Nanotechnology for Energy Efficient Building Materials 

Prof. Surendra P Shah  

Concrete is a ubiquitous material used in the construction industry. It is a nano-structured multi-phase, composite material which ages over time. The binding material in the cement paste, the Calcium-Silicate-Hydrate (CSH) the 'glue' that holds concrete together, is itself a nano material.

Nanotechnology offers interesting new opportunities in the construction sector through the development of energy efficient, ultra high strength, extra durable, extremely lightweight construction materials. Preceded by the IT and software revolution, Nanotechnology and Science are expected to usher a new paradigm shift in all spheres of technology including infrastructure and construction. In a nutshell, Nanotechnology is today recognised as a revolutionary technology that can help address key needs relating to energy, environment, health, shelter and agriculture in developing countries. It has been estimated that $ 1 trillion worth of products worldwide will incorporate nanotechnology in key functional components by the year 2015. To achieve this goal, the Govt. of India has launched an ambitious mission mode programme with a budgetary allocation of Rs. 1000 crore under which several major research initiatives have been initiated.

Nano-modification of cement is an emerging field. Synthesis and assembly of materials in the nano-meter scale offers the possibility for the development of new cement additives such as novel super-plasticisers and nano particles. It is now possible to manipulate the fundamental structure of cement phases to control concrete properties, performance and durability. Nano-modification also provides crucial information for predicting the service life of concrete more accurately and insights on improving it further.

To give a broader view on the latest development and emerging trends in cement industry, Indian Concrete Institute (ICI), Karnataka Bangalore Chapter in collaboration with UltraTech Cement Limited recently organised a technical lecture on 'Improving Performance of Concrete through Nano-Modification' in Bangalore.

 Prof. Surendra P Shah

Prof. Surendra P Shah, an international expert in the field of Civil Engineering and Material Science gave his insight views on Nano-Engineering and spoke about manipulation of concrete using this science. He is presently Walter P Murphy Professor (Emeritus), Centre for Advanced Cement-Based Materials, North Western University, Evanston, IL 60306, USA, and heads various renowned international bodies. His diversified expertise includes nano-technology, fibre reinforced concrete, high

Prof. Surendra P Shah in an exclusive interview with Built Expressions talks about Nanotechnology for Energy Efficient Building Materials. 

Growth of Concrete

Having observed the growth of the concrete over the years, Prof. Shah says, "At present, we have witnessed lot of development in concrete technology compared to last 20-30 years. For instance, we have built world's tallest building Burj Khalifa which is 830 m height. Historically, this development is possible because over the years, we have learnt to characterise and manipulate the concrete at micro level."

Further he adds, “To be precise, concrete is a porous material and to make it very strong and flowable, sub micron size particles such as silica fumes and super-plasticisers are added in the mix. Such inclusions and additives, gave rise to a new concept of High Strength Concrete (HSC) with about 70-80 MPa compressive strength. To recall, the development of HSC was first started in Chicago for high rise structures.

In the process, we realised that in addition to high strength and workability parameters, durability and longevity of concrete is also very important in aggressive conditions especially with structures like Bridges etc. We called it as High Performance Concrete. Subsequently as technology evolved, addition of Fibres to the inherently weak and brittle cementitious matrix to get improved ductility and toughness came into picture giving rise to Fibre Reinforced Concrete. However, the latest in the field is the Ultra High Performance Concrete with a compressive strength of up to 200 MPa.” UHPC technology has a unique combination of superior technical characteristics including ductility, strength, and durability obtained by manipulating the micro-structure of concrete matrix.

Manipulation of Concrete Properties

The next step was clearly to manipulate nano structure because; even the cement based hydration product is heterogeneous at nano scale. There are several aspects of applying Nanotechnology. Firstly, one needs to understand the material better. There are many tools available for this purpose but regularly used are; Atomic Force Microscopy (AFM) and Nano-indentation. Nano Cements, for example are being developed as cements containing well dispersed nano sized particles of cement and mineral admixtures. The nano sized particles would be evenly distributed among the larger particles of mineral admixtures and with such dispersion even lower cement content should be able to provide desired binding between aggregates and admixture particles. Here, strength is not a barrier but one of the barriers in terms of high rise and long transportation is the constructability. Other overwhelming need is the sustainability where the material leaves lesser carbon footprint. For example, sustainability in cement production can be achieved by lowering the clinker component of cement and using more of usable industrial waste. The fact is that, the world needs more quantity of concrete especially in expanding economies like China and India. But, it is important to adopt a sustainable method to produce this material. The group effort should be to use nanotechnology to make concrete more sustainable and constructible.  

Speaking about one of his areas of research Prof. Shah says, "My present focus of research is Self Consolidated Concrete (SCC) where the concrete flows like water. The advantage of SCC in high rise structures and in congested areas is that you can pump the concrete both vertically and horizontally. If SCC flows like water and is delivered as water then the formwork has to be designed for hydrostatic pressure which then becomes very expensive. Concrete but then is not water-it is like blood. It flows and then it coagulates. SCC being a thixotropic material, is time-dependent. In this process we find that there is a continuous decrease of apparent viscosity with time under shear and the subsequent recovery of viscosity when the flow is discontinued. We did a lot of research and we are still continuing. Efforts should be made to understand the concept of thixotropy. One way to increase the thixotropy is to add a small amount of Nanoclay.

Nanoclay

Nanoclay has been a very important material for many other industries especially in polymers. These are purified, exfoliated and comparatively not very expensive. In one of the recent experiments, Nanoclay was used in the manufacturing of concrete pipes and it was observed that this addition of nano clay gave rise to a very smooth internal surface finish. This is certainly one of the solutions to achieve better constructability. Nanoclay is added to the concrete at the mixing stage itself. It is a very small quantity say, about 0.5 % by weight of cement.

Prof. Shah says, "Nanoclay is still in the research and development stage but people have already applied without really knowing the science. With time, people will see its application growing. Lot of work has been done in China with Nanoclay and they found out that it improves long term and hardening properties, although they do not understand the application of Nanoclay." We have also observed that there is no effect of Nanoclay after couple of hours, he added.

Nanosilica

When questioned about the durability and longevity of the structure with the use of nano material Prof. Shah explains, "We are aware that we want to consume less cement. One way is to use pozzolonic material and most common is fly ash. In India, Fly ash is used to replace about 20% of cement but can it be increased to 50%? On the other hand, if we use 50% of fly ash, then the reaction, the rate of hydration and hardening slows down and with the modern construction, it is not accepted. So, there are many ways to accelerate it. We have been looking at a

Citing an example of the groups' joint project in Bilbao, Spain Prof. Shah says, "The group have a project from the European Union of using concrete for storing nuclear waste. One of their concerns was the calcium leaching out of concrete. They found out that Nanosilica was much more beneficial in reducing the calcium leaching. Nano-indentation also showed that CSH is stiffer with the presence of Nanosilica."

Carbon Nano Tubes (CNT)

Explaining the role of CNT, Prof. Shah says, "We never use concrete by itself; it is reinforced with a reinforcing bar. The reinforcing bar does not do anything until concrete cracks. CNT as nano particles act as reinforcements for cementitious matrices improving their flexural strengths, fracture toughness and other properties. These are essentially cylindrical tubes with diameter of just a few nanometres and several millimetres in length. They have a tensile strength 100 times that of steel and can be manipulated. Technically, A CNT is a sheet of graphite rolled up into a tube structure and can be described as; 

• Single walled (SWNT) - Where a single sheet has been rolled up with diameter close to 1nm.
• Multi walled (MWNT) - Where a number of sheets have been rolled up with diameter ranges from 10-80nm 

When asked whether it reduces the percentage of steel reinforcement used in concrete Prof. Shah says, "For structural purpose, you don't want building to collapse. Here, it is not the failure consideration. But it is how you make concrete last longer. Majority of concrete failure is not through earthquake or hurricane. It is due to lack of day-to-day maintenance, cracking, repairing, and corrosion and so on. So, this is where CNT comes into picture." He further adds, "We have found that when we add 0.5 % of CNT, it does remarkable things. It imparts significant improvement to flexibility and fracture toughness characteristics.

Energy Consumption Reduction

• Use less cement but not concrete
• Prolong service life
• Develop ultra high performance material
• Develop multifunctional materials for reducing energy consumption
• Apply global holistic LCA/LCC model
• Recycle demolition waste concrete