Elsevier

Construction and Building Materials

Volume 37, December 2012, Pages 707-715
Construction and Building Materials

Influence of nano-silica agglomeration on microstructure and properties of the hardened cement-based materials

https://doi.org/10.1016/j.conbuildmat.2012.08.006Get rights and content

Abstract

Influence of nano-silica agglomeration on microstructure and properties of the hardened cement-based materials was investigated by using precipitated silica (PS) with very large agglomerates and fumed silica (FS) with much smaller ones as nanoscale additives. The results showed that the addition of either PS or FS refines pore structure of the hardened cement paste. However, the SEM observation revealed that the pozzolanic C–S–H gels from the agglomerates cannot function as binder. There even exists interfacial transition zone between the agglomerates and the bulk paste. The nano-indentation test indicated that the large agglomerates may become weak zones due to their low strength and elastic modulus. It is proposed that the microstructure improvement have nothing to do with the seeding effect, but result from the water-absorbing, filling, and pozzolanic effects. Through PS addition, compressive strength of the mortars and their resistance to calcium leaching and chloride penetration were enhanced. However, these improvements were less significant than those with FS addition. The reason is that much more fillers are provided whereas much fewer weak zones are introduced in the mortar with FS addition than that with PS.

Highlights

► Pozzolanic C–S–H gels from agglomerates cannot function as binder. ► There exists interfacial transition zone between agglomerates and bulk paste. ► Large agglomerates may become weak zones due to low strength and elastic modulus. ► The microstructure improvement has nothing to do with seeding effect. ► It is resulted from water-absorbing, filling, and pozzolanic effects.

Introduction

The durability and mechanical properties of high performance concrete are mainly dependent on the gradually refining microstructure of the hardened cement paste (HCP) and the interfacial transition zone (ITZ) through incorporating mineral admixtures, such as blast-furnace slag, fly ash, and silica fume (SF) [1], [2], [3], [4], [5]. The fine particles in these admixtures are believed to act through a triple mechanism [1], [2], [3], [4], [5], [6], [7], [8]: (1) they tend to physically fill the void space between the larger particles, which is otherwise occupied by water that is not free to contribute to fluidity; (2) with time they react chemically with calcium hydroxide (CH) to produce additional material such as pozzolanic C–S–H gels, combine some water in their products and reduce in this way the porosity of matrix and ITZ; and (3) the pozzolanic C–S–H gels act as seeds to provide nucleation sites for cement hydration products, causing pore as well as grain refinement of matrix and ITZ.

In recent years, there has been a growing interest in the use of nano-silica in concrete. It is believed that nano-silica exhibits much finer particle sizes and higher pozzolanic reactivity than silica fume, thus can act as fillers, pozzolan and seeds more effectively [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. However, the nano-particles in powder are often in an aggregated (firmly-held clusters) or agglomerated (loosely-held clusters) form with final grain size from submicron to as high as 100 μm due to their very high specific surface area and energy [20]. Even in the well-dispersed colloidal dispersion, the nano-particles still exist as aggregates with grain size in submicron [11]. By using colloidal silica sol, it is often assumed that the mono-dispersed nano-particles can act as fillers and seeds much more effectively than those agglomerates in powder or dispersion [13], [14], [15], [16]. Nevertheless, our investigations revealed that the silica sol will gel or coagulate immediately when the cement is mixed into the water containing sol [21] due to rapid increase of ionic strength in paste [22]. As a result, no matter what source of nano-silica is used, it is the behavior of final agglomerates, rather than that of individual nano-particles, which controls the filling, pozzolanic and seeding effects on cement hydration and microstructure improvement.

In a previous paper, influence of nano-silica agglomeration on properties of fresh cement pastes has been investigated by using precipitated silica (PS) with very large agglomerates and fumed silica (FS) with much smaller ones as nano-scale pozzolanic additives [23]. In the present study, we mainly focused on its influence on microstructure improvement of the hardened cement-based materials through SEM observation, nano-indentation test and MIP analysis. The mechanical properties and some durability index were tested to verify the dependence of microstructure improvement on properties of the hardened cement-based materials. The mechanisms of the influence of nano-silica with various agglomerate sizes on microstructure and property enhancement were also discussed.

Section snippets

Materials

Ordinary Portland cement (namely 52.5 grade) conforming to Chinese standard GB175-2007 was used. The physical and mechanical properties are shown in Table 1. A naphthalene-based superplasticizer (SP) with solids content about 30% and a commercial single-graded quartz sand with 0.3–0.6 mm in size were used to prepare pastes and mortars.

Commercially available PS and FS were used as nano-scale additives. The nanometer feature of the primary particles and their agglomeration characteristic of the

SEM observation

Fig. 1, Fig. 2, Fig. 3 present SEM photographs of the HCP with and without nano-silica addition after curing for 28 and 180 days. The results confirmed an improved microstructure in the HCP with nano-silica introduction. In blank sample, the voids among cement particles have been occupied by the hydration products after curing for 28 and 180 days, but many connected capillary pores were observed. With PS and FS incorporation, the HCPs showed denser formations of hydration products than the blank

Conclusions

Based on the microstructural analyses and experimental results presented in this study, the following conclusions can be drawn:

  • (1)

    Through SEM observation, an obvious microstructure improvement of the HCP and the ITZ in mortar was found by adding nano-silica, regardless of its agglomerate size. Nevertheless, addition of FS with much smaller agglomerates showed a more significant improvement than that of PS with much larger agglomerates.

  • (2)

    It was found that C–S–H gels from pozzolanic reaction of the

Acknowledgements

The authors gratefully acknowledge the financial support from Scientific Bureau of Zhejiang Province, China under Grant 2009R50022 and Infrastructure Technology Institute at Northwestern University under Grant DTRT06-G-0015. The first author would also like to thank China Scholarship Council (CSC) for its financial support during his co-operative research at Northwestern University.

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