engleski

Calcium Aluminate Cement Mass Balance Model Based On CaAl2O4 Hydration

During the cement setting and hardening, microstructure of material and amounts of individual phases (hydraulic cement minerals, water and formed hydrates) are changing continuously. Knowledge of the hydration reactions and microstructure evolution is important from the viewpoint of safe usage, good durability and functionality. Hydraulic hardening of calcium aluminate cement (CAC) is primarily due to the hydration of CA (cement notation: C=CaO, A=Al2O3, S=SiO2, H=H2O), but other compounds may also participate in the hardening process especially in long term strength development. The most hydraulic phases are CA and C12A7, while C2AS, C2S and ferrite phase (C4AF) are considered to have no significant reactivity at early ages (first 48 hours). C12A7 is present in much smaller quantities (~2-5%) than CA (40-60%) and gives similar products as CA hydration, so its hydration can be treated together with CA hydration (CA quantity increased by C12A7). As typically 70% - 90% of the heat evolved is liberated in the first 24 h, simplified thermodinamic (non-kinetic) modeling of early age hydration process based only on CA hydraulicity seems reasonable. The hydration of CAC is highly temperature dependent, yielding CAH10 as main products at temperatures less than 20 oC, C2AH8 and AH3 at about 30 oC and C3AH6 and AH3 at temperatures greater than 55 oC. CAH10 and C2AH8 are known to be metastable at ambient temperature and convert to the more stable C3AH6 and AH3 with consequent material porosity and permeability increase and loss of strength. The conversion is accelerated by temperature and moisture availability. In this work three main reactions schemes of the CA hydration at different temperatures are applied to predict the composition of cement paste. The model enables the quantification of volume fractions of the various hydration products, free water, unreacted cement and chemical shrinkage as a function of the degree of hydration. The model input variables are mass fraction of CA and C12A7 in cement, water to cement ratio, functions of individual reactions partition during the hydration, while stoichiometry of individual reactions, densities, and molar masses of reactants and products are known. Constant total volume of cement paste is assumed during hydration. The developed model, written in MATLAB® is capable to deal with simultaneous hydration reactions, both in saturated and in sealed conditions. Constant or variable individual reaction partition during the hydration can be also handled by the model. In this way a relatively simple and robust cement hydration model obtained is suitable for engineering purposes.

Calcium aluminate cement; hydration; modeling

nije evidentirano