Gypsum is aded to the line Solution for One Purpose, So that The Solution Applic to the Surface Drys Faster. AFTER ALL, EVEN A Thin Layer of Lime Solution, Freezes for a Very Long Time. TheREFORE, IF Gypsum Is Added to it, The Process Is Going On, Much Faster. Thirty Minutes Later, It Will Completly Harden and You Can Proceded to the Next Operations.