I am building a stone spring box on my farm. The base will be a solid granite, and I'm using granite stone found here on the property. The box will be approximately 4' x 8' and a max of 3' tall. Most of the wall will be below grade when finished.
Can I add portland cement to type S mortar mix to make the mortar waterproof? I've also been told to mix a bag of thinset with 2 bags of type S.