I have some leftover masonry cement. Can I use it in place of Portland cement?

Masonry cement cannot be used in place of Portland cement. Specially formulated and manufactured, masonry cement is a blend of Portland cement and plasticizing materials – along with other property-enhancing admixtures – for use in brick, block and stone masonry construction. A mixture containing masonry cement will not set up and gain strength in the same way that Portland cement will.