Controlling the cohesion of cement paste by the use of polyelectrolytes: towards a sustainable cement – BRIDGE

The project consisted in two complementary approaches, one based on Monte Carlo simulations and the other combining various experimental characterization methods. The main part of the work was done on C-S-H suspensions used as model for cement. However, some tests were also conducted on cements.
Monte Carlo simulations were used to point out the more efficient polymer structure (linear or branched, polymerization degree ... ) for bridging C-S-H nanoparticles. These numerical results were supplemented by experimental data obtained using the following techniques: (i) determination of the adsorbed amounts of polyelectrolyte by the rest method; (ii) acoustophoresis to determine the zeta-potential variations upon polycations adsorption to the C-S-H nanoparticles; (iii) measuring of the yield elastic deformation of pastes by rheometry; (iv) atomic force microscopy to measure the interaction forces between C-S-H surfaces (v) measuring of the compressive and flexural strengths by conventional mechanical testing.

A first occurrence of ductility enhancement was observed for C-S-H at low calcium content i.e., like what happens in new cements obtained by replacing a part of the clinker by industrial by-products (Secondary Cementitious materials). In this case, the elastic deformation was multiplied by ten, which, to our knowledge, has never been observed before in the field of cementitious materials.
A significant improve in ductility was also obtained under conditions of ordinary cement (calcium content approximately twenty times the first case) with polymers of special structure. This result required a collaboration with Bozzetto Group, international leader in the synthesis of organic additives for the polymer developments.

The results obtained in the BRIDGE project encourage su to apply for a new ANR project in order to continue this field of research. The aim of the new application is to apply the results that have been obtain with model systems to cements. The fundamental purposes are twofold. First, the polycations influence on cement hydration will have to be investigated. Second, the relationship between mecanical properties at a macrosize level and microstruture of the cement paste will be studied. This project is expected to give rise to new greener composite cement with novel mechanical poperties.

F. Brunel, M. Turesson, C. Labbez, I. Pochard, S. Gauffinet
2013 International Concrete Sustainability Conference, 8 mai 2013, San Francisco (USA)

Improved mechanical properties of hybrid organic-inorganic materials
F. Brunel, M. Turesson, I. Pochard, C. Labbez, S. Gauffinet
European Materials Research Society 2013 Spring meeting, 30 mai 2013, Strasbourg (France)

Controlled interaction in C-S-H/polycation composites: effect on mechanical properties
F. Brunel, M. Turesson, S. Gauffinet, C. Labbez, I. Pochard
1st International Conference on the Chemistry of Construction Materials, 7-9 octobre 2013, Berlin (Allemagne)

Colloidal Behavior of C-S-H Nanohydrates
C. Labbez, F. Brunel, I. Pochard, A. Nonat
NANOCEM Workshop, 3-4 Octobre 2011, Lausanne (Suisse)

Concrete, i.e. mix of Portland cement and aggregates, is the most used material in the world for housing and infrastructure construction. In addition to its good compressive strength properties, this omnipresence is explained by the low cost and the worldwide availability of cement constituents that happened to be the main elements of the earth's crust (Si, Ca, O). However, the manufacturing of Portland cement clinker is responsible for about 5 percent of human-caused emissions of the greenhouse gas carbon dioxide. The environmental impact of concrete can be significantly lowered by improving its compressive and tensile stress properties since it would allow to both improve the durability and reduce the use of concrete. Two ways can be envisaged. The first, already explored, consist in reducing the porosity of the final material, and thus its compressive strength, by lowering the added water in cement paste thanks to the use of the fluidifying properties of anionic polyelectrolytes, i.e. superplasticizers. The second, would be to improve the weak tensile strength of hydrated cement which comes as a result of the very short range of the cohesive forces between the cement nano-hydrates. As suggested by preliminary experiments and simulations at a mesoscopic scale, this should be made possible by bridging cement nano-hydrates via the use of cationic polyelectrolytes. Indeed, the bridging force is more long range and should in principal lead to a more ductile material. Surprisingly, in the scope of the cement chemistry, works following this route has never been prospected before. In the present proposal we shall pursue this route. To this end, we shall conduct experiments as well as performed and further the development of simulation tools conducted under the ongoing project “Adsorption of polyelectrolytes on mineral oxides”. The experiments will involve, adsorption, electrophoretic, rheology and atomic force measurements on cement/polycation systems. The simulations will be based on a combination of an isotension and a grand canonical ensemble for handling the non-trivial full equilibrium between a bulk solution containing long chain polymers and the confined solutions between two cement particle surfaces. The newly developed simulation tools will be made available to the scientific community through the open source project faunus (http://faunus.sourceforge.net/). This research should provide the base for the development of new greener hybrid materials with improved mechanical properties and, more generally, for a better understanding of colloid/polyelectrolyte systems.