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Chemical collection and stock control for UK schools, universities and hospitals

Calcium sulfate (CAS 7778-18-9): storage and disposal

Calcium sulfate turns up in school, college and university stores as the dihydrate and as plaster, usually in a paper sack, a tub or a bottle that has been refilled over several terms. It is not classified as hazardous on the sources we checked, yet it remains a chemical waste that needs an accurate description. The form matters most: dihydrate sits quietly, while plaster behaves very differently once water reaches it.

CAS number
7778-18-9
Form
Solid
Storage class
General
Usual EWC code
16 05 09

What to do this week

  1. Check the label is readable and write down the opened date if it is not already there.
  2. Check it is stored as general stock and apart from the items listed under "Do not store with".
  3. If you no longer need it, add it to your disposal list so it goes on the next collection.

Safe storage in a school or lab

Plaster arrives in a paper sack or a thin tub, and neither keeps damp out for long. Once a sack has been opened and folded over, the plaster at the top starts taking moisture from the air and stiffens into lumps, and a bag stored near a tap or on a floor that gets mopped can set solid. Move it into a rigid, tight lidded container, separate from the dihydrate, and label both. Keep the pair on a high, dry shelf in a general store, dated on arrival, and apart from food grade supplies. School prep rooms often hold several part-used tubs from different suppliers, so combine nothing and record each by its own label.

How to tell it has deteriorated

Dihydrate looks the same year after year: a fine white powder or small crystals. Plaster is the one that changes. A sack that feels stiff, rattles or has hard nodules inside has taken in water, and powder that no longer runs freely will not behave as a fresh batch. A fully set block in the bottom of a tub is the extreme case. Split paper, a lid stuck down with white crust and a label that has lifted from a damp surface all point to storage that was too humid. Check the date of receipt as well, since undated tubs from earlier years are the usual find.

How it must be disposed of

By specialist collection only. It never goes down the drain or into general waste. It is normally packed in a non-regulated or low hazard laboratory pack with compatible chemicals.

Plaster sets hard when it meets water, which is why this stock must not go down a sink or drain, where it can block pipework. It must not go into general waste or a skip either, and nobody should try to treat it first. Collection is by the specialist route, and waste code 16 05 09 is the entry for it. When you describe the stock, say which form each container holds, whether the plaster has already set, and how much is left. A set block in a tub is described as set plaster, not as loose powder. Dihydrate and plaster can travel in the same lab pack if their containers are sound and closed.

Waste codes and transport details for Calcium sulfate
EWC code, laboratory chemicals16 05 09
Hazardous wasteNo

The code depends on where the waste came from and what it is mixed with. Use the waste code finder to see why.

Common quantities we collect

Usually held as: dihydrate, plaster.

Part-used bottles and full containers are both fine. Tell us the size and how many, and an estimate is enough for a first price.

Where it is commonly held

Questions

Can set or lumpy plaster still be collected?

Yes. Lumps and set blocks are collected like loose powder. Describe the condition on the label, keep it in its original container with the lid on, and do not try to break it up or dissolve it beforehand.

Is calcium sulfate classed as hazardous waste?

It is not classified as hazardous on the sources we checked. It remains a chemical waste, so it still needs to be described accurately and collected by the specialist route. Check your own supplier information for the product you actually hold.

Why does our plaster go lumpy in the tub?

Plaster takes moisture from the air, and once a tub or sack has been opened the damp gets in. A tight lid and a dry shelf slow the process. Tubs that have already stiffened are best put forward for collection.

Guidance only. Check your own risk assessment and your supplier's safety data sheet.

Hazard data source: PubChem GHS Classification, retrieved 31 Oct 2026. Formula CaO4S. EC number 231-900-3.

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