Failure and frustration - ideas/solutions please
Finished a gazing ball a few weeks ago that my sis fell in love with, but it needed a sturdy stand due to the strong winds in her area. Decided I wanted to use a green glass vase as the stand part, so I made a stepping stone to mosaic. Got a bag of "Type S portland mortar mix" which the guy at HD said was the same as Portland cement ( it is composed of sand and Portland cement). Used a greased cake pan for the mold and poured a layer about 1" thick, let it cure for a couple of weeks, poured a second layer of same thickness and centered the top of the vase in it so the mix came up the inside and outside of the vase to hold it in place. Unmolded it today after it had set for a week. Found 2 problems and don't know what caused them:
Two vertical cracks in the second layer that started at the juncture with the bottom layer and went to the top of the 2nd layer. Cause??
Even after letting it set, I can easily sand all parts (sides, top and bottom) of the stepping stone with a piece of 150 grit sand paper. Cause??
A few taps with a hammer and that 2nd layer just fractured and came off the bottom layer, leaving the piece that was inside the vase relatively intact but with cracks all over it. Vase is still intact so I can still use it to make a stand, but I'm really frustrated about the failure of the layers to stick together, the ease of sanding it and the cracks.
Ideas on what caused the problems?? How I can avoid them on future stones??