The white deposits are salts, which are bleeding through from the grout joints or from the mortar. An acid wash will be required to remove the salts. If the deposits are heavy, try scraping them off before washing the slate with acid.
Several acid products are available for this purpose. Ask your local stone supplier.Once the salts are cleaned apply a good quality sealer to the slate. The sealer should help keep the salts from returning.
TIP To prevent these salts from coming back, make sure all the grout joints are sealed. It may be necessary to reseal or re-caulk them.
Article ID: 620
Created: January 21, 2022
Last Updated: January 21, 2022
Author: Pam Freitag
Online URL: https://sr-manual.com/kb/article/the-slate-steps-on-our-front-porch-have-developed-a-hard-white-deposit-coming-from-the-grout-lines-how-can-we-clean-it-off-620.html