Hot and Cold pHun!
Cement Hydration and pH Evolution during Curing
Objective: Students will calculate the amount of water that reacts during hydration when cement becomes concrete under differing curing temperatures and observe the pH change that occurs in the curing process.
Water is the reactant that makes concrete hard. The degree of hydration, or maturity, of the concrete determines the porosity of the concrete. Porosity is the amount of empty space in the concrete. Low porosity concrete has high strength and lasts for a long time.
During the hydration of the cement, the compound calcium hydroxide, Ca(OH)2, is produced. Calcium hydroxide is a basic compound (alkaline). Bases are caustic (eats skin tissue), feel slippery on your skin (like soap), and have a bitter taste (don't taste it!). Calcium hydroxide is a mild base, but can irritate sensitive skin, so be careful!
Bases also affect indicators (chemicals that change color). There are many different indicators. Universal indicator is a mixture of different indicators so that colors can be achieved all along the pH scale. The pH scale is a numerical range used to determine the acidity or alkalinity of a substance. The scale ranges from 1 to 14. pH's from 1 to 6 are acidic. A pH of 7 is neutral. Values ranging from 8 to 14 are basic.
The environmental conditions, such as temperature and humidity, under which the concrete is cured can also affect the concrete's processing and properties. Cooler surroundings results in concrete hydrating at a slower rate.
Time: 100 - 120 minutes (2 1/2 class periods)
Materials and Supplies for Groups of 2-3:
- Fresh cement
- mixing container
- stir stick
- pH paper
- 3 glass or plastic petri dishes (see Teacher's Notes)
- plastic wrap or Ziploc bags
- graduated cylinder
- releasing agent (PAM cooking spray, cooking oil, aluminum foil, plastic wrap)
General Safety Guidelines:
- Prolonged exposure may cause severe chemical burns. Wash exposed skin after contact.
- Gloves should be worn to protect the skin, especially if your skin is sensitive.
- All heating must be done in a oven only.
- Prepare a batch of fresh paste using 100 g of cement and 40 mL of water.
- a. Place a releasing agent (PAM cooking spray, aluminum foil, plastic wrap, cooking oil) in all three petri dishes. Be sure to cover the entire inside of the dish.
b. Mass the three petri dishes and record in the data table.
- a. Fill each petri dish 1/4 full with the paste and level it off.
b. Mass each dish separately and record in the data table.
c. Wrap two dishes in plastic wrap or place in Ziploc bags. Set one aside at room temperature. Set the second in a refrigerator.
d. Put the third dish into a sellable container with water. Be sure the water level is well below the edge of the dish. Tightly seal the container and place it in an oven set at 40[[ring]] C.
- Using pH paper, test the pH of tap water and any leftover fresh concrete paste. Record your values on the data page.
- Allow forms to cure 24 hours.
- Remove the concrete samples from their controlled environments.
- Mass each dish separately and record in the data table.
- Determine the mass of original water by using the equation found in the data and calculations section.
- Determine the mass of the original cement by using the equation found in the data and calculations section.
- Place each sample in an oven set at 100 -110 [[ring]] C. Allow the samples to remain in the oven for 24 hours.
- Remove the samples from the oven and allow to cool.
- Weigh each petri dish and then determine the mass of only the cement paste.
- Determine the mass of water driven off in the oven by using the equations in the data and calculations section.
- Calculate the mass of water combined during hydration using the equations in the data and calculations section.
- Calculate the percent of water reacted using the equations in the data and calculations section.
Data and Analysis:
pH of mixed cement _________________
pH of tap water ____________________
|mass of petri dish|
|mass of concrete and dish|
|mass of concrete and dish|
|mass of sample|
|mass of original water|
|mass of original cement|
|mass of dish and dry sample|
|mass of dry sample|
|mass of water driven off|
|mass of water combined in reaction|
|% of water reacted (hydration)|
- Which curing environment yielded the greatest percent of water reacted?
- a. What is the relationship between curing environment and amount of hydration?
b. Do your results support your answer?
- Which concrete sample would you hypothesize to have the greatest strength?
Explain your reasoning.
- What proof do you have that hydration is a chemical change?
- Suppose you came across a container filled with a substance that looked like concrete. When you tested it with pH paper, the paper turned red. Was the substance concrete?
Explain your reasoning.
Notes for Teacher:
- Students may use an universal batch or make their own mixtures to study water to cement ratio effect on pH and hydration.
- If plastic petri dishes are used, place a few sheets of paper on the oven rack before setting the dishes in the oven. Then they shouldn't melt. If plastic dishes are used, a releasing agent is unnecessary since the dish is disposable.
- Students may investigate the effect of admixtures. Calcium chloride can be used to accelerate setting (2% CaCl2.2H2O by weight of cement). Sugar can be used to retard setting (0.1% by weight of cement). Commercial accelerators and retarders can be obtained from your local ready mix company.
- An alternative experiment is to vary the amounts of water available for hydration and determine the results. Samples are all kept at room temperature. One is covered with wet paper towels and plastic sheeting, one with plastic sheeting only, and one is covered with only a wet paper towel. The test specimens would then be treated as described in Day 2.
- Students could perform the same procedure on cylinders of different ages to prove that hydration continues over a long period of time.
Hydration of the sample in the oven is accelerated by a warm environment. Therefore, the sample in the oven will use more water in its hydration products. So it will have the highest water reacted and the cooler environment in the refrigerator will have the least.
Answers to Questions:
- a. Colder environment results in less hydration occurring, more free water is available.
b. Answers will vary
- Oven sample, because more hydration could have occurred.
- The pH paper changed color and new products were formed, because water was retained, not evaporated.
- No. pH paper changes to blue, indicating a base is produced during the formation of concrete. A change to red indicates an acidic change, not basic.