Chemical Composition of Portland Cement
Chemical Composition of Portland Cement: Cement is mostly composed of lime, silica, alumina, and iron oxide as raw materials. At high temperatures, these oxides react with one another to form more complex compounds. Along with the rate of cooling and fineness of grinding, the relative proportions of these oxide compounds influence the different properties of cement. The oxide composition of ordinary Portland cement is approximated in Table 1.
Multiple standards specify the chemical requirements for cement as follows:
As previously said, when raw materials are exposed to high clinkering temperatures, the oxides found in them combine to form complex compounds. The description of the main compounds is mostly dependent on the work of R.H. Bogue, which is why it is referred to as “Bogue’s Compounds.” Table 2 lists the four compounds that are often referred to as main compounds.
It should be remembered that for the sake of convenience, abbreviated notations are used. CaO is abbreviated as C, S is abbreviated as SiO2, A is abbreviated as Al2O3, F is abbreviated as Fe2O3, and H is abbreviated as H2O. The following are Bogue’s proposed calculations for estimating the percentages of major compounds.
C3S = 4.07 (CaO) – 7.60 (SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3CaO.SiO2)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF= 3.04 (Fe2O3)
The oxide denoted by brackets denotes the proportion of the same in the raw materials.
Along with the four main compounds, the kiln produces a large number of small compounds. These minor compounds have no effect on the properties of cement or hydrated compounds. Two of the minor oxides, K2O and Na2O, which are referred to as alkalis in cement, are important. This point will be addressed later in the discussion of the alkali-aggregate reaction. Table 3 illustrates the oxide composition of standard Portland cement and the related measured compound composition.
The most common compounds responsible for strength are tricalcium silicate and dicalcium silicate. They together account for between 70% and 80% of cement. The average C3S content of modern cement is around 45%, while C2S content is around 25%. The total amount of C3A and C4AF in current cements has decreased significantly. Although a comparatively minor improvement in the oxide content of the raw materials has a significant effect on the measured quantity of compounds in cement.
To produce a cement with a specified compound composition, it becomes critical to strictly regulate the oxide content of the raw materials. Increases in lime content above a certain point make it more difficult to mix with other compounds, and free lime forms in the clinker, causing cement to be unsound. Increased silica content at the cost of alumina and ferric oxide would make it more difficult for the cement to fuse and shape clinker.
Cements with a high overall alumina and ferric oxide content are advantageous for producing cement with high early strengths. This may be attributed to the fact that these oxides are responsible for the complete mixing of the total amount of lime available to form tricalcium silicate.
The advent in science and technology has allowed us to recognize and comprehend the microstructure of cement compounds both before and after hydration. The X-ray powder diffraction technique, X-ray fluorescence method, and the use of a strong electron microscope capable of magnifying the unhydrated or hydrated cement crystalline or amorphous structure have all aided in revealing the crystalline or amorphous structure.
Le Chatelier and Tornebohm also noted the presence of four distinct types of crystals in thin sections of cement clinkers. Tornebohm coined the terms Alite, Belite, Celite, and Felite to refer to these four crystal types. Tornebohm’s description of the minerals in cement was found to be identical to Bogue’s. As a result, Bogue’s compounds C3S, C2S, C3A, and C4AF are sometimes referred to in the literature as Alite, Belite, Celite, and Felite.