Abstract
The calcium leaching behavior of cement paste and silica fume modified calcium hydroxide paste, exposed to hydrochloric acid solution, is reported in this paper. The kinetic of degradation was assessed by the changes of pH of hydrochloric acid solution with time. The changes of compressive strength of specimens in hydrochloric acid with time were tested. Hydration products of leached specimens were also analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric (TG), and atomic force microscope (AFM). Tests results show that there is a dynamic equilibrium in the supply and consumption of calcium hydroxide in hydrochloric acid solution, which govern the stability of hydration products such as calcium silicate hydrate (C-S-H). The decrease of compressive strength indicates that C-S-H are decomposed due to the lower concentration of calcium hydroxide in the pore solution than the equilibrium concentration of the hydration products. Furthermore, the hydration of unhydrated clinker delayed the decomposition of C-S-H in hydrochloric acid solution due to the increase of calcium hydroxide in pore solution of cementitious materials.
Highlights
Calcium leaching, which is a degradation mechanism consisting in a progressive dissolution of the cement hydrates as a consequence of the migration of the calcium ions to the pore solution, may severely damage structures like dams, nuclear waste containment structures, pipes, water storage tanks, and tunnels
This indicate that the concentration of calcium hydroxide in the pore solution is lower than the equilibrium concentration, resulting the decomposition of hydration products such as calcium silicate hydrate (C-S-H) and C-A-H
And 7, it can be seen that the bound water content and calcium hydroxide content of the sample in the hydrochloric acid solution are lower than that of control sample at 10 days
Summary
Calcium leaching, which is a degradation mechanism consisting in a progressive dissolution of the cement hydrates as a consequence of the migration of the calcium ions to the pore solution, may severely damage structures like dams, nuclear waste containment structures, pipes, water storage tanks, and tunnels. The calcium leaching decreases with increasing silica fume content[4], which refine the pore structure through pozzolanic reaction, provide resistance to the leaching process from porosity generation[9,16]. Tests results showed that hydrochloric acid attacks the clinker materials of Portland cement forming some soluble salts, mostly with calcium, which are subsequently leached out. This way the porosity of cement mortar is increased. It is showed that the hydrochloric acid resistance of a mortar is related best to the cement content and the formation of layers of ferric hydroxide possibly can influence the leaching processes. The object of this paper is to investigate the extents of chemical degradation by the action of hydrochloric acid on cement paste and silica fume modified calcium hydroxide paste (SF/CH system)
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