We have chosen to use Plaster of Paris in this set of experiments because of its ubiquitous use and low human health risks. Plaster is not an ideal substance as it tends to degrade over time as gypsum will slowly dissolve in acidic water. It is satisfactory to learn the engineering processes, but may not have a practical purpose beyond this point.
Portland cement or some form of concrete or cement grout may be considered as an extension of this engineering experience or can substitute for Plaster of Paris. Researchers have shown the potential for cement as a water filter material. See http://www.biosandfilter.org/biosandfilter/index.php/item/269 and http://www.watersanitationhygiene.org/forum/phpBB3/viewtopic.php?t=1046
To examine the risks of using either material, please read below.
Plaster of Paris gets its name from the gypsum deposit at Montmarte in Paris, France. Gypsum (CaSO4· 2H2O) is a soft mineral, about 2 on the Mohs hardness scale. To form Plaster of Paris, gypsum is pulverized and is heated in large ovens. Half of the water is driven-off in this reaction:
2 CaSO4·2H2O → 2 CaSO4·0.5H2O + 3 H2O (released as steam)
When water is added to Plaster of Paris CaSO4·0.5H2O, it reforms gypsum, CaSO4·2H2O. This hardened form of gypsum is useful in creating wall coverings, decorative sculpture, and plaster casts for broken limbs.
Pure Plaster of Paris has few safety concerns although two cautions are suggested.
1) Always encourage careful dispensing of any powdered substance and avoid inhaling any material whatsoever. Particulate matter, no matter the source, may affect the sensitive tissues of the respiratory system. Always dispense powders using careful technique so as to avoid creating a dust cloud.
2) Plaster of Paris and water produce heat. Normally, with the amounts suggested in this experiment, the heat is unimportant. If large volumes of Plaster of Paris are mixed, avoid contact with the skin as temperatures can cause burns.
Portland Cement can be used as a substitute for Plaster of Paris. If you choose to use this substance, please note the further cautions here.
Portland Cement (the grey powder that when added with sand and gravel and water makes concrete) gets its name from a stone quarried on the Isle of Portland in Great Britain. The cement was a chemical invention suited for connecting bricks and stones together for building.
Portland Cement is a more complex substance. It is made primarily from limestone and clay with additions of aluminum, iron, and magnesium in various oxide forms. At its basic level, powdered limestone (calcium carbonate) is added to clay (silicon dioxide) and heated to 2700 degrees in the presence of other minerals. A large quantity of calcium silicate is formed as well as a number of other compounds.
When Portland cement is reconstituted with water and allowed to cure, complex crystalline structures form. Insoluble calcium carbonate (like seashells) forms with other compounds. The outcome is a hard, rock-like substance that has a very complex and largely unexplained structure.
Portland Cement such that one would purchase at a lumber supply has safety concerns:
1) Always encourage careful dispensing of any powdered substance and avoid inhaling any material whatsoever. Particulate matter, no matter the source, may affect the sensitive tissues of the respiratory system. Always dispense powders using careful technique so as to avoid creating a dust cloud. Portland cement is alkaline and can cause eye and respiratory irritation. It should not touch the skin or any mucous surface of the body.
2) Because it is alkaline (pH 13), it can also burn the skin. Avoid prolonged contact. Wet cement is caustic.
3) Portland cement and water produce heat. Normally, with the amounts suggested in this experiment, the heat is unimportant. If large volumes of cement are mixed, avoid contact with the skin as temperatures can cause burns.
4) If Portland cement is used as a substitute in these experiments, students should wear appropriate eye wear, gloves and mask over the mouth and nose. Dispensing of cement should by with implements only with no contact to the skin until cured.
If you have read the cautions and are prepared to guide students through safe use of Portland Cement, then click here to see how to modify the process to accommodate this substance.