Nano-scale characterization of elastic properties of AFt and AFm phases of hydrated cement paste

Abstract

ABSTRACT: Hydrated cement paste is an amorphous composite material consisting of few crystals, each of which contributes to the strength and stiffness of cement-based materials. Among the crystals present in the cement paste, the nanostructures of AFt and AFm phases are highly correlated to each other at different stages of the hydration process. Since ettringite and kuzelite are known as the most prominent crystals in the AFt and AFm family, respectively, molecular dynamics simulations are used in this study to investigate their main elastic properties. An effort is also made to examine the effects of the size of the simulation supercell as well as the forcefield of choice on the predicted values. An in-depth understanding of the nanostructure and mechanical characteristics of the crystals under consideration is expected to lead to a nano-engineered concrete with enhanced durability and structural performance.the strength and stiffness of the net paste. Calcium aluminate trisulphate hydrates (AFt) and calcium aluminate monosulphate hydrates (AFm) are two of the most important phases present in the HCP. While these two phases contain several water mol-ecules in their interlayer, the Al/Ca ratio of AFm is higher than that of AFt. Among various crystals that can represent these two phases, ettringite and kuzelite have been identified as the most common phases of AFt and AFm, respectively.There are only few experimental tests and simula-tion studies to investigate the mechanical character-istics of the AFt and AFm phases. Except Speziale et al. (2008), which estimated the elastic properties of natural ettringite, including all the adiabatic elastic constants and acoustic velocities using the Brillouin spectroscopy in the ambient condition, no experimental results are available in the literature regarding the elastic properties of kuzelite. On the other hand, the nano-scale investigations are lim-ited to the study carried out by Manzano (2009), where the mechanical properties of both ettringite and kuzelite were quantified by Lattice Dynamics (LD) simulations. Although Manzano (2009) can be considered as a comprehensive study in terms of covering a variety of crystals present in the HCP, there are some important simulation issues that still need to be addressed. One of the issues is to evalu-ate the efficiency of various force fields in captur-ing the expected mechanical characteristics. Since choosing a proper forcefield is a critical step underlying all forcefield-based simulations (e.g., Kalinichev et al. 2007, Pellenq et al. 2009, and Dharmawardhana et al. 2013), the current study utilizes Molecular Dynamics (MD) simulations to 1 INTRODUCTIONThere is a growing interest in the identification of the nano-scale properties of cement-based materials, such as concrete and mortar (Ulm et al. 2004, Allen et al. 2007, Mondal et al. 2007, K onsta-Gdoutos et al. 2010, and Zanjani Zadeh & Bobko 2013). Since cement-based materials are considered as one of the most widely used con-struction materials throughout the world, devel-opment of time and cost-efficient strategies to examine their nano structures is an important step forward to better understand the associated macroscopic engineering properties. In the case of concrete for example, the outcome of such efforts can be directly utilized to improve the durability and performance of reinforced concrete structures during their service life. Because of the fact that binders plays an important role in the stiffness and strength properties of cement-based materials, investigation of the elastic properties of the nano-structure of binders provides invaluable informa-tion on the overall mechanical behavior of this group of materials.Portland cement, which is commonly used in Reinforced Concrete (RC) structures, reacts with water through the hydration process and binds the aggregates by forming a complex material called Hydrated Cement Paste (HCP). Despite several research efforts, some fundamental questions about the stoichiometry and morphology of this essential building block of concrete have remained unanswered. The HCP is a heterogeneous com-posite material, which can be characterized by its constituent phases, each of which contributes to