What is retarder paper, and how is it used?
Retarder paper is a production material that enables producers to efficiently cast exposed-aggregate units without using liquids. Paper manufacturers mechanically coat a special laminated paper with varying concentrations of a retarding admixture. (This concentration level is typically reported as grams per square meter.) Manufacturers typically offer two paper thicknesses: paper that's coated for semi-dry casting and paper that's impregnated for products that are wet-cast or produced on press slab machines.
Producers place sheets of retarder paper on the bottom of their forms or molds. For best results, one paper manufacturer suggests placing concrete in the middle of the paper, on its chemically treated side. The paper transfers the retarder to the fresh concrete it directly contacts. For wet-cast operations, the paper manufacturer recommends that concrete cure 18 hours if unheated, before stripping. If heated, they recommend letting the mix preset for a minimum of 3 hours before subjecting the concrete to heat.
They recommend not exceeding 130° F during curing. After curing, the producers remove the retarder paper and pressure-wash the surface to a uniform exposed finish. Producers are advised to try several production trials with varying paper grades to achieve the desired effect. Retarder concentration will vary since it's based upon the top-sized aggregate in the mix, the mix design, and the producer's curing method.