Addition of Flyash (FA) to cement matrix has become more common and getting accepted; a limit of 35% has also been suggested in BIS Code on FA based Portland Pozzolana Cement (PPC). But, cements with FA content of about 25% are commonly produced in cement plants in India. Any cement composite with such flyash content is likely to have significant porosity for considerable period after casting due to dilution of cement content in the binder portion and slower pozzolanic reaction of FA as compared to hydration reactions of cement. Thus, if a matrix with water-binder (w/b) ratio of 0.50 is considered, its actual water-cement (w/c) ratio would be about 0.67. This high w/c ratio can be expected to cause high of porosity and reduced permeability for considerable duration which may extend often over several weeks, sometimes even months, after casting, till pozzolanic reactivity of FA is large enough to cause refinement of micro- structure. Meantime, the nano-silica (nS) is now available commercially and it would be interesting to know whether it can improve the porous FA-cement matrix. Another mineral admixture, silica fume (SF), though much costlier than FA, is also used widely recently to achieve improved performance from cement matrices. SF also refines microstructure of cement matrix because of its high fineness and efficient pozzolanic activity. But, the nano-silica (nS) being finer (particles being nano-size) and chemically more reactive than SF, it would be educative to know whether nS can impart any benefit to the hardened cement matrix containing SF also. Considering this, present experimental work was taken up on use of 'nano-silica solution' (nSS) to study permeability of cement matrix with FA and SF. It was observed that hardened (i.e. cured) cement mortar containing 25% FA (nomenclated as 25FAM) in its binder portion can be treated with nSS to achieve significant reduction in permeability to water. This effect was much less in cement mortar containing 5% SF (nomenclated as 5SFM). The nano size silica particles present in nSS can act efficiently and physically as a very fine filler for the pores present in the matrix, and perform chemically by reacting fast with free lime available in the matrix to form secondary C-S-H gel. This effect can be checked by measuring the rate of water absorption using which it is possible to compute the 'Coefficient of Absorptivity'. The numerical value of this parameter was found to reduce as much as by 60% in 25FAM matrix on external treatment by nSS, but, in case of matrix of 5SFM, there was only a marginal effect. Thus, the nSS treatment technique investigated is practically more useful to improve durability of cement matrices containing FA rather than those containing SF.
Rajamane N. P., Head, Shyam Samarpan, BTech Student, Subhajit Saha, BTech Student, and Jeyalakshmi R., Professor, Dept of Chemistry, SRM University, Kattankulathur. (TN)