Heat Capacity of Portland Cement

Brief History of Portland Cement

• Cement was developed by the Romans who discovered that lime, volcanic ash and water formed a solid that was extremely durable and could be used to construct buildings. The circular dome of the Pantheon was constructed using their knowledge of cements. Unfortunately, when the Roman Empire fell into disarray the knowledge of cement making was lost and was not rediscovered until the 19th century. Joseph Aspdin in 1824 patented a process which mixed lime and clay with water to form what he call portland cement because it resembled the limestone found on the Isle of Portland.

  Portland cement is now produced in almost every single country in the world today. It is produced in such large quantities because it is used in all areas of construction. It is much easier to mold cement into desired size and shaped slabs than to carve such shapes out of stone. Due to its heat capacity, it is also an excellent material to keep energy cost low in extreme conditions.

Composition of Portland Cement

• Portland cement is a mixture of lime (CaO), silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3), where Ca is calcium, O is oxygen, Si is silicon, Al is aluminum and Fe is iron. Lime typically comes from limestone or chalk deposits. Clay and slags are the source of silica and alumina, while iron oxide is a byproduct of iron smelting. Portland cement is composed of these components in the following percentages by mass: lime 61 to 69 percent, silica 18 to 24 percent, alumina 4 to 8 percent and iron oxide 1 to 8 percent. The final component to create cement that can be poured and set for construction purposes is water. Water is added to the powder mixture to form a slurry which sets as water is incorporated by chemical reactions into the final product. Portland cement is classified as a hydraulic cement because the final hardened product is not due to water evaporating but by chemical reactions.

Definition of Heat Capacity

• Portland cement has a high heat capacity, however, before discussing the specific heat capacity of cement it is best to define the concept in general. Heat capacity is defined in chemistry and physics textbooks as "the amount of heat required to raise the temperature of an object by 1 K at constant pressure, or at constant volume" as referenced by Oxtoby et al., 1999. The variable used to represent heat capacity in a mathematical formula is C. K represents the temperature scale of Kelvin which is the standard scale used in chemistry and physics. One degree Kelvin is the same as one degree Celsius, however 0 C is equal to 273 degree K.

  The mathematical formula for heat capacity is:

  q = C x delta T

  where q is heat, delta T is the change in temperature and C as stated before is heat capacity.

  Typically heat capacity for a substances have been calculated based on experiments and are listed in tables in text books or reference books.

Specific Heat Constant of Portland Cement

• The specific heat constant (HC) of Portland cement is: 0.84 KJ/Kg/K according to the Engineers Technical Toolbox and the Portland Cement Association.

Calculating Heat Capacity of Portland Cement

• Using the specific heat constant (HC) referenced in Section 4, you can calculate the amount of heat needed to raise the temperature 1 degree of a piece of Portland cement of a known mass. To do this you multiply the mass of the concrete by the heat capacity constant (0.84 KJ/Kg/K).

  q = HC x mass of cement x Temperature change

  where q is the amount of heat, and the mass of the cement is in Kg and the temperature change is in Kelvin. In this case the temperature change is 1 degree.

  If need to know how much energy is required to raise a mass of concrete by a specific temperature difference, say 10 degrees, then you will multiple the mass of the concrete mass, by the temperature difference by the heat capacity constant.

  q = HC x mass of cement x Temperature change (10 degrees)

  Because the Kelvin temperature scale and Kg are not typically units of measure in the United States the specific heat capacity of Portland Cement is converted into units more typically used is 0.20 BTU/lb/degree F.