Calgon is used for water treatment.

$\mathrm{Na}_{6}\left(\mathrm{PO}_{3}\right)_{6}$ or $\mathrm{Na}_{6} \mathrm{P}_{6} \mathrm{O}_{18}$

Order of abundance of element in earth crust is

$\mathrm{O}>\mathrm{Si}>\mathrm{Al}>\mathrm{Fe}>\mathrm{Ca}>\mathrm{Na}>\mathrm{Mg}>\mathrm{K}$

So second most abundant element in earth crust is Si not Ca.