(649j) The Effect of Select Organic Compounds on Hydration of Portland Cement: Experimental and Molecular Dynamics Study

A large sector of industrial chemicals are amphiphilic organic compounds. When dissolved in water, amphiphlies are attracted to the air-water, i.e. nonpolar-polar, interface. In the concrete industry, amphiphilic compounds are used to reduce shrinkage cracking in portland cement concrete by reducing the surface tension of the pore water within the cement microstructure. The concrete pore solution contains sodium, potassium, calcium and to a lesser extent other ions, largely due to the chemistry of portland cement. Assembly and concentration of the amphiphilic compound at the air-water interface in the presence of various ions is critical to the understanding of compound effectiveness. In addition to that, amphiphilic compounds are not chemically or physically inert and can negatively interfere with the progress of cement hydration and at the same time be effective surface tension reducing agents. Side effects such as hydration retardation lead to delayed strength development or worse. The influence of the amphiphilic compounds on the cement product phases is a complex multi-parameter problem.

To understand the effect of organic compounds on hydration of cement, a combined experimental and computational approach was used. This study summarizes ongoing simulations and experiments using three compounds: butyl glycolate, 2-butyoxyethyl acetate, and hexylene glycol. Molecular dynamics simulations were performed to simulate the interactions of the test compounds with the (001) surface of the portlandite crystal (calcium hydroxide) and (040) surface of the tricalcium silicate crystal (the most abundant species in ordinary portland cement). In addition, further molecular dynamics simulations were performed to investigate the interactions of the test compounds with simulated pore solution. To measure the adsorption isotherm of chelating agents - calcium glycolate and calcium acetate - experiments were performed by equilibrating aqueous solutions of the agents in the presence of various amounts of solid phase calcium hydroxide. In addition, isothermal calorimetry, gas chromatography and surface tension experiments were also used to elucidate the process and support the computational work.