"Unbuffered solution" means that OH
- ions only come from the dissociation of Ca(OH)
2 (those from the dissociation of water are negligible). Then K
sp = 4x
3, where x is the concentration of calcium ions and [OH
-] = 2x.
In fact, there is one more equilibrium Ca(OH)
2 CaOH
+ + OH
- in the calcium hydroxide solution, which is not taken into account by the solubility product. As a result, the concentration of calcium ions calculated from the actual pH value of the solution is slightly lower (a few%) than half of the concentration of hydroxide ions. With your K
sp value, nonsense comes out of your incorrect calculation.