Cement Ingredients Percentage | Cement Chemical Composition
What is Cement?
Cement may be defined as the adhesive substance that unites the different fragments or masses together. The different types of concrete are mostly made through variations in cement. Who invented cement First?
Joseph Aspdin first invented cement and named it Portland Cement. Portland cement is made by mixing calcareous and argillaceous, silica, alumina, and other iron-bearing materials grinding the mixture into a fine powder. The proportion of quantities is determined and burning those materials at high temperature (1400 degree centigrade) in rotatory kilns.
As the material moves along the kiln due to high temperature, the water first evaporates, and then CO2 is liberated from the Calcium carbonate. This dry material’s reaction starts from there on until, in the hottest part, the mixture is converted to liquid form. Balls of around 3-25mm diameter known as clinkers form. As the clinkers cool down, it is ground into powder form, and this powder form is called Portland cement.
According to some American and British standards, no material other than gypsum, water, and grinding aids to be added after burning. The reason behind adding gypsum to the cement is to prevent the flash setting of cement. A single unit of the kiln can produce as much as 6200 tons of clinker each day. This cement’s powder form is packed in a bag and transported to different sites for construction purposes. Each country has its own cement usage, but china is the largest consumer of cement.
Cement Ingredients Percentage
The raw materials of cement are lime, silica, alumina, and iron oxide. In the kiln, certain interactions between these compounds occur, resulting in other complex products. After the reaction cooling process occurs, and as the cooling rate is not constant and this variable rate of cooling affects the extend of crystallization and the amount of amorphous material present.
Cement Chemical Composition
The following table shows the four main constituents of cement, their chemical formulae, and abbreviations.
The first two compounds, i.e., tricalcium silicate and dicalcium silicate, are the most significant components. These are responsible for the hydrated cement paste’s strength. Apart from providing strength, they also contribute to the silicates’ atomic arrangement, crystal form, and hydraulic properties.
The third component is the Tri-calcium aluminate, whose presence is undesirable. It also contributes less to the strength except at an early age and only when sulfates attack hardened cement paste. In the earlier phase, C3A facilitates the reaction between lime and silica.
Then the last component, Tetracalcium aluminoferrate, which is present in a very minute amount. This compound, although it doesn’t affect the behavior so much, it reacts with gypsum and form Calcium sulfoferrite and accelerates the hydration of the silicates.
These four are the main component, which affects the behavior of cement significantly. But there are certain other minor components, like MgO, TiO2, Mn2O3, etc. They are not more than a few percent of the overall amount of cement.
The two minor compounds, namely, Na2O and K2O, react with some aggregates, and these reactions cause disintegration in the concrete.
Cement Ingredients Percentage/Constituents of Portland Cement
The following table shows the typical composition of oxide in cement. The other part of the table shows the Portland cement chemical composition percentages ranges of different compounds
|Lime (CaO)||60 to 67%|
|Silica (SiO2)||17 to 25%|
|Alumina (Al2O3)||3 to 8%|
|Iron oxide (Fe2O3)||0.5 to 6%|
|Magnesia (MgO)||0.1 to 4%|
|Sulphur trioxide (SO3)||1 to 3%|
|Soda and/or Potash (Na2O+K2O)||0.5 to 1.3%|
Cement Hydration Process
The chemical mentioned above compounds react together and form different products. These chemical reactions are termed hydration. These reactions are mostly exothermic. When fully hydrated at some given temperature, the quantity of heat (joules per gram) of un-hydrated cement is known as the heat of hydration. There are many methods to calculate this hydration heat; one of them is ASTM C 186-05.
During Ordinary Portland cement hydration process, around 50% of the total heat is liberated during the first three days of reaction, and 75% in 7 days and 90% takes six months to release.
Therefore, this is a slow process whose rate decreases over time. The temperature at which these reactions occur is sometimes far more important than the total heat releases from the reactions. So, the temperature to be kept under check for the proper execution of these reactions.
The cement comprises around 7-10% of the concrete. The primary purpose of cement is bonding in concrete. American Society for Testing and Materials (ASTM) has classified types of cement into five categories, and they are as follow:
- Type I cement: A most common type of cement is a general-purpose cement. type I cement is used in conditions where Sulphate exposure is minimum.
- Type II cement: Is used for conditions where there is low sulphate exposure.
- Type III cement: This cement gives strength at an early age, so it is used in situations where initial strength is used.
- Type IV cement: The cement sets quickly, and its usage is in applications where fast setting time is required, like in the case of dams and where mass concreting is carried out.
- Type V cement: The cement is highly resistant against sulphate attack. The usage is in the application where is there is exposure to a high level of sulphate.
Apart from these fundamental types of cement categorized by ASTM, there are other types of cement-based on properties and types of material used. And they are used for different purposes.
Different Types of Cement
Rapid Hardening or High Early Strength Cement
This cement gains strength faster than OPC. In just three days, this type of cement develops seven days’ strength of OPC, with the same water-cement ratio. Even though their initial and final setting times are the same, this cement is more finely grounded. This cement emits more heat of hydration, and they are not suitable for mass concreting. Their usage is in works that require immediate strength, like repair works.
Quick Setting Cement
This cement sets faster than OPC. It has an initial time of 5 minutes and a final setting time of 30 minutes. They are used in underwater construction. The mixing and placing should be faster to avoid setting before proper laying.
Low Heat Cement (LHC)
They release comparatively lower heat of hydration, so they are suitable for mass concreting, as in the case of dams. They have a lower percentage of Tri-calcium Aluminates (5%) and higher di-calcium silicate (46%). They develop strength slowly, need longer curing, and the structure utilization is also delayed, so they are not preferred for ordinary structures.
Air Entraining Cement
This cement is a derivative of OPC, made through the addition of a small number of air-entraining agents, i.e., resins, oils, fats, etc. The air is entrained in the form of little bubbles during the hydration process. Using this type of cement, the resulting concrete is more plastic, making it more workable and more resistant to freezing. The addition of these air-entraining agents affects concrete strength, so their amount should be kept as low as possible, i.e., it should be less than 5%.
Sulphate Resistant Portland Cement
This cement is made by decreasing the amount of tri-calcium aluminate and is kept below 5%. The cement has higher resistance against sulphate attack. It is used in structures which are likely to be attacked by suphates, like severe alkaline conditions in bridges, canal, siphons, etc. They develop less heat of hydration, and they are usually costly.
Portland Pozzolana Cement
This is cement made through the combination of OPC clinkers and Pozzolana (calcined clay, sukhi, fly ash, etc.). They release less heat in hydration and also offer resistance against sulfate. They are suitable for mass concreting and marine works. Their ultimate strength is higher than OPC.
Other than the types mentioned earlier of cement, there are certain different types of cement-like white cement, colored cement, high alumina cement (HAC), etc. The important characteristic of cement is that it should not expand; if there is expansion, then stress is generated, which causes harmful effects.