Performance evaluation of nano-silica concrete

. In this paper, the study of the influence of nano-silica (nano-SiO 2 ) on the properties of the interface between CSH gel and cement particles and its effect on nano-mechanical properties of the products at the interface zone was examined. In this paper M50 grade SCC mixes were developed using 5% micro-silica and various percentages of 0.5%, 1.0% and 1.5% nano-SiO 2. For 1.0% nano-SiO 2 addition to M50 grade SCC mix, the compressive strength is maximum. Similarly concrete quality using non-destructive techniques, water absorbtion capacity and porosity are also assessed.


Introduction
Concrete is a composite material consisting of many phases in micro and macro scale level. It is strong due to the heterogeneous nature acquired due to (1) interfacial transition zone between aggregate and matrix, (2) interaction between sand and paste matrix and (3) collaboration among CSH gel, large crystals of Ca(OH)2 , unhydrated cement particles and pores formed due to high w/c ratio. Interface between coarse aggregate, fine aggregate and cement paste is understood well with micro observational studies but the phase between the unhydrated cement grains and CSH gel was not been explored effectively especially in concretes with high w/c ratio due to availability of high amount of unhydrated cement particles. To enable a uniform distribution of nano-SiO2 in paste, colloidal nano-SiO 2 (CNS) was used instead of nano-SiO 2 powder. The SiO 2 content of CNS was larger than 99% by weight, and the pH value was 11.

Micro-silica and Nano-silica
Nanosilica demonstration that presence of nanoparticles in cement increases the CSH gel chain length thereby increasing the stiffness of CSH. This highly stiff CSH prevents calcium leaching mechanism improving the durability of concrete.
Micro-silica and nano-silica conforms to IS 15388:2003. Micro-silica and nano-silica falls under category of Silica fume. The tables below physical and chemical properties of micro-silica and nano-silica. Silica fume is available in the form of micro-silica and nano-silica. Micro-silica is about 100 times smaller than cement size whereas nano-silica is about 1000 times smaller than cement.  Micro-silica mainly acts as a filler densely packing the voids created by the CSH gel and un-hydrated cement particles. Due to the presence of high reactive silica in micro-silica which reacts with calcium hydroxide to form additional hydrates which adds to the strength and stiffness of the concrete. Nano-silica has higher pozzolanic reactivity than micro-silica and also helps in lowering the water demand and reduces the capillary pores due to improved particle packing and developed intermolecular forces which may make concrete little stiff to work with. Due to addition of micro-silica there is decrease in setting times of the cement paste this reduction is observed more when nano-silica is used. The objective of the present work is to evaluate performance of nano-silica in self-compacting concrete (SCC) of M50 grade. Nan Su mix design principles are adopted to arrive at the quantities required for M50 grade SCC.

Effect of dosage of nano-SiO2
To quantify the amount of nano-SiO 2 to be used in concrete, it is admixed with cement mortar cubes and tested for compressive strength. The amount of Nano-SiO 2 added are 0.5%, 1.0% and 1.5% by weight of the cement.

Fig. 1. Optimum dosage of nano-SiO2
For 1% addition of nano-SiO 2 by weight of cement, the strengths obtained in cement mortar cubes is maximum so for further examinations this is considered as the optimum percentage of nano-SiO 2 is adopted.

Consistency
A flow table test was conducted as per IS 5512-1983 to determine the flow for the nano-SiO 2 admixed mortars. As the percentage of nano-SiO 2 was increased in mortar specimens there is a gradual increase in consistency was noted due to improved homogeneity of the cement matrix in nano-SiO 2 admixed cementitous materials. The initial setting time of nano-SiO 2 mixed samples drastically decreased due to the ultra-fine size of nano silica particles.

Mix quantities
Super plasticizers and mix quantities are revised subjected to satisfaction of flow properties as per EFNARC Guidelines. 5% Microsilica is added to attain the desired compressive strength.
Based on the Nan-Su mix design the quantities that are obtained for M50 grade is listed below:

Fresh properties on SCC as per EFNARC guidelines
The following table presents the various workability tests based on EFNARC guidelines

Compressive strengths of nano-SiO2 SCC mixes
The following table presents the compressive strengths of nano-SiO 2 SCC mixes mixed with various dosages of nano-silica. Compressive strength found to be more for 1.0% addition of nano-SiO 2. The percentage of nano silica is restricted to 1% due to workability issues. This seems to be controlled by the particle size distribution and the high specific surface area of nano-SiO 2 in the presence of poly-carboxylate (PCE based) superplasticizer. With a further increase in nano-SiO 2 content, the pozzolanic reaction and the resulting hydrated products formation increases, but along also increases the micro-cracking caused by the selfdesiccation effect, resulting in harmfully disturbing its strength development.

Rebound hammer and Ultrasonic pulse velocity tests
Fly ash has very low initial pozzolanic reactivity, but this activity considerably amplified after incorporating a small percentage of nano-SiO 2 . The use of fly ash creates a highly porous structure at early days of curing but inclusion of nano-SiO 2 creates a more condensed microstructures, even during early ages of 1-3 days due to positive effect of nucleation of nano-silica. The following table presents the quality of M50 grade SCC made with 1.0% addition of nano-SiO 2. based on combined rebound hammer and ultrasonic pulse velocity tests. Rebound numbers obtained indicate the superior surface hardness than reference SCC and also the USPV measurements were greater than 4.5km/sec which denotes that nano-silica based SCC is classified as excellent concretes in terms of strength and durability point of view due to improved pore structure of concrete.

Water absorbtion capacity and porosity
Usage of nano-silica will accelerate the hydration process resulting in additional hydration products due reaction with portlandite formed during 1-3 days of curing. So the microstructure of concrete is densified due to both filler and pozzolanic effect particularly at the interface between aggregate and cement paste and between unhydrated cement grains and hydrated products forming a homogeneous and uniform structure. Distribution of pores are refined and their size is reduced drastically. The pozzolanic activity of nano-SiO2 is superior to that of micro-silica because nano-silica consumes more CH crystals reducing the size and orientation of CH crystals at the interface. Figure below presents the water absorbtion capacity and porosity of M50 grade SCC made with 1.0% addition of nano-SiO2.