Types of construction sand in Zanzibar: plaster, mortar and concrete sand

Three jobs, three sands

Sand does different work in each mix. In concrete it is the fine aggregate that fills the gaps between the stone and contributes to strength, so it wants to be sharp and well graded from coarse to fine. In blockwork mortar it has to give the mason a bed that spreads, holds the block and does not bleed water out, so workability matters more than grit. In plaster and render it has to be finer and cleaner still, or the finish tears, scratches and shrinks as it dries.

Using a single all-purpose load across a whole building is one of the most common false economies on Zanzibar sites. The concrete ends up under-strength, the mortar joints crumble, and the plasterer spends days fighting a finish that will not close.

Sand for concrete

• Sharp, angular grains rather than rounded ones — they lock together and bond better with the cement paste
• Well graded: a spread of particle sizes, not all fine or all coarse
• Low silt and clay content — excess fines raise water demand and weaken the mix
• No organic matter, roots or shell fragments that leave voids and stains

Sand for blockwork mortar

• Slightly finer and more workable than concrete sand so the bed spreads evenly
• Consistent between loads, or the mix changes halfway up the wall
• Clean enough that the joint does not powder when it dries
• Free of salt — corrosive to any reinforcement or ties within the wall

Sand for plaster and render

• The finest and cleanest of the three; coarse particles scratch the trowelled face
• Low in silt to limit shrinkage cracking as the coat dries in the sun
• Screened where necessary to remove shell, pebbles and debris

The Zanzibar factor: coastal sourcing and salt

Sand on an island comes from somewhere near the sea, and that is where the risk lies. Salt-contaminated sand carries chloride into the mix, where it attacks reinforcement from the inside and shows up years later as spalling concrete and rust staining. Beach sand also tends to be uniformly fine and rounded — poor for strength, poor for mortar. Source matters, and it is worth asking where a load came from before it goes into a structural mix.

What to check when the tipper arrives

• Rub a handful between your palms: excessive staining or a slick feel points to clay and silt
• Look for shell, roots, leaves and lumps of clay
• Compare the colour and grain with the previous load — a sudden change usually means a different source
• Have it tipped on clean ground or a sheet, not on soil or old rubble that will contaminate it
• Keep plaster sand separate from concrete sand; do not let the crew take from whichever pile is closest

Ordering sand for a Zanzibar site

Tell us what each load is for — concrete, mortar, plaster or screed — rather than ordering a single quantity of 'sand'. Give the approximate quantity or the wall and floor areas it has to cover, confirm where the truck can tip and where the pile will sit, and we will supply against the use and sequence the loads with the blocks, cement and aggregate on the same programme.