One of the distinctive advantages of shotcrete is its fluidity: when the homogeneous mix is applied pneumatically, it adheres to even rugged surfaces easily while retaining the early resistance to compression to provide structural support from the get-go.
A family of chemical polymers is behind the material’s fluidity: plasticizers and super-plasticizers. Also known as water reducers, they help reduce the total water-to-cement ratio, giving it a more ‘liquid’ consistency without the need to dilute the mix with water.
These additives, which are typically added during the mixing process, allow the concrete mix to remain malleable until application without losing its consistency.
Plasticizers and super-plasticizers have a temporary dispersing effect, which allows for a thorough hydration of each cement particle, improving the mix’s fluidity or rheology.
Eggs with your concrete? Roman recipes for concrete ‘fluidity’
Now in their third generation, plasticizers have played an important role over the course of history. Strange though it may seem, many authors lend weight to the theory that the Romans, enthusiastic proponents of concrete and its manifold applications, often used egg whites as a plasticizer.
This mix, combined with volcanic fly ash to create the type of hydraulic concrete seen throughout the Bay of Naples in Italy, makes for an extremely hardy and weatherproof material, including to highly corrosive salt water.
Plasticizers: first steps
The beginning of the XX century heralded the arrival of the first generation of plasticizers. Lignosulphonates, a byproduct derived from wood processing, are still often used nowadays to produce a workable mix with only basic raw materials.
These additives, known as Mid-Range Water Reducers [MRWR] attach themselves to the surface of a cement particle, which carries both positive and negative charges. Plasticizer polymers, which are negatively charged, counterbalance the positive charges on the cement surface, making the entire surface appear fully negative.
This triggers a physical effect that causes the now negatively charged cement particles to repel each other, creating a dispersing effect that allows for greater water permeation. This mix is now more ‘workable’ without the need to add more water, and allows for a reduction in the overall amount of water needed, cutting the water-cement ratio by around 10%.
One of the key issues with these additives is that they delay the curing process, which may in turn create further difficulties. An inability to cure within a specific timeframe means that an enormous amount of hydrostatic pressure could accumulate in a formwork column over a prolonged period, causing the formwork to burst.
The second generation: Plasticizers 2.0
A new generation of polymers hits the market towards the middle of the century, capable of delivering a greater reduction of the water-to-cement ratio of around 25%. Polysulfonates such as naphthalene and melamine have a similar working mechanism to the first generation of plasticizers, delivering an electrical dispersing albeit of greater intensity.
As we saw, these polymers adhere to the cement particles, charging them negatively which creates a repulsion between akin particles, permitting water to flow and hydrate the mix better.
That same repulsion activity also triggers major air occlusion, increasing the workability of the mix but simultaneously creating pockets of air that diminish its resistance and compromise its structural integrity.
This type of polymer may also pose other challenges, as it offers a very narrow window of ‘workability’: once the cement is hydrated, it tends to generate a crust-like byproduct that renders application difficult.
Super-plasticizers: the third generation
Super-plasticizers are the additives with the most comprehensive range of benefits, including a water-to-cement ratio reduction of around 40%.
As opposed to the two others above, polycarboxylates or High Range Water Reducers (HRWR) act on the base of sterical rather than electrostatic repulsion. A key steric effect is steric hindrance, which prevents a chemical reaction from taking place: in this case it prevents cement particles from agglomerating.
Polycarboxylates are complex co-polymers which can be engineered to fulfill many different functions, and consist of a negatively charged ‘backbone’ molecule with polymeric side chains.
Many of these additives can be combined together, and mixed with other types such as air-entraining, accelerating and retarding additives, categories which we will discuss further along in this blog.
Sources:
- “Could a 2,000 year old recipe for cement be superior to our own?”, Henry Grabar, The Atlantic Citylab, June 2013, Last accessed 14/12/16
http://www.citylab.com/tech/2013/06/could-2000-year-old-recipe-cement-be-superior-our-own/5800/
- The Electrostatic Potential of Highly Filled Cement Suspensions Containing Various Superplasticizers, O. Blast & D. Honert, International Concrete Abstracts Portal, 2003, Last accessed 14/12/16
https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&ID=12907
- “Aditivos Plastificantes”, CivilGeeks.com, 2011, Last accessed 14/12/16
http://civilgeeks.com/2011/12/10/aditivos-superplastificantes/
- “Super-plastificantes: Última Tecnología en Aditivos Para Concreto”, 360gradosenconcreto.com, 2011, Last accessed 14/12/16
http://blog.360gradosenconcreto.com/superplastificantes-ultima-tecnologia-en-aditivos-para-concreto/
