Mortar, Concrete and Grout Preparation
Core concepts
Mortar bonds units and fills joints; concrete carries load; grout fills cavities and cores. All three are batched, mixed, placed and cured to a specification, and all fail if the water content is wrong.
Mortar
Cement:sand by volume. Typical mixes: 1:3 or 1:4 for below damp-proof course, retaining walls and harsh exposure; 1:4 to 1:5 general walling; 1:5 to 1:6 for internal, lightly loaded work. Lime or a plasticiser may be added for workability but never to compensate for poor sand. Use clean, well-graded sand free of clay and salt, and clean water. Damp sand bulks by up to about 25 percent, so batch by volume after allowing for bulking, not by loose shovels. Mix on a clean hard surface or in a mixer; cement mortar should be used within about two hours in hot weather. Never add water to mortar that has begun to set.
Concrete
Know nominal mixes (1:2:4 is roughly grade C20) and the importance of the water:cement ratio: the wetter the mix, the weaker the concrete. Test workability with a slump test, compact with a poker to remove voids (honeycombing is a placing fault, not a mix fault), and cure for at least 7 days by keeping it damp. Cubes are tested at 7 days (an early indicator, roughly two thirds of strength) and 28 days for acceptance.
Grout
A flowing cement-sand-water mix, often with a plasticiser, used to fill hollow block cores and grouted cavities. It must flow without segregation and is placed in lifts.
Traps in questions
Adding water at the barrow, guessing ratios, using dirty water, over-mixing, skipping curing, and assuming a 7-day cube result is the final strength.
Worked rule of thumb
Bulk density of cement is about 1,440 kg/m3, so one 50 kg bag occupies about 0.035 m3. At 1:4, that bag needs roughly 0.14 m3 of sand, i.e. four gauge boxes each holding the same volume as the bag.