Grout

Frequently asked questions

Why is masonry grout deliberately so wet?

Because it has to flow into a narrow cavity full of reinforcement and fill it completely without being vibrated everywhere, and because the dry masonry around it will take the excess water back out. Masonry grout is specified at a slump far higher than would ever be acceptable in structural concrete — the standards call for something close to pourable — and that would normally mean a weak, high water-cement-ratio material. It does not, because the blockwork is absorbent: it draws water out of the grout almost immediately, and the grout in place ends up at a much lower effective water content than it was poured at. That is why grout and concrete cannot be substituted for each other. Concrete at grout's slump is weak concrete; grout at concrete's slump will not fill the cells it was poured into.

What does non-shrink mean, and why does it matter under a baseplate?

It means the material does not lose volume as it sets, and under a baseplate that is the entire requirement. A steel column's baseplate transfers its whole load into the foundation through the thin layer between them, and that layer has to be in continuous contact with both faces. Ordinary cementitious material shrinks as it cures — a small percentage, which under a baseplate means a gap, and a gap means the load concentrates on whatever high spots remain. Non-shrink grouts are formulated with expansive agents to compensate, and they are placed from one side so air is pushed out ahead of the flow rather than trapped under the plate. Using ordinary sand-cement mortar under a baseplate is a structural substitution, not an economy.

Sanded or unsanded tile grout — and why does the joint width decide?

Sand is there to control shrinkage, and the joint width decides whether it fits. UNSANDED grout is a fine cement paste for joints below about 3 mm — it flows into narrow gaps and does not scratch a polished or soft glazed tile, but in a wide joint it shrinks and cracks because there is no aggregate holding it. SANDED grout carries fine aggregate, which resists shrinkage and is the right choice for wider joints, and it will scratch marble, polished stone and some glass tiles unless a sample is tested first. Epoxy grout is a third family, chemically set rather than cementitious, which is what a laboratory, a commercial kitchen or a heavily used shower gets — far harder to work with, and effectively impervious.