GC Mishra and KN Bhattacharjee observe that if the MgO content is <2.0 per cent in the raw meal it is incorporate into the crystal structure and works like good mineraliser by improving the burnability, promoting the absorption of free lime and improve the formation of C3S and C4AF.
With depletion of high cement grade limestone, presently Indian cement industry is facing an acute cement raw material problem for smooth plant operation and manufacture of higher grade cement. Although India is bestowed with huge imestone resources the most of the limestone deposits in India presently available for cement manufacture are either marginal grade or low grade, whereas the demand for high grade limestone has been increased in recent years for anufacture of higher grade cements. Some of the cement plant starts with a simple limestone deposit with more or less uniform quality of limestone, with the consumption of the high grade limestone, the deposit converted in to intricate and left with very low heterogeneous grade some times dolomitic with high Magnesium oxide (MgO) content.
Hence a serious thought is essential not only for detailed exploration of limestone deposits to convert the resources to reserves (as per UNFC), but also for development of a cost effective dry beneficiation technique for up gradation the low and sub-marginal grade limestone in India and use of huge high MgO limestone to meet the increase demand of limestone in cement manufacture.
Scenario of limestone deposits
Limestone being the prime raw materials for cement manufacture the growth of the cement industry depends on the availability of cement grade limestone. India is having huge limestone deposits distributing through out the geological stratigraphic horizon starting from Archaean to Tertiary formations. Thus, the quality variation of limestone from deposit to deposit is very wide in India. The geographical distribution of limestone deposits in India is also not uniform. Some of the States do not have any limestone deposits. It has been observed that more than 95 per cent of cement grade limestone deposits are concentrated in 10 states, although the cement grade limestone is occurring in 23 States in India.
The limestone deposits are not reported from Punjab, Mizoram, Goa, Sikkim and Tripura, whereas Haryana, Manipur, West Bengal, Andaman and Nicobar Islands have very meager reserves not suitable for large capacity cement plants. Based on the prima-facie availability of freehold cement grade limestone deposits, there is very limited scope for further addition of cement manufacturing capacity in Kerala, Tamil Nadu, Bihar, Uttar Pradesh and Odisha. However, the States of Andhra Pradesh (including Telengana), Assam, Gujarat, Himachal Pradesh, Karnataka, Chattisgarh, Meghalaya and Rajasthan have potential for further creation of additional cement manufacturing capacity.
The qualitative and quantitative assessment of limestone deposits have been carried out by various central government, State Directorate of Geology and Mining and private companies. Based on the exploration data generated by these agencies at present the total cement limestone resource estimated by Indian Bureau of Mines (IBM) is 1,24,539.551 MT.
As per the quality of limestone, except limestone occurring in the Northeast, Gujarat, a few patches of Andhra Pradesh (including Telengana), Rajasthan and Madhya Pradesh most of the deposits are low grade, either having high silica content (SiO2 16-20 per cent) or high magnesia containing 6-12 per cent MgO or even more. These limestone can not use for manufacture of cement as such with the require beneficiation. Specially the production of sound cement from high MgO limestone is the biggest challenge.
Further, out of total limestone resources 1,24,539.91 MT, 30 per cent falls under the forest cover, restricted area and eco-sensitive zone, not permissible for exploitation. As a result further 34677.19 million tonnes of limestone are not available for the cement industries, although some of the deposits are best cement grade limestone. From the available remaining 89862.361 limestone resources cement grade limestone reserves is only 8,948.926 MT (UNFC Code (111), (121) and (122)) as per UNFC classification of mineral deposits.
Although the consumption ratio of limestone to cement has gone down in India due to production of more blended cement, the limestone quantity requirement for cement manufacture has increased 1.5 times in last 15 years. Further the demand for high grade limestone has been increased due to production of more 43 and 53 grade cements. It has been observed that the demand of high grade limestone increase the mines rejects further. The limestone requirement for production of 71.28 MT of clinker during 1997-98 was 106.92 million tonnes and in the year 2013-14 it has enhanced to 300.00 MT. From an estimate it has been seen that for production of 500 MT of Clinker (as anticipated) by the end of 2020 the limestone requirement will be around 700 million tonnes. Hence the detailed exploration and establishment of the limestone deposits , use of high silica and high MgO limestone in cement manufacture is the need of the hour.
If this trend persist the annual limestone requirement for production of 500 MT (as anticipated) of clinker by the end of the year 2020 will be around 700 MT. With this rate of consumption the present available limestone reserves of India can sustain only 40 years. (12th plan report)
Effect of High MgO in Portland cement
For production of Portland cement the limiting value of MgO in case of cement grade limestone is 05.0 per cent, however the preferable value is 3.5 per cent to control the autoclave expansion of the cement under the prescribed value of BIS. In IS 269:2015 the limit of MgO in case OPC 33, OPC 43 & OPC 53 is 6 per cent however in case of sleeper grade 43 & 53 it is 5 per cent. As a result the production of Porland cement become more stringent from the high MgO limestone.
During the pyroprossing around 02.0% MgO will dissolve in the clinker liquid predominantly in C4AF and contribute the liquid phase in rotary kiln. That is why some times change in MgO content of kiln feed can cause ball or coating ring formation. Above 2 per cent MgO remain as solid phase as Periclase. Due to slow hydration of the periclase mineral present in the cement makes its unsound. When the MgO is more than 2 per cent the rapid heating and cooling is beneficial for development of small periclase crystals in the clinker. These reactive periclase crystals are hydrated faster than the cement hardening. Therefore this do not cause any unsound by later expansion.
The hard burnt MgO in cement reacts with H2O very slowly to form Mg(OH)2 that causes volume expansion of 118 per cent. This volume expansion is after the cement is hardening thus makes the cement unsound and causes cracks in concrete. The higher MgO in cement retards the initial hydration of the cement and increase the setting time. During the hydration of the cement the solubility product Mg(OH)2 is much smaller than Ca(OH)2, hence the Mg(OH)2 precipitated earlier than Ca(OH)2 . The formation of Mg(OH)2 reduces the Ca(OH)2 saturation ratio, thus delaying the initiation of maximum of Ca(OH)2 saturation ratio. When MgO hydrate in high alkali medium the Mg(OH)2 with tiny crystals precipitates around the cement grains to forming a protective layer, hence retarding further hydration of the cement grains. MgO also gives some darker colour to the clinker.
Measure to control MgO
The causes of the cement expansion by the crystalline MgO ( periclase) may be due to the following factors ( Dreizer 1981)
i. MgO Content in Raw Materials
ii. Chemical & Mineralogical composition of raw materials
iii. Raw meal preparation and fineness
iv. Pyroprocessing of the Cement Clinker
v. Size and distribution of the periclase in the cement
vi. Cement Fineness
vii. Cement Storage
viii. Cement Additives
Screening the limestone during crushing and blending with low MgO Limestone some times reduced the over all content of MgO in raw material. Grinding the raw meal finer for better burning. It has been observed that quartz and dolomites are weekly magnetic with relative attractivity of 0.37 and 0.22 respectively, where as the calcite is nonmagnetic with relative attractivity of 0.03. Hence, with high magnetic separator the silica and dolomite can be effectively separated from limestone. In case of electrostatic separation, it is not effective to separate the calcite and silica as they are having almost same relative empirical conductivity (voltage-10,920).
Whereas the relative empirical conductivity of Dolomite (voltage-8,268) is much lower than the calcite and quartz, as a result the dolomite crystals can be effectively separated from limestone by electrostatic separation methods.
The magnesia content more than 5 per cent is undesirable in cement grade limestone as it produce unsound cement. In most of the cases it has been found that the dolomitization in limestone alter the calcite in such a way that magnesia limestone nearly defies any economic separation. However, removal of dolomite crystals from limestone is possible through air cyclone effectively. A detailed study on establishment of cost effective beneficiation technique for separation of dolomite (MgO) from limestone is essential.
A study of clinker cooling reveals that under slow cooling, only 1.5 per cent of MgO is retained in solid solution and rest crystallized as larger periclase crystals and makes the cement unsound. The rapid cooled clinker forms smaller alite and belite crystals, exhibits faster strength growth during hydration, and is able to accommodate the hydration of periclase (MgO) considerably. Various studies say that increasing the fineness of cement and addition of fly ash effectively stabilises MgO up to 10 per cent in Portland cements.
With the lower Lime Saturation Factor (LSF) (0.94to 0.97) the high MgO in raw meal does not have any influence on the free lime generation during pyro-processing. Whereas incase of high LSF (0.97-1.00) any increase of MgO in raw meal result more free lime generation and results unsound cement.
The free MgO decrease in cement clinker as the iron content in the raw meal increase. It has been observed that a cement containing 5.0 per cent MgO and 2.3 per cent Fe2O3 had the autoclave expansion of 1.28 per cent whereas another cement containing 4.95 per cent MgO and 3.6 per cent Fe2O3 exhibit the autoclave expansion of 0.15 per cent. The iron content in the raw meal should in the following ratio to control the autoclave expansion.
Fe2O3 = 2+ (0.2xMgO).
Further the ratio between (Alumina +MgO)/Fe2O3should be maximum 2.7 for production of sound cement. When the MgO: Fe2O3 >1.53 the autoclave expansion of the cement is in danger zone. When the ratio of MgO: Fe2O3 < 1.40 probability of expansion failure decrease rapidly and MgO: Fe2O3 < 1.20 is the limiting value for production sound cement.
During pyroprocessing the C4AF is act as an excellent stabilizer for MgO and capable of transforming the considerable amount of MgO into nun expanding compounds. The autoclave expansion due to MgO may be control by lowering the alumina modulus of the raw meal. This will enhance the C4AF content in cement and less C3 A. The C3A should maintain 6-8 per cent in the cement clinker to control the MgO in form expansion. The higher MgO content in raw meal consumes extra thermal energy also.
In case of hard burning even the raw meal having 2.5 per cent MgO fails in autoclave expansion. Whereas the light burning raw meal containing 5 per cent MgO does not fail in autoclave expansion as it hydrates rapidly before the cement sets.
The higher SO3 in the Portland cement clinker also prevented the autoclave expansion in case of high MgO limestone. it has been observed that in case of 5 per cent MgO containing clinker if the SO3 is 2.17 per cent the autoclave expansion is 0.44 per cent. When the SO3 content in the cement decreased to 1.01 per cent the autoclave expansion increase to 4 per cent. From the above it can be concluded that in case of high MgO limestone the petcoke with high sulfur is suitable. It has been reported that the use of 0.4-0.8 per cent fluorspar (CaF2) in raw mix as mineralizer reduced the size of periclase to 1-7 micron in the cement clinker.
By the fast cooling of the clinker, the MgO retained in the solution in the glass form. This reduce the expansion of the cement due to periclase. On the other hand in case of the slow cooling clinker the more periclase (MgO) crystallized out of the melt and larger crystals were formed . These large crystals of MgO are responsible for the failure of autoclave expansion of the cement.
The smaller size of periclase crystals in the cement does not contribute to autoclave expansion as the smaller periclase crystals are predominantly located in the interstitial phases. In an experiment it has been observed that cement clinker containing 6.5 per cent (5.4 per cent free periclase) MgO ground with 2 per cent SO3 containing and free lime (CaO) of 0.47 per cent. When the particle size distribution of this cement is 80 per cent of the periclase is <5.0 micron size and 20 per cent periclase is size of 5.0-15.0 micron the autoclave expansion of the same cement found only 0.47 per cent.
Coarser the ground of the cement have always exhibited a greater amount of the autoclave expansion in case of clinker having high MgO content. Therefore, incase of high MgO clinker the cement should be ground finer to control expansion. It has been observed when a high MgO content clinker ground to 225 m2/kg was showed the autoclave expansion of 7.06 per cent. When the fineness of the same cement enhanced to 350 m2/kg the autoclave expansion dropped to 1.49 per cent. With the increase of fineness of the cement to 400 m2/kg the autoclave reduced to 0.24 per cent.
Longer the curing of cement concrete decreases the autoclave expansion considerably. In an experiment it has been observed that the autoclave expansion of a cement containing 5 per cent MgO and 0.22 per cent free lime in case of 3 days curing cement concrete is 0.48 per cent and expansion of the same cement in 7 days curing reduced to 0.22 per cent.
Alternate for production of cement
The production of the blended cement or composite cement are the best alternative for cement production in case of high MgO clinker mix in stead of ordinary Portland cement. In case of the Portland slag cement and composite cement the limit of MgO in cement has been enhanced to 8 per cent. The unsoundness of the cement can deduced by adding 60-70 per cent GGBFS or fly ash 15-30 per cent in OPC. The higher content of MgO even 5-15 per cent can be tolerated with the addition of the above pozzolanaic materials. Hydration of high MgO cement under autoclave conditions cause rapid formation and crystallization of Mg(OH)2 creation of larger pore sizes. This result in loss of mechanical strength and higher expansion value.
Under the ambient water curing precipitation and distribution of C-S-H gel into finer net work causes a homogeneous morphology and development of smaller pores. The resultant higher mechanical strength associated with partial hydration of MgO yields reduced expansion. High MgO cement paste containing fly ash also showed considerable pore refinement and improved hydrate morphology favouring volume stability under both autoclave and embient water curing.
In recent years, Sufoalluminate Belite cements receiving a lot of interest, as it can be manufacture from various industrial wastes and low grade limestone, environment friendly and requires less thermal energy for manufacture than the Portland clinker.
Raw mixes for C4A3S clinker differ from the Portland cement as they contain significant amount of sulphate. Therefore, the reactions and product are quite different from those normally found in Portland cement production. The total lime requirement for production of such cement is less than 50 wt% as against the about 65 wt% for Portland cement. This indicates that for production of Sufoalluminate Belite cement the low grade limestone are also suitable. The Sulfoaluminate Belite clinker can be produced by burning limestone, bauxite and gypsum at around 12000C. For production of Sufoalluminate Belite cement the Lime Saturation Factor (LSF) requirement is low around 0.67, which reduces the consumption of limestone stone in manufacture.
As the consumption of lime is less, for manufacture of this type of cement it generates around 35 per cent less CO2 than the Portland cement. The requirement of thermal energy in production of Sufoalluminate Belite cement is also much less than the Portland cement as it is manufactured at 12000C. It has been found that the Sufoalluminate Belite is having very good dimensional stability, sulfate resistance, compressive strength development and better resistance to atmospheric carbonation comparable to commercial Portland cement. Presently these cements are produced commercially some of the countries such as China, Japan, Russia successfully. These cements are best suitable for the construction in the coastal areas owing to their sulphate resistance property.
The Sulfoaluminates Belite Cement contains C4A3S as main component together with Calcium Sulfate, Dicalcium Silicate (C2S), Tetracalcium Iron Aluminate (C4AF), Calcium Sulfosilicate (C3S2F), Calcium Aluminate (C3A,CA,&C12A7) and Silicoaluminates (C2AS, CAS3). The mineralogical composition of the Sufoalluminate Belite cement are quite different than the Portland cement ( Table -2) as it contains relatively high concentration of C2S, C4A3S and C4AF. Thus these cements shows different properties during the hydration and capability to control periclase effect during hydration to make sound cement.
The Sufoalluminate Belite is chemically different markedly from the Portland Cement. The rapid setting of sulfoaluminate cement is mainly due to quick conversion of C4A3S to hydration product during early age hydration. On hydration the gypsum react with C4A3S and from ettringite (C6AS3H32) to regulate the technical properties of Sulfoaluminate Belite cements.
4 CaO3Al2O3SO3+8CaSO4+6CaO+93H2O ? 3 (CaOAl2O3CaSO431H2O)
The formation of the ettringite is very fast in this case, as a result in reduced workability of the cement and required retarder. The C2S present in this cement add the strength and durability. The mortars of such cements also has comparable compressive strength and total porosity when compared with Portland Cement. The mortar prepared from the Sufoalluminate Belite cement release less quantity of Ca(OH)2 than the Portland cement mortar on hydration, thus reduces the porosity of the concrete.
The higher content of gypsum in the Sufoalluminate Belite cement also decrease the carbonation of concrete. The Sufoalluminate Belite cement exhibits better protection for the steel reinforcement corrosion. Presently these cements are produced commercially some of the countries such as China, Japan, Russia successfully. These cements are best suitable for the construction in the coastal areas owing to their sulphate resistance property.
The influence of MgO on the composition structure and properties of alite calcium strontium sulphoaluminate cement were investigated. The results show that when the mass fraction of MgO is 1-5 per cent the early strength of cement can be enhanced significantly. The optimal content of MgO in cement clinker is 2 per cent and the compressive strength of the cement at 3, 28 days are 64.3 and 103.6 MPa respectively.
The suitable amount of MgO can promote the formation of cl.5Sr2 S A3, while the formation of cl.5Sr2 S A3 can be hindered if the content of MgO is excessive. The existence of MgO can improve the formation of C3S, increasing the mechanical properties of the cement. Compared to Portland cement the Calcium Strontium Sulphoaluminate cement has higher capacity to dissolve MgO which indicate that the low quality high magnesium limestone can be effectively used in cement production.
From the above analysis it can be concluded that fort use of high MgO limestone in Cement production the following measures can be taken.
Screening of dolomitic limestone and blending with low MgO limestone to control the MgO in the Raw meal . It should not be more than 5 per cent limestone.