Experimental study on strength and microstructure of mortar in presence of micro and nano-silica

https://doi.org/10.1016/j.matpr.2020.06.167Get rights and content

Abstract

Sustainable construction practices require cementitious materials with high strength that is strongly dependent upon the nature of binding materials and pore structure. The physico-chemical properties of these materials can be tailored suitably by preferential substitution of cement by materials having comparatively small particle size resulting in improved pore structure. This study is aimed to investigate strength and microstructure of the preferentially substituted cement mortars with incorporation of microsilica (MS), nanosilica (NS) and their combined use at 3, 7 and 28 days of curing. The substituent MS (5.0–20%) and NS (0.5–1.25%) have been used at a water binder ratio of 0.5. The specimens were analyzed for the fresh (consistency, setting time, flow) and hardened (compressive and split tensile strength) properties and a correlation between compressive and split tensile strength was obtained. Mortar containing NS was found to develop better strength as compared to the mortar containing MS. The optimum usage of MS with incorporation of NS was further found to increase the strength of mortar significantly. SEM-EDX was used for the analysis of the microstructure of the specimens and the correlation between Ca and Si content was used to analyze the cement matrix. The findings show that the optimized usage of micro and nano silica can give beneficial effects to improve the fresh properties as well as strength with dense microstructure.

Introduction

Cementitious materials are the composite materials produced from Portland cement with paramount importance in all the construction fields due to their extensive applications. Day-by Day increasing world population and tremendous technological & industrial advancement leading to massive infrastructure requirement has further increased the demand of cement [1]. The production of cement and fine aggregates is not only highly energy intensive, but also pose a threat to the environment due to release of harmful pollutants including particulates and greenhouse gases [2]. As per estimation, cement clinker production is one of the main sources of global pollution generating greenhouse gases (GHGs) with identification of construction industry as one of the major contributors of GHGs emissions. With the ever-increasing demand of cement, the emission is expected to become more significant [3]. Further, strength, durability and maintenance cost of construction material are another important issue of prime concern. Sustainable development demands new techniques that must involve control and reduction of the GHG emissions and energy use along with substitution of cement by supportive cementitious materials (SCM) such as Pozzolans [4]. The conventional pozzolans include volcanic ash [5], diatomaceous earth [6], silica fume [7], bottom ash [8] and fly ash [9]. However, with tremendous growth in nanotechnology, various nanomaterials such as carbon nano tubes [10], nano-SiO2 [11], [12], [13], [14], nano-TiO2 [15], [16], nano-Al2O3 [17], [18], nano-metakaolin [2], [19], nano-CuO [20] and nano-Fe2O3 [1], [21] have also been explored recently. These materials are used in cement mortar/concrete to preferentially replace cement so as to obtain the required properties of the more economical products along with the better performance of the cementitious materials.

Due to finer particle size, nanoparticles possess high pozzolanic activity and also act as filler, resulting in homogeneous, dense and compact microstructure. It drastically improves the physico-chemical and mechanical behaviour of the cement structures [22]. The ultra-fine nanoparticles invade the voids present in the cement matrix resulting in refinement of the pore size, reduction of the pore volume and disconnection of the capillary mechanism of the pores. Consequently, there is a decrease in water permeability capacity of cement matrix along with simultaneous increase in the chemical resistance of the cementitious structures. In cement mortar, the nanoparticles create a micro dense structure which leads to extremely compact cement mortars and improves the mechanical characteristics of the cement mortar [23]. Literature reports better results for the systems with incorporation of nanoparticles in presence of microsilica. Micro silica can exhibit both cementitious and pozzolanic behaviour and is quite worthwhile in amending the fresh and hardened behaviour of cement mortars and concrete [24]. The nucleation effect of nanoparticles facilitates hydration reaction involving formation of CSH gel with the development of microstructure accounting for closing of cracks. The fresh properties such as setting time also gets lessened due to extensive reaction of nano silica particles resulting in early hardening [25].

The fresh and hardened properties of mortar not only depend upon the application techniques, water/binder ratio but also upon the nature, concentration and particle size of the binder and additive particles. Nanosilica can be used either as dry powder or in the colloidal form [26]. Colloidal nanosilica is in aqueous suspension form that consists of amorphous hydroxylated silica nanoparticles with particle size lying in the range of 1–500 nm. The particles of colloidal nanosilica show lesser segregation and better dispersion in cement mortar and is considered to facilitate the production of CSH gel with high stiffness [27]. Although the reports are available on the individual use of micro silica & nano silica to modify the behaviour of the cement mortar yet not much data is available on the use of amorphous silica nanoparticles on behaviour of cement mortars. This study is aimed to understand the outcomes of preferential substitution of cement by amorphous nanosilica nanoparticles in reference to fresh, hardened and micro structural properties of cement mortar in presence and absence of microsilica and to design the statistical model for prediction & validation of the various studied properties at the desired curing ages.

Grade 43 Ordinary Portland Cement was used for preparation of the samples used in the study. Fine aggregates in the form of Standard Ennore sand and potable water was used to prepare all specimens. Colloidal NS of average particle size 40 nm and microsilica with average particle size 0.20 µm were used with physical properties listed in Table 1, Table 2 respectively. The % content of MS and NS was varied from 5.0% to 20% and 0.5% to 1.25% respectively at a water/binder ratio of 0.5 as listed in Table 3. The fresh properties viz consistency, initial and final setting time of the cement were determined confirming to IS 4031-2019 part 4 and 5 respectively [28], [29]. Flow of mortar specimens was determined by using Flow table as per IS: 5512-1983 [30]. Compressive strength of mortar cubes was determined in accordance to IS 2250-1981 [31] and the split tensile strength was determined in accordance of IS 5816-1999 at 3, 7 and 28 days of curing [32]. The microstructural analysis has been investigated through SEM-EDX studies.

Section snippets

Normal consistency

Cement paste is characterized in terms of Normal Consistency that further deduces the water demand and workability of cement paste. Normal Consistency of controlled mix (CM) paste was examined and then compared with that of the cement pastes with preferential substitution of cement by micro silica and/or nano silica as represented in Fig. 1. The consistency of cement pastes M1, M2, M3 and M4 increased by 8.0%, 14%, 23% and 26% respectively as compared to CM. Further as the percentage of MS

Conclusion

The study of effect of preferential substitution of cement by MS and/or NS on fresh as well as hardened properties & microstructure of cement mortars in comparison to control mix has been carried out. The consistency, initial and final setting time of the specimens were found to increase while the flow was found to decrease in content of MS as well as that of NS but the effect was more pronounced in case of ternary blends. An increase in compressive and split tensile strength was obtained with

CRediT authorship contribution statement

Rishav Garg: . : Conceptualization, Methodology, Data curation, Investigation, Resources, Writing - original draft. Rajni Garg: Formal analysis, Software, Writing - review & editing. Manjeet Bansal: Project administration, Validation. Yogesh Aggarwal: Supervision, Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (35)

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