61st Annual Report on Research 2016

We developed new force field parameters for cement minerals, continuing with tricalcium aluminate, which is the fastest hydrating phase in Portland cement. This work is the first to accurately examine structural, surface, and hydration properties, as well as organic interfaces by simulations of the aluminate up to the large nanometer scale and has been used to explain cleavage and hydration properties (Dalton Transactions, 2014, 43, 10602). The insights were then applied in a combined collaborative experimental-theoretical study of blocking/retarding effects of aluminate ions during the hydration of calcium silicate phases, showing that the action of aluminate retarders is strongly pH dependent (manuscript under review in Cem. Concr. Res.). New multiscale simulation approaches of minerals, their cleavage, and stress transfer were explored in a collaborative effort to understand clinker comminution and the role of organic grinding aids (Adv. Appl. Ceram. 2015). Along with earlier work before this PRF grant, these efforts have been applied industrially for computational screening of grinding aids on a commercial scale and guiding the development of commercial grinding aids at Sika AG. For this work, Hendrik Heinz and his colleagues received the 2016 Sandmeyer Award from the Swiss Chemical Society, which is annually given to a team of researchers for outstanding advances in applied chemistry.

This grant further supported three invited review papers on the adsorption of biomolecules and polymers on silicates, glasses, and oxides, on advances in computational design of inorganic-organic interfaces, and on surface decoration of nanoparticles. Mechanisms, predictions, and opportunities by molecular simulation and experimental design are described in these contributions (Curr. Opinion of Chemical Engineering 2016, Chem. Soc. Rev. 2016, and Surf. Sci. Rep. 2017).

As a core effort in this project, the structure and aqueous surface properties of crystalline calcium silicate hydrate (C-S-H) phases (tobermorites) has been investigated. We obtained exciting new insight into the surface characteristics of these important crystalline analogs of gel-like C-S-H. Different calcium-to-silicate ratios as a function of crystallographic direction endow the (h k l) tobermorite 14 A surfaces with specific adsorption and retardation properties. The mechanism of adsorption of common ionic polymers, such as polycarboxylate esters, includes the migration of calcium ions to the surface, followed by ion pairing with the polymer and conformation adjustments. Detailed molecular models of the tobermorites and full validation of their properties will be published separately (manuscript in preparation).

Together with experimental collaborators at U Akron, we used these models and explained the working mechanism of comb-like polycarboxylate superplasticizers in cement hydration as a function of molecular architecture (manuscript submitted). This paper introduces quantitative correlations between copolymer architecture and setting properties of cement using zeta potential and adsorption measurements, calorimetry, fluidity tests, as well as detailed molecular simulations of the organic-inorganic interfaces. The synthesis of the copolymers was carried out with better control over polydispersity and definition compared to prior studies, and a consistent molecular mechanism of the adsorption of the polycarboxylate ethers on calcium silicate hydrate surfaces is proposed. Specific ion pairing interactions and flat-on versus tightly packed upright conformations of the copolymer were identified as a function of the density and length of side chains, consistently explaining zeta potentials, observed adsorbed amounts, retardation of the hydration reactions, and dispersion forces between colloidal particles. The new working mechanism is supported by multiple independent measurements for a range of polymer architecture. It is also explained for the first time how one set of molecular parameters controls adsorption and retardation of the hydration reaction while different molecular parameters affect the reduction of interparticle forces and fluidity. In summary, this work introduces design principles for polymeric modifiers for cement materials from the molecular scale that rationally explain important cement properties. The proposed working mechanism is more predictive and conceptually sound in comparison to prior theories, which neglect critical interactions of the polymer with the cement surface as well as the aqueous surface chemistry of calcium-silicate-hydrate surfaces. The mechanisms further aid in the understanding related particle dispersions and alternative (greener) cements.

Furthermore, a computational study is in progress that explains in detail the effect of superplasticizer backbone length, degree of acrylate ionic groups, and side chains on adhesion to specific (h k l) facets of tobermorite 14 A, a crystalline model mineral of cement surfaces. Collaborative work with partners at ETH Zurich also aims at exploring the role of superplasticizers on amorphous calcium-silicate-hydrate minerals. This task is more challenging as various microenvironments must be taken into account which, however, is also critical to improve the quality of computational guidance for design of cement materials.

The project supported mainly a PhD student and benefitted from many in-kind contributions. We involved also an undergraduate student at CU Boulder who is about to design an automated surface model builder for calcium silicate hydrates as a function of a given C/S ratio, water content, and NMR-based topology of oligomeric silicate chains. This resource, once automated, will be a helpful tool for various researchers to conveniently build suitable molecular models of C-S-H ready for molecular simulations. Our force field parameters for cement minerals are fully integrated into the INTERFACE force field, which is the most comprehensive platform for molecular modeling of construction materials (and inorganic materials in a broader sense). It is easy to access via website and extensions for a graphical user interface to further simplify usage by a broader community are in planning. Recently, a new CemFF database has been established at EPFL (Switzerland) and a collaborative review paper on cement force fields with contributions by major groups and lead by our development team has been submitted (April 2017).

The work in this project has been presented at more than 15 national and international meetings to-date including several invited talks and posters. Hendrik Heinz was elected as a fellow of the Royal Society of Chemistry in 2016. We expect several further impactful contributions from this grant that will leverage future research efforts in this area and funding.