[Alan Wheeler]

Roverhi,... I have been doing some homework for you, in regards to your very important question on "how do you make new cement stick to old"? OK... there are two important aspects to this. First is in further reply to your "concrete path" description. It's important to remember that you aren't dealing with 'concrete' as such as found in a path. The reason why old and new concrete will not join well, is because the main medium in 'concrete' is the stones. The cement is bonding the stones together and as there is not a lot of cement in the concrete mix ratio, cement will not or can not bond to itself very well. Thus a concrete slab will crack at a join. The cement mix in boat construction is a different story. The cement is of very proportion to the sand mix. The cement over time will indeed continue to chemicaly bond together as it cures.
But here is the important point No.2 in a repair. PVA is used as a bonding agent. Yep, that is simple white wood glue. The old cement edge is kept wet with water and the PVA as the new cement is applied. Small repairs can have the PVA mixed into the cement mix as well. The finished result is a strong water tight bond that continues to get stronger with future curing. One such repair done on a local boat here, was surveyed about 10yrs later and no one could find were the repair was carried out.