Joseph J. Biernacki

Tennessee Tech University

Discerning the mechanism of interaction for organic molecules used as admixtures in portland cement

Ojas Chaudhari

Modern concrete contains not only cement but frequently also includes a variety of organic compounds that are deliberately added to modify one of any number of fluid or hardened properties. How does the molecular structure of organic molecules impart physical effects and how might knowledge of such be used to systematically deign compounds with targeted properties?  Recent research using computer aided molecular design (CAMD) [1, 2] has shown that quantitative structure activity relationships (QSAR) can be used to identify compounds that are effective for a given application.  This work, however, intends to focus on the actual mechanism of functionality rather than further application of the QSAR correlative approach.

One of the important properties of cement is the rate of the hydration reaction. Set retarders are one class of organic compounds which delay the rate of cement hydration and which may also increase the later age compressive strength of the concrete. Prior research, however, focuses more on the effect that set retarders have on mechanical properties of concrete rather than chemical mechanisms.

For example, glycolic acid, a well-known hydration inhibitor, is thought to interfere with calcium hydroxide precipitation, a by-product of tricalcium silicate hydration, and thus obstruct the reaction pathway.  Tricalcium silicate (alite) is the most abundant constituent of ordinary protland cement and its hydration is largely responsible for early strength development in concrete.  Little actual experimental evidence, however, in support of this or other hypothesis is available.  Just how does glycolic acid or other set retarders interact with anhydrous or hydrate surfaces or reaction intermediates? What is the effect of different molecular groups on the hydrated surface?

The present work is a combined experimental and molecular-scale computational study focused on elucidating the mechanism of glycolic acid inhibited cement hydration.  A force field-based method using CLAYFF and UFF is being applied to study surface binding energies. The recently developed semi-empirical force field CLAYFF includes flexibility of surface OH groups from calcium hydroxide and allows energy-momentum transfer between interfaces. UFF is versatile force field that allows approximate parameterization for a wide range of atoms including calcium.

The primary focus of this in-vacuo study is to calculate the interaction energy (binding energy) of various set retarders (such as glycolic acid) on the calcium hydroxide surface. In addition, the interaction of organic molecules with calcium hydroxide surface in a native aqueous environment will also be investigated. The principle focus is on the structure at and near solution-solid interfaces and adsorption of aqueous organic compounds on the calcium hydroxide surface. The interaction energy on the calcium hydroxide surface is being compared for different retarders using the generated models.

Along with computational work, the experimental plan involves micro-scale testing and lab scale testing of retarders in the cement paste. Micro-scale testing includes imaging using scanning electron microscopy (SEM) to characterize the effect of retarders on microstructure development. X-ray diffractrometry is being used to analyze the composition of hydrated cements and isothermal calorimetry, compressive strength and shrinkage behavior of mortars with 1% retarders are part of the lab-scale characterization.

[1] H. M. Kayello, N. K. R. Tadisina, N. Shlonimskaya, J. J. Biernacki and D. P. Visoc Jr., An Application of Computer-Aided Molecular Design (CAMD) Using the Signature Molecular Descriptor – Part 1. Identification of Surface Tension Reducing Agents and the Search for Shrinkage Reducing Admixtures, J. Am. Ceram. Soc., 97(2), 365-377 (2014).

[2] N. Shlonimskaya, J. J. Biernacki, H. M. Kayello and D. P. Visoc Jr., An Application of Computer-Aided Molecular Design (CAMD) Using the Signature Molecular Descriptor – Part 2. Evaluating Newly Identified Surface Tension Reducing Substances for Potential Use as Shrinkage Reducing Admixtures, J. Am. Ceram. Soc., 97(2), 378-385 (2014).



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