrete that has pre-mixed with sand (not small
stones). I mention this just in case this matters.

Thanks in advance for any tip.

Jay Chan