The use of fly ash & Granulated slags as mineral component in Blended Cements mainly Portland Pozzolana Cement & Portland Slag Cement has been now a common feature, these cements today dominate the Products Menu of all Cement Groups in the country. It has now been completely accepted that the blended cements provide several distinct advantages over OPC especially in terms of enhanced durability of blended cement concretes .
Every manufacturing Cement Plant /Groups are striving to achieve the lowest possible Clinker factor in PPC/PSC , without affecting the performance of Cement and Resultant Concrete . The Country still has a dominating market for Bag - Cement .
Practically every Cement Group today have a Normal PPC / PSC Brand , along with a premium high strength brands of the two Blended Cements claiming it to be competitive to OPC 43/53 concrete performance . The author discusses the various aspects which can help achieve the lowest clinker factor and still have the PPC/PSC with good performance of Cement in Concrete .
The Clinker Factor Reduction of course helps to
In a running plant the approach has to be by attempting small gradual changes to clinker composition and assessing the impact of the changes on kiln Performance & clinker Quality . The changes to be attempted could be indicated through data analysis. In each plant the QC & Process has a detailed analysis data of the Day Average clinkers along with its lab ground Cement Test Results .It is also suggested to test at least one spot clinker per day for chemical parameters and physical tests of Lab ground Cement . From the analysis data :
For The Lab Ball mill grinding of average/spot clinker with fixed Time , it is preferable to select a grinding time to achieve Blaine’s of around 300 - 320 M2/kg with the residue on 45 microns of the Cement in range of 18 to 20% , at this fineness, the clinker is observed to depict changes in clinker reactivity in terms of changes in 1 Day strengths of cements (± 3 to 5 MPa) . At lower grinding Blaine’s (of around 250 M2/kg ) , presently practiced by many Cement Plants , one does not observe the changes in clinker reactivity as the changes in 1 Day compressive strengths is only ± 1 MPa .
Typically Clinkers with Good reactivity are observed to show 1 Day strengths in Lab ground Cements of 30 to 35 MPa ( Higher values being observed when clinker alkali sulphates are high ) , the achieved Blaine in the fixed grinding time is also indicative of clinker grindability.
In a plant , If the clinker free lime is frequently observed to periodically fluctuate from low to high values during the day the possible reasons could be , Process / Quality fluctuations , some of which are summarized below
In plants where clinker MgO is higher (3.5 to 4.5%) , besides having the above target modulii Values , the minimum clinker lime targeted should be between 62.5 to 64% for good clinker reactivity (lower value of CaO , for higher MgO in clinker ) If the Plants raw Materials have low MgO , and the quarry limestone is siliceous , use of low silica dolomitic limestone or choice of low silica correctives such as low silica Iron Ore / Bauxite /Laterite etc is always advantageous . Use of fluoride based Mineralizers (CaF2, SSF etc ) , are observed to substantially improve the clinker reactivity , however from cost benefit angle presently it is not attractive .
Selection &Optimisation of Type & % use of Gypsum (for detailed article “The role of gypsum in Cement .refer ICR Vol 34 November 2019 No 4 , pg 48 -52) The sulphate content (%SO3) of the cement should be such that it is consumed for the initial reactions and not later than 24 hrs after mixing with water.
(Fig 1 & 2 )
The optimum quantity of Gypsum (%SO3 usage )would depend mainly on:
As and when there is decreased availability of fly ash , during such occurrences the Plant tends to reduce % fly ash and decrease the fineness of PPC so as to maintain the Mortar quality norms of the Product. However such actions would affects the PPC performance in Concrete at the applications sites , it also increases the variable cost of the Product , due to increased clinker factor .
For such instances the plant should use lower purity Gypsum (kept in stock ) which would help to relatively maintain the clinker factor of PPC. Use of cement additive would help in such cases . In general use of anhydrite Gypsum (synthetic / Natural ) , is advantageous for PPC / PSC as it increases the early strengths and there by assists in increasing % Fly ash / % slag in PPC/PSC
Characteristics of Fly ash / Slag used & Optimization of PPC /PSC
For an effective optimisation of PPC/PSC for achieving lower clinker factor , we need to look at some of the important Characteristics of the fly ash / Slag (GBFS), which are discussed in brief ahead .
Combustibles in fly ash The carbon or combustible contents of fly ash can be determined by loss-on-ignition (LOI, a measure of carbon mass). The high-carbon fly ash tends to use more water thus affect the compressive strength characteristics of the resultant PPC and resultant Concrete . The Fig.3 depicts the physical properties of PPC (Indian standards) produced with 20% of high carbon (12.5%) and low Carbon (2.5%) fly ash.
Chemico–Mineralogical properties of fly ash. In India the chemico - mineralogical characteristics of dry fly ash produced, has been observed to vary. The mineralogy of fly ash has 15 -30% Mullite, 15-45 % Quartz, 1-5% Magnetite, 1-5% Hematite and around 25 – 50 % of amorphous glassy alumino - silicate phase. The ternary phase diagram of SiO2-Al2O3-CaO (Fig 4) indicates the relative positions of different materials . As the lime in fly ashes increases their relative position moves towards the center of the ternary diagram so is the reactivity .
The amorphous phase in fly ash is the reactive part in fly ash responsible for the pozzolanic hydration reaction. The crystalline phases of fly ashes such as Mullite, ?-quartz, hematite, magnetite are non hydraulic , while in Class C Fly ash the calcium alumina silicate glass crystalline calcium aluminate phases present in some of the Class - C fly ashes are cementitious in nature .Thus the chemico-mineralogical properties would determine the reactivity of the fly ash and their by influence the %clinker factors in PPC .
Fig.3: Effect of high carbon content in fly ash on compressive strength
Fig.4: SiO2-Al2O3-CaO system showing relative positions of cement & cementitious materials
% Use of CPP Fly ash (Crushed Coal fired (AFBC/CFBC) boilers ) .Many Cement Plants , have Captive Power Plants : Crushed Coal fired AFBC/CFBC Boilers (Fig 5 ) , this fly ash constitute 10 to 40% of Total fly ash used for PPC manufacture , depending on generation of CPP Fly ash as well as consistency of supply of Dry Fly ash, the % use of CPP Fly ash increases . This CPP fly ash produced at lower over board Temp. show higher water demand and show much lower early strengths in PPC . There by Increasing the Clinker Factor for achieving quality compliance at Early ages . It has been observed that lower is the percentage use of CPP fly ash lower would be the clinker factor achieved in PPC .
Use of Pond ash : Pond ash has higher absorbed & adsorbed moisture , the wet storage agglomerates the finer size particles , it also has varying percentage of bottom ash which is pumped to the same fly ash ponds . Use of Pond ash affects the water demand and early strengths of PPC there by tends to increase the clinker factor .
Pulverised coal fired
Crushed Coal fired (CFBC/AFBC)
High temperatures (1300 to 1500 oC)
Low temperatures 850 – 900 oC
In both the above cases use of suitable Cement additive combination could help to reduce the water demand of PPC , thus would help achieve Lower clinker factor in PPC , such cement additives would also enhance the performance of PPC Mortar & Concrete .
Fineness (Particle size Distribution of fly ash ) received from Power Plants. The fineness of the fly ash available from a given source varies considerably depending on the ESP fields from which it is collected , as well as other operational parameters of the thermal plant. On an average the fineness of fly ash in terms of residues on 45 microns ranges from 12 to 50 %. . In the inter grinding Mode (closed circuit system) , It is suggested to add the fly ash at Ball Mill Inlet and not at the inlet of dynamic separator . By adding at the ball inlet (even in the combo circuit ) during grinding the coarser sintered fly ash particles get ground finer and concentrate in the finer fractions of Cement (<20 microns ) which helps to increase % fly ash of PPC and also helps reduce the water demand , which shows improved performance in Mortar as well as in resultant concrete . Typically PPC fineness should be 360 to 380 M2/kg , more importantly the fly ash should be mainly below 20microns , the residue on 45 micron can be between 6 to 10 % in an intergrinding mode , further lowering of % clinker factor in PPC could be achievable through use of Cement additives .
Characteristics of Granulated Blast Furnace slag (GBFS) & PSC optimisation .
As can be seen from the ternary diagram in Fig 4 as the CaO in materials increases the hydraulic activity of the material increases , thus Granulated blast furnace slag (GBFS ) is cementitious showing its own hydraulic potentials , The hydraulic property of different materials with increase in C/S ratio is indicated below .
The influence of GBFS properties on reactivity is tabulatedbelow .
GBFS properties influencingreactivity
Chemical composition – Major elements
CaO controls dissolution,
7.5 - 20
ettringite formation,at early ages
Not as effective as CaO
Bulk density, kg/l
Low bd, higher reactivity, but high % moisture
Melilite: Gehlenite (C2AS2) ,Akermanite (C2MS2),Merwinite: C3MS2
Merwinite in basic slags, indicates high reactivity
85 to 96
Due to aging & false Blaine“
The Alumina content has strong influence on GBFS reactivity, especially at early ages. Ideal Al2O3 Content being: 13-14%. , as the TiO2 content in GBFS increases the slag becomes denser and harder to grind and its reactivity is observed to decrease
As is known , typically the reactivity of GBFS slag increases with increase in Fineness
Fig 6 Effect ofincreased fineness on reactivity of ground GBFS
In most of the Modern Plants manufacturing PSC , the slag and OPC is ground separately and interblended to produce PSC . The % slag in PSC is more governed by the Particle size distribution of the ground slag and also ofcourse on the clinker Quality . It may be noted here that the OPC component in the Portland Slag Cement is the alkaline activator .
For achieving lowest clinker factor , typically in the inter blending mode , OPC (clinker +Gypsum) should be ground to around 380 to 400 M2/kg (with 10 to 12 % residue on 45 microns ) while slag ( preferable slag + Gypsum ) should be ground to around 70 to 75% passing 20 microns with Blaine’s surface of around 400 – 420 M2/kg . Many plants today have achieved by inter blending mode % slag up to 64 to 67% with clinker factor of PSC at around 27 to 29% and the quality of PSC manufactured is competitive in Market
In the inter-grinding mode with Clinker +Slag + Gypsum ground together , the clinker (with Gypsum) gets preferentially ground leaving slag relatively coarser , proper optimization of the distribution of slag in desired size fractions of Cement with or without cement additives could help achieve a clinker factor of around 35 to 40 % .
There are grinding aids (combined with strength accelerators ) , used for slag grinding , as well as for OPC grinding , for slag grinding with hot air the grinding aids should not be steam volatile. The strength accelerators along with grinding aids should be chloride free . Achieving the lowest clinker factor in PSC is advantageous firstly from cost economics and also because it substantially increases the Cement volumes , from clinker which is beneficial for a sold out market .
Use of Cement Additives.
The last and final step in achieving Lowest Clinker factor in PPC & PSC would be , use of Cement additives . These are substances added in very small proportions typically < 0.25 % to the cement mill to improve
When Cement additives are used , in general the additives reduce the inter-particle forces , reduced coating of grinding balls and mill internals , due to creation of like charges on cement particles , there is decreased agglomeration , much improved flowability , higher generation of fines better dispersion of particles in separator feed and reduction of mill filling level (decrease of residence time) in VRM grinding actions need to be taken to have stable Bed formation on the table .
Commercially available Cement additives from national and international suppliers are a formulated mixture of compounds in aqueous solutions (40 to 50 % water ), these mixtures are formulated as per the requirements of the customer, as per the cement components used by the customer and as per the desired objective of usage of the additives , which could be like increased thru puts , improved early and /or later age mortar strengths , improved concrete performance of the resultant Cements etc .
There are many Cement plants / cement groups in the world , who have developed the cement additive formulation themselves so as to avoid a black box - Cement additive from suppliers . These plants procure the individual compounds (in their purer form, and create their own optimized formulations at site with adequate dilutions with water , for use in Cement grinding.
It has been reported in literature and also substantiated by a number of detailed evaluations of different cement additive formulations in market , that the cement additive formulations are a combinations of Different Chemical compounds , typically composed of :
1.Accelerator/s for the hydration reaction of Cements which are dependent on the acceleration effect desired in Mortar compressive strengths at early or later ages, the choice of the materials is also dependent on clinker quality and blending components (fly ash / slag) or a mix of both.
2.Water reducer / workability / wet-ability / enhancer, which would show impact on the resultant Cement mortars & concrete. Some of the compounds (retarders) like polysaccharide derivatives, gluconates etc show an initial retarding action towards hydration which result in reducing the water Requirements for the Cements thus act as water reducers, or it could be some appropriate polymeric molecules which show improved wet-ability and reduce water demand.
3. Grinding aids : Compounds that work as Grinding Aid i.e. which would enhance Mill thru-put on one hand as well as would increase the early strengths due to the higher fines generation/ or activation of cement components . These compounds could be like alkanol-amines such as TIPA , DEIPA , TEA etc or could be compounds like glycols and other poly-ols , depending on whether it is a PPC or PSC manufacture
Thus such an optimal combination of Cement additives (either from suppliers or as formulated at site), evolved for PPC/ PSC /PCC are observed to be grinding aid cum accelerator combinations , which enhance productivity of Cement grinding on one hand and also help achieve lower clinker factor in PPC/PSC/PCC) and many a times show a much improved concrete performance of the blended Cements at application sites .
These cement additives are also used to create specific niche properties of the Blended Cements (premium brands ) which are the USPs of such Cement Products , and are useful for customers to build a durable concrete structure with increased service life . Such properties could be water repellency, water proofing, much reduced water demand, self-compacting, self- leveling in concrete etc .
Cement additives could be used to overcome in-efficiencies of clinker and grinding circuits, however this would actually not realize the complete benefits from using Cement additives. Thus as discussed above a judicious optimization, in the steps indicated above, would definitely help achieve lowest clinker factor in PPC/PSC/PCC
Authored by S A Khadilkar
Consultant & Advisor
Former Director Q&PD ACC ltd Thane
email : firstname.lastname@example.org
Blog : khadilkarcement.com