Mortar, Grout, and Core Fill

Three wet materials go into a masonry wall, and they get treated as interchangeable far more often than they should be. One bonds the units together, one fills the cores around reinforcement, and the third belongs in a form rather than inside a wall.

The three products side by side

	Mortar	Grout (core fill)	Standard concrete mix
What it does	Bonds units together, beds and levels each course	Fills the hollow cores, encases vertical reinforcement and bonds it to the units	Structural mass inside a form: footings, slabs, piers
Where it goes	The 3/8 in bed and head joints	Inside the cores, after the wall is laid and the mortar has cured	Not in the cores
Consistency	Stiff. Holds its shape on a trowel and carries a unit without squeezing out	Fluid. 8 to 11 in slump, so it flows around bars without voids	Firm. Typically 3 to 5 in slump
Largest aggregate	Fine sand only	Sand, or pea gravel up to about 3/8 in	3/4 in and larger
Governing standard	ASTM C270	ASTM C476	Neither of the above
How it is sold	Bagged dry mix (60, 70, 80 lb) or site-mixed from cement, lime and sand	Bagged core fill for small jobs, ready-mix by the cubic yard above roughly a yard	Bagged mix or ready-mix by the cubic yard
Planning quantity	1 bag per 30–37 CMU, or per 35–40 brick	0.20–0.55 cu ft per block, by width	Whatever the form holds

The rest of this guide takes those columns in turn.

Why standard concrete mix does not belong in a core

A core in an 8-inch block is roughly 5 inches square, narrowed further by the webs and by a reinforcing bar sitting in the middle of it. Concrete mix carries coarse aggregate up to 3/4 inch at a 3 to 5 inch slump, stiff enough to hold a shape. Dropped into that space it arches across the opening and hangs up on the bar, and what reaches the bottom is the paste that ran past it. You get voids exactly where the wall needs continuous material, and you cannot see them. A bag of 4,000 PSI concrete mix is stronger on paper than most masonry grout and will still leave you with a worse wall.

Masonry grout is built around the opposite priority: aggregate capped at pea gravel or smaller, placed at an 8 to 11 inch slump, wet enough to be self-levelling. That much water would ruin a slab. It does not ruin a core, because concrete masonry units are absorptive. They pull free water out of the grout after placement, so it stiffens and gains strength in place at a far lower water content than it had in the bucket.

The high slump is a placement requirement rather than sloppiness, and it is only safe because it is being poured into something that drinks. Do not stiffen grout up to look more like concrete, and do not thin concrete mix out to look more like grout.

Mortar: how many bags a wall takes

Mortar quantity is driven by the number of joints, not by wall thickness. At standard 3/8 inch joints, one 80 lb bag of premixed mortar lays:

• 30–37 concrete blocks, assuming face-shell bedding, which is how CMU is normally laid.
• 35–40 bricks, assuming a full bed under single-wythe modular brick.

A wall 24 ft long by 8 ft high is 192 sq ft, which at 1.125 blocks per square foot is 216 blocks. At 33 blocks per bag that is 6.5 bags, so seven, and eight if you would rather not stop mid-course. The concrete block calculator runs the block count and the bag count together, and the brick calculator does the same for brick.

Three things move those figures:

• Joint thickness. Mortar volume scales with it directly. A 1/2 inch joint holds a third more than a 3/8 inch joint (0.5 ÷ 0.375 = 1.33), taking an 80 lb bag from roughly 33 blocks down to 25.
• Bedding. Face-shell bedding puts mortar on the two face shells only, about 2.5 inches of the block’s 7-5/8 inch width. Full bedding adds the cross webs and consumes around 40% more. The first course on the footing is normally fully bedded whatever the rest of the wall gets.
• Spillage and board life. Mortar dropped off the trowel, and mortar that stiffens past use on the board, are real losses rather than rounding errors. The 30–37 range absorbs some of it, which is why the low end is safer on a first masonry job. Our waste factors guide sets this against the allowances on other materials.

Mortar types N, S and M

ASTM C270 classifies mortar into five types. Take every other letter of the phrase MASON WORK and you have them, in descending order of strength.

Type	Minimum Compressive Strength	Where It Is Specified
M	2,500 PSI	Below grade, foundations, retaining walls, paving
S	1,800 PSI	Most exterior and structural CMU, walls in bending
N	750 PSI	Above-grade exterior veneer, interior partitions
O	350 PSI	Interior non-loadbearing, repointing soft historic masonry
K	75 PSI	Historic restoration only

The concrete block calculator covers which type belongs where. Two things sit behind that table.

Stronger is not better. Mortar is designed to be the sacrificial element. It is softer than the units so that movement and freeze cycling crack the joints, which can be raked out and repointed, rather than the units, which cannot. Specifying Type M on an above-grade veneer that called for Type N reverses that relationship, and on soft older brick it will spall the faces off the units.

Those strengths describe a laboratory specimen. ASTM C270 allows mortar to be specified either by proportion or by property, but not both at once. A mortar specified by proportion, the usual case for site-mixed cement, lime and sand, is not required to demonstrate the strengths above at all. The number classifies the mortar. It does not predict what a sample cut from your wall would test at.

Core fill: cubic feet per block

Grout quantity depends on block width, the only dimension that changes between CMU sizes. Every standard block has the same 8 by 16 inch nominal face.

Nominal Width	Grout per Block	Grout per Sq Ft of Wall
6 in	~0.20 cu ft	~0.23 cu ft
8 in	~0.32 cu ft	~0.36 cu ft
10 in	~0.44 cu ft	~0.50 cu ft
12 in	~0.55 cu ft	~0.62 cu ft

The right-hand column is the one to order against. Walls get measured in square feet, and converting to a block count first only to convert back is an extra step that invites an arithmetic slip. Multiply wall face area by that figure and you have the grout volume directly. The concrete block calculator works the other way round, from a block count, because that is what it needs for the block order itself.

Those figures assume every core is filled. A 200 series block is the 200 mm metric equivalent of the 8-inch row, and derived from it, full core fill works out to roughly 0.11 m³ per m² of wall face. Metric core geometry varies between manufacturers more than the imperial sizes do, so check the published figure for the unit you are buying.

Core volumes are not perfectly consistent within a nominal size either. Two-core and three-core units differ, and lightweight units often have thicker webs. Add roughly 10% before ordering, because cores are not clean rectangles and grout keys into open head joints and any gap it finds.

The number most estimates get wrong: partial grouting

Where the reinforcement goes, how much of it there is, and which cells get filled are structural decisions. They come from the drawings or from an engineer, not from a quantity estimate. What follows assumes you already have a layout and are working out the grout it implies.

Almost every residential block wall is partially grouted: the cells holding vertical bars, plus any bond beam course, and nothing else. Applying the table above to the full block count is how estimates end up double or triple what the job needs.

Take that same 24 ft by 8 ft wall in 8-inch block, 216 blocks, bars at 32 inches on centre, one bond beam course at the top.

Fully grouted, for comparison: 216 × 0.32 = 69.1 cu ft, or 2.56 cu yd.

Actually grouted:

1. Bars. 288 inches ÷ 32 = 9 spaces, so 10 bars counting both ends.
2. Grout per cell per vertical foot. A block holds 0.32 cu ft across two cores over 8 inches of height, so one core is 0.16 cu ft per course. At 1.5 courses to the foot, that is 0.24 cu ft per vertical foot.
3. Reinforced cells: 10 × 8 ft × 0.24 = 19.2 cu ft.
4. Bond beam course. A solid course is 0.32 cu ft per block, or 0.24 cu ft per linear foot; bond beam units with the webs cut away run close to the same, so call it 0.25. Across 24 ft that is 6.0 cu ft, less the 10 reinforced cells already counted in that course (10 × 0.16 = 1.6), giving 4.4 cu ft.
5. Total: 19.2 + 4.4 = 23.6 cu ft, or 0.87 cu yd.

That is 34% of the fully grouted figure. At the 0.6 cu ft yield of an 80 lb bag it is the difference between 40 bags and 115, and on the ready-mix side between a one-yard short load and a three-yard order. Add the 10% allowance and you land at 26 cu ft, about 44 bags or one yard delivered.

The rebar calculator gives the bar count once the spacing is fixed, and for a wall retaining soil rather than just standing up, the retaining wall calculator handles block, grout and reinforcement together.

Why grout goes in lifts

Fluid grout in a tall core behaves like any other liquid in a column: it pushes outward with a pressure that rises with depth. What is containing it is a stack of units held together by mortar that, a day or two after laying, has very little of its eventual strength. Fill a full-height wall in one go and the pressure at the base can blow out head joints, push the bottom courses out of plumb, or float units off their beds. Grout weeping through the joints is the visible version, but a joint that opened and resealed itself under pressure is a permanent weak spot whether or not anything leaked.

• Let the mortar cure first. A day is a common minimum on small residential work, and specifications often ask for more. Grouting onto joints laid the same morning is asking for trouble.
• Place in lifts. Around 5 feet is the conventional maximum for ordinary grouting. Taller pours are permitted under specific conditions, but the allowable pour and lift heights depend on grout type, the clear dimension of the cell and whether cleanouts are provided, so work to the number on your drawings.
• Consolidate, then reconsolidate. Grout is vibrated or puddled as it goes in to drive out voids, then reconsolidated once the block has pulled water out and the grout has settled, typically half an hour or so, to close the gap the settlement leaves.
• Stop each lift below a bed joint, not level with one, so the next lift keys into it rather than forming a cold joint at the weakest plane in the wall.

Brick versus block: why brick drinks more mortar

Brick joints are the same 3/8 inch as block joints, and brick still takes several times the mortar per square foot of wall. The reason is joint length.

Per square foot of wall face, modular brick has about 6 linear feet of joint: 4.5 feet of bed joint from 4.5 courses to the foot, plus 1.5 feet of head joint across 6.75 bricks. The same square foot of 8-inch block has about 2.25 linear feet: 1.5 courses of bed joint plus 0.75 feet of head joint across 1.125 blocks. Brick has roughly 2.7 times the joint, laid on a full 3-5/8 inch bed rather than block’s 2.5 inches of face shell, into a more absorptive unit.

Run it through the bag figures. The 192 sq ft wall above took seven bags as block. The same area in brick veneer is 1,296 bricks, which at 37 per bag is 35 bags. Same wall, same joint width, five times the mortar. Pricing a brick job off a block rule of thumb leaves you short by a factor that no waste allowance will cover.

Bags or ready-mix

Mortar is a bagged material in almost every residential case, mixed in small batches through the day because it stiffens on the board. Grout is the one worth pricing both ways: bagged core fill makes sense below about a cubic yard, which is already around 45 bags of 80 lb to move and mix by hand, and above that ready-mix is usually cheaper and far more consistent batch to batch. The concrete calculator converts a grout volume into either bags or yards so you can compare, including whatever short-load fee applies below three or four yards.