The influence of Metal Impurities on the Properties of Recycled Concrete

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1 The influence of Metal Impurities on the Properties of Recycled Concrete Wonjun Park 1,a, Takafumi Noguchi 2,b and Hironori Nagai 3,c 1 Dept. of Arch., Graduate School of Eng., The Univ. of Tokyo, Japan, Dept. of Arch., The Univ. of Tokyo, Japan, Dept. of Arch., The Univ. of Tokyo, Japan, a jooney@bme.arch.t.u-tokyo.ac.jp, b noguchi@bme.arch.t.u-tokyo.ac.jp, c nagai@bme.arch.t.u-tokyo.ac.jp ABSTRACT Recently, recycle of construction waste has become more important for the sustainable building material like recycled aggregate (RA) due to the disposal of waste matter and carbon dioxide (CO 2 ) emission. However, RA from demolished concrete treatment could bring about several problems due to the various impurities mixed in waste. Therefore it is significantly important to determine the influence of some impurities which could affect properties of recycled concrete using RA, because perfect removal of all impurities is impossible. This paper especially evaluated the properties of concrete containing two types of metal impurities, i.e. aluminum and iron. The effects of the metal impurities on the concrete properties were evaluated through various experiments. As the result, it was found that aluminum contained in RA would adversely affect both mechanical properties and durability even with very low content of impurity. Also this paper suggested the necessity of inspection method for risk reduction of the aluminium impurity in RA. KEYWORDS: Construction Waste, Metal Impurities, Recycled Aggregate, Recycled Concrete 1. INTRODUCTION RA from construction and demolition waste treatment could bring about several problems due to the various impurities mixed in waste, i.e. plastic piece, wood, wastepaper, tile, ceramics, brick, etc., when RA was used in concrete mixing. The earlier studies were based on the various impurities, and the upper limit of the each impurity content in RA has been enacted as Japanese industrial standard (JIS). But the investigation of impurities in construction waste reported that two types of metal impurities, i.e. aluminum and iron which are not specified in JIS A 521 were also found in concrete waste. (Kensuke et al. 6) Table 1 reports the limitation of the impurities contents in RA in JIS A 521. (Tokyo Metropolitan Government. 2 and 8) Table 1. The upper limit of the impurity content in RA (JIS A 521) Upper limit Upper limit Classification Contents of impurities Classification Contents of impurities (%) (%) tile, brick, ceramics A 2. D inorganic board.5 asphalt concrete lump B glass piece.5 E plastic piece.5 C gypsum wood, wastepaper.1 F plasterboard piece asphalt lump.1 The sum total of all impurities amount 3. Note :The upper limit is the mass ratio. 225

2 Therefore, this paper especially evaluated the properties of concrete containing two types of metal impurities, i.e. aluminium and iron because the effects of the metal impurities on concrete properties have not been evaluated experimentally. Also this paper suggests the necessity of inspection method for risk reduction of the metal impurities involved in RA. 2. METHODS 2.1 Metal Impurities Table 2 reports the metal impurities that were used in experiments of this research. And the experiment levels and specimen marks are shown in Table 3. The quantity of the metal impurity with coarse aggregate 1kg is shown in Table 6. Table 2. The types of impurities Materials Contents Metal Impurities Manufacturing method Flake of Steel Fe Used metals were cut and sieved after removing wire and metal fittings membrane and rust of metal surface using Flake of Aluminium Al sandpaper. aluminium chassis Table 3. Experiment levels and test specimens Impurity Mixing ratio Impurities size (%) 1 (mm) Specimen NON.1,.2,.3, 1.25~2.5 Al-T-( 2) Aluminium.4,.5, 2.5~5 Al-S-( 2).1,.25,.5,.75, 5~ Al-M-( 2) 1, 2, 4 ~ Al-L-( 2) 2.5~5 Fe-S-( 2) Steel.5, 1, 2, 4 5~ Fe-M-( 2) ~ Fe-L-( 2) Remarks 1:mass proportion with coarse aggregate 1kg 2:impurity content(%) 2.2 Mixing design Table 4 presents the concrete mixing design involving the metal impurities. And Table 5 reports the test methods and levels of this research. Table 4. Concrete mixing design f c Slump Air W/C G max S/a W unit Mix proportion(kg/m 3 ) (MPa) (cm) (%) (%) (mm) (%) (kg/m 3 Admixture ) C S G C.3% Table 5. Test methods Fresh concrete Hardening concrete Test level Specimen size (cm) Standard method Impurity Air contents - JIS A 1128 Aluminium Bulk density Compressive strength Elastic modulus Acceleration carbonation Curing condition Temperature: Humidity: RH6% - Steel: Φ JIS A 18 Steel temp. 4, 5%brine atomizing Aluminium Aluminium: temp. 4, steam atomizing 4 Aluminium 5% CO 2 density, Temp., RH6% 226

3 Table 6. Quantity of the metal impurity with coarse aggregate 1kg (upper: aluminum, lower: iron) Size(mm) Al-T( ) Al-S(2.5-5) Al-S(2.5-5) Al-M(5-) Al-L(-) Weight-Ratio(g-%).5g-.5%.5g-.5% 2.5g-.25% 5g-.5% 5g-.5% S(2.5-5) Fe-M(5-) Fe-L(-) Fe-S(2.5-5) Fe-M(5-) Fe-L(-) 5g-.5% g-1% g-1% g-2% g-2% g-2% 3. RESULTS AND DISCUSSION 3.1 Concrete properties with steel impurity 3.1.1Bulk density Figure 1 presents the bulk density of concrete with the steel impurity content. It was found that the higher is the content of steel impurity, the bigger is the bulk density of concrete Mechanical properties 2.2 Test of compressive strength was conducted to identify the influence of steel impurity on 2.1 mechanical properties of concrete. The test results of Compressive strength are show in Figure 2 and Figure 3 shows the results of elastic modulus. The results indicated that the higher is Figure 1. Bulk density the content of the steel impurity, the bigger are the compressive strength and elastic modulus. Compressive Strength(Mpa) NON Fe-S-.5 Fe-S-1 Fe-S-2 14Day Fe-S-4 Fe-M-.5 Figure 2. Compressive strength Fe-M-1 28Day Fe-M-2 Fe-M-4 Fe-L-.5 Fe-L-1 Fe-L-2 Fe-L-4 bulk density(t/m 3 ) Elastic Module(kMpa) NON NON Fe-S-.5 Fe-S-.5 Fe-S-1 Fe-S-1 Fe-S-2 Fe-S-2 Fe-S-4 Fe-S-4 Figure 5. Elastic modulus 14Day Fe-M-.5 14Day Fe-M-.5 Fe-M-1 Fe-M-1 28Day Fe-M-2 28Day Fe-M-2 Fe-M-4 Fe-M-4 Fe-L-.5 Fe-L-.5 Fe-L-1 Fe-L-1 Fe-L-2 Fe-L-2 Fe-L-4 Fe-L-4 227

4 3.2 Concrete properties with aluminium impurity Bulk density Table 7 reports the bulk density of concrete with the aluminium impurity content. Each value represents the average of experimental observations with three specimens. Concrete with aluminum impurity at 28 days showed lower bulk density. Table 7. Bulk density (unit: t/m 3 ) Mixture-1 Mixture-2 Mixture-3 Mixture-4 NON 2.36 NON 2.35 NON 2.32 NON 2.34 Al-S Al-L Al-T Al-T Al-T Al-M Al-S Al-L Al-T Al-T Al-T Al-M Al-S Al-L Al-T Al-S Al-T Al-M Al-S Al-L Al-S Al-S Al-T Al-M Al-M Al-S Al-S Al-L Al-M Al-S Al-S Al-L Al-M Al-S Al-L Al-M Al-S Al-L Mechanical properties Figure 4 presents the residual ratio of the compressive strength and the elastic modulus of concretes with impurity size and proportion. The results indicated that aluminum impurity had effect on reducing the compressive strength of concrete. Decreasing tendency of the compressive strength was confirmed all over the specimens. This is due to the high amount of hydrogen gases generated by aluminum in concrete which result in the reduction of the compressive strength. Particularly, the smaller the impurity size is and the larger content is, the obvious decreasing tendency of compressive strength was. 1.2 Residual Ratio (%) T( mm) S(2.5-5mm) M(5-mm) L(-mm) NON Impurity Contents (%) (a) Residual ratio of compressive strength Residaul Ratio (%) T( mm) S(2.5-5mm) M(5-mm) L(-mm) NON Impurity Contents (%) (b) Residual ratio of elastic modulus Figure 4. Residual ratio of mechanical properties of concrete 228

5 3.2.3 Chemical reaction of aluminum in concrete The reaction of aluminum with calcium hydroxide in aqueous solution to produce hydrogen has been already studied. Calcium hydroxide (Ca(OH) 2 ) is generated by hydrates in cement minerals, i.e. alite (3CaOSiO 2 ) and belite (2CaOSiO 2 ). The chemical reaction between aluminum alloy in RA and alkaline concrete also could be explained by mechanism of the generation of Hydrogen gas by chemical reaction Eq. (1) and Eq. (2). (Lluís et al. 7 and Tsuji. 7) xca(oh) 2 + yal + zh 2 O Calcium aluminate + 3H 2 (1) where calcium aluminate means 3CaO Al 2 O 3, 12CaO 7Al 2 O 3, CaO Al 2 O 3,CaO 2Al 2 O 3,CaO 6Al 2 O 3. Ca(OH) 2 +2Al + 2H 2 O CaO Al 2 O 3 + 3H 2 (2) Figure 5 shows the internal and external observation of specimens. For concrete containing aluminum impurity, hydrogen gas generated by aluminum alloy was observed around the aggregate and surface of specimens. The results show that the mechanical properties of concrete could be also affected by hydrogen gas generated by aluminum alloy. (a) around aluminium (b) around aggregate (c) hydrogen-layer (d) hydrogen-foam Figure 5. Internal and external observation of specimens 1.6 NON L(-mm) M(5-mm) S(2.5-5mm) T( mm) According to JIS A 531, expansion tests of mortar were also 1.2 conducted to identify the affection of.8 hydrogen gas generated by aluminum impurity. Figure 6 presents the results of mortar expansion. Although the results.4 showed an indefinite tendency with the aluminium impurity contents and its -.4 Impurity Content(%) size, the results indicated that Figure 6. Expansion rate of mortar aluminum impurity in RA could have effect on expansion of mortar, even with very low content of impurity Accelerated carbonation Expansion Ratio(%) 229

6 It is known that the process of concrete carbonation is actually a long-term reaction. Therefore, an accelerated carbonation test has to be used to carbonate them. The concrete specimens were exposed to an environment where three variables could be controlled, i.e. relative humidity at 6%, constant temperature, and 5% concentration of CO2. Figure 7 presents the carbonation depths according to the curing period, i.e. 1, 2, 3, 4 and 8 weeks using regression analysis. The results show that there is a strong relation between the contents of aluminum impurity and the carbonation depth. Carbonation Depth(cm) NON Al-T-.25 Al-T-.5 Al-T-.75 Al-S-.25 Sl-S-.5 Al-S Time(days) Figure 7. Accelerated carbonation depth 4. PROPOSAL OF INSPECTION METHOD FOR THE ALUMINIUM IMPURITY IN RA For suggestion as a risk evaluation of aluminium impurity in RA, water substitution method using aqueous Ca(OH) 2 solution was conducted as Table 8. Figure 8 presents the volume of hydrogen gas with the impurity size and its contents. The results indicated that it could be the inspection method to reduce the risk of aluminum impurity in RA. Table 8. Substitution method using aqueous Ca(OH) 2 solution Ca(OH) 2 solution Test method Remarks Ca(OH) 2 Water ph Ca(OH) 2 solution + Aluminium impurity ml + Contents (.1%,.5%,.3%).1%=1.g T-.3% T-.5% T-.1% (a) Impurity size: mm M-.3% M-.5% M-.1% S-.3% S-.5% S-.1% (b) Impurity size: 2.5-5mm L-.3% L-.5% L-.1% (c) Impurity size: 5-mm (d) Impurity size: -mm Figure 8. Volume of hydrogen gas by water substitution method 5. CONCLUSION This paper examined the influence of metal impurities in RA on the properties of recycled concrete. The results indicated that the higher is the content of steel impurity, the bigger are the compressive strength and elastic modulus. In addition, the aluminum impurity contained in RA would adversely affect not only mechanical properties but also durability even with very low content less than.1%. Finally, this paper suggests the necessity of inspection method for risk reduction of the aluminum impurities in RA such as a water substitution method which could be adopted at construction field or RA production. 23

7 REFERENCES Bureau of Urban Development Tokyo Metropolitan Government, 2. Construction Waste Recycling Law. Tokyo Metropolitan Government. Tokyo Metropolitan Government, 8. Plan of the Construction Waste Recycling Law. Kensuke, K., Rie, M., Hideki, T., Takashi, M. and Takashi, I., 6. Investigation on properties and influences of impurities in construction waste. Architecture Institution of Japan, No.6, pp.66. Lluís S., Jorge M., Maria M., and Juan C., 7. Synergistic hydrogen generation from aluminium, aluminium alloys and sodium borohydride in aqueous solutions. International Journal of Hydrogen Energy, Vol.32, pp Tsuji, Y., 7. Recycle concrete JIS product. Japanese Standards Association, pp