Futuro in Ricerca 2013

2014 - 2017
Title: 
Advanced nanostructured materials for eco-sustainable cements: investigation of the structural properties and innovative strategies for their improvement
Principal Investigator: 
Silvia Borsacchi
Project type: 
National
Labs: 
MaRS-Lab

Abstract

With a worldwide production of about 3 billion tons/year, cement is the fundamental binder of the most important building materials used in the modern society. Due to the enormous volumes involved, the production process of cement is responsible for the 5% of the CO2 of human origin. 40% of the emitted CO2 is due to the energy consumption of the process, while the remaining 60% directly arises from the calcination of limestone and clays to obtain the mix of calcium silicates and aluminates that constitutes cement. Considering the increasing worldwide attention paid to the environmental sustainability issues, the need of developing innovative technologies for reducing CO2 emissions has recently prompted the scientific research to look for alternative binders that can be produced with very low CO2emission. One of the most promising technologies, worth of economic and scientific investments, is based on the possibility of producing a cement based on "highly reactive periclase" (MgO), starting from magnesium silicates. When hydrated in the presence of a silica source, periclase forms a colloidal gel (magnesium silicatehydrate, M-S-H) with mechanical properties comparable with those of the hydrated phase formed in the common Portland cement, the calcium silicate hydrate, C-S-H. Considering the abundance of the natural reserves (the earth crust is composed by about 70% magnesium orthosilicate), magnesium silicates couldsubstitute limestone as starting material for the production of eco-friendly cement, leaving the extraction and the manufacture expenses almost unchanged.In this context, the objectives of this project are: 1) the development and the preparation of innovative and eco-sustainable cement formulations based on reactive MgO and 2) the study of the composition-structure-properties relation, aimed at providing the fundamental scientific bases for developing the optimal formulations for the final applications and proposing innovative strategies to improve their mechanical and functional properties. At present the production of MgO based cement is active only in pilot plants; to plan an industrial scale-up it is crucial to characterize the reaction kinetics, the nature of the hydrated products and the micro/nano structure of the obtained phases. Within the activity of the project we will prepare samples containing MgO/SF (silica fume), Portland cement and MgCO3 (necessary to improve the efficiency of the process) in variable proportions and their structural, kinetic and applicative features will be investigated and related to the composition. The study of the reaction kinetics will disclose the parameters that control the nucleation and growth of the hydrated phases, the duration of the induction period, the setting and hardening times and the corresponding heat release. The study of the evolution of the porosity, the transport properties, the chemical shrinkage and the mechanical resistance will detail the macroscopic features and the applicability of each formulation. In parallel, the structure of the M-S-H binder phase atvariable Mg/Si ratio will be studied at the atomistic and mesoscopic level by using both experimental and theoretical approaches. Furthermore, the mechanical properties at the nanoscale will be investigated and put in relation with the structural studies: this will provide the fundamental insights for the prediction of the characteristic of this material as a function of the compositional changes.In the second part of the project we will focus on the solution of some drawbacks associated with the use of these formulations: inferior mechanical properties, scarce compatibility between M-S-H and C-S-H and between M-S-H and the steel structures. To overcome these limitations, nanotubolar natural microfibers, known ashalloysites, will be included into the formulations. The use of microfibers to increase the modulus of rupture of the cement pastes is a consolidated practice. Thanks to their monodimensional morphology and to their alumino-silicate nature, the halloysite microfibers can improve the mechanical properties of MgO based cements, without introducing additional compositional heterogeneities. Moreover, these structures (with inner and outer diameter of about 15 and 50 nm, respectively) can be selectively functionalized on their external surface, becoming "cross-linker" between M-S-H and C-S-H, or on their internal surface, making them valuable and tunable "nanocontainers" for the release of anti-corrosive or passivant agents for the stabilization of reinforced concrete. This approach is entirely innovative andopens the way to the “nanoengineering” of cement formulations, to produce functional materials by means of bottom-up strategies.