THE LEACHING EFFECT OF CONCRETE IMMERSED IN AMMONIUM NITRATE SOLUTION

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1 THE LEACHING EFFECT OF CONCRETE IMMERSED IN AMMONIUM NITRATE SOLUTION U. Schneider and S.-W. Chen Institute of Building Construction and Technology, Vienna University of Technology, Austria Abstract This paper deals with of the leaching process of cement based materials and the effect on the mechanical properties. The process induces mainly a total leaching of Ca(OH) 2 and a progressive decalcification of hydrate products of the hardened cement paste. Ammonium nitrate solution is very aggressive on concrete due to the rapid leaching. In the present work the behavior of ordinary concrete and high strength concrete under attack of ammonium nitrate solutions was studied over a period of more than 1 years. The concrete specimens were immersed in a variety of concentrations including 1%, 5% and 1% (NH 4 ) 2 NO 3 solutions. Phenolphthalein was used as chemical agent for measuring the leaching depth of the specimen. The Ca 2+ -concentration, the leaching depth (penetration depth), the porosity, the specific density, the mass changes and the strength of the concrete specimens were measured. The influence of concentration of the leaching process and the initial strength of OPC and HPC was studied and discussed. 1. INTRODUCTION Concrete constructions are used in a wide variety of applications. If concrete is exposed to aggressive salt solution, its microstructure can be attacked by a variety of chemical reactions. For example, rainwater, groundwater and seawater can degrade the concrete by dissolving soluble constituents such as calcium hydroxide. The concrete must withstand the attacks of solutions to which it is exposed. Predicting the service life of concrete under such conditions is thus a subject of considerable interest, and was discussed in a recent review. Empirical and physical models of degradation in service life many concrete constructions are simultaneously subjected to attacks by a combination of chemical corrosion and mechanical stresses. The investigation of simultaneous effects of chemical attacks and mechanical stresses was initiated and originally studied by Schneider and co-workers in 1984 [2, 3]. The aim of the research was to develop an empirical and physical model describing the degradation of concrete under simultaneous mechanical and chemical stresses. It was reported that several media, such as ammonium and sodium sulfate, sodium nitrate, magnesium sulfate and sodium chloride tended to cause stress corrosion, if a low bending load (2 to 5% of load of failure) is applied to concrete specimen being immersed in aggressive solution. It is well known that ammonium nitrate solutions act corrosive on cementitious materials, which leads to a leaching process according to the following gross reaction: 2 NH 4 NO 3 + Ca (OH) 2 Ca(NO 3 ) NH H 2 O (1) 54

2 The reaction products are calcium nitrate and ammonia, both of which are easily dissolved in water. Furthermore, the dissolution of calcium hydroxide in the ammonium nitrate solution is higher than that in pure water. The leaching effect results in a decalcification and dissolution of other products of the hardened cement paste and leads to a reduction of the phvalue of the solution. Consequently, steel reinforcement corrosion may occur after the nitrate solution has penetrated the concrete cover reaching the steel cover. The experimental work indicated that the deterioration and damage of the cement-based material is being intensified and accelerated, when the material suffers under a leaching attack superimposed by mechanical bending tensile stresses. For our work the cements CEM II 32.5 und 42.5, natural siliceous sand and gravel were used. As additive for the high performance concrete a super plasticizer based on melamine sulphonate, a retarder based on lignosulfonate and silica fume were used. 2. TEST METHOD In the present work, ammonium nitrate solutions were chosen as leaching medium. The research program was designed with varying types of cement-based materials (hardened cement paste, ordinary concrete and high strength concrete), different concentrations of solutions and different bending loads were used to investigate the effect of stress corrosion (Fig. 1). The loaded direction of the immersed specimens is the same as the loading direction of the specimens to bending and compressive test. Prisms, 4x4x16 mm, were used as test specimens in this study. The concrete mixtures were designed to have compressive strengths of 4 MPa (ordinary concrete) and 8 MPa (HPC without silica fume), 95 MPa (HPC with silica fume). After 24 hours of concrete placement, the moulds were stripped. The specimens were cured under water for 28 days until testing. At the age of 28 days the bending and compressive strengths were determined and used as reference strengths for the concrete. Thereafter, the test specimens were immersed into the aggressive solutions and simultaneously subjected to different bending loads with load levels of 3%, 4% and 5% of their initial bending strengths using a special 4-point-bending load system (Fig. 1). The concentrations of the ammonium nitrate solutions applied varied between 1% ([NH + 4 ]=225 mg/l, [NO 3 ]=775 mg/l), 5% ([NH + 4 ]=1125 mg/l, [NO - 3 ]=3875 mg/l), 1% ([NH + 4 ]=225 mg/l, [NO - 3 ]=775 mg/l) and.1% ([NH + 4 ]=1125 mg/l, [NO - 3 ]=3875 mg/l), respectively. The concentrations were kept constant by regular concentration measurements and exchange of the solutions. level arm compression bar agitator load dispatcher prism specimen exchangeable loading weight solution support platen container ground stopper Figure 1: Loading apparatus for stress corrosion tests 55

3 3. RESULTS AND DISCUSSION 3.1 Leaching effects of ammonium nitrate solution on cement-based materials Leaching effect decalcification Leaching of concrete means that the ammonium nitrate neutralizes the hardened cement paste and dissolves the calcium hydroxide and other hydration products in the concrete. As cementitious materials are based on calcium and silica oxides, its hydration products consist mainly of calcium-hydrate, Ca-Al-hydrate and Ca-Si-hydrate. The leaching effect is essential a decalcifying effect, i.e. Ca 2+ - ion are dissolved and NO ions penetrate into concrete XRF EDX analysis Concrete in ammonium nitrate solution is subjected to a leaching attack, which leads to a dissolution of the calcium hydroxide in cement-based materials. In this analysis the specimens of HPC (C95) were immersed separately into water saturated with Ca(OH) 2 and solution with 1%NH 4 NO 3 for 56 days. Afterwards the specimen were analyzed with Energy dispersive X- ray spectroscopy (EDX) in order to study the distribution of the iron Ca 2+ from the surface into the pore system of the specimen. The results are shown in Fig cpm - counts per minute HPC C95 in 1%(NH4)2NO3 after 56 days distance from the surface (mm) Figure 2: Ca 2+ distribution of a HPC specimen C95 immersed in 1%(NH 4 ) 2 NO 3 solution after 56 days The results show: The Ca 2+ quantity of the specimen in ammonium nitrate solution is apparently increased with the increasing of distance from the surface. The Ca 2+ concentration in the surface of the specimen is nearly 5% lower than the in 5 mm depth Color changes of the specimen due to leaching effect of the ammonium nitrate The decalcification results in a change of color of the specimen immersed for a long time in an ammonium nitrate solution, the color changes from dark grey to light grey. Fig 3 shows the specimen of OPC C4 immersed in a.1% NH 4 NO 3 solution for 325 days (more than 8 years) (light grey). 56

4 Figure 3: The color changes of concrete after 8 years corrosion in a.1% ammonium nitrate solution (light grey) Penetration depth of the ammonium nitrate solution The decalcification results in a neutralization reaction of the specimen. This implies that the ph-value decreases significantly in the leached part of the specimens. NH 4 NO 3 = NH NO 3 - dissociation (2) NH OH - = NH 3 + H 2 O neutralization (3) In our work the penetration depths of the fractured sections were measured by means of phenolphthalein solution after the mechanical tests. Fig. 4 shows the sections of the prisms of the HPC C95 after being sprayed with phenolphthalein solution. The red area in the centers of the specimen shows the material sections with a ph-value above 8.2 (Phenolphthalein solution 1%, in ethanol, indicator ph 8,2-9,8), these sections were defined as unaffected. Figure 4: Sections of the specimens of the HPC C95 in the NH 4 NO 3 solutions with different concentrations after 182 days (top: c=1%; below: c=1%) 57

5 depth of penetration, mm 12 C6 in 1%NH4NO3 1 C8 in 1%NH4NO3 C95 in 1%NH4NO3 8 C8 in 1%NH4NO3 6 C95 in 1%NH4NO3 C6 in 1%NH4NO time of immersion, days Figure 5: Depth of penetration of the OPC and HPC specimens immersed in 1% and 1% NH 4 NO 3 solutions up to 495 days The results show that a higher concentration of the solutions leads to a deeper penetration after a defined period of immersion. The increase of the concentration of the ammonium nitrate solution from 1% to 1% leads to penetration depths of the HPC C95 from 2.36 to 7.5 mm within 182 days (Fig. 5). According to the latest results the depth of penetration of the OPC C4 immersed in.1%nh 4 NO 3 solution after 325 days the penetration depth was mm. For HPC C8 in.1%nh 4 NO 3 after 376 days the penetration depth is 6.1 mm, but for the HPC C95 in.1%nh 4 NO 3 after 299 days the penetration depth is 5.5 mm. The experimental results show that the relation between the depth of penetration and the time of immersion can be described by a simple root law: D(t) = α (type of concrete, concentration of solutions) t 1/ (4) with D(t) Depth of penetration, mm α : Coefficient, mm/days.5 t : Time of immersion, days; 28 t 36 On the basis of the experimental results the coefficient α are.58 for C8 and.5 for C95, which immersed in 1%NH 4 NO 3 solution and.19 for C95 immersed in 1%NH 4 NO 3 solution. The derided empirical formulae and the obtained results are shown in Fig. 6. There is a little difference between the calculated and measured values. The results show that the higher the concentration of solution is, the higher is the α-value and the lower the strength of concrete is the lower the α-value. 58

6 14 depth of penetration C(t), mm C8; 1% α=,58 C95; 1% α=,5 C95; 1% α=,19 C(t)=,58t 1/2 Sy=1,3 mm C(t)=,5t 1/2 Sy=,32 mm 2 C(t)=,19t 1/2 Sy=,34 mm time of immersion t, days Figure 6: Depth of penetration of the HPC C8 and C95 specimens immersed in 1% and 1% NH 4 NO 3 solutions, comparison of formulae with the obtained results The leaching effect dissolution Mass loss of the concrete specimens immersed in ammonium nitrate solutions Leaching effect neutralizes the hardened cement paste and dissolves the hydration products of the concrete. This effect leads to a loss of mass of the concrete specimens. Table 1 shows the mass loss of the concretes immersed in different ammonium nitrate solutions. For example, the mass loss with HPC C95 in NH 4 NO 3 -solutions for 18 days was determined: 1.6% in 1%NH 4 NO 3 solution, 3% in 5%NH 4 NO 3 solution and 4.4% in 1%NH 4 NO 3 solution respectively (Table 1). The mass of the specimens in.1%nh 4 NO 3 solution for one more year indicates no clear change, but after more than 8 years (299 days) the mass loss reached about 2%. In comparison it was measured that the mass loss of the OPC C4 immersed in into.1%nh 4 NO 3 solution for 325 days (ca. 9 years) reached about 4%, however for the HPC C8 after 1 year (367 days) is about 3% (Table 1). Table 1: Mass loss of the concrete specimen immersed in NH 4 NO 3 solutions with different concentrations (in percent by weight) day C4 C8 C95 1% 5% 1%.1% 1% 5% 1%.1% 1% 5% 1% 28 -, , , , ,

7 The negative values mean the increase of weight, i.e. the immersed pressmen absorb water and salt from solution Loss of density of the specimens immersed in ammonium nitrate solutions The specific density of the leached part (not red part shown in Fig.4) and no leaches part (the red part in the Fig. 4) were determined. The results show the specific density of the leached part is lower than the not leached part (Table 2). Table 2: specific density of the specimen immersed in (NH 4 ) 2 NO 3 solutions Specimen Solution Immersion (days) Surface layer (g/cm³) Inside (g/cm³) HCP CEM II 32,5 (W/C=,4) 1%(NH 4 ) 2 NO ,37 2,44 OPC C4,1%(NH 4 ) 2 NO ,5 2,53 HPC C95,1%(NH 4 ) 2 NO ,51 2, The porosity of the hardened cement paste in NH 4 NO 3 - solution 6 5 Porosity, % Water 1%NH 4 NO 3 5%NH 4 NO 3 1%NH 4 NO 3 Figure 7: Porosity of the hardened cement paste immersed in ammonium nitrate solution with different concentration after 28 days of storage The porosity of the sample of the hardened cement paste with w/c=.3 (CEM II 42,5) was measured by mercury porosimetry (PASCAL 14 44). The samples were with the size of about.,5x,5x1, cm were immersed in water and ammonium nitrate solutions for 28 days, thereafter dried at 15 C for 24 hours and tested by Hg-porosimetry. The test results show, that the porosity of the hardened cement paste immersed in ammonium nitrate solution for 28 days is higher compared the storage under water (p=17.96%), i.e. storage in 1%NH 4 NO 3 the porosity reached p=2.19%, which is an increase about 12%, for storage in 5%NH 4 NO 3, a porosity of p=38.9, was measured indicating 1% increase; storage in 1%NH 4 NO 3 gave p=49.7%, nearly 2% higher as water storage. The penetration of nitrate solution has significant effect on the pore structure of the hardened cement paste (Fig. 7). 6

8 Bending strength reduction of the concrete in NH 4 NO 3 - solution Relative bending strength, ß/ß 1,2 1,8,6,4 c=1%; L=,2 c= 5%; L= c=.1%; L= Time of immersion, days Figure 8: Bending strength reduction of the OPC C4 immersed in ammonium nitrate solution with different NH 4 NO 3 concentrations Relative bending strength, ß/ß 1,2 1,8,6,4 c=.1% c=1%,2 c=5% c=1% Time of immersion, days Figure 9: Bending strength reduction of the HPC C8 immersed in ammonium nitrate solution with different NH 4 NO 3 concentrations The experimental results of the relative bending strengths of the OPC C4, HPC C8 and C95 immersed into the NH 4 NO 3 -solutions with different concentrations are shown from Figs The results indicate a significant influence of concentrations on the time dependence of the mechanical properties of concretes. The storage time of the strength reduction by about 2% is ca. 2 days for C8 in 1%NH 4 NO 3 and ca.182 days for C8 in 1%NH 4 NO 3 (Fig. 9). The relative bending strength of the C4 in.1% NH 4 NO 3 solution for 325 days are.48 (Fig. 8). But relative bending strength of HPC C8 in the same solution for 367 days are.58 (Fig. 9). 61

9 Relative bending strength, ß/ß 1,2 1,8,6,4 c=.1% c=1%,2 c=5% c=1% Time of Time Immersion, of Immersion, days days Figure 1: Bending strength reduction of the HPC C95 immersed in ammonium nitrate solution with different concentrations High strength of concrete results generally from high compactness, i.e. a dense concrete is less sensitive to leaching. The time for a strength reduction by about 5% is ca. 56 days for C4 (Fig. 8) and ca. 98 days for C95 immersed in 1%NH 4 NO 3 solution (Fig. 1). That means the resistance of HPC is about two times higher than those of OPC. The bending stresses for HPC were about 46 N/mm² and for OPC 2 N/mm², i.e. the residual strength of HPC is two times higher than that of OPC. 3.2 Stress corrosion of concrete - combination of external stresses and leaching on concrete 1,2 Relative bending strength, ß/ß28 1,8,6,4,2 c=,1% ; L= c=5% ; L= c=5% ; L=3% c=1% ; L= c=1% ; L=3% c=1% ; L=4% c=1% ; L=5% time of immersion, days Figure 11: Relative strength of HPC C95 immersed in NH 4 NO 3 solutions with different concentrations and load levels 62

10 The stress corrosion is a special corrosion effect which occurs in many materials. It occurs if a combination of a corrosive environment and mechanical stresses is simultaneously applied. The strength of the specimens of HPC C95 immersed in the.1% NH 4 NO 3 solution increased slightly for 5 to 6 weeks, thereafter the strength decreases little by little, after 299 days the strength reduction was about 3%, by contrast to that, the strength of the HPC C95 immersed in the 1%NH 4 NO 3 solution starts to decrease clearly immediately after immersion. After 28 days of immersion the strength of the HPC C95 was reduced by about 1%. After 5 months immersion the strength measured was only half of its initial strength (Fig. 11). An external load applied to specimens being immersed in a 1%NH 4 NO 3 solution, accelerated significantly the rate of deterioration i.e. the chemical attack increased the loss strength significantly. The attack is called stress corrosion which was discovered by Joffrey (1972) and is known to occur in many materials like metal (steel), glass, ceramic and salt. The HPC C95 in the 1%NH 4 NO 3 solution under a load level of 5% lost 4% of its strength in only 5 days, whereas about 12 days were required for the same loss of strength of unloaded specimens (Fig. 1). It is seen from the tests that stress corrosion is much more destructive than the pure chemical corrosion on cement-based materials [2-4]. The experimental results indicate that the simultaneous effect of external stress and leaching on mechanical behavior clearly depends on the strength of concrete, the type and concentration of leaching media and the load level of the external stress and the level of concentration of the aggressive solution (Fig. 11).[3, 4]. Relative bending strength ß(t)/ß 1,2 1,8,6,4,2 HPC C95 in 1%NH 4 NO 3 L=5% f(t) = t 1/3 -,3t Sy =.42 L= f(t) = t 1/2 +.2 t 2/3 Sy =.49 L=; test result L=, Eq (5) L=3%; test result L=3%; Eq.(6) L=5%;test result L=5%; Eq.(6) L=3% f(t) = t 1/3 -.32t Sy = Time of immersion, t days Figure 12: Relative strength of HPC C95 immersed in 1%NH 4 NO 3 solution with different load levels, comparison of formulae with the obtained results The experimental results indicate that the simultaneous effect of external stress and leaching on mechanical behavior clearly depends on the strength of concrete, the type and concentration of leaching media and the load level of the external stress and the level of concentration of the aggressive solution (Fig. 11).[3, 4]. The experimental results show that the relation between the relative bending strength and the time of immersion can be described by a formula [5]: 63

11 f(t i ) = ß i /ß 28 = f(type of concrete, concentration of solutions, load level) t n (5) For chemical corrosion (L=): f (t) = a 1 + a 2 t 1/3 + a 3 t 2/3 ( t 365) (6) For stress corrosion (with external load): f (t) = b 1 - b 2 t 1/3 + b 3 t ( t (98-182) (7) with f(t) Relative bending strength a, b: Coefficient t : Time of immersion, days The calculated values from the formulae (6) and (7) for HPC C95 immersed in 1%NH4NO3 solution under different load levels are shown in the Fig. 12 in comparison with the test values. In all cases the residual standard error (Sy) are less than.5. 4 CONCLUDING REMARKS Ammonium nitrate solutions are aggressive mediums for cementitious materials, which lead to a leaching processes and strength losses starting from the specimen s surface. The leaching process leads to a decalcification and the dissolution of the hydration products in the concrete, so that the Ca 2+ distribution and the ph value in leaching area decrease, the weight, density and strength of the concrete reduce. The leaching process due to ammonium nitrate depends on the concentration of the solution, the concrete strength, the duration of immersion. The simultaneous effect of an external stress during exposure in the aggressive medium increases and accelerates the deterioration and enhances an early failure of the concrete. The effect of stress corrosion on concrete immersed into ammonium nitrate solution depends on the concentration of the solution, the strength of the concrete and the load levels, i.e. on the applied external stresses. A high strength of concrete and a lower load level and a low concentration of the aggressive medium lead to a higher resistance against stress corrosion and enhance the service life of the concrete. REFERENCES [1] A.R.C. Westwood, J. Mats. Sci. 9 (1974), [2] U.Schneider and E. Nägele, The Influence of Mechanical Stresses on the Corrosion of Cementeous Materials, Int. Conf. on Fract. Mech. of Concrete, Lausanne, 1985 [3] U. Schneider, E. Nägele, F. Dumat, Stress Corrosion Initiated Cracking of Concrete. Cement Concrete Research 6, (1986) [4] U. Schneider, S.-W. Chen, Deterioration of high - performance concrete subjected to attack by the combination of ammonium nitrate solution and flexure stress. Cement and Concrete Research, Volume 35, Issue 9, September 25, pp [5] S.-W. Chen, Untersuchungen zur Chemischen Korrosion und Spannungskorrosion von Hochleistungsbeton. Dissertation, TU Wien, Oct