Effect of Nanosilica Additions on Belite Cement Pastes Held in Sulfate Solutions

Abstract

Fly Ash Belite cement (FABC) pastes with and without nanosilica additions have been prepared and maintained in sulfate solutions (Na 2 SO 4 0.5 M) for 180 days. The mechanical performance and the changes in microstructure have been monitored at 28, 90 and 180 days by compressive strength, X-ray diffraction (XRD) and 29 Si MAS NMR measurements. We have found that, unexpectedly, and contrary to what happens in Ordinary Portland Cements (OPC), the addition of nanosilica particles induces an initial fall in the compressive strength of the samples. Only in samples maintained for long time (180 days), do the nanosilica addition improve the mechanical properties. Our XRD and 29 Si NMR experiments have revealed that although the nanosilica additions trigger the consumption of Belite phases, this is not always accompanied by formation of longer Calcium-Silicate-Hydrate (C-S-H) gel structures. Only at long time (180 days), and due to a mechanism that seems to be controlled by the pH of the samples, do the nanosilica additions lead to high polymerized C-S-H gels.Keywords: cement-based materials, nanomaterials, mechanical properties, microstructureThe authors gratefully acknowledge the financial support by the Spanish Government (Projects no. MAT 2002-04023-CO1-CO2-CO3 and MAT2005-03890). (1) IntroductionBelite cements (BC) are nowadays receiving a great deal of attention by the scientific community due to the low CO 2 emissions produced during their fabrication. Recently a new pathway based on the additions of nanoalumina has been explored to achieve faster strength gains. 6 Although in Ordinary Portland Cements (OPC) the beneficial effect of adding nanoparticles has been studied for some time, 7-10 in the case of Belite cements, to the best of our knowledge, no other work that previously mentioned Ref.[6] has been presented.In OPCs, the nano-SiO 2 and nano Al 2 O 3 particles serve as surplus source of Si and Al atoms so that C-S-H gels can grow further and yield improved mechanical properties. 7 Whether something akin occurs in Belite cements is unclear. On the one hand, Belite cements develop pore-solution with lower pHs (about 11-12) than that of OPCs (about 13-14) during the hydration process, fact clearly connected to their lower calcium content (C 2 S versus C 3 S) and low (or lack of) portlandite (Ca(OH) 2 ) formation, among other things. This difference is of extreme importance if one is aware that the pH governs most of the aggregation/disaggregation mechanisms involved in nanoparticle solutions. On the other hand, the microstructure of FABC cement pastes is noticeably different to the one found in OPC pastes. Differently to the case of OPCs, where the C-S-H gel is the only hydrated product which contains Si atoms, the microstructure of FABC cement pastes also contain alumino-silicates and zeolite-like products which incorporate Si atoms into their structure. 11This works aims precisely to investigate the effect of nanosilica additions on the mechanical properties and the microstructure changes of FABC pa...

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