The substance not likely to contain $CaCO_3$ is

Portland cement contains '$X$' to enhance the setting time. What is '$X$'?

Plaster of Paris hardens by:

Assertion $(A)$: $MgO$, $CaO$, $SrO$, and $BaO$ are insoluble in water.
Reason $(R)$: In aqueous medium, the basic strength of $MgO$, $CaO$, $SrO$, and $BaO$ increases with an increase in the atomic number of the metal.
The correct option among the following is:

Which hydroxide of the $II^{nd}$ $A$ group metals has the lowest solubility product $(K_{sp})$ value at $25\,^{\circ}C$?

The difference in the number of water molecules in gypsum and plaster of Paris is: