(279b) Semi-Empirical AM1, PM3 and PM6 Calculations On the Metakaolin Molecular Structure: Practical Application in Geopolymer Cement Production

Portland cement, the basic ingredient in concrete, is possibly one of the most common construction material used in the world. However, portland cement production results in the generation of green house gases from both energy consumption and chemical changes associated with the production process. In general, production of one ton of portland cement results in approximately one ton of the green house gas, CO₂,released into the atmosphere. In an effort to reduce our dependence on traditional portland cement-based concrete, ongoing research is concentrating on development of new low environmental impact alternative cements. Geopolymer cement concrete is one recently developed alternative. Geopolymer concrete uses different raw materials and different production processes than portland cement, which enable a five to six times reduction in green house gas emissions.

The process for geopolymer production includes mixing an anhydrous alumniosilicate material with alkaline solution such as sodium or potassium hydroxide. As such, large volumes of industrial by-products and waste such as fly ash or blast furnace slag can be used as potential sources of the alumniosilicate materials required in geopolymer cement production. However, metakaolin, a modified naturally occurring material, is a preferred raw material due to its high rate of dissolution in the alkaline solutions. The formation of geopolymer consists of three-stages: (1) dissolution; (2) reorientation; and (3) polycondensation. These, three stages take place almost simultaneously; therefore it is difficult or impossible at this time to conduct experimental studies on the isolated steps.

Understanding the dissolution of metakaolin is of great importance and represents a first step toward developing efficient geopolymeric cements. Dissolution of metakaolin in alkaline solution was investigated using quantum mechanical semiempirical methods. Calculations were performed using the AM1 (Austin model 1), PM3 (Parametric method 3) and PM6 (Parametric method 6) semiempirical methods on a metakaolin structure proposed by Yunsheng and Wei [1].

The assumed metakaolin structure consists of a SiO₄ and AlO₄ tetrahedron (single six-membered rings). The semi-empirical calculations were performed on six-membered ring clusters. Heat of reaction calculations were performed in both local (aqueous) and alkaline (NaOH or KOH) environments. Heats of formation were obtained for reactants and products using the mentioned semiempirical methods, and the difference between products and reactants was used to compute (estimate) the heat of reaction. The ion paring reactions between broken six-membered SiO₄ and AlO₄ rings, anion and alkaline cation, are considered to elucidate the dissolution processes. The recently developed PM6 semi-empirical method was successfully used to study dissolution of ring clusters in both environments.

Thus estimated heat of reactions in the local environment suggested that the AlO₄ ring is more easily dissolvable than the SiO₄ ring. Furthermore, the use of an alkaline environment is expected to accelerate the dissolution process with high exothermic energy values. Finally, SiO₄ and AlO₄ ring clusters demonstrate contraction after dissolution in the highly alkaline solution, which indicates that defect structure of metakaolin will contract during dissolution. Such observations can be used to understand the dissolution process of metakaolin and are expected to forward the development of geopolymeric cements made from waste and by-product materials in the near future.