The fight against water is a real war. We have discussed how vital for concrete is to be protected from the aggressions that water brings. In this post we will shortly talk about two great weapons which will significantly contribute in winning this war.
There are several other ways to achieve this goal and we'll discuss about some of them in the near future, but for the now, let's focus only on these easy, common, reliable methods.
The first weapon is “Low w/c ratio”
It is pretty easy to understand that when w/c ratio is lower, permeability is also lower. It´s because having more cement in a given volume leads to having more hydration products and a consequent capillary porousness reduction (more detail in my post “Concrete permeability”).
However, must be said that sometimes lowering w/c ratio is not a viable decision, or maybe it is not enough to obtain the requested permeability level.
That´s why a second “weapon” is needed.
The second weapon “micro pozzolana”
Silica fumes are a by-product of the silicon and ferro-silicon alloys industry and have high micro pozzolanic qualities.
Their shape is micro-spheres like, with dimensions around 0.01 mm, that is 50-100 times smaller than cement. Silica fumes have a pozzolanic effect, that is, they react with lime (which is a poduct of cement hydration), just like natural pozzolana of ashes but with higher efficiency.
The small size allows the positioning of the particles inside and in between of the capillary porousness and the pozzolanic qualities allow the formation of additional hydration products which occupy other voids.
Therefore, concrete permeability drastically drops and at the same time the concrete mechanical performance and resistance to chemical aggression rises increasing durability (read my post “Concrete durability”).
As said, similar result can be achieved using other sources of pozzolana. The pozzolanic cement for example can be efficiently used in replacement of the traditional Portland cement. As known the values of Fck/Rck are measured at 28 days but that doesn´t mean that chemical reactions are going to stop the 28th day. Not at all. Hydration is a process that last a lot depending on several factors but one of these is the choice of cement. Please check this simple graph here.
As you can see Pozzolanic and blast furnace cement will continue to be active after the 28th day way more than the Portland cement, which means that there will be more hydration products in concrete pozzolanic/blast furnace based after months or years in comparison with Portland based concrete. Sure, the permeability values will never be comparable with Portland+silica fumes but in a “contest” between Portland and pozzolanc/blast furnace cement these last two will always win.
Last but not the least, flying ashes could be a more economic choice when it comes about adding micro pozzolana to Portland cement. Fly ashes behave just as silica fumes since they react with lime producing hydration elements with their ramifications and physically occupying voids due to their micro diameter. They are not as effective as silica fumes especially because of their dimension, that is some 10 times bigger, and their chemical activity is lesser performing as well.
However must be said that, both silica fumes and flying ashes require specific concrete mix design since their adding comes with a superior demand of water (more surface getting wet and more water for the hydration process), so in order to keep the w/c ratio constant, countermeasures must be taken. Conversely, pozzolana will amplify the compression strength at 28 days and more, contribute significantly in having more viscous mixes, harder to be washed out and to segregate (see the posts “Concrete compaction” and “Concrete segregation”), helping the rheology (concrete flow ability) being so a good allied in design of Self Compacting Concrete (see my post "Self compacting concrete") and “cooling” down the temperature during cement hydration which means lesser shrinkage phenomena (please read the post “Concrete shrinkage part 1”). On top of that setting time is extended (which is a plus during the warm season) and all the pozzolana actively contribute in significant reduction of alkali-aggregate reaction aka “pop-out" you can read about that in my free booklet “Concrete flooring tips and tricks” available for download after registration as member of P-Concrete forum.
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