Elkem Microsilica is a superfine pozzolanic material. It is one hundred times finer than cement – or GGBS or FA. The silicon dioxide content is normally over 90% making it highly reactive. When mixed into a normal concrete, the small size and pozzolanic reaction fills the voids of the concrete increasing the bond and filler effects while the strength is increased. Thus concrete with low permeability to water and hazardous materials can be produced, increasing the durability and lifetime of structures. Using microsilica in combination with GGBS or FA can further improve the plastic and hardened qualities and produce highly economic and long lasting concretes.
Microsilica concrete has been used increasingly across the globe for the last 40 years and in the harsh Gulf environment since the early 1980s. Starting off in the UAE, progressing through Bahrain, Kuwait, Oman and Saudi Arabia and up into Qatar, the use of microsilica has given the opportunity to use concrete where it would previously have been thought impossible – or certainly not economic.
Now, the use of microsilica is widening beyond the Gulf Region into areas like Africa where it would never have been thought possible to produce such durable, yet economic, high performance concretes. Let’s take a look at the development of high performance Elkem Microsilica Concrete in such a tough environment.
The Early Days
Back in the 1980s, a number of projects began to use microsilica concrete, in order to get a better lifetime from the structures. The Middle East is by far the worst possible environment for placing concrete – especially high performance concrete where long-term durability is paramount. Unfortunately, it is that very environment that requires high performance concrete in order to achieve the long lifetime for the structures. A difficult “Catch 22”.
The requirements for good concrete to be durable in the region meant very low water permeability, very high chloride resistance, high sulfate resistance, yet good long duration workability combined with low heat and resistance to thermal effects. While microsilica concrete could give all of these, the use of a low water cement ratio, non-bleeding, high strength concrete made many concerned about the ability to actually handle and place this material. However, concrete technology persevered and the use of microsilica concrete has become synonymous with high durability and long lifetime.
Major projects started using the material in the early 1980s: such as the Alba no.4 potline (Bahrain) – high electrical resistance and physical durability; Al-Khobar power and desalination plant (Saudi Arabia) for ultra-high chloride resistance; Precast Operations (Dubai) started to run through the 1980s and into the 90s. In 1993, the Gulfs first ‘skyscraper’, the Baynunah Tower in Abu Dhabi was built using 80MPa, zero water permeability microsilica concrete. More projects followed: The Burj Al-Arab, Dubai Airport extension, the 2nd Manama-Muharraq Crossing in Bahrain, LNG projects in Qatar, more power stations in Saudi Arabia and the huge Shioba Oil Pier in Kuwait.
By the year 2000, high performance microsilica concrete was a necessary part of construction in the Gulf. The world’s tallest building, the Burj Khalifa stands not only on foundations of triple blend microsilica concrete, OPC+FA+MS, but the columns throughout the building are the same high performance material. Not only that, despite the environmental drawback of extreme heat, the concrete was pumped over 600 metres vertically, by one pump, for placing.
The next world’s tallest building, The Kingdom Tower in Jeddah, is using the same high performance concrete.
As with all concrete, Microsilica Concrete needs the usual precautions to ensure best results.
Over the years a number of questions have been put forward concerning the use of microsilica in the hot environment of the Gulf: Problems with cracking, worries about heat evolution, mixing and handling, curing and similar.
Many of these are easily addressed by remembering the fact that ALL concrete needs to be cured properly – to get maximum performance – and microsilica concrete is no different.
Curing is a necessity when using such high performance concrete. All normal forms of curing can be used – sometimes combinations of methods are more effective. Ponding, mist spraying, wet hessian, plastic sheeting, membrane compounds etc.etc. are all effective – dependent on the situation. As the mix is designed for the purpose, so the curing must be designed for the situation. The Baynunah Tower was cast with 80MPa concrete using a 10% microsilica dosage. However, the curing regime was specified at 14 days wet curing. This was done and the concrete had NO cracking at all.
Microsilica is not a product that can be just dumped into a concrete mix and a perfect mix will come out. Mix design is very important. The concrete should be designed for purpose – and certainly not overdesigned. The average dosage of microsilica across the Gulf is about 8%. Higher percentages, when used for specific performance qualities, need better design of the mix to prevent bulking, high water demand and handling difficulties. It is recommended that concrete designers use particle packing software to get the best blend and mix design for high performance concrete.
Concern over Heat Evolution is a very misleading point. Just because the microsilica can increase the strength – and improve the rate of gain of strength – does not mean that it is going to evolve huge amounts of heat. The heat of hydration is similar to cement (kilo for kilo). Therefore, if you modify the mix to achieve a certain strength – reducing the cement content say, from 450 to 350 kg/m3, then adding 35 kg of microsilica, the resultant mix will have an overall loss of heat by some 65kg cements’ worth. Equally, microsilica can be used to gain early age strength in very high replacement concretes – where 50 to 70% of the cement is exchanged for FA or GGBS. The pozzolanic action of the microsilica will give an early strength, structural integrity, and reduce the permeability of the fragile concrete, while not increasing the heat by more than a few kilos worth of cement.
Self Compacting Concrete
If anyone thought that microsilica concrete was sticky and difficult to move, then they haven’t ventured into the world of SCC. This is not a new concrete. It is merely a method of mix design and control that enables much easier placement of concrete. Microsilica is the perfect addition for creating SCC. Using particle packing methodology mixes can be designed with superb flow characteristics and no segregation. This reduces the use of ‘fillers’ such as limestone dust and high dosages of chemical admixtures in the concrete. The microsilica also gives much better hardened properties to the concrete. This is also true for SCC’s made with large replacement levels of FA or GGBS that are modified with microsilica additions.
To learn more about SCC visit the Elkem Materials website:
and download EMMA, the free particle packing technology software.
Here and Now
Microsilica concrete is currently being used across the Gulf Region in a number of highly visible and prestigious projects. Jordan, Egypt, Lebanon are now using microsilica concrete – just as Saudi Arabia, the UAE, Oman, Kuwait, Bahrain and Qatar.
The use of highly durable, high performance microsilica concrete in and around the Gulf Region has increased, year on year, as the improved qualities of the concrete are seen. Many millions of m3 of microsilica concrete have been placed in the harsh, unforgiving environment of the Gulf over the last 25 years. A large number of papers have been written concerning the technical benefits of microsilica in concrete for the Gulf Region. The best testimony to the benefits of microsilica concrete is simply that the Gulf is by far the biggest consumer of microsilica concrete in the World.
In the harsh climate of Africa, where durability is needed in concrete, there is a way to design and produce the high quality concrete to give the lifetime economics, and that, is using Elkem Microsilica.
Suggested web-sites for more information:
www.elkem.com/concrete (Elkem Materials ‘concrete’ pages)
www.silicafume.org (The Silica Fume Association)
www.concrete.org (The ACI web-site)
Original source: Elkem
