Origin of cement modulus:
Even though Portland cement was manufactured by practical experience and process, when you see the chemical analysis, there exists a relation between the percentages of lime with the percentages of combination of silica, alumina and iron oxides.
The oxide ratios was later developed into formulas called cement modulus.
Lets discuss some important modulus by which we can mange any cement divison.
Silica ratio:
It’s the ratio of silica to the flux (iron+alumina).
This silica ratio varies between 1.5 to 5 depending upon the cement type, but for conventional portland cement it was advisable to maintain silica ratio between 2.1 to 2.7.
More the silica ratio, then lesser the coating tendency in kiln due to low liquid formation resulting hard burning of clinker with increase in fuel consumption, also causes slow setting and slow hardening of cement.
Also increase in silica modulus causes cement to have a property of increase in resistance to chemical and atmospheric attack and a higher strength.
Lesser the silica ratio, then more the liquid formation and coating tendency in kiln and improves good burnability of clinker with less fuel consumption.
A decrease in silica ratio causes increase in liquid content in rawmix.Thus a low silica ratio rawmix burns easily and quickly in shorter time resulting good nodulisation of clinker.
The fuel consumption of cement kiln depends on silica modulus/ratio
So if any kiln is dusty,then it means the rawmix has high silica modulus or too low liquid content.So reduce the silica modulus to improve the kiln and clinker nodulisation condition.
In some cement plants,silica ratio or modulus was expressed as:
Silica modulus ={C2S+C3S}/{C4AF+C3A(+C2F)}
Alumina modulus
It shows the factor or ratio or proportion of iron oxide and alumina. It was also called alumina ratio.
Generally the alumina ratio for portland cements to be maintained between 1.5 to 2.5 to have a good liquid phase composition in the clinker.
A high alumina ratio cement has fast setting of cement. So more gypsum was required for such cements to delay the setting time.
For example Ferro cements has alumina ratio of 1.5 range.
A too low alumina ratio has low heat of hydration, low shrinking and slow setting properties. Also if alumina ratio was below 0.7, then the resulting clinker has only C4AF compounds without C3A compounds. Such kind of cement is called Ferrari cement.
This alumina modulus decides the kiln burning zone reaction time.If alumina modulus was maintained between 1.6 to 1.8, then liquid phase gets generated at lower temperature in the kiln resulting reduced fuel consumption.Also improved nodulisation genetrates at the entrance of the burning zone.
Alumina modulus does not affect C3S and C2S formation.
A high alumina modulus causes hard burning of clinker due to low liquid phase resulting high fuel consumption.
In some area the alumina modulus was called iron modulus in cement.
Hydraulic modulus
It’s the ratio between percentage of lime to percentages of combination of alumina, iron oxide and silica.
The hydraulic modulus to be maintained between 1.7 to 2.3.
If hydraulic modulus was more than 2.4, then the concrete made from the cement has expansion and shrinkage problems even at normal or high/low temperatures ie., the cement loses its volume stability.
Also the clinker is hard to burn resulting increase in fuel consumption. The cement liberates more heat when mixed with water (more heat of hydration) resulting chances of cracking if not cured properly.
The cement has lower initial strength and so the setting and strengthening of concrete takes time resulting increased time to demould /remove concrete frame work.
The cement also loses the property of resistance to chemical attack. So that kind of cement cannot be used at construction of bridges as cement was exposed to water/sea water.The cement cannot be used at places where it was exposed to water, ice ,salt and chemicals.
If hydraulic modulus of raw mix is less than 1.7, then the resulting cement will have lower strength.
Hydraulic modulus was used for compositional effect of ash replacement on calcinations. It was also used to predict hydraulic activity on strength development. If you are using flyash or any other ash, you must monitor hydraulic modulus compulsory to avoid strength and expansion problems.
So monitor hydraulic modulus while producing special cements and flyash added cements like PPC cements as these cements are exposed to salt, water, chemicals and ice.
Hydraulic modulus was not used in most of the cement plants nowadays as only lime saturation factor, alumina modulus and silica modulus are used mostly. After the discovery of lime saturation factor, the usage of hydraulic modulus was declined as hydraulic modulus not applies for varying conditions and materials under modern cement manufacturing process.
