Oh, that's a question and a half Herbie! Ratios vary depending upon the application, and nobody seems to agree exactly what they should be. Also, the specifications (usually shown as a "C" plus a number) have changed recently, so two systems are now being used side-by-side, both employing a C-rating, but each meaning something different. It's very confusing,
The amount of water you add also has an effect on final strength, and the rule of thumb is that the mix should have the consistency of warmed marzipan when ready. Too sloppy and the mortar/concrete will crack as it dries.Here are some ratios for Concrete Mixes:
C7.5 (low strength) 1:3:6 or7 (Cement/Sand/Coarse Aggregate)
For general non-structural use – bedding in kerbs, posts, stabilising underground pipes etc.
C10 to C15 (medium strength) 1:4:6 to 1:4:5 (Cement/Sand/Medium Aggregate)
Used in typical house foundations, footings for garden walls, load-bearing areas etc.
C20 (strong) 1:2:4 (Cement/Sand/Medium Aggregate)
Used as a footing mix in house construction in softer ground. Also as the slab foundation to floors, bases for caravans and pathways, hard landscaping.
C25 (stronger) 1:1.5:3 (Cement/Sand/Medium Aggregate)
Can be used for foundations to larger houses and for creating floors. Can also take light traffic. Also suitable for lining pools and fosse septic.
C30 (very strong) 1:2:3 (Cement/Sand/Fine Aggregate)
A general-purpose, easy-to-remember mix for many hard-wearing applications.
C35 (industrial strength) 1:1.5:2.5 (Cement/Sand/Fine Aggregate)
Structural concrete for major construction work and roadways
C40/45 (atomic bunkers) Major civil engineering projects, bridges, skyscrapers and unlikely to be needed for domestic use!Here are some ratios for Mortar:
General Purpose Mortar, for re-pointing a stone wall: Use a mix in the ratio of 1:3:0.25 (Cement/Sand/Hydrated Lime*)
General Purpose Mortar, for re-pointing a brick wall: Use a mix in the ratio of 1:4 (Cement/Sand)
Specialist Mortar for Soft Limestones, such as Tuffeau. 1:1:5 (Chaux Blanche (hydraulic, hard)/Aerienne Chaux (soft)/sand)
(* The addition of Lime makes the mortar more pliable and less likely to crack. Alternatively, add a "plasticiser", which is usually available in sachets or a plastic tub from builders' merchants.)
This is probably the best page I’ve found for explaining all this:http://www.pavingexpert.com/mortars.htm