A flexible cloth that can be set to become rigid or semi-rigid, the cloth comprising: a first face; a second face separated from the first face by a space; linking fibres extending between the first and second faces (10, 12) that maintain the first and second face (10, 12) in a spaced-apart arrangement; and a compacted powder material located in the space between the first and second faces (10, 12), which material is capable of setting to a rigid or semi-rigid solid on the addition of a liquid, wherein the linking fibres are configured to be self-supporting in that they are sufficiently resistant to bending under forces tending to crush the fabric before addition of the powder material, in order to maintain the spacing between the faces (10, 12) when the powder material is placed onto the first face, and wherein both the first and the second faces (10, 12) are substantially impervious to the powder material but at least one of them is pervious to liquid and characterised in that the amount and type of reagent in the compacted powder material, and the volume and compaction of the powder material are such that: MV − OV = x * LV where: MV = the maximum volume of the space within the cloth per unit area of the cloth; thus MV includes both the volume of empty space in the cloth before addition of the powder material, and an additional volume resulting from any expansion of the space due to the pressure exerted by the swelling of the powder material during the addition of the liquid or during the setting of material; OV = the volume of the space within the cloth that is occupied by the particles of the powder material, which volume does not include the volume occupied by voids within the powder material per unit area of the cloth; LV = the volume of liquid per unit area of the cloth that results in the maximum long term 28 day compressive strength, of the fill blend when set; and x = a factor between 0.65 and 1.1. a first face; a second face separated from the first face by a space; linking fibres extending between the first and second faces (10, 12) that maintain the first and second face (10, 12) in a spaced-apart arrangement; and a compacted powder material located in the space between the first and second faces (10, 12), which material is capable of setting to a rigid or semi-rigid solid on the addition of a liquid, wherein the linking fibres are configured to be self-supporting in that they are sufficiently resistant to bending under forces tending to crush the fabric before addition of the powder material, in order to maintain the spacing between the faces (10, 12) when the powder material is placed onto the first face, and wherein both the first and the second faces (10, 12) are substantially impervious to the powder material but at least one of them is pervious to liquid and characterised in that the amount and type of reagent in the compacted powder material, and the volume and compaction of the powder material are such that: MV − OV = x * LV where: MV = the maximum volume of the space within the cloth per unit area of the cloth; thus MV includes both the volume of empty space in the cloth before addition of the powder material, and an additional volume resulting from any expansion of the space due to the pressure exerted by the swelling of the powder material during the addition of the liquid or during the setting of material; OV = the volume of the space within the cloth that is occupied by the particles of the powder material, which volume does not include the volume occupied by voids within the powder material per unit area of the cloth; LV = the volume of liquid per unit area of the cloth that results in the maximum long term 28 day compressive strength, of the fill blend when set; and x = a factor between 0.65 and 1.1. MV = the maximum volume of the space within the cloth per unit area of the cloth; thus MV includes both the volume of empty space in the cloth before addition of the powder material, and an additional volume resulting from any expansion of the space due to the pressure exerted by the swelling of the powder material during the addition of the liquid or during the setting of material; OV = the volume of the space within the cloth that is occupied by the particles of the powder material, which volume does not include the volume occupied by voids within the powder material per unit area of the cloth; LV = the volume of liquid per unit area of the cloth that results in the maximum long term 28 day compressive strength, of the fill blend when set; and x = a factor between 0.65 and 1.1.