My wife is replacing mortar around the pool. Yesterday she put some down, she is using the bonding agent, and then this morning it was cracked in various places. Can someone explain to me how this occurs? Thank you in advance.

Clarke

A bonding agent between two different mortar / concrete surfaces only works over a large surface area. For example: You have a slab, you apply bonding agent on top and then pour a second slab on top. The bonding agent will help the two stay together because there is such a large surface area.

Different for cracks. There is really no way to permanently seal a crack - especially if it is under some stress or movement. Mortar only has strength in compression so it doesn't do much to hold the two sides of a crack together. Add to that is that a thin mortar layer will tend to crack as it cures. There is just not much strength there to keep it together.

About the only thing that will seal a crack in concrete is a concrete caulk. This works better because it will adhere to the concrete and will also flex and compress with any movement. However, the caulk rarely matches the look of the existing concrete.

Jaybee, I liked your reply to Clarke. Very informative.
And, to amplify what you said about caulk rarely matching the existing concrete, from first-hand experience, I can tell you that it looks even worse after a rain. I have a concrete patio with a long crack down the middle which I filled with urethane, concrete, caulk. After a rain, the crack looks like the river Nile (the concrete gets dark, while the caulk remains light).

I have 'cheated' and swept on a coating of dry mortar on top of the still wet caulk. Add enough so that you can gently tap some of the mortar in to the caulk with your finger. While this doesn't help hold the crack together it will blend better for color. Still a temporary fix though.

You may want to try hydraulic cement, just pay close attention to the directions.