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)
IntroductionFrom Nano-technology, a new siliceous material, which is pozzolanic in nature, known as nano-silica (nS) is commercially available now. Many studies on its use directly as an admixture into cement concrete mix are extensively reported from 1 to 9. However, its use as a microstructure improver of already hardened cement matrix is not widely reported, though there was report recently on its utility to improve properties of recycled aggregates [Swapna, 2001]10. This paper examines the possibility of modifying the matrices of porous cement composites containing FA and SF, by incorporation of nS solids after the matrix has been cured through soaking of the oven dried composites in nSS.
Portland cement is a very common cementing material in the construction field. It reacts with water resulting in formation of free lime (CH) and C-S-H gel [Neville, 1996]11. The C-S-H gel provides the binding action in the cement composites (such as mortar and concrete), but the porosity is generated by presence of capillary and gel pores (besides entrapped and entrained air) which contribute towards permeability to external aggressive agencies such as water, oxygen, CO2, chlorides, sulphates, etc [Mehta, 2001]12. The use of mineral admixtures (MAs), (such as fly ash, silica fume, rice husk ash, metakaolin, ground granulated blast furnace slag, etc) in binder portion of cement composites is becoming common now as these MAs not only reduce actual cement content but also improve considerably the microstructure thereby enhancing the durability of the cement composites such as concrete, mortar, etc. However, some of these MAs such as fly ash (FA) act more as a filler initially without much cementing action and only after considerable curing period, their contribution towards microstructure improvement comes into effect. This is partly due to slow chemical reaction between lime generated during cement hydration and the siliceous portion of the MA. When the cement replacement level of FA is higher, say 25%, the capillary pores do remain for long duration and in some cases, even the products of complete cement hydration would not be able to fill the capillary pores generated at this high w/c ratio [Powers, 1967]13. However, SF being much finer and more amorphous siliceous than FA, can create faster pozzolanic reactions and hence the matrix would become refined faster leading to quicker improvements in microstructure. It may be noted here that 'cementing efficiency factor, k' for FA is often much less 1 and that of SF is usually more than 1, often in the range of 2 to 3. Usually, silica (SiO2) react with free lime available in the hydrated cement matrix to form secondary C-S-H gel which act as pore filling as well as pore-size refining material. [Taylor, 1999; Larrard, 1999, Hewlett, 1999]14,15,16. Therefore, the nano-silica (nS), due to its extreme small size (much smaller than SF), besides being amorphous, can act both physically and chemically in the hardened cement matrix to improve its degree of impermeability. This aspect was taken up for study presented in this paper and the degree of permeability of a cement mortar with 25% fly ash content and also of another cement mortar with 5% SF, were measured by determining the 'coefficient of water absorptivity' (COA) which is based on rate of water absorption by capillary action for 60 minutes as suggested well-known and highly respected cement scientist, T C Powers13. The test data indicated the high potential for external treatment of the matrix by nS solution (nSS) for FA containing matrix and the matrix with SF seems to get less benefitted.
Scope of Present StudyThe nano-silica (nS), a product of modern Nano-technology (NT), was chosen to examine whether it can improve the degree of impermea- bility of a porous cement matrix containing mineral admixtures such as FA and SF. For this, a high water-binder ratio of 0.5 was chosen to produce a porous mortar matrix wherein 25% of cement was replaced by Class F fly ash and in another mix, 5% SF was utilised. It may be noted here that the value of 25% represents the average FA content of common Portland Pozzolana Cements (PPCs) available in the market today in India and the value of 5% represents the common SF content used in most of the concrete mixes. After curing the mortar specimens, they were oven dried and then impregnated with 'nano-silica solution' (nSS) by simple soaking. The nSS contained 30% nS solids. The permeability of nSS treated specimens was evaluated by determining the COA which required water absorption measurements on oven dried specimens. The test data indicated higher beneficial nature of nSS treatment on FA contacting matrix as compared that with SF.
Significance of StudyNano-technology (NT) has been a recent development and is getting applied to many varieties of fields. There have been many highly successful NT based applications which could have been almost impossible without utility of nano sized particles. This is due to extra-ordinary properties of materials when their particle sizes are in nano-scale. However, for civil engineering applications, nano-silica (nS) seems to have very high potential, since, Portland cement generates free lime which can be utilized with very high efficiency by nS, as there is always a chemical affinity between Ca(OH)2 and SiO2 to form secondary calcium-silicate hydrates (C-S-H) which are both pore filling and refining compounds in a cement matrix. This was found to occur in the high volume fly ash containing cement mortar studied in this paper.
The test data suggest that nS can be used to improve durability of existing concrete structures which always contain free lime in their matrix. The nS when used in solution form can be employed to develop high degree of impermeability in ferrocement members which are usually very thin with very small cover (often 3 to 5 mm only) to the embedded steel mesh reinforcement. The nSS treatment applied externally could be a very useful process to improve the concrete in cover portion of the precast elements. The recycled concrete aggregate could benefit tremendously from the nSS treatment presented in this paper.
Details of Experimental WorkMaterial Used
These are described in Tables 1a to 1f.
Preparation of Cement Mortar
|Table 1a Properties of sand|
|Sl. No||Property||Fine aggregate|
|2||Water absorption||0.50 %|
|4||Bulk density||1550 kg/m3|
|Table 1b Properties of Portland Cement (OPC)|
|2||Normal Consistency (%)||31|
|3||a) Initial Setting Time (minutes)
b) Final Setting Time (minutes)
|4||a) Soundness Le Chatelier Expansion (mm)
b) Soundness Autoclave Expansion (%)
|5||Compressive Strength (MPa) at 28 Days||56|
|9||Loss on Ignition (%)||1.31|
|Table 1c Properties of superplasticisers|
|Chemical base||Carbolic acid ester (CAE)|
|Nature||Free flowing liquid|
|Recommended dosage for Portland cement||0.1 – 1.0 kg/100 kg cement.|
|Portland cement||Base chemical = CAE|
|Type of surfactant||Non-ionic, stearic|
|Table 1d Specifications of Silica Fume|
|Particle Size||100 times finer than cement|
|Bulk density||540 kg/m3|
|Surface area||20,000 m2/kg (nitrogen adsorption method)|
|Average diameter||about 0.1 mm|
|pH||6.7 (20 grams in 80 ml of deionized water)|
|Crystallinity||Essentially amorphous (XRD)|
Preparation of Test Specimens
Both mortars containing FA and SF, (25FAM and 5SFM) were self-compacting type in freshly mixed stage and the mix could be easily filled in the plastic cylindrical moulds (of size: 50mm dia * 100mm height). Compaction of the mix in the cylindrical moulds was carried out by gravity due to self-weight only and no external application of compacting effort such as vibration was necessary.
The specimens were cured with wet gunny bags immediately after casting so that there is no loss of moisture from the specimens. After 3 days of ambient temperature curing, the moulds containing test specimens were placed in hot water bath (maintained at about 90-95°C) for further period of 3 days to affect accelerated curing. Later on the specimens were demoulded by carefully stripping of the plastic moulds (Photo 1).
|Table 1e Properties of Fly Ash|
|Sl No||Components||IS 3812 (2003) Specifications||Fly ash Used|
|1||SiO2 + Al2O3 +Fe2O3||70% min||98%|
|6||Total sulphur as SO3||2.75% max||-|
|7||Alkalis as Na2O||1.5 % max||0.54%|
|11||Bulk density||-||950 kg/m3|
|12||Fineness (Blaine)||-||3435 cm2/gm|
|Table 1f Properties of nano-Silica Solution (nSS)|
|3||Form||Colloidal transparent solution (aqueous medium)|
|5||nS solid content in the solution||30%|
|6||Sizes of nS solid particles in nSS solution||30%|
|7||Active Nano Content||30- 32%|
|8||Particle Size||5- 40 nm|
External Treatment of nSS
The specimens were oven dried at about 105°C for 48 hours and then soaked in nSS for 16 hours (Photo 2). Weighs of specimens were determined to compute the quantity of nSS and by that the nS solids impregnated into the matrix.
|Photo 1: Careful demoulding operation||Photo 2: Specimens ready for soaking in nano-Silica solution|
Measurement for Water Absorption
|Table 2 Mix details and test results|
|Ingredients of binder||%SF||5||0|
Nominal specimen sizes
|Compressive strength, fc||MPa||29.8||19.2|
|Oven Dry Density (ODD)||g/ml||2.054||1.975|
|Before nSS treatment|
|Water Absorption for 1 hour=WA||%||4.4||6.6|
|Saturated Water Absorption = SWA||%||5.8||7.9|
|Coefficient water absorption = COA =( (Q/Asur)2 )/t||10-6 cm/sec2||2.2||4.7|
|After nS treatment of cured mortar specimens|
|Absorption of nSS after soaking for 16hrs||%||3.8||4.0|
|Water Absorption for 1 hour=Wans||%||4.4||4.2|
|Saturated Water Absorption =SWAns||%||4.9||5.3|
|Coefficient water absorption = COAns =( (Qns/Asur)2 )/t||10-6 cm/sec2||2.3||1.9|
|Reduction in WA due to nSS =100*(WAns-WA)/WA||%||0.7||-36.1|
|Reduction in SWA due to nSS =100*(SWAns-WA)/WA||%||-14.9||-33.3|
|% Reduction in COA =100*(COA-COAns)/COA||%||1.3||-59.1|
Both types of specimens, control and nSS treated, were oven dried at about 105°C for 48 hours and cooled to ambient temperature. Then, the specimens were weighed and soaked in water for a period of 60 minutes. The specimens were taken out of water and weighed after wiping the surface with dry cloth to achieve surface dry condition. The quantity of water penetrated in 60 minutes through capillary suction was used to compute the COA using the following formula (suggested by Powers)13:
COA = (Q/A)2 / t, cm2/sec
Where, Q = Water penetrated in 60 minutes, cm3
A = Total surface area of cylinder through which water penetrates = (π/4 d2) × 2 + πdh)
d = Dia of specimen, cm
h =Height of specimen, cm
t = 60 minutes = 3600 seconds = Period of water soaking
Discussion of Test Results (Table 2, Figs 1 and 2)In case of 25FAM specimens before nSS treatment, it was observed that an average of about 25 ml of water was absorbed in 60 minutes in each cylindrical specimen and this was amounting to water absorption of 6.6% (by mass) and COA of 4.7 x 10-6 cm2/sec. In contrast, the average water absorption of nSS treated specimens was 4.2% indicating about 36% reduction of water absorption due to nSS treatment; the COA computed for nSS treated specimens was 1.9 x 10-6 cm2/sec which is about 60% less than the COA of untreated specimens. There was significant reduction of about 36% in saturated water absorption. Thus, nSS treatment seems to be highly effective in FA containing cement matrix.
|Figure 1: Effect of nano-Silica treatment on Coefficient water absorptivity (COA)||Figure 2: Effect of nano-Silica treatment on saturated water absorption|
In case of 5SFM specimens before nSS treatment, it was observed that an average of about 18 ml of water was absorbed in 60 minutes in each cylindrical specimen and this was amounting to water absorption of 4.38% (by mass) and COA of 2.24 x 10-6 cm2/sec. The average water absorption of nSS treated 5SFM specimens was at about 4.41% indicating almost negligible change in water absorption due to nSS treatment; the COA computed for nSS treated specimens was 2.27 x 10-6 cm2/sec which is only marginally different from the COA of untreated specimens. However, there was about 15% reduction in saturated water absorption. Thus, as the specimen properties were almost same before and after nSS treatment, it seems that the external application of nSS to SF containing cement matrix is largely ineffective from permeability considerations.
The above discussion shows that nSS was able to reduce considerably the water permeability of the mortar matrix containing fly ash. This could be attributed to both physical and chemical nature of nSS treatment. The nSS being similar to water in viscosity, was able to penetrate easily into the matrix even by simple soaking and fill the physically the capillary pores of the cement matrix. The nS solids present in nSS were being nano in size could be accommodated in the pores of the matrix easily. Moreover, the hardened cement mortar matrix has free lime generated as a result of cement hydration and this could easily chemically react with SiO2 of the nS solids due to the very high surface area of nS resulting in formation of secondary C-S-H gel which now acts as a filler in capillary pores. Thus, both physical and chemical actions of nS reduces porosity and causes pore size refinement. This leads to reduced permeability of the matrix thereby increased resistance to penetration of water. This was confirmed by reduced water absorption and decreased COA value for nSS treated specimens. Similar observations are made on saturated water absorption measured before and after nSS treatment.
However, it was seen that nSS was able to influence considerably the water permeability of the mortar matrix containing silica fume. The nSS being similar to water in viscosity, was able to penetrate into the matrix, but, it could not change substantially the microstructure as the SF had already reacted very fast enough to make the microstructure quite dense, by that, leaving little scope for further improvement. The free lime of hardened cement mortar matrix (generated as a result of cement hydration) had easily chemically reacted with SiO2 of the SF (due to its high surface area) to form secondary C-S-H gel which had already acted as an efficient filler in capillary pores, before external nSS treatment itself. Thus, both physical and chemical actions of SF, rather than the nS solids, had reduced porosity and pore size refinement. This has lead to almost no contribution from nS when nSS had penetrated into the matrix. This was confirmed by only marginal change in water absorption and COA values before and after nSS treatment.
The amount of nSS absorbed by the mortar in the present study was about 4% (by mass) for 25FAM and 3.8% (by mass) for 5SFM mortars, which is equivalent to penetration of same amount of nS solids into both the matrices. This small quantity of nS solids was sufficient to effectively reduce the permeability of the matrix containing FA, though, it was almost ineffective in matrix containing SF.
Concluding RemarksThe nS is an amorphous SiO2 chemical with very high specific surface areas and is made of nano-size particles. Its availability in colloidal solution form, though highly expensive at present, enables engineers to examine possibility of using it to reduce the permeability characteristics of existing concrete structures, besides any cured cement based composites such as ferrocement, precast concrete products, etc. This was confirmed in the present study by utilizing a basic porous fly ash containing cement mortar mix (with fairly high w/c rate of 0.50) which was impregnated with nS solution (nSS) by simple soaking for 16 hours. This nS treatment of cement mortar was able to reduce the one hour water absorption from 6.6% to 4.2% a reduction of 36%; this is related to reduction in 'coefficient of water absorptivity' (COA) from 4.7 x 10-6 to 1.9 x 10-6 cm2/sec, a decrease of about 60%. Thus, there was considerable increase in improvement of microstructure of cured mortar matrix due to entry of nS solution. The reduced permeability can be attributed to excellent filler effect of nS solids and the pore-filling effect of secondary C-S-H gel formed due to efficient/pozzolanic reactivity of amorphous nS solids with free lime available in the hydrated cement matrix. But, due to presence of efficient pozzolana of SF in cement mortar containing SF, this matrix was already refined considerably and thereby there was little scope for nSS for contributing towards micro structure.
The tests indicate that nSS can be used to improve generally the microstructure of any hardened cement composites near the exposed surface, thereby reducing the rate of entry of aggressive agents [such as O2, CO2, H2O, C1-, SO4- etc] resulting in enhanced durability. Thus, any precast concrete products and ferrocement units have potential for getting benefitted easily by nS treatment.
The nS treatment described in this paper is simple and practically easy to adopt. It is useful to reduce permeability thereby improve durability of concrete components in bridge structures including roads, pavements, paver blocks, etc.
AcknowledgmentThe cooperation and help received from the staff of various laboratories of SRM University in the preparation of this paper are gratefully acknowledged. Hearty thanks are due to Dr T P Ganesan, ProVC, Dr R Annadurai, HOD, Prof M Lakshmipathy, Prof. P. T. Ravichandran, & Dr K Sathyana- rayanan Civil Engg Dept, for their guidance and encouragement given at various stages of the project.
Notations AbbreviationsCH = Calcium hydroxide = free lime = Ca(OH)2
COA = Coefficient of Absorptivity
C-S-H = Calcium silicate hydrate
FA = fly ash
FA CM = Fly ash cement mortar
MA = Mineral admixture
nS = nano-silica
nSS = nano-silica solution
NT = Nano-technology
PPC = Portland Pozzolana Cement
SP = Superplasticiser
w/c ratio = water-cement ratio
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