Method of forming a decorative layer on a precast concrete board

Method of forming a decorative coating on the surface of precast concrete board which comprises:

• pouring a concrete mixture consisting essentially of cement, aggregate and water into a mold, smoothing the surface of the resulting unhardened concrete, spraying an hydraulic coating material on the unhardened concrete having a good workability in the same mold at a coating density from about 4 to about 6 kg/m.sup.2, and immediately thereafter heat-curing the so-coated concrete at a temperature between about 20.degree. C and about 90.degree. C for from about 3 to about 5 hours, said hydraulic coating material consisting essentially of a coating composition whose viscosity is between about 5,000 and 50,000 cps. and which is composed of (a) 100 parts by weight of Portland cement, (b) from about 2 to about 20 parts by weight of curing agent composition consisting essentially of at least one inorganic salt selected from the group consisting of alkali metal carbonate, alkaline earth metal carbonate, alkali metal chloride, alkaline earth metal chloride, and alkali metal silicate, and (c) an effective quantity of at least one additive selected from the group consisting of water proofing agent, cement dispersing agent, defoaming agent, crack-preventing agent, aggregate, coloring pigment and pigment-dispersing agent.