Rock Cycle - Rocks (6B)

  • Exploring how to make cement.
  • Distinguishing between adhesives and cements.
  • adhesion
  • cement
  • concrete
  • hydration
  • plaster of Paris
  • flour
  • concrete mix
  • wax paper
  • Styrofoam tray
  • water
  • mixing bowls
  • spoon
  • Swift-GH microscope

Students make concrete and adhesives.

Pouring concrete 


The name "cement" generally refers to a chemical "binder" or adhesive. Adhesives bond materials as they dry out of an organic solvent or by a polymeric chemical reaction. The chemical and physical processes of adhesion, and therefore of cement, are not completely understood.

In contrast, in geology cementation refers to the hardening and welding together of sedimentary fragments such as sand by the precipitation of minerals in the spaces between the fragments. The particles are not glued together, but held in place by the precipitated minerals. The most common mineral is quartz, but calcite, other carbonates, iron oxides, and clay minerals also form cement.

The concrete used in the building industry forms the same way as geological cements. Concrete is made of rocks, sand, and cement. It is one of the most inexpensive, widely used, and durable building materials available. Man-made geological cements have been used for a long time. Cement dates back more than 5,000 years to the construction of the Great Pyramids in Egypt. The cement used by the Egyptians was a "calcined" gypsum (dried to remove water), which today is commonly known as plaster of Paris. The Mycenaeans and Phoenicians realized that the calcined gypsum cement was not very strong, and found that a lime-based ("lime" = CaO) cement was much stronger and durable. This lime-based cement is very similar to the cement in modern concrete. The most impressive use of concrete in the ancient world was by the Romans, who improved the strength of the cement and used it to construct most of their famous monuments and buildings.

In 1824, Joseph Aspdin developed a process for measuring out limestone and clay, converting them to powder, heating the powder, and then grinding them into another powder finer than flour. He found that if he mixed this cement with water and aggregate, i.e., a mixture of sand and rock, the resulting material had nearly twice the strength of many natural mineral cements. This cement was named "Portland cement" because the limestone was mined on the English island of Portland.

Modern concrete is mixed in proportions that are expressed in three numbers. For example, 1:2:3 means one part Portland cement, two parts of clean sand, and three parts gravel or pebbles (often called aggregate).

A common misconception about concrete is that it hardens or dries through evaporation. Actually, concrete hardens because of chemical reactions that take place when water is added to the dry concrete mixture. These produce new chemical compounds that lock in water between their molecules. This process, known as hydration, forms crystals that bind the concrete mixture together. These new crystals fill in the spaces between the aggregate, cement, and sand. The speed of hydration depends on the temperature, amount of water, and the fineness of the cement.

The process of hydration takes curing time. The concrete mixture can lose up to half of its strength if it is not kept wet during the first 7 days after pouring. The full hydration time for common cement is about 28 days. The longer the curing time, the more hydration takes, place and the stronger the concrete becomes. Concrete thus becomes stronger as it grows older. In this lab, the students learn the difference between cementation and adhesion. This will emphasize that cementation is the process used in creating sedimentary rocks.


  1. Before lab, prepare laboratory materials for each student group.
  2. Have the students make adhesive and cement mixtures, following the worksheet instructions. The students will see that the plaster of Paris and concrete form solids by hydration. The flour and water paste is an adhesive formed by evaporation of the water.
  3. Tell the students to measure the mixtures accurately or the end product may not solidify properly. If the students add too much or too little water, make sure that they record their observations. You can then discuss the "failures" and "successes" and their causes.
  4. The concrete mix might take a day to completely dry. Have students observe it over time. Looking at the mixture with a microscope will help.

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