Science 7–10 · Year 8
Making conglomerate and sandstone: cement, compaction and time
Science understanding: Earth and space sciences (NSW Stage 4 focus area: Change)
The idea
Loose sediment becomes rock when a cement grows in the pores between grains, a process the class can run in days that nature runs over millions of years.
Safety card
Hazards
- plaster dust (irritant) and the heat of setting
- hot water for dissolving sugar
- flying fragments when a dried rock is struck with the rock hammer
Controls
- mix plaster in a ventilated room and avoid breathing dust
- warm rather than boiling water
- never put plaster down a sink
- safety glasses for everyone near the crush test, with the sample wrapped in a cloth before it is struck
Note
Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) of the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update), or through RiskAssess; keep the Safety Data Sheet for every chemical with the class register.
What you need
- plaster of Paris, 75 g (a quarter cup) per group, with the manufacturer's safety directions
- gravel and sand mixed in the first paper cup (Geoscience Australia gives no fixed amount); a second paper cup; 30 mL water; a paddle-pop stick
- for the sugar sandstone: 50 mL warm water, sugar to saturation, a quarter cup of sand, a paper cup
- for the salt comparison: the same with table salt
- 1 electronic balance and a rock hammer or a heavy weight for a crush test after drying
How to do it
- Conglomerate: mix 75 g plaster into 30 mL water until smooth, then stir in the gravel and sand a spoonful at a time, with plenty of gravel, stopping while the paste still coats every grain (30 mL of water and 75 g of plaster make only a small amount of paste, and too much sediment leaves loose grains); leave to set for a few hours, then tear the cup away and rub the surface to expose the grains (Geoscience Australia method).
- Sugar sandstone: dissolve sugar into 50 mL warm water until no more dissolves, quarter fill a cup with sand, pour in just enough solution to cover it, and leave to dry for several days.
- Salt sandstone: repeat with salt instead of sugar.
- Record the mass of each cup daily as the water evaporates; the rock is done when the mass stops falling.
- Compare the three 'rocks' for how much force a thumb or a weight needs to break grains off, and relate the cement to natural quartz or calcite cement.
What you should see
The plaster conglomerate sets hard within hours and warms slightly as it sets (an exothermic reaction, as the GA sheet notes); pebbles are locked in a matrix like a real conglomerate. The sugar sandstone loses mass for several days and ends as a weakly cemented sand that crumbles under a thumb; the salt sandstone, as the GA sheet reports, is more crumbly again. The learner knows it worked when the daily mass readings flatten and the dried rock holds together when the cup is peeled away.
What changes
- What you change
- cement type (plaster, sugar, salt)
- What you measure
- drying time and strength of the model rock
- What you keep the same
- sand amount
- cup size
- drying place
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Sediment turns to rock by drying out; it is cemented by minerals deposited from water in the pores.
- Sedimentary rocks form fast because sand piles up fast; the cementing takes far longer than the deposition.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-CHG-01SC4-WS-01SC4-WS-04
- Australian Curriculum v9AC9S8U04AC9S8I02
Sources
The pages the author read to write this activity.