Within SmartCast, two main routes to achieve Active Rheology Control (ARC) and Active Stiffening Control (ASC) have been studied, as schematically summarized in Figure 1.
The first route is based on the concept of magnetorheological fluids (MRF). Magnetizable solid particles are added to the cementitious material, e.g. magnetizable nanoparticles (MNP) consisting of Fe3O4 or more traditional cement-replacing materials like fly ash with some magnetic response. Upon applying a magnetic field, the magnetizable particles can form clusters, inducing stiffening on demand. Alternatively, the acceleration of the magnetizable particles due to the electromagnetic forces temporarily gives the cementitious material some more liquid-like properties. Variable magnetic fields can be applied to actively control rheology, using the magnetizable particles as internal actuators. As a conceptual illustration, Figure 2 shows the window of opportunity for active magneto-rheological control of a cement paste containing magnetisable nanoparticles (MNP). The evolution of the storage modulus in absence of magnetic field is shown as the central curve. When a constant magnetic field is applied, a controlled faster stiffening is obtained as shown by the top curve. In case of a varying magnetic field, a controlled reduced stiffening is obtained as shown by the staggered lower curve. In principle, the area in between these two extreme curves, is the potential field of application for active rheology control for the considered cement paste with MNP.
The second route is the development of switchable superplasticizers. This technology further builds on the working principle of the latest generation of superplasticizers, which are comb copolymers consisting of a backbone with negative charges in view of adsorption onto the surface of cement grains and of side chains in view of providing steric hindrance to disperse cement particles. Instead of using classical polycarboxylate ethers (PCE), additional functionalities are given to the superplasticizers by incorporating responsive elements into the polymer architecture.
One example is a comb copolymer consisting of a classical backbone with negative charges (MAA) and of side chains of a new type, namely TEMPO. This side chain on the one hand can provide steric hindrance, and on the other hand can be triggered by an electric signal to become positively charged. Upon electric triggering, the positive charges of TEMPO+ will counterbalance negative charges form the backbone, reducing the adsorption capacity to the cement grain surface. In this way, a controllable superplasticizer is obtained, enabling active rheology control by moderating the adsorption capacity through external electric signals. The potential of the newly developed switchable superplasticizer is conceptually illustrated in figure 3. Implementing the newly developed admixture, initially without external control signal, leads to a reduced flow curve compared to the reference paste, showing the plasticizing effect. Upon application of a redox signal, pTEMPO is given a positive charge, resulting in pTEMPO+. This is influencing (reducing) the adsorption to the cement grains, and thus the plasticizing effect, as illustrated by the higher flow curve. The dotted area conceptually shows a potential window of operation for the active rheology control by means of the redox-switchable plasticizer.
Another example of a switchable superplasticizer is a magnetizable polymer composite consisting of polymer chains linking MNP (by means of a chelator) to the surface of a cement grain (by means of electrostatic adsorption). These polymers act as a superplasticizer, due to adsorption on the cement surface and steric hindrance by the side chains. By means of an externally applied magnetic field, the MNP will try to cluster in a similar way as the previously mentioned responsive mineral additions, now also involving the cement particles to which the polymers are electrostatically adsorbed. This can provide a strong active stiffening effect.
The SmartCast project successfully reached the intended goals. A laboratory proof of concept has been successfully obtained at paste level, both for the approach using responsive mineral additions and the approach applying switchable responsive plasticizers. This shows the great potential of active rheology control (ARC) and active stiffening control (ASC) of cementitious materials. Nevertheless, further technological and valorisation studies are still needed (and have been initiated) to bring the new technology to the market.
