Bricks, Masonry, Asphalt and Concrete - iron railings

I recently bought and inslalled new iron railings on my front stoop. I drilled through the sidewalk at the base of the steps to set the rails about 6" into the ground. they are lag bolted to pillars at the top of the porch. the holes that I drilled in the sidewalk are small leaving an inch or so around the railing post to recement. As the holes are small I was going to make my cement mix watery in order to penetrate these holes to the bottom. Will this cause a problem? and is there a particular type of cement that I should use? Thank all of you for any info and advice you may make.

They make a cement that set up in five minutes that is made for railings. pack the cement in real tight with a trowel or spoon.