The properties of cement are the qualities and ingredients that make it one of the most versatile and valuable building materials in existence. The properties include the physical properties as well as the chemical properties of cement.
Physical properties include elements like cement’s compressive strength and binding ability that allow builders to create strong and enduring structures. The chemical properties are related to how the ingredients in cement contribute to its impressive physical qualities.
Physical Properties of Cement
The chemical components of cement play a significant role in its physical properties. Cement companies make certain adjustments to the ingredients of cement when they want to create specialized types of cement that have unique properties.
The fineness of a cement mixture refers to the particle size distribution of the cement powder. This property affects the cement’s performance and workability. The fineness is measured in square meters per kilogram (m2/kg) or square centimeters per gram (cm2/g). A higher specific surface area indicates finer particles and a lower specific surface area indicates coarser particles.
The finer the cement particles, the greater the interaction between water and the cement particles. This enhances the chemical reactivity and subsequent production of calcium silicate hydrate (CSH) gel. CSH gel forms the matrix that creates the hardening and strength of cement.
The soundness of cement refers to its ability to retain its structure once it has set and hardened. It measures the resistance of cement to volume changes like shrinkage and expansion that can occur because of delayed hydration or unstable chemical compounds. Some unstable chemical compounds include excess lime or magnesia that can react with water over time causing expansion.
Manufacturers ensure the soundness of cement by selecting the proper mix of raw ingredients to maintain a balanced chemical composition. This allows them to ensure proper kiln temperatures in the clinker production phase and grind the particles to an acceptable level without excessive heat generation.
The consistency of cement refers to its ability to flow and maintain workability when mixed with water. This property relates to the plasticity and fluidity of the cement paste. Different chemical compounds within the cement mixture affect the consistency of the cement because of their rate of hydration and rheological properties. For example, tricalcium silicate is quite reactive and requires a high amount of water for good consistency. Dicalcium silicate has a lower reaction time and does not require much water to achieve optimal consistency.
The strength of a type of cement refers to its ability to withstand pressure from external forces while maintaining its structural integrity. This critical property details the cement’s load-bearing capacity and durability. Experts determine the strength of cement by subjecting cylindrical specimens of cement or concrete to a compressive load until a failure occurs. Tricalcium silicate gives a cement early strength gain and dicalcium silicate supports long-term strength.
The setting time of cement describes how long the cement takes for freshly mixed cement paste to reach a solid, firm state. Cement makers measure setting time in two categories: the initial set time and the final set time. The initial set time should not be too fast because this inhibits good workability. The final set time should not be too long as this can slow progress on projects.
Initial average set times are between 30-45 minutes. The average final set time ranges between 7-10 hours. These times will be different for specialized cement like rapid-hardening cement or low-heat cement. The presence of tricalcium silicate leads to faster set times. The addition of dicalcium silicate can delay setting times.
Heat of Hydration
The heat of hydration property refers to the amount of heat that is released during the hydration process. During this process, exothermic chemical reactions occur that can impact the workability of the cement. Heat production in cement is beneficial in cold climates but not in warm climates where too much heat generation can cause cracks and/or shrinkage that will impact the strength and durability of the structure.
It is common for cement manufacturers to list the potential heat hydration for their cement, though these limits will vary according to environmental conditions.
Loss of Ignition
The loss of ignition (LOI) property refers to the amount of weight lost when the cement is subject to high temperatures. This property measures the amount of volatile compounds and organic materials in the cement mixture. To conduct this test, manufacturers expose a quantity of cement to a specified temperature, usually around 900o-1000o C. After all the volatile compounds have burned, the inorganic materials remain. The weight loss is determined as a percentage of the weight of the initial cement sample.
This property helps cement makers to evaluate the purity and quality of the cement composition which affects the strength development, durability, and resistance of the cement to chemical attacks.
The bulk density property refers to the mass of cement per unit volume. This is expressed in terms of kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cm3). This measures the density of cement when it is loosely packed without compaction. The average bulk density range for regular cement is from 1000-1600 kg/m3.
This property measures the density of cement to the density of water at a specific temperature. The specific gravity of regular cement averages between 3.1-3.16. The specific gravity measurement helps engineers to accurately measure the cement with the correct proportions of aggregate and water to achieve the concrete density they require.
Chemical Properties of Cement
The chemical properties of cement are determined by its composition. The four main chemical compounds in cement are tricalcium aluminate, tricalcium silicate, dicalcium silicate, and ferrite. These compounds undergo chemical changes during hydration which impact cement’s physical properties.
- Tricalcium Aluminate (C3A) – This compound reacts with water to start the hydration process. C3A also leads to strength development in cement.
- Tricalcium Silicate (C3S) – C3S is responsible for rapid hydration and hardening.
- Dicalcium Silicate (C2S) – This chemical compound helps cement gain long-term strength.
- Ferrite (C4AF) – Ferrite is a fluxing agent which helps reduce the melting temperature of the raw materials in the kiln which aids the manufacturing process.
- Magnesia (MgO) – A small amount of magnesia helps cement gain strength. Too much magnesia in the mix will make cement expansive and unsound.
- Sulfur Trioxides (SO3) – Sulfur trioxide is present in cement in the form of gypsum. It can act as a setting time accelerator. Too much SO3 can make cement expansive and unsound.
- Iron Oxides (Fe203) – This compound adds strength and hardness to the cement. It also gives the cement color.
- Alkalis – The presence of alkalis can help to increase the speed of hydration but also slow down the setting time of cement.
- Alumina – Cement with high alumina content can withstand very cold temperatures.
- Silica Fumes – Silica fumes help to improve the compressive strength, abrasion resistance, and bonding strength of cement. An excessive amount of silica fumes can delay the setting time of cement.