What is dry pack mortar made of?

The usual ratio

Most dry pack beds are mixed somewhere between four and six parts sand to one part Portland cement, by volume. Pre-bagged mud bed mixes are usually at the richer end, around four to one, and they take the guesswork out of the batch: every bag is the same, which matters when a floor takes many batches.

More cement is not better. A rich mix shrinks more as it cures and is more likely to crack or curl at the edges. Too little cement and the bed stays sandy at the surface and will not hold the thinset or the membrane on top. The tile and stone methods published by the Tile Council of North America, and the instructions on the bag, set the range a given job should use.

The right sand

Clean, washed, sharp sand with a mix of grain sizes, the kind sold as concrete sand. Its angular grains lock together when packed, which is what lets a bed hold a slope without slumping.

Play sand and fine masonry sand are too round and too fine: they pack poorly and leave a weak bed. Sand with clay or dirt in it is worse, because the fines coat the grains and keep the cement from binding them. Wet sand from an open pile changes the water in every batch, so it is covered or allowed for.

How much water: the squeeze test

Dry pack is mixed until it looks like damp sand, not mortar. Take a handful and squeeze it hard: it should hold its shape as a ball, leave the palm barely damp, and break into clean pieces when dropped. If water runs out between the fingers, it is too wet; if it crumbles in the hand, it is too dry.

Too wet, and the bed slumps, will not hold a slope and shrinks more. Too dry, and it will not compact, leaving soft spots and a surface that dusts. The mix is turned over until every grain is coated, ideally in a mixer, so there are no dry pockets.

Bond coat, additives and mesh

Over a clean concrete slab, a bonded bed is laid on a bond coat: a cement slurry or a bonding agent brushed into the slab just ahead of the mud, so the bed grabs it. Over wood or over a membrane, the bed is unbonded instead and sits on a cleavage membrane with metal lath or wire mesh in it, so it acts as one slab and does not crack when what is under it moves.

Some crews replace part of the water with a latex or acrylic admixture, which makes the bed stronger and stickier. It is useful in thin areas and on vertical work. The admixture is used the way its maker says, because too much of it makes the mix sticky and hard to pack.

Mixing and placing it

Batches are mixed small enough to be placed before they start to set, usually within the working time on the bag. The mud is spread between screed guides set to the finished height and slope, packed hard with a float or a tamper, struck off with a straight edge and floated tight. It is never trowelled wet like plaster: the strength comes from the packing.