Outdoor microcement and pools

The question we get is always the same: does microcement work on a terrace or a pool coping in Mallorca? The honest answer is yes, with an exterior system applied as one, and with the right expectation of what will happen over the years. Interior microcement laid outside yellows, wears and stains within a couple of summers. Well sealed exterior microcement behaves like any other continuous outdoor surface on the island: it ages through the sealer, and the sealer is renewed.

What changes compared with indoors

Three things. The material: exterior microcements contain a higher proportion of resin and tougher aggregates, to cope with expansion and abrasion from sand. The sealer: indoors a standard water-based polyurethane is used; outside you need an aliphatic polyurethane, which does not yellow under ultraviolet radiation, in two full coats. And the schedule: indoors the sealer is checked after five years or more; on a south-facing terrace, after two or three.

Sealer with a UV filter

The sun is what wears an outdoor floor most in Mallorca, ahead of water. An aromatic polyurethane, the usual one indoors, yellows and turns brittle after a few months of exposure. An aliphatic one keeps its tone and elasticity for years, and it is the one we always use outside, including on covered porches, because reflected light counts too. In summer it is applied first thing in the morning or at the end of the day: on a surface above 35 degrees the sealer dries before it levels and leaves roller marks.

Anti-slip: class 3 where it is walked on wet

Smooth sealed microcement is slippery when wet, just like polished porcelain. On pool copings, outdoor showers, stairs and ramps, Spanish building regulations call for floors with slip resistance class 3, and microcement reaches it when a fine aggregate, usually silica or polymer, is added to the last coat of sealer. It feels slightly rough, much like a fine sandstone, and it is cleaned in the same way. On terraces that do not usually get wet, a matt finish without aggregate, or with a smaller dose, is enough. We compare it with other materials in the guide to non-slip pool surrounds.

Water, falls and drains

Microcement does not tolerate standing water. A terrace with no fall, or with the fall towards the wall, ends up with a dark ring wherever the water sits after each rain. Before applying, we check that the fall reaches 1 or 2 percent towards the drain and that the drains take the water away; if not, it is corrected with a levelling layer before the microcement, not after. On terraces over rooms, microcement does not replace waterproofing either: it goes on top of it, and if the terrace leaks, that has to be solved first. We explain it on the page on terrace waterproofing.

Salt, chlorine and pools

Within two kilometres of the coast, from Portals to Cala Ratjada, salt settles on every surface and crystallises in the pores. A sealer in good condition keeps it out; a spent sealer lets it in, and white efflorescence appears. Chlorinated pool water does the same on the coping and the pool steps. That is why for pools we ask for a rinse with fresh water now and then and a check of the sealer every season.

The pool shell is a different job. Microcement under permanent immersion needs a specific system, with cementitious waterproofing underneath and an epoxy or polyurethane sealer suitable for immersion, applied with the shell dry and with a curing time of one to two weeks before filling. The water chemistry has to be kept balanced; a low pH over a long period attacks any cementitious coating.

Thermal movement and joints

An outdoor slab expands and contracts with every cycle of day and night, and in Mallorca the difference between the surface at midday in August and before dawn in January is more than 50 degrees. The expansion joints in the slab are there to absorb that movement, and microcement respects them: they are cut in the same place and sealed with an elastic sealant the colour of the floor. Covering them guarantees a straight crack in the first winter. On large terraces with no joints, we say so at the visit, because the substrate is not suitable as it is.

What to expect over the years

Well executed outdoor microcement behaves like this: in the first year it does not change; from the second, in sunny areas and walkways, the sheen dulls and water starts to darken the surface instead of running off; between the second and the fourth year it is time to sand it lightly and apply two new coats of sealer, without touching the microcement. Done in time, the cycle can be repeated indefinitely. Left too long, water and salt get into the mortar, stains appear and the recovery work is greater. It is the same logic as resealing stamped concrete, and that is why we raise it from the first quote.