Role of Cement Type on Carbonation Attack
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carbonation of two hydrated ordinary portland cements of alkali content 1.03% or 0.43% Na2O equivalent and hydrated calcium aluminate cement (0.1% Na2O equivalent) was studied in a semi-dynamic atmosphere of 100% CO2, and 65% relative humidity at 20 ± 1 °C, for a period of 100 days. The changes of the microstructure before and during the carbonation were characterized by x-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. The kinetics of the process was evaluated from the total CaCO3 content by means of thermogravimetric analysis. The changes of the mechanical flexural strength were also studied. The pore solution was collected and analyzed before and after different periods of time. The results were compared with those obtained under natural carbonation conditions. The results showed that the alkali content of cement does not influence the kinetics of the process when the carbonation is accelerated. In the case of natural carbonation, an induction period is produced in the ordinary portland cement of low alkali content and calcium aluminate cement. The carbonation rate of calcium aluminate cement is the slowest for accelerated and natural carbonation.
I. INTRODUCTION
There is a general consensus that ordinary portland cement (OPC) is more resistant to carbonation than calcium aluminate cement (CAC) if the different buffering effects of their hydrated phases and alkali contents are considered. In the absence of carbonation or any other attack, which causes a pH depletion, the pH of the pore solution of an OPC can reach values near 14.1 In the case of CAC, little data exist about the composition of its pore solution. According to previous works of Gaztan˜aga et al.,2,3 the pH of the paste pore solution of a Spanish Fondu CAC, one month after mixing, reached values between 12.3 and 12.7 depending on the water-to-cement ratio. When the mechanism of carbonation is dissolution, as in the case of OPC, during the attack, the solubility of portlandite maintains the pH of the pore solution at 12.5. In contrast, in the case of CAC, hexagonal CAH10 will dissolve first due to its high solubility product (Ks ⳱ 10−7.6) in comparison with the cubic C3AH6 (Ks ⳱ 10−22.3). In the former case, the pH of dissolution of hexagonal will be 11.35 and in the latter case 10.8. In other words, the dissolution of CAH10 caused by the neutralization of the pore solution will maintain the pH of the pore solution at a value of 11.35.4 Few comparative studies have been carried out on equivalent OPC and CAC samples. Smolczyk pointed out that the residual unreacted material is independent 1834
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J. Mater. Res., Vol. 17, No. 7, Jul 2002 Downloaded: 14 Mar 2015
of the type of cement and the water-to-cement ratio, reaching values in the range of 25–30% of the total CaO content of the cement.5 Gaztan˜aga et al. studied the influence of the alkali content on the carbonation of two OPCs of different alkali content and CAC pastes (water/cement ⳱ 0.4).6 The carbonation was carried out after a cu
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