Which metal has $2$ electrons in the outermost orbit?

Thermal stability of $BaCO_3, CaCO_3, SrCO_3$ and $MgCO_3$ decreases in the order of :

$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms $Y$. On passing an excess of $CO_2$ through $Y$ in water,a clear solution $Z$ is obtained. On boiling $Z$,compound $X$ is reformed. The compound $X$ is

The setting of Plaster of Paris is:

The substance not likely to contain $CaCO_3$ is

Portland cement is manufactured using which of the following?