Effect of Sodium Silicate Properties in Alkali-Activation of Mexican Blast Furnace Slag
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Effect of Sodium Silicate Properties in Alkali-Activation of Mexican Blast Furnace Slag O. F. Cortés-Salmerón1, M. L. García-Chávez1, T. A. García-Mejía1 1 Facultad de Química, Universidad Nacional Autónoma de México, Avenida Universidad No. 3000 Colonia Universidad Nacional Autónoma de México, 04510, Distrito Federal, México
ABSTRACT The present work is a study on alkali activation of Mexican blast furnace slag, using sodium silicate. The aim is to produce an optimal specimen, homogeneous without carbonation, and with small fraction of crystalline phases, similar to CSH, which provide mechanical properties suitable to use in the construction industry. The samples were prepared using sodium silicate activator solutions with modulus (SiO2/Na2O) of 1.25, 1.5, and 1.75. The weight percentage of Na2O in the activator solutions was added at 4, 6 and 8% relative to the slag weight. The prepared samples were stored in sealed molds, at room temperature (20°C), during 7 days. The X-ray diffraction has revealed the presence of an amorphous phase, semi crystalline clinotobermorite phase and signals of calcium carbonate for the samples of 4 and 6 % of Na2O; in contrast with the 8% Na2O, where the latter signals almost disappeared. The specimen selected as optimal was prepared with an activator concentration of 8% of Na2O /Slag, and SiO2/Na2O of 1.25. A specimen under these optimal conditions was prepared with accelerated curing (40°C, humidity, 48 hours), and a compressive strength test was attained, with an average value of 52 MPa at 3 days.
INTRODUCTION Alkali activated slags (AAS) have been studied as an alternative for Portland cement since the 1940’s1. Due to their calcium content, the main binding phase in this compounds is a silicate hydrate gel with low calcium content, and phases similar to tobermorite 14Å and 11Å, formed by layers of calcium oxide chains and dreierkette type silicon oxide chains2. These silicon oxide chains can also be substituted with tetrahedral Al atoms, which opens the possibility of cross-linking between layers3 (Figure 1). Other phases that may be formed are CA-S-H gel with alkali cation substitution ((C,N)-A-S-H) and N-A-S-H gel. The latter, in the presence of calcium, is converted into C-A-S-H gel at high pH, and is not stable at low pH4,5. Conditions for the activation, including activator nature and concentration, slag grain size, and curing regime, have been studied seeking to attain high compressive strength values6–8. However, these compounds are more susceptible to carbonation than Portland cement, probably due to the fact that the alkaline hydroxides present in the pore solution tend to absorb CO2 from the air and gas streams to form carbonates9. Studies on carbonation have established that after activating blast furnace slag with sodium silicate, the calcium silicate hydrate gel formed during the activation is decalcified because of carbonation, reducing the cohesion in the matrix, resulting in a loss of mechanical strength10,11. Therefore, in these systems is important study the beh
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