Effect of injection of NaOH solution on pozzolanic and hydration reactions in low-calcium fly ash cement paste

Abstract

Alkali activation at normal temperature by an injection of NaOH solution into fly ash cement paste was investigated. It was reported that the injection of NaOH solution from the age of 1 month accelerated the pozzolanic reaction in cement paste with 40% replacement with fly ash. However, this injection was not effective in accelerating the pozzolanic reaction in cement paste with 20% replacement with fly ash at the age of 2 months. Moreover, it affected the production of Ca(OH)2 adversely in the cement paste in the short term after injection. The present study aims at investigating the effect of injection of NaOH solution from the age of 6 months on reactions in cement paste with 0% (FA0), 20% (FA20) and 40% (FA40) replacement with fly ash so that the cement hydration and pozzolanic reaction should be accelerated more effectively from directly after the injection. In addition to NaOH solution at a concentration of 0.1mol/L, water or no solution was also injected into the paste through an installed syringe for comparison. As a result, the injection of solution increased the Ca(OH)2 content in FA0, whereas it reduced the Ca(OH)2 content in FA20 and FA40 at the ages of 8 and 10 months. The injection of NaOH solution or water from the age of 6 months increased the consumption of Ca(OH)2 by the pozzolanic reaction in FA20 and FA40 more than that of no solution. According to the porosity, the volume ratios of pores ranging 20-330 nm in diameter to the total pore of FA20 and FA40 activated by water or NaOH solution were also reduced. In conclusion, the injection of NaOH solution from the age of 6 months was effective in promoting the cement hydration in FA0 and accelerating the pozzolanic reaction in FA20 and FA40.