I'm planning to finish out my half basement and was wondering how best to seal the gap between the poured concrete floor and concrete block walls. After the floor was poured the concrete apparently shrunk leaving a gap that varies from about 1/8 inch to 1/4 inch around the entire perimeter.

My first thought was to brush dry hydraulic cement into the gap and then spray it with water. As the cement hardens it should expand and seal the gap. But then I was afraid that temperature changes might cause the floor to expand/shrink breaking the seal.

My second thought was to try and force clear silicone sealant into the gap which would remain flexible should the floor expand or contract. But I can foresee a problem in getting enough sealant into the rather small opening to effect a permanent seal.

My objective in sealing this gap is mainly to prevent entry of water which happened once when the perimeter drain system failed due to grass clogging the discharge pipe. Sealing the gap would also prevent entry of ants, which IÂve had some problems with, as well as reducing the possibility of radon entry.

Any suggestions on how best to do this?