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5
Lesson 5: So, You Think Concrete Dries
Out?
Age: Grades 7-12
Subjects: Science
Skills: Description, proportioning,
observation, small group work
Duration: 2 class sessions
Setting: Laboratory or classroom
Key Vocabulary: Hydration, mass, evaporation,
tricalcium silicate, dicalcium
silicate, tricalcium aluminate,
tetracalcium aluminoferrite, gypsum
Objective
Students will learn 1) The chemistry of cement 2) That concrete
hydrates and the difference between hydration and drying 3) Principle
of conservation of mass
Method
Students will learn about the hydration process by creating samples
of concrete and weighing them before and after hydration. They will
see that the samples lose no weight to evaporation because of the
chemical reaction between cement and water.
Background
Concrete is made by the combination of cement, water, and aggregate
of various sizes to make a workable slurry that has the consistency
of a thick milk shake.
| Name |
Percent by
Weight |
Chemical Formula |
| Tricalcium silicate |
50% |
3Ca0 SiO2 |
| Dicalcium silicate |
25% |
2Ca0 SiO2 |
| Tricalcium aluminate |
10% |
3Ca0 Al2 O3 |
| Tetracalcium aluminoferrite |
10% |
4Ca0 Al2 Fe2 O3 |
| Gypsum |
5% |
CaSO4 H2O |
The binding quality of portland cement paste is due to the chemical
reaction between the cement and water, called hydration. Portland
cement is not a simple chemical compound, it is a mixture of many
compounds. Four of these make up 90% or more of the weight of portland
cement: tricalcium silicate, dicalcium silicate, tricalcium aluminate,
and tetracalcium aluminoferrite. In addition to these major compounds,
several other play important roles in the hydration process. Different
types of cement contain the same four major compounds, but in different
proportions.
The cement in concrete needs water to hydrate and harden. Even though
the chemical reactions may be complete at the surface of the concrete,
the chemical reactions at the interior of the concrete take much
longer to complete. The strength of the concrete keeps growing as
long as the chemical reactions continue.
When water is added to cement, the chemical reaction called hydration
takes place and contributes to the final concrete product. The calcium
silicates contribute most to the strength of concrete. Tricalcium
silicates are responsible for most of the early strength (first
seven days).
The original dicalcium silicate hydrates, which form more slowly,
contribute to the strength of concrete at later stages. The following
word equations describe the production of concrete.
Tricalcium silicate + Water
(yields)
Calcium silicate hydrate + Calcium hydroxide + heat
Dicalcium silicate + Water
(yields)
Calcium silicate hydrate + Calcium hydroxide + heat
Of the five chemical reactions important for providing the strength
for concrete the above reactions are the most important.
The two calcium silicates, which constitute about 75% of the weight
of portland cement, react with water to form two new compounds:
calcium hydroxide and calcium silicate hydrate. The latter is by
far the most important cementing component in concrete. The engineering
properties of concrete—setting and hardening, strength and
dimensional stability—depend primarily on calcium silicate
hydrate gel. It is the heart of concrete.
When concrete sets, its gross volume remains almost unchanged, but
hardened concrete contains pores filled with water and air that
have no strength. The strength is in the solid part of the paste,
mostly in the calcium silicate hydrate and crystalline phases.
The less porous the cement paste, the stronger the concrete. When
mixing concrete, therefore, use no more water than is absolutely
necessary to make the concrete plastic and workable. Even then,
the water used is far more than is required for complete hydration
of the cement. The water-cement ratio (by weight) of completely
hydrated cement is about 0.22 to 0.25, excluding evaporable water.
Materials
Use a 40 lb bag of mortar mix, polystyrene cups, and a scale. Mortar
is a concrete mix that uses sand as the only aggregate in the mixture.
Procedure
A common expression is, “Don’t walk on the concrete
until it dries!” One of the easiest ways to show that the
hardening (curing) of concrete is not due to drying (the water becomes
a part of the chemistry of concrete) is to use the principle of
the conservation of mass.
Using polystyrene cups, weigh out 500 grams of mortar mix (sand
and cement mixture), add 75 grams of water. Mix until the lumps
disappear from the mortar mix. Weigh again to find the total weight
and the added amount of water as a check. Set the cup with the concrete
mixture aside as well as another polystyrene cup of water filled
to about the same level as the cup with the concrete. Weigh the
cup of water. Set the cup of water next to the cup of concrete.
This cup of water is used to indicate how much water evaporates
from the surface of the concrete before complete hardening occurs.
Wait about 24 hours and weigh each cup again. The concrete will
have lost only a fraction of the water to evaporation from the exposed
surface. You can compare the loss of water from the concrete with
that of the cup of plain water that also lost some water due to
evaporation. Both of these amounts of water are small when compared
to the original amount of water added to the concrete that does
not evaporate to make the hardened concrete. By comparing the amounts
of original ingredients to the weight of the final concrete it is
clear that the concrete does not dry out.
The concrete mixture may lose a little more water than the cup with
the water only. If you look carefully you’ll see that the
concrete’s surface is not smooth. This rougher surface area
makes it possible for water to evaporate faster than the water in
the cup alone. Again, this amount of water is negligible when compared
to the water added into the concrete mixture that went into the
chemical reaction to make the hardened material.
Discuss with students what efforts workers in the construction industry
might take to eliminate evaporation from the surface of newly poured
concrete.
Given this information, the expression given by the teacher at the
beginning of this investigation needs to be more accurately stated.
Ask the students to write a more accurate directive. |
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