The key ingredient difference
Concrete is a mix of cement, sand (fine aggregate), and coarse aggregate (crushed stone or gravel) — the coarse aggregate is what gives concrete its structural compressive strength, letting it bear significant load once cured. Mortar is cement and sand only, with no coarse aggregate — it's designed to be workable and adhesive rather than load-bearing on its own.
Because mortar has no coarse aggregate, it also uses a smaller "dry volume" expansion factor than concrete when estimating material quantities — commonly 1.33 for mortar versus 1.54 for concrete, since there's one less material's worth of voids to account for during mixing.
What each one is actually used for
Concrete goes into anything that needs to carry structural load: foundations, slabs, beams, columns, and footings. Mortar goes into binding masonry units together (brickwork, blockwork, stonework) and into plastering (the finish coat over walls) — jobs where adhesion and workability matter more than raw compressive strength.
A worked example — mortar mix ratios
For 1 m³ of wet mortar with a 10% wastage allowance, a 1:4 (cement:sand) mix needs about 8.4 cement bags and 1.17 m³ of sand. A leaner 1:6 mix for the identical volume needs only about 6 cement bags, with correspondingly more sand (1.25 m³) — richer mixes (more cement relative to sand) are used for high-strength or waterproofing-critical applications, while leaner mixes suit general brickwork and plastering.
What happens if you use the wrong one
Using mortar where concrete is needed (e.g. a load-bearing footing) risks structural failure, since mortar alone isn't engineered to bear that kind of load — this is a genuine safety issue, not just a cost or workability question. Using concrete where mortar is needed (e.g. for plastering) is wasteful and produces a poor, overly coarse finish unsuited to the fine, smooth surface plastering requires — coarse aggregate has no place in a plaster coat.
Choosing the right mix ratio within each
Just as concrete has different mix ratios for different strength needs (see the concrete material guide), mortar mix ratios also vary by application — richer mixes (1:3 or 1:4) for structural masonry or areas needing more strength/water resistance, leaner mixes (1:6) for general brickwork and non-critical plastering. Always follow your structural engineer's or local building code's specification for load-bearing or safety-critical work rather than defaulting to a generic ratio.