Concrete vs Mortar vs Grout vs Screed vs Render

The one-line version

The quickest way to keep them apart is by what is in the mix and what the mix has to do structurally. Coarse aggregate is the single biggest tell: concrete has it, mortar does not, and that one difference explains most of the rest.

Stone is what gives concrete its compressive strength and its dimensional stability. Take the stone out and you get a material that is far more workable and far more prone to shrinkage - which is exactly what you want for a bedding layer a few millimetres thick, and exactly what you do not want for a slab.

Concrete: the structural one

Concrete is the only material of the five that is normally a structural element by itself. Its coarse aggregate carries load in compression, the cement paste binds it, and reinforcement - where present - carries the tension the concrete cannot.

Because it is structural, it is the one specified most carefully. Nominal volumetric ratios cover general and non-structural work, running from lean blinding mixes up to the top of the by-volume range; above that, structural concrete is a design mix proportioned by weight from tested materials to a target strength.

Concrete is also the material where placement and curing matter most, because it is thick. Compaction removes trapped air, joints control where the inevitable shrinkage cracking happens, and curing keeps moisture available for the hydration that develops strength over weeks rather than hours.

Mortar: the deliberately weak one

Mortar is the bedding and bonding material between masonry units. It has no coarse aggregate, and it substitutes lime for some of the cement - lime brings workability, water retention and the tolerance for movement that lets a wall crack at its joints rather than through its bricks.

It is mixed to a stiff, plastic consistency: firm enough to hold a brick where the bricklayer puts it, wet enough to bond. That consistency is doing real work, and it is the most visible difference from grout, which is poured.

Mortar is graded into types by proportion, from a below-grade type at the strong end to conservation mixes at the soft end. The counter-intuitive rule - pick the weakest type that does the job, not the strongest available - is covered in detail in the mortar types guide.

Substituting concrete for mortar fails immediately and obviously: the coarse aggregate will not let a joint close to the right thickness, and the mix has none of the water retention that bond depends on.

Grout: the one that has to flow

Masonry grout fills the cores of hollow block and the cavities of reinforced masonry. Its defining property is that it must flow - placed at a high slump so it consolidates completely around reinforcing bar and into every corner of the space, with no voids left behind.

That is why it is proportioned from cement and sand rather than cement and lime, and why a coarse version adds pea gravel for wider grout spaces such as bond beams while a fine version stays aggregate-free for narrow cores. The choice between them is about the size of the space being filled, not about strength - both sit at similar strength levels in the reference.

The high water content sounds like it should be a strength problem, and in ordinary concrete it would be. Grout gets away with it because the surrounding masonry is absorbent: it draws water out of the grout after placement, so the grout ends up denser than it was poured. Substituting a stiff concrete mix defeats the entire mechanism - it will not consolidate, and the voids it leaves are exactly where the reinforcement was supposed to be bonded.

Note that masonry grout is a different material from tile grout, which is a thin-joint finishing product and shares only the name.

Screed and render: thin layers with one job each

A floor screed is a cement and sharp sand layer laid over a structural slab to give a flat, level surface for a floor finish. It is not a structural layer, which is why it is proportioned simply and laid semi-dry - compacted and trowelled rather than poured. A bonded screed sits thin and rich directly on the slab; a floating screed goes over insulation or a membrane, is laid thicker, and is often lightly reinforced against curling at the edges.

Render is the coating on the outside of masonry, and it is built in coats with one governing rule: each coat must be softer than the coat beneath it. That way, shrinkage stresses point inward into the wall rather than outward at the surface, and the finish does not pull itself off. The scratch coat is the richest, the undercoat is next, and the thin finish coat is the softest and most workable of the three.

Both fail in the same characteristic way when the rule is broken. A screed that is too rich, laid too thin or dried too quickly curls and debonds; a render coat harder than its undercoat crazes and eventually detaches in sheets. In both cases the material has been asked to be strong when what it needed to be was compatible.

All the proportions referenced here are typical planning values for general work. Structural and load-bearing elements must follow your local building code and a qualified engineer's specification.

• Bonded screed: thin, rich, laid directly onto and bonded to the slab.
• Floating screed: thicker, over insulation or a membrane, often lightly reinforced.
• Render: coats get progressively softer from the wall outward - never the reverse.
• All three of screed, render and mortar are thin layers whose enemy is shrinkage, not load.

What actually goes wrong when they are swapped

Substitutions between these five are common on small jobs, usually because one bag is already open. Each swap has a predictable failure mode, and none of them shows up on the day.

Concrete used as mortar cannot be worked into a joint and has no water retention, so bond is poor. Mortar used as concrete has no coarse aggregate, so it shrinks heavily and has no aggregate interlock to carry load. A stiff mix used as grout leaves voids around reinforcement. A render coat harder than its undercoat detaches. A screed mixed too rich or too thin curls.

The pattern behind all five is the same: each material is proportioned for a specific combination of workability, shrinkage behaviour and strength, and strength is rarely the property that was limiting. Choosing by "which is strongest" is what produces most of these failures.

Frequently asked questions