I'm having a pool installed now... water is going in as I type! 21 x 40 inground vinyl liner pool. Last week we had a bad heat wave (massachusetts). The pool company usually uses vermiculite for the hard bottom. But they didn't like how the vermiculite was mixing last week so they went to concrete/cement. They kept saying it's more expensive and stronger this way... they didn't want to do it but had no choice and ate the cost. Then the deep end sides went on last Friday... using a different concrete. Over the weekend, we poked around and went in it and it was hard, crumbly, and not smooth, bumps everywhere. Today they sanded it around the sides and went over the cracks with a vermiculite and within 2 hours, had the liner on and in goes the water.
I heard vermiculite bottoms are softer and better! Cement tends to crack and create sharp edges and can do a number on the liner. We expressed concern but they say they are experts and not to worry about it... we have an overbuilt solid pool.
Anyone have experience with any of this???