23.20 - Manufacture of refractory products
This class includes the manufacture of intermediate products from mined or quarried non-metallic minerals, such as sand, gravel, stone or clay. This class includes: - manufacture of refractory mortars, concretes etc. - manufacture of refractory ceramic goods: . heat-insulating ceramic goods of siliceous fossil meals . refractory bricks, blocks and tiles etc. . retorts, crucibles, muffles, nozzles, tubes, pipes etc.