Research On Carbonation Characteristics Of High Volume Mineral Admixtures Concrete

With wide use of mineral admixtures, the carbonation of concrete containing high volume mineral admixtures is becoming an obviously limiting factor for the rational use of mineral admixtures in concrete. Is the anti-carbonation ability of high volume mineral admixtures concrete really not satisfactory? How are the carbonating characteristics of high volume mineral admixtures concrete? To answer those questions above, this paper conducted the following researches:1 The influence of the pH value and reaction temperature upon the soluble SiO₂ content in six different kinds of fly ash was studied by the Boiling Reflux method. The results show that the proportion of the soluble SiO₂ in fly ash suddenly increases when the alkalinity of solution and reaction temperature is appropriate, and the activity of fly ash obviously increases. Fly ash is no longer an admixture with low activity in high alkaline environment and its pozzolanic reaction would consume a lot of alkaline matter reserved in the concrete.2 The alternation of Ca（OH）₂ content of fifteen types of composite binder pastes was studied by Thermogravimetric method （TGA/DTG） at different humidly curing periods during 2 years. The results show that even in the binder paste with highest water-binder ratio and highest mineral admixture content, there is still more than 4% Ca（OH）₂ remained in the paste, which is enough to saturate the pore solution. Therefore, Ca（OH）₂ deficiency will not happen in the composite binder containing high volume mineral admixture. The study also discovers that pozzolanic reaction clearly happens in the paste with high volume admixtures and high water-cement ratio. Therefore, the adding proportion of mineral admixture and water-binder ratio should be restricted for the concrete with high anti-carbonation ability requirement.3 The carbonation process of 15 kinds of concrete after four different types of humidly curing periods in early stages exposing in natural indoor environment were studied during 3 years. Thermogravimetric method （TGA/DTG） and X-Ray Diffraction method （XRD） were applied to analyze the constitution of paste during carbonation. Isothermal adsorption and mercury intrusion porosimetry method （MIP） were used to analyze the pore structure of concrete during carbonation along the carbonating direction in concrete. The results show that the water-binder ratio and curing time in early age are the most important factors influencing the carbonation of concrete. There is a rapid carbonation period in the young-age concrete controlled by the CO₂ reaction rate. In later period, concrete becomes denser and its carbonation rate turns gradually to be controlled by the diffusion of CO₂. It is found by TGA that there is a great deal of Ca（OH）₂ remained in the carbonated concrete. The criterion of carbonation in early period is actually the wrapping rather than the exhausting of Ca（OH）₂ in cement paste.4 Further researches indicate when the temperature and the relative humidity in environment are suitable, the process of carbonation almost ceases and the unhydrated cement grains remained in carbonated area in concrete and pozzolanic particle would re-hydrate. For the concrete mixed with only pure Portland cement, the re-hydrate process in the carbonated area would result in a re-alkali process and the reduction of carbonation depth. For the concrete containing mineral admixtures, when the re-hydration rate is faster than the rate of pozzolanic reaction, the concrete would also be re-alkalied. Otherwise, the reserved alkali would be consumed and the carbonation depth of concrete would be intensified without the corrosion of CO₂ due to so called“self-neutralization”.