It is hardly necessary to say that when there is a choice, the strength and quality of the cementing material should be in proportion to the importance of the part the concrete has to play. Thus fat lime concretes would be objectionable almost anywhere, except as filling in the spandrils of arches. Hydraulic lime, or cement, is advisable for concrete in nearly all situations. Eminently hydraulic limes should be used for concrete foundations in damp ground, and in the absence of cement for subaqueous work of any kind. Portland cement concretes are adapted for all position . especially for work under water, or where great strength is required; also in situations where the concrete has to take the place of stone, as in facing to walls, copings, etc. For work to be executed between tides, where the concrete is required to set quickly but not to attain any great ultimate strength, Roman or Medina cement may be used with advantage. When, for the sake of its strength, Portland cement concrete is necessarily used under water, it must be protected by canvas covering or other means from any action which would wash it away before it had time to set.

When concrete is likely to be exposed to great heat, as in fireproof floors, gypsum has been used as a matrix.