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A method of manufacturing a porous cementitious product, which method comprises: forming a cementitious premix; casting the premix in a desired configuration; generating gas bubbles within the premix; and curing the premix, wherein gas bubbles are generated and/or collapsed at specific locations wit

A method of manufacturing a porous cementitious product, which method comprises: forming a cementitious premix; casting the premix in a desired configuration; generating gas bubbles within the premix; and curing the premix, wherein gas bubbles are generated and/or collapsed at specific locations within the premix in order to produce a porosity profile along a cross-section of the product such that the product comprises a relatively low density core region and higher density outer regions.

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1. A method of manufacturing a porous cementitious product, which method comprises in the following order: (a) forming a cementitious premix including sand as aggregate by mixing premix components with water;(b) mixing the cementitious premix formed in (a) with a heat-activated gas-generating agent

1. A method of manufacturing a porous cementitious product, which method comprises in the following order: (a) forming a cementitious premix including sand as aggregate by mixing premix components with water;(b) mixing the cementitious premix formed in (a) with a heat-activated gas-generating agent and then immediately casting the premix in a desired configuration in a formwork, wherein the premix is sufficiently viscous to achieve gas bubble retention but not so highly viscous so as to inhibit formation and migration of gas bubbles, and wherein the formwork is underfilled by from 30 to 50% based on the volume of the formwork;(c) allowing gas bubbles to be generated within the premix by reaction involving the heat-activated gas-generating agent thereby causing the premix to expand in the formwork;(d) collapsing the expanding premix at its upper surface by contacting with a formwork lid or by subjecting the upper surface to trowelling, screeding and/or rolling; and(e) curing the premix,wherein gas bubbles are generated and/or collapsed at specific locations within the premix in order to produce a porosity profile along a cross-section of the product such that the product comprises a relatively low density core region and higher density outer regions. 2. A method according to claim 1, wherein the lid of the formwork is adapted to allow gas dissipation when gas bubbles collapse at the premix/lid interface. 3. A method according to claim 1, wherein, after casting but before curing of the premix, the upper surface of the premix is subjected to trowelling, screeding and/or rolling in order to cause collapsing of expanding premix. 4. A method according to claim 1, wherein, after casting but before curing of the premix, the formwork is vibrated vertically and/or laterally at an appropriate frequency and amplitude in order to achieve an even distribution of premix within the formwork, to control the cross-sectional bubble distribution and/or to improve smoothness of the product surfaces. 5. A method according to claim 1, wherein a formwork is used for shaping the premix and wherein the formwork is vibrated vertically and/or laterally at an appropriate frequency and amplitude in order to achieve an even distribution of premix within the formwork, to control the cross-sectional bubble distribution and/or to improve smoothness of the product surfaces. 6. A method according to claim 1, wherein, after casting but before curing of the premix, a sparging lance is used to inject gas at selected locations into the premix. 7. A method according to claim 6, wherein the sparging lance comprises an elongate hollow member having a series of holes through which gas may be injected into the premix. 8. A method according to claim 7, wherein the sparging lance is moved through the premix during gas injection to provide a distribution of bubbles appropriate to achieve the desired porosity profile. 9. A method according to claim 1, wherein, after casting but before curing of the premix, the premix is sufficiently viscous to achieve gas bubble retention but not so highly viscous so as to inhibit bubble formation. 10. A method according to claim 1, wherein the viscosity of the premix is controlled by varying the premix temperature, by blending of fine materials into the premix to obtain desired particle graduation for optimal flow properties and/or by incorporation into the premix of a superplasticising admixture. 11. A method according to claim 10, wherein the viscosity of the premix is controlled by incorporation into the premix of a superplasticiser. 12. A method according to claim 1, wherein the strength to density ratio of the cementitious product is controlled by varying the extent to which gas bubbles that have been generated in the premix are retained. 13. A method according to claim 1, wherein the strength to density ratio of the cementitious product is controlled by varying the degree of confinement of the premix as it expands due to generation of gas bubbles within the premix. 14. A method according to claim 1, wherein the strength to density ratio of the cementitious product is controlled by selection based on premix strength. 15. A method according to claim 1, wherein after casting but prior to curing of the premix, an upper surface of the premix is finished by cutting, trowelling, screeding or rolling. 16. A method according to claim 1, wherein the premix is a high strength premix that on curing would yield a product having a compressive strength of from 60 to 120 MPa (in non-gassed form). 17. A method according to claim 16, wherein the premix is used to manufacture a cementitious product having a dry density of from 1000 to 1500 kg/m3 and compressive strength of 10 to 25 MPa. 18. A method according to claim 16, wherein the cementitious product has a 1-day strength of from 75-90% of its 28-day strength. 19. A method according to claim 18, wherein curing of the premix takes place using heat at atmospheric pressure. 20. A method according to claim 16, wherein the cementitious product has residual water content of from 12-15% by weight. 21. A method according to claim 1, wherein the cementitious product exhibits a flexural strength of from 3-4 MPa for compressive strengths of from 15-20 MPa for product densities of from 1300-1500 kg/m3. 22. A method according to claim 1, wherein the cementitious product has a thermal conductivity of from 0.3-0.6 W/m·K for product dry densities of from 900-1300 kg/m3. 23. A method according to claim 1, wherein high shear mixing is used to vary the premix temperature and/or the premix rheology thereby allowing the viscosity of the premix to be controlled prior to casting. 24. A method according to claim 1, wherein the cementitious product is manufactured in the form of a flat slab, wall panel, roofing tile, block-work system or paver. 25. A method according to claim 1, wherein the formwork includes surface relief in order to produce a patterned surface on the product. 26. A method of manufacturing at least two cementitious products which are formed from a single cementitious premix and which have a different ratio of strength to density, which method comprises forming each cementitious product in accordance with the method claimed in claim 1 and wherein the strength to density ratio of each cementitious product is controlled by varying the degree of confinement of the premix as it expands due to generation of gas bubbles within the matrix. 27. A cementitious product obtained by the method as claimed in claim 1. 28. A method according to claim 1, wherein the premix is confined in the formwork in order to contribute to the desired porosity profile. 29. A method according to claim 1, wherein the cementitious product has a consolidated, dense outer skin.