Effect of the ITZ on the Leaching of Calcium Hydroxide from Mortar
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457 Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials Research Society
10.6/1.2 mm 170% - 881 kg/m3
(gravel (silica)
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gravel (silica) 1.2/2.5 mm 30% - 377 kg/m3 Normal Portland Cement 629 kg/m 3 315 kg/m3 water Table 1 : Mortar composition
T
Chemical composition of cement SiO 2 A120 3 Fe2 0 3 CaO MgO S03 Na2 0 20.2 4.9 3 63.4 0.67 3.2 0.25 Table 2: Chemical composition
The mixtures were poured into cylindrical moulds (11x22 cm). The demoulding was done after 24 hours, then the samples were cured for 27 days immersed in water saturated in lime at 20"C (t VC). Samples Because of the slow kinetics of leaching, we have been obliged to work with samples of small sizes. The samples used are cylinders whose diameters are 10,12 and 14 mm with a ratio h/41 = 2 (h height of the sample). After curing, the samples are extracted from the test pieces by means of a diamond tipped core lubricated with water. For each sample dimension, two series of samples have been made, the first one which has been immersed in the aggressive solution (treated series), the other one which has been kept in an endogenous environment (control series). Leachin2 process The aggressive environment used is a solution of ammonium nitrate NH 4NO 3 (concentration =50%.). This process induces mainly a total leaching of the lime and a progressive decalcification of CSH. The leaching process leads to an uniform degradation of the samples. The peripheral degraded zone delimited by the dissolution front of calcium hydroxide, is more porous than the bulk sample and also less resistant. The chemical attack of the ammonium nitrate leads to the development of a soluble calcium nitrate, a not very soluble nitro-aluminate of calcium and an ammoniac emanating NH 3 1. Because of this emanation, the solutions of ammonium nitrate work like diluted acids. The degradation is governed by a diffusion mechanism and can be described by the Fick's law relating the degraded thickness to the square root of time ZT. Figure 1 shows the linear variation of the degraded thickness, measured out with a microscope, and the loss of mass of the sample in relation to the square root of immersion time in the aggressive solution.
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Fig. 1: Variation of degraded thickness (la) and the loss of mass (1b) in relation with the square root of immersion time in the solution of NH4NO3 (50%) The variation of the thickness of this degraded zone is the same for the samples of mortars and the samples of cement. Mechanical tests Both the treated samples and the control samples have been subjected to a compressive load in order to measure their compressive resistances. The device used is a Hounsfield press with a maximal capacity of 5OkN. The displacement speed is controlled during the load. The force applied on the 458
sample and the longitud
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