Method and plant for the manufacture of building products
A plant and method for the manufacture of cementitious products, viz. cast concrete roof tiles (T), the plant comprising a mixing station ( 2 ) where the constituents of a cementitious composition, less an air entraining agent, are admixed under atmospheric pressure and further mixed under a partial vacuum of 200 to 340 mBar after the addition of an air entraining agent, the plant also comprising a molding station ( 4 ) where the tiles (T) are cast in a gang-mold ( 6 ) before being cured in a conventional autoclave for up to 10 hours at 45 degrees C. The cured concrete roof tiles (T) are de-molded after curing and conveyed to a surface preparation zone ( 12 ) where a primer coat of a water based epoxy and a surface coating of a water based emulsion paint are applied to at least an upper surface in use of the cast concrete roof tiles (T). The admixed cementitious composition has a Ford Cup flow of between 40 and 60 seconds and a slump test of between 180 and 220 mm. In an alternative method, instead of applying a primer coat, the upper surfaces, in use, of the roof tiles T are subjected to abrasion by grit-blasting to remove a thin layer from the upper surfaces thereby facilitating the subsequent application thereto of a surface coating.
1. A method for the manufacture of a concrete roof tile comprising the steps of:
a) admixing a cementitious composition comprising: water, sand, pigment, reactive fine filler, cement and additives;
characterized in that in step a):
a1) the water, sand, pigment, reactive fine filler, cement and an air entraining agent are mixed under atmospheric pressure; and,
a2) a super-plasticiser is then added to the cementitious composition, which is thereafter mixed in a second step in a partial vacuum;
b) feeding the mixed cementitious composition into a reusable mold to form green-state roof tile;
c) curing the green-state roof tile in a curing station;
d) de-molding the cured roof tile,
e) conveying the cured roof tile through a surface preparation zone where the upper surface, in use, of the roof tile is prepared to facilitate subsequent application thereto of a surface coating;
f) conveying the cured roof tile through a coating zone where the cured building roof tile is provided with a surface coating on its prepared upper surface, and
g) the surface preparation of said upper surface includes the application of a primer coat thereto.
2. A method according to claim 1 , wherein the cementitious composition is mixed in the second step under a partial vacuum of between 240 and 320 mBar.
3. A method according to claim 1 , wherein the step of feeding the mixed cementitious composition into a re-usable mold is selected from filling the mold under pressure and filling the mold by gravity.
4. A method according to claim 1 , wherein the filled mold is conveyed to a curing zone where the green-state roof tile is subjected to heating at between 40 and 60 degrees Centigrade for a period of between 8 and 12 hours.
5. A method according to claim 1 , wherein the mold is prepared after the removal of the cured roof tile for its re-use by cleaning and applying a release agent.
6. A method according to claim 1 , wherein the primer is a water-based primer.
7. A method according to claim 6 , wherein after application of the water based primer coat to the cured roof tile, it is conveyed to a first curing zone in which the primer coat is heated to effect cross-linking of the primer coat and to reinforce the surface of the cured roof tile to provide a receptive surface for the subsequently applied surface coating.
8. A method according to claim 7 , wherein the surface coating comprises a water-based emulsion.
9. A method according to claim 8 , wherein the cured roof tile is conveyed to a second curing zone in which the water based emulsion surface coating is heated to reduce a tendency for the cured roof tile to be damaged during packaging.
10. A method according to claim 6 , wherein the roof tile is selectively warmed up in a first heating zone before the application of the water based primer coat to said upper surfaces, and the primer coated roof tile is selectively warmed up in a second heating zone before the application thereto of the water based emulsion surface coating.
11. A method according to claim 1 , wherein said upper surface is additionally subjected to abrasion.
12. A method according to claim 6 , wherein said primer is a water-based epoxy.
13. A method according to claim 1 , wherein said primer coat reinforces loose particles and penetrates release layer residue on the surface of the cured roof tile to provide a receptive surface for a subsequently applied surface coating.
14. A method according to claim 7 , wherein said primer is a water-based epoxy.