Introduction

Concrete is a composite building material made up of water, cement, and an aggregate, such as sand or gravel. A composite material is much stronger than its individual components and, unlike other mixtures its components stay uniquely identifiable. Cement, when mixed with water, forms a paste that binds or glues the aggregate together into a solid mass. Water is the key ingredient, because water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which cement and water produce crystals that interlock and bind together.

When water is added to cement, the pH rapidly rises to 12 due to the release of calcium ions, hydroxide ions, and a large amount of heat. Heat occurs during hydration due to the making and breaking of chemical bonds. Time is an important factor in determining concrete strength, because it takes a great deal of time for all of the bonds to form. Stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period, lasting from one to three hours. The evolution of heat slows dramatically in this stage. During this period, the concrete is in a plastic state, which allows the concrete to be transported and placed without any major difficulty. This is particularly important for contractors who must transport concrete to the job site. It is at the end of this stage that crystals begin to form and initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present. Most strength tests are commonly performed after the concrete has cured for 28 days.

The properties of concrete can be altered by the addition of admixtures. Admixtures are substances other than the key ingredients, which are added during the mixing process. Some admixtures add fluidity to concrete without adding excess water. Some admixtures speed up or delay the setting time. Other admixtures improve strength or durability. The selection and use of an admixture are based on the need of the concrete user.

In this activity, you will use a Vernier Stainless Steel Temperature Sensor to graphically profile the hydration cycle for curing concrete. You will also test the effect of admixtures (sugar and salt) on normal concrete.