Cement vs concrete: what’s the difference, and which bag to buy

The short answer

• Cement (portland cement) is a fine grey powder. Mixed with water it hardens through a chemical reaction. On its own it is a binder, not a building material.
• Concrete is cement mixed with sand, gravel (or crushed stone) and water. The cement paste glues the stones together into a hard, strong mass.
• Mortar is cement (often with lime) mixed with sand but no gravel. It is the “glue” between blocks and bricks.
• Stucco is a cement-sand-lime plaster spread on walls in coats.

So a sidewalk is concrete, not cement — it just contains cement. When someone asks how much cement they need for a slab, they almost always mean concrete.

What’s in the bags at the store

How far each goes, from the same data sheets: concrete mix and 5000 make about 0.60 cu ft per 80 lb bag; fast-setting about 0.375 cu ft per 50 lb bag; sand/topping mix about 0.66 cu ft per 80 lb bag; mortar mix lays up to 13 blocks or 37 bricks per 80 lb bag; and five to six 94 lb bags of portland cement make a cubic yard of concrete with sand and gravel (Quikrete’s 5000 mix reaches 5,000 psi at 28 days).

For almost every DIY job, the bag you want is concrete mix — the one that only needs water. The yields are on the concrete bag yield chart, and the most common slab question is answered on how much concrete for a 10×10 slab.

Mixing your own from cement

If you buy portland cement, you also need sand and gravel. Quikrete’s Portland Cement data sheet gives a typical concrete mix of 1 part cement, 2 parts sand and 3 parts gravel by volume, and says five to six 94 lb bags of cement go into each cubic yard of concrete. Both are worked through on what is the concrete mix ratio and how many bags of cement in a yard of concrete.

Compare that with premix: a yard of concrete is about 45 bags of 80 lb concrete mix — see how many 80 lb bags of concrete in a yard. Mixing your own can be cheaper where sand and gravel are sold in bulk nearby, but premix is simpler and more consistent for small jobs.

Why the difference matters

• Strength. Cement paste alone shrinks and cracks; the stone in concrete carries load and controls shrinkage. Pouring “just cement” into a post hole makes a weak, brittle plug.
• Thickness. Concrete mix with gravel needs room for the stones — Quikrete’s standard mix is for pours 2 in thick or more. For thin layers, sand mix or a resurfacer is the right product — see self-leveling concrete coverage per bag.
• Laying units. Concrete is too stiff and stony to lay block or brick; mortar is made for that, with lime for workability.
• Weight. All of these are heavy. A cubic yard of concrete weighs about two tons — see how much does a yard of concrete weigh.

One thing they share: safety

Cement in any form is caustic once wet. Quikrete’s data sheets warn it causes severe skin burns and serious eye damage, and OSHA notes cement burns may not hurt until hours later. Dry cement and mix dust is also a lung hazard. Wear waterproof gloves, eye protection and a dust mask when mixing, and wash off any wet concrete promptly. More in concrete safety.