Cement, identified as one of the eight core sectors in India, is an energy and resource intensive Industry. Limestone is the primary constituent of Cement, which occurred naturally and gets mined out scientifically by the Cement companies. A typical composition of Cement includes calcareous materials (or calcium carbonate) and argillaceous substances (silicates of alumina) crushed and ground together above 1500º C to form a uniform powder (in the dry process) or a homogenous paste (through the wet process).1The calcium component of the cement is derived from limestone.
Material chemistry plays a very important role in maintaining a desired quality/grade of Cement. End use performance of the Cement material also depends upon chemical reactions that take place upon addition of water. These reactions are also affected by the fineness of the grinding of cement and presence of mineral impurities.2
Quality of cement is largely determined by the quality of input material and the control processes adopted during clinkerisation and grinding. Clinker characteristics are hence described by various chemical parameters, mainly as follows:
Clinker production primarily requires source of lime (CaO), silica, alumina and iron oxide. Limestone is the fundamental source of lime in clinker making. Lime Saturation Factor (LSF) is a measure of actual amount of lime in raw meal/clinker to the theoretical lime required by the oxides of silica, alumina and iron oxide. Generally, LSF of clinker falls within a range of 92 to 98, whereas, LSF of limestone is often maintained between 102-104 by way of blending high grade limestone and medium/low grade limestone.
Clinker with LSF close to 1.0 or above indicates residual free lime content within it, which results in higher burn of clinker, difficulty in grinding and higher energy consumption. Excess free lime is also responsible for undesirable volume expansion, increased settling time and reduced strength of Cement material.3 When coal, fly ash, blast furnace slag, etc. are added to kiln, presence of ash content and other composition also need to be monitored to check the impact on LSF. Hence, it is important to measure and control LSF for optimal results with clinker manufactured. Standard formula to measure LSF is:
Alumina modulus, ratio of alumina oxide to iron oxide, generally helps in determining temperature at which liquid phase composition of clinker starts inside a kiln. It also signifies the colour of clinker formed. AM value of 1.6 is optimum for clinker formation and nodulisation. At this value fuel consumption is also optimised.Higher the AM value, lighter the colour of clinker with fast setting of cement. More gypsum is required for such cements to delay the settling time. Too high AM causes hard burning of clinker resulting high fuel consumption. Too low AM value leads to low heat of hydration, low shrinking and slow setting properties.4AM = Al2O3/Fe2O3 Typical range for AM is 1 – 2.5.5
Silica Modulus, ratio of oxides of silica to the oxides of alumina and iron, determines the ratio of solid content to the melt content. Higher SM leads to better resistance properties against chemical and atmospheric factors with improved strength. It, however, needs to be balanced as too high SM results in poor clinkerisation, more fuel consumption and slow setting/hardening of cement. If any kiln is dusty, it signifies that the rawmix has higher silica ratio or too low liquid content, which needs to be balanced and controlled.6
Lesser the SM, more will be the liquid formation, which ultimately improved clinker burnability with lesser fuel consumption. Too low SM also results in more melt formation leading to thick kiln coating and lower melting.SM = SiO2/(Al2O3 + Fe2O3) SM varies between 1 to 5 depending upon the cement type. Generally it is advisable to maintain SM value of 2.1 to 2.7.7
The above three factors, broadly clubbed together as Cement Modulus are prime quality determinants for cement manufacturing. In addition to these, several other parameters are measured and controlled by the Cement plant operators to meet the desired results. These are mainly – loss of ignition, liquid content, degree of calcination, sulphur to alkali ratio, free lime, raw meal to clinker factor, etc.
This brief article demonstrates that cement making is a pure work of chemistry and precision. It needs highly skilled operators and state of art plant and machinery for best quality at optimised energy and resource consumption. Over time, the Indian Cement Industry has mastered this skill with its persistence and business leadership. Indian Cement sector is front runner in energy efficiency and environmental friendly practices. Careful execution of plans and rigor with manufacturing processes are one of the many reasons behind the success story of the Indian Cement Industry.