THE VALUE OF COLLOIDAL SILICA FOR ENHANCED DURABILITY IN HIGH FLUIDITY CEMENT BASED MIXES

Transcription

1 THE VALUE OF COLLOIDAL SILICA FOR ENHANCED DURABILITY IN HIGH FLUIDITY CEMENT BASED MIXES Jansson, Inger (1), Skarp, Ulf (1) and Bigley, Carl (2) (1) Eka Chemicals AB, Sweden (2) Consultancy, New Zealand ABSTRACT This research focuses on the effect of adding small amounts of (nanometric) colloidal silica (CS) to highly fluidised Portland Cement mixes. The mixes discussed were designed to highlight the effects different sources of Portland cement would have in terms of yield values and plastic viscosity with time as a measurement of the stabilising effect of the CS addition. CS dosages used were sufficient to eliminate all segregation and bleed characteristics. At these levels of addition, CS could be classified as an admixture to the definitions of EN480. The mechanism we are proposing is that the addition of the CS to fluidised Portland cement systems results in the accelerated generation of and increased amount of C-S-H gel (Calcium Silicate Hydrate). This greater quantity of C-S-H gel thereby contributing to much greater stability in the fluid state and, with some cement sources, greater early strength. CS nanoparticles preserve their integrity during the hydration processes. The silica particles behave as highly active nucleation sites for ions released from dissolving clinker phases. The significant rates of both Alite and Belite hydration brought about by CS addition suggest a rapid formation of C-S-H gel on the active CS surface. The addition of polycarboxylate superplasticiser will generally retard the rate of formation of C-S-H gel from both the Alite and Belite phases. It is this effect that causes many of the fresh state stability problems that can (to a large extent) be overcome through the addition of small amounts of CS. Keywords: Colloidal Silica, stabilisation, segregation, early strength development 1. INTRODUCTION The purpose of this paper is to pull together several new areas of research undertaken or directly sponsored by EKA Chemicals AB. This research focuses on the effect of adding small amounts of (nanometric) colloidal silica (CS) to highly fluidised Portland Cement mixes. The mixes discussed were designed to highlight the effects different sources of Portland cement would have in terms of yield values and plastic viscosity with time as a measurement of the stabilising effect of the CS addition. At these levels of addition, CS could be classified as an admixture to the definitions of EN

2 The definition of CS in this work is nanometric, monomeric, amorphous silica particles in aqueous dispersion ranging from 3 to 40 nanometres in diameter. Generally the CS used comprised a range of particle sizes. Further, the surface of the CS nanoparticles can be modified by removal of some of the Silicon groups and replacement with Aluminium. The main focus of the work however is with unmodified particles. The mechanism we are proposing is that the addition of the CS to fluidised Portland cement systems results in the accelerated generation of and increased amount of C-S-H gel (Calcium Silicate Hydrate). This greater quantity of C-S-H gel thereby contributing to much greater stability in the fluid state and, with some cement sources, greater early strength. Further, this effect is seen with many different cement sources which generate a possibility that this technology could be utilised as a means to iron out local differences in cement sources for the production of highly fluid Portland cement based materials. 2 THE MECHANISMS AT WORK It is known from our previous research [ref 1] that the CS nanoparticles preserve their integrity during the hydration processes in spite of the high alkalinity. Clinker dissolution rapidly results in a saturated pore solution and it is this high ionic concentration that prevents the silica particles from dissolving. The high monomeric silica activity resulting from cement dissolution is responsible for this alkalinity and depresses the rate of CS dissolution. Instead, the silica particles behave as highly active nucleation sites for ions released from dissolving clinker phases. The significant rates of both Alite (C3S) and Belite (C2S) hydration brought about by CS addition suggests a rapid formation of C-S-H gel on the active CS surface. In this way CS addition acts as both an accelerator and active site to promote the formation of greater amounts of C-S-H gel early on in the cement hydration process. The stearic hindrance effects of adding polycarboxylate admixtures to concretes and mortars are now well known and quantified with many new generation commercial products tailored to produce different effects as a way to push concrete performance for particular end uses. Such applications could include extended working times for readymix or high early strength for precast concrete operations. However, when we consider a concrete or grout superplasticised with a polycarboxylate theory dictates that the polycarboxylate component will generally retard the rate of formation of C-S-H gel from both the Alite and Belite phases. This is demonstrated by the appearance of their respective spectral signatures [ref 2]. C-S-H gel formation is one of the main stabilisers in fresh and particularly high workability concretes and grouts where the interaction of the C-S-H gel with both cement and aggregate particles determines concretes resistance to segregation and bleed in the fresh state and can lead to durability issues in the hardened state. This effect can (to a large extent) be overcome through the addition of small amounts of CS. These effects are consistent with the various measurements we have used to observe the effects of CS addition on highly fluid Portland cement based mortars. The mechanism we propose in this paper is as follows. The addition of colloidal silica admixture to a highly fluid cement mix results in the early formation of more C-S-H gel. This results in a better developed earlier age cement gel structure providing more resistance to segregation and bleed together with plastic settlement and other disruptions caused by movement. CS nanoparticles act as a site for the accelerated formation of C-S-H gel and result in the earlier formation of a functioning gel structure. This in turn leads to: 762

3 Increased number of adsorption sites for superplasticiser Less agglomeration and increased cement particle dispersion Less internal/interparticle friction (lubricating C-S-H sheath) No problematic increase on heat of hydration at early ages [ref 3] Less particle blocking especially in coarser/poorly graded cements, which would also allow you to use CS to reduce filler contents in concretes and grouts [ref 4] 3. WORKED EXAMPLES AND EXPERIMENTAL WORK By keeping the admixture component constant we can focus on different cement types from around the globe and demonstrate the effect of CS additions into the highly fluid mortar mix. We wish to demonstrate that all cements are not the same, not a universal grey powder but that a CS dispersion can give some control over this. The following cements were used in our experimental work Golden Bay 42.5N OPC ex N Zealand. Cementa Cem II/A-LL 42.5N OPC Type G oil well cement with no C3A component 3.1 Golden Bay Cements manufactured in NZ, parts of Australia, Indonesia, Malaysia and several other locations are quite different to the norm seen in Europe. In straightforward concrete we observe a reactive clinker with rapid hydration exhibited by the C2S/C3S phases. It appears that this reactivity is markedly checked by using polycarboxylate superplasticisers. Such cements develop strength quite quickly and only needs grinding to surface areas around 340 blaine to achieve required strengths (to NZS ). Further, very reactive, high lime class C fly ash is added typically at 3% plus limestone filler at 2%. The high lime content further accelerates strength development. Various grinding and milling aids are added to further the advantages. We end up with a cement that will set and develop the required strength but is often very difficult to use with admixtures. When we come to wanting to make SCC or other high fluidity products we find it is not always possible with conventional technologies. Without superplasticisers these cements exhibit quite rapid dissolution of both the Alite and Belite phases. Once we start to look at SCC and especially when using polycarboxylate superplasticiser the retardation effect on the silicate phases is quite pronounced. Such cements will form less C-S-H at a given time than a euro cement and stability in terms of bleed and segregation is very hard to control. The rates of consumption and effects on certain admixture types are also a big problem to maintaining constant workability with time. With Golden Bay cement a rapid early consumption of polycarboxylate is often seen (within the first 40 minutes) resulting often in the need for high doses to maintain workability. This is in part due to a lack of soluble sulphate being available at early ages which further complicates the picture with this cement. 763

4 Compression Strength (Mpa) Control Cembinder 50 Mix CB8/CBG50 (10/90) 1 day 7 days Slump 62-8 minutes The mix contain Cembinder produced a stable fresh state 10 mortar with constant workability 5 over the 62 minute period of the 0 test. The control shows a drop in -5 workability, although not as -10 pronounced as can be seen with -15 other superplasticiser products. The control mix suffered fresh state instability (bleeding and segregation) not seen in the mix Added Colloidal Silica with Cembinder 50, but to some extent in the other mixes Mix CB8/CB50 (28/72) Mix CB8/CB50 (50/50) Cembinder 8 Cembinder 36 containing CS. In the case of Golden Bay cement, it was Cembinder 50 that gave the best results in the fresh state together with a maximisation of early age compressive strength at 7 days. These experimental results correlate well to what we observe in the field. 3.2 CEM II The relatively high Al2O3 and Fe2O3 contents suggest reasonable levels of both C3A and C4AF to provide initial adsorption sites for polycarboxylate superplasticisers to work. However a low initial availability of sulphates in the 40 minutes of mixing causes rapid polycarboxylate consumption. This is compounded by the limestone filler content that further increases the rate of consumption for no gain in workability. The Alite and Belite phases seem readily retarded by the presence of polycarboxylate. 1 day 7 days Slump 62-8 minutes The control mix demonstrates the large increase in workability often seen on site between batching and delivery to the customer This cement also has a very high water demand and at 35 0 these observed slumps would not expect to see any signs of bleed and segregation. The poor early age workability can be partially Added Colloidal Silica explained by a lack of available soluble sulphate and slow to hydrate Alite and Belite phases. This results in a rapid early consumption of polycarboxylate on the C3A sites whilst providing the superplasticiser with nothing much to adsorb onto to improve workability. Addition of CS is clearly seen to overcome this problem and gives the cement a uniform workability over the 62 minutes of the test duration. The CS also gives the added benefits of enhanced 24 hour strengths as predicted by our theory. Compression Strength (MPa) Control Cembinder 50 Cembinder 8 Mix CB8/CB50 (50/50) Mix CB8/CBG50 (10/90) Slump difference (cm) Slump difference (cm) 764

5 3.3 Class G cement The class G cement is a coarse ground modified Portland cement. Less Alumina and more iron favour increased levels of C4AF and low C3A for improved soundness in high sulphate environments. These cements are generally interground with starches, chrome lignins and cellulose ethers to provide for extended setting times at higher temperatures. These additives have the side effect of greatly reducing the solubility of available sulphates at early ages which adversely affects the rate of consumption of polycarboxylate superplasticisers. The retarding action reduces the rate of hydration of the Alite and Belite phases and the resultant slow formation of C-S-H gel leads to problems of bleed and segregation. In many cases certain additives have to be used on site to check bleed and segregation in critical well cementing operations. Because of the environments in which these cements function they are designed around low C3A contents to give excellent sulphate resistance. Often through the incorporation of interground retarders they also exhibit very unreactive Alite and Belite phases to give long working times at higher temperatures. There is an interest in moving to new polycarboxylate technology from many of the oil companies but the unreactive silicate phases plus lack of C3A makes them very much prone to segregation and bleed; a very critical issue in the oil well industry. From our experimental work you can see the addition of CS gave much improved performance. The optimum CS in this case is Cembinder 36 which allowed for much improved amounts of water reduction or increased workability for a given dose of superplasticiser over the control. The beneficial effect of CS addition on early age compressive strengths can also be seen with again Cembinder 36 proving optimum. Yield value (Pa) w/c (class G) = 0,323, 0,6% SP of PC-type, 4% Cembinder CB 36 CB 88 CB 75 CB 50 Plastic viscosity and yield values have also been investigated (figure 3), and can show Cembinder 36 has the least impact on these properties over measured time, 60 minutes (critical for pumping large volumes underground) whilst maintaining fresh state stability Bingham plastic viscosity (Pas) 4. CONCLUSIONS The practical, physical observations would be that adding CS to a fluid concrete or grout will result in greatly reduced bleed and segregation problems and often with the added benefit of accelerated early strength development. We also observe much lower plastic viscosity and yield values than it is possible to achieve with more conventional VMA s (viscosity modifying additives). This usually results in much higher dosages of superplasticiser for a required workability, and often with the problem that this workability will only be available for a limited timeframe as the mix continues to thicken. This effect is not seen with CS. 765

6 Our observations and experiments have been brought together here and examined in the context of cement chemistry and what role we believe the CS particle takes in this process. The enhancement of durability comes from a combination of greatly enhanced stability whilst in the fluid state together with an increase in effective cement paste volumes at early ages giving the resultant concrete a more rapid build up of cement gel structure. We wish to demonstrate that all cements are not the same, not a universal grey powder and that when performance is pushed, as with the incorporation of modern, high performance polycarboxylate superplasticisers differences in chemistry and hydration characteristics bring about quite noticeable changes in performance. A CS addition can give some control over this with respect to stability control at high workabilities, maintaining a given workability with time and enhancement of early age compressive strength. Such technology may also be useful in the activation of low energy Belite cements. The latest generation polycarboxylate superplasticisers have gone a long way to pushing the performance of modern concretes to provide much better customer solutions. It is now the differences between sources of cement that are the obstacle to further technological steps forwards and we believe in this area CS can be a benefit. Different CS products may be needed with different cement sources but the net result is that you can iron out many of the differences between cements and produce a more uniform response of workability versus time whilst maintaining stability in a concrete or grout in the fresh state. We would like to be able to relate specific CS particle size to given properties in particular cements but this generates a need to reclassify cement not just on the basis of chemical analysis and Bogue composition but on respective rates of hydration of the various mineral phases, sulphate availability with time and the rates of release of alkali. This will form the basis of our future investigations. REFERENCES [1] Björnström, J., Martinelli, A., Matic, A., Börjesson, L., Panas, I., Accelerating effects of colloidal nano-silica for beneficial calcium-silicate-hydrate formation in cement Chemical Physics Letters 392 (2004) [2] Björnström, J., Panas, I., Antagonistic Effect of Superplasticizer and Colloidal Nano-Silica in the Hydration of Alite and Belite Pastes Not yet published, part of PhD thesis Göteborg s University [3] Collepardi, M., Ogoumah Olagot, J., Skarp, U., Troli, R., Influence of amorphous colloidal silica on the properties of self-compaction concretes SCC Island In Proceedings of the 3 rd International Symposium on SCC, 2003 Island. [4] Skarp, U., Jansson, I., Engstrand, J., Bigley, C., Omari,M.,Wirje, A., A new concept for improving durability in Self Compacting Concretes SCC Japan In Proceedings of the 2 nd International Symposium on SCC, 2001 Japan. 766