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Answer by Dr Mohammed Alhaj (12-10-2010)

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 No one can give one exact factor for the Durability and sustainability of green concrete in green building.

The Durability and sustainability of green concrete or green cement depend on many factors:

In my PhD book that had been published in 2010, I discussed the above factors.

1-The surrounding environment of the building ( temperatures, humidity of air and salinity of surrounding soil)
2- The concrete cover, water/cement ratio, the total cement and pozzolana content, the type of pozzolana, the finess of cement and pozzolana, the type of admixtures, the type of aggregate and sand used in the concrete and the type of water used in the mix and curing.
We can had more service life for the green concrete in the building by increasing the total of cement and pozzolan content and reducing the water to cement ratio and using clean and unsalted water or re-treated water. 

3-the concrete cover is very important. As we increased the cover depth we can increase the service life of the green concrete in green building. More concrete cover means more depth for the concrete and more distance and time will be need it for the sulfates ions, chlorides ions, carbonation and acids.

4-The curing time is very important for the green concrete and for the sustainability of concrete.
Green concrete is more sensitive to curing time than normal Portland cement because it had slower reaction than than the Portland cement.

In my PhD book that had been published in 2010, I discussed the above factors.
I found the corrosion and deterioration of my samples had increased after short time by reducing cement content and increasing water content and reducing cover depth with less curing.
I had sever corrosion with 90% pozzolana after few days only because I had high water/cement ratio and less curing days. Even with good water/cement ratio i had corrosion after few months only due to the short time of curing.

We had some cracks and deterioration problems with king Fahd causeway bridge in Bahrain , even that they had 50% of slag because they did only short curing than required by the designer.

5- The type of pozzolan and % of pozzolana activity is very important for the durability of concrete.
A- Silica fume - that had been used in KAUST -had very fine particles that had very high pozzolan activity (150%). This will increase the early strength and increase the shrinkage cracks and temp. cracks which will increase the porosity and permeability, Thus the service life of the concrete will be reduced. (unless many precautions had been taken)

B- Super-pozz (very fine fly ash) that had been used in Dubai Tower- had around 115% pozzolana activity comparing to Portland cement. This will increase the early strength and increase the shrinkage cracks and temp. cracks which will increase the porosity and permeability, Thus the service life of the concrete will be reduced.  (unless many precautions had been taken)

C- Natural pozzolana, medium quality of fly ash and Slag had fine particles that had medium pozzolan activity (less than 80% ). This will reduce the early strength, increase the ultimate strength after 30 or 60 days, and reduce the shrinkage cracks and temp. cracks which will reduce the porosity and permeability, Thus the service life of the concrete will be increased. All roman sustainable building had used fine particles of natural pozzolana that had low early strength and high ultimate strength  for 2000 years.


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According to GHD, DUBAI TOWER that been built in 2010, had 100-years service life ( The green concrete in the tower contain 25% imported pozzolan from south Africa and Europe).
I-35 Bridge in Minnesota, USA that had been built in 2009 had 100-service life ( The green concrete in the bridge contain 85% of local slag and fly ash ). It is the only one in the world that had 85% pozzolana, whilst the city of MASDAR, UAE had only 60% of slag imported from Japan. Many green building in USA had 30% to 70% local Pozzolan. Roman building that last for 2000 years had used 70% natural pozzolana from Italy and 30% heated Lime.
There is only one green building in the world that had been built on 2009 at south America using 60% fly ash + cement had more than 900-years service  life and crack free. This building with 2 stories had no steel in it .

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The adding of local pozzolana( volcanic ash = NATURAL POZZOLANA - ASTM 618 TYPE N)  to cement in cement factory or ready mix concrete in Saudi Arabia and Arabian Gulf States by 50% will achieve the following :
1- Reduce 50% of the energy consumed in the cement factory.
2- Reduce Co2 emission of cement factory by 50%.
3-Reduce mixing water in the green concrete mix design by 10-15%.
4- Save 50% of cement clinker for the next generation.
5-Reduce the heat island effect of the green building that used green concrete.
6-Get LEED green credit since we replace more than 30% of renewable pozzolana in the concrete.
7- Improve the chemical and physical properties of green concrete by increase the C-S-H gel and thus reduce the porosity and permeability. This will increase the durability of green concrete and thus the sustainability of green building.
8-Increase the ultimate strength of green concrete by increasing the reaction between the silica oxide from pozzolana CaO from cement. OPC Cement had 62% of CaO and 22% of SiO2, whilst the the Natural Pozzolana in Saudi had 44% of SiO2 and 10% of CaO. The addtion of 50% of Natural Pozzolana to OPC cement in Saudi Arabia will help us getting workable and dudrable green concrete, hence IP cement will have equal amount of 35% of SiO2 and 35% of CaO.
9- The addtion of 50% Natural Pozzolana to cement will reduce the health and environmental risks of OPC cement and improve the outdoor and indoor environment for the green building.

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Please refer to my book for the durability and sustainability of green concrete.
http://www.amazon.com/Corrosion-Behavior-Durability-Various-Pozzolans/dp/3838338359

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Product Description

This work examines the corrosion behavior of reinforcing steel (RC) embedded in concrete mixes of various types by incorporating both Ordinary Portland Cement (Type I) and Sulfate-Resistance Portland Cement (Type V) and Blends with Fly Ash (PFA) or Slag (GGBS). The RC specimens were exposed to wet-dry cycles of chloride and/or sulfate solutions. Theory predicts that under such conditions the electrode potentials of the steel can suffer the formation of anode and cathode regions that depend on cement type and the local environment, so localized corrosion may occur. The final results in this book concludes that using two concrete types in the same structure will lead to the preferential formation of anode and cathode sites. Moreover, this has implications for the durability of real structures whereas the steel, either through design or repair, is embedded in more than one concrete mix type. Furthermore, the results showed that green concrete - containing either 30% Pozzolan or 70% Slag ? had the best overall quality, durability and better performance than Portland cement concrete. This book will help professionals in Corrosion Control, Green Building and Sustainable Development.

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Salam
Dr Mohammed Alhaj

www.dralhaj.com

m@dralhaj.com

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