Cement, Energy and Environment

21 v. 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% MgO and 0.22% free lime in case of 3 days curing cement concrete is 0.48 % and expansion of the same cement in 7 days curing reduced to 0.22 %. 5.0 Alternate for Production of Cement from High MgO Limestone : The production of the blended cement or composite cement are the best alternative for cement production in case of high MgO clinker mix instead 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%. The unsoundness of the cement can be deduced by adding 60-70% GGBFS or fly ash 15-30% in OPC. The higher content of MgO even 5-15% can be tolerated with the addition of the above pozollanic 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 network 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 C 4 A 3 S 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 1200 0 C. 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% less CO 2 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 1200 0 C. 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 C 4 A 3 S as main component together with Calcium Sulfate, Dicalcium Silicate (C 2 S), Tetracalcium Iron Aluminate (C 4 AF), Calcium Sulfosilicate (C 3 S 2 F), Calcium Aluminate (C 3 A,CA,&C 12 A 7 ) and Silicoaluminates (C 2 AS, CAS 3 ). The mineralogical composition of the Sufoalluminate Belite cement are quite different than the Portland cement (Table- 2) as it contains relatively high concentration of C 2 S, C 4 A 3 S 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. A comparison of chemical composition and properties of typical Portland cement and sulfoaluminate belite cement are as given in table-3. Table-2 Mineralogical Composition of Portland Cement and Sulfoaluminate cement Clinker Minerals (%) Type of Cement Sufoalluminate Belite Cement Portland Cement C 3 S none 67.60 C 2 S 50.30 5.10 C 3 A none 9.20 C 4 AF 19.50 10.60 C 3 A 4 S 20.20 None CS 9.70 5.90