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The process suggested for Plaster of Paris requires that "holes" be added to the gypsum matrix so that percolation is made possible. Portland cement (concrete) has natural porosity that can be controlled by simply adjusting the proportion of water, aluminum, and sand or other particles used in the mix. The greater amount of aluminum, water, and added materials, the greater porosity of the outcome.
Suggested procedure
See Cement information before starting.
Be sure to have students record procedures and data results in the STEM Notebook.
Prepare a series of plastic cups, each with 75 grams of Portland Cement.
There are several experiments that can follow:
a: Testing the variable of aluminum
1) To each cup use 50 ml of water and incremental amounts of aluminum powder from .2 to .8 grams.
2) Add the aluminum powder to the water and mix. Then add the cement spoon by spoon as with the plaster process.
3) Mix thoroughly for at least 1 minute or until the mixture starts to thicken.
4) As in the process adopted in the Plaster of Paris examples, use the same techniques to make a filter pot with paper cup, washers and wooden cover.
5) Portland Cement sets in a few hours, but takes longer to cure. Allow pots to dry over at least three days.
6) When cured, pots can be rinsed and then arranged for experimentation.
7) When the pots are cured they no longer provide a health risk and can be handled without concern. They have much more permanent chemical structures and will not degrade like plaster ones.
8) Run percolation rate tests as done with plaster filter pots.
b: Testing the variable of amount of water.
1) Repeat the series above by selecting one value of aluminum for each trial, say 0.4 g.
2) Vary the amount of water used in each mixture starting at 40 ml to 70 ml in 5 ml increments.
3) Repeat the process above starting at #3.
c: Testing the addition of other particles.
1) Choose the most promising combination of aluminum concentration and water amount to conduct the next set of trials. You might start at 0.4 g of Aluminum and 60 ml of water depending on your results above.
2) Choose different materials to add to the mix such as fine particle sand, coarse sand, perlite, activated charcoal, or other material. Use a standard volume of this material in each mix, say 1 tablespoon. We would not use weight as a measure here as all of the mentioned materials have greatly varying densities and would represent a great variance in volume if we used mass.
3) Set up a series as before and follow the process at #3 above.
d: Testing for variable volume of added materials
One additional test that can be done is to choose one of the materials above and create a series of tests using a different volume of added particles. What is the effect of 1, 2, 3 tablespoons of sand, for example on a .4g aluminum, 100g Portland cement, 60 ml water mix?
Going Further
Having collected some experimental data, students can be challenged to consider upscaling their most promising formula and processes. The test size that is used to determine percolation, turbidity and bacterial tests is not practical for use for humans. A much larger concrete vessel must be made.
The challenge for students at this point is to create a model for a larger mold and real test of the system. This part of the engineering task requires a great degree of thinking, brainstorming and discussion. This is an ideal time to form teams to analyze the results of the class and to make recommendations on the next course of action.
Ideally, student teams will create larger concrete vessels using different kinds of forming material. The results of these experiments have a great potential for communicating to a wider audience of interested engineers and lay people alike the potential impact of student research. This authentic outcome of this engineering study is what separates and elevates the budding engineer from simply learning about science and engineering to actually doing science and engineering and ultimately contributing to society.
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