Cement, Energy and Environment

hydration products are not identical. Without going into the details of chemistry, it can be stated that GGBS itself is an essentially glassy material that hydrates but it requires an alkaline medium to hydrate, while the reactive SiO in fly 2 ash needs adequate Ca(OH) 2 from OPC (clinker + gypsum) hydration to react with. The clinker component provides the basic hydration requirements for both GGBS and fly ash to work but there is an 'induction period' for the above hydration mechanisms to trigger as shown in Fig.5. This induction period for GGBS and fly ash is due to the non-porous nature of the reactive quenched glass particles in them as compared to rice husk ash or nano- particles of condensed silica fume. Once the induction period is over, both GGBS and fly ash continue to contribute to strength. Thus, the late age strengths of PPC and PSC are higher than that of OPC. If under these chemical conditions, one attempts to obtain higher levels of early age strength in PSC and PPC, there is hardly any alternative than to increase relatively the clinker proportion to hydrate during the induction period of the SCMs, which means that the SCM amounts used will have to be reduced. This measure is obviously an undesirable step from both the durability and sustainability angles. In order to avoid the reduction of SCMs in cement, a possible process alternative that may be taken recourse to is to enhance the lime content in clinker with a view to forming more alite or C S 3 phase so that the early age strength is achieved. The results in most cases have not been satisfactory even with good quality clinker. In this context, some experimental results of enhancing the lime content in clinker and intergrinding of high proportions of GGBS and fly ash are shown illustratively in Table 3. Achieving high early strengths with high levels of clinker substitution was a Fig.5. Relative age of strength contribution by different SCMs Parameters Clinker with 21.9% SiO 2 & 67.0% CaO; corresponding cements with SSA 400 m 2 /kg and 3.5% SO 3 : test results of mortar compressive strength in MPa Clinker with 21.7% SiO 2 & 69.8% CaO; Corresponding cements with SSA 400 m 2 /kg and 3.5% SO 3 ; test results of mortar compressive strength in MPa Clinker with 21.7% SiO 2 & 69.8% CaO; corresponding cements with SSA 400 m 2 /kg and 3.5% SO 3 ; test results of mortar compressive strength in MPa Age, days 1 3 7 28 56 1 3 7 28 56 1 3 7 28 Control OPC 18 40 52 65 68 25 42 60 78 79 20 28 34 49 PPC with 35% fly ash 10 20 25 35 43 10 25 34 46 51 15 23 27 43 PSC with 50% GGBS 11 23 34 62 69 12 25 45 59 65 20 28 35 48 Table 3. Illustrative comparison of response of clinkers with different lime contents to interground SCMs Note: the values in the table are rounded off to the nearest integers 21