COMPLETELY RECYCLABLE CONCRETE OF AGGREGATE- RECOVERY TYPE BY USING MICROWAVE HEATING TECHNOLOGY

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1 COMPLETELY RECYCLABLE CONCRETE OF AGGREGATE- RECOVERY TYPE BY USING MICROWAVE HEATING TECHNOLOGY Noguchi, T. (1), Kitagaki, R. (1), Nagai, H. (1) and Tsujino, M. (2) (1) The University of Tokyo, Japan (2) Shimizu Corporation, Japan ABSTRACT Concrete is the most widely consumed construction material, which is expected to serve as a wastebasket for industrial by-products but will generate a lot of waste in the future while currently most concrete waste is recycled as road bottoming and backfilling. Closed-loop recycling systems for concrete, therefore, will be one of the key issues in establishing a resource-recycling society. Several methods to retrieve aggregates from concrete with high quality have been eagerly developed in Japan. However, some methods produce approximately 10 times amount of CO2 emission to retrieve aggregates comparing with virgin aggregate production, and another cannot control the quality of the retrieved aggregates, which might cause the decrease of concrete performance. In this research, the new method is developed, which accomplishes low CO2 emission in retrieving process and low water absorption of the retrieved aggregate, as well as better mechanical performance of concrete than concrete with virgin aggregates owing to coating the surface of the aggregates with compounds of a dielectric material and pozzolan, which respectively contribute to being heated selectively and efficiently from the outside by microwave radiation so as to easily recover the original aggregate and reinforcing the bond strength between the aggregates and cement matrix. Keywords: Recycled aggregate, Mechanical properties, Microwave heating, Dielectric material, Silica fume, By-product powder 1. INTRODUCTION The total material input of Japan ranged from 1.8 to 2.2 billion tons annually in recent years [1], of which 0.8 to 1.1 billion tons were accumulated every year in the form of buildings and civil structures. The production of concrete, a primary construction material for forming modern nations, amounted to 300 to 500 million tons. In other words, concrete accounts for nearly 20% of Japan s total material input. Incidentally, the construction industry s consumption of steel and wood, two other primary construction materials, amounted to approximately 25 and 6 million tons, respectively, both of which are far less than concrete consumption. Furthermore, the amount of waste in Japan totaled approximately 579 million tons in 2005 (industrial waste: 422 million tons). Waste from construction accounted for 18% (76 million tons) of total industrial waste. Moreover, in 2005, construction waste accounted for nearly 25% (6 million tons) of the 24 million tons of industrial waste destined for final disposal sites. As concrete lumps account for approximately 42% (32 million tons) of total construction waste, approximately 6 percent of total waste in Japan therefore consists of 333

2 concrete lumps. As stated above, concrete accounts for large percentages of both resource input and waste discharge. The scarcity of the residual capacity of final disposal areas in Japan, which has become increasingly serious in recent years, also alerts us to formulate a resource-recycling society. With the aim of solving the construction waste problem, the Japanese Ministry of Land, Infrastructure and Transport (formerly the Ministry of Construction) formulated the Action Plan for Construction Byproducts (Recycling Plan 21) in 1994, the Promotion Plan for Construction Waste Recycling in 1997, the Construction Material Recycling Act and the Law on Promoting Green Purchasing in Thanks to such active and continual policies, construction waste discharge decreased, with the recycling ratio of concrete lumps exceeding 98%. Recycling of concrete lumps for road bottoming has particularly been promoted as a national policy. An enormous amount of demolished concrete lumps, however, will be generated in the near future from concrete structures mass-constructed during Japan s rapid economic growth era. Moreover, road construction is decreasing and the method of road repair is expected to shift from replacing to milling and applying an overlay. These trends will lead to an imbalance between the supply of demolished concrete and the demand for road bottoming. Also, the volumetric reduction of future infrastructures based on population estimation and the extension of the service life of the existing stock by increased succession, utilization, and conversion will keep on reducing the amount of new construction of structures and concrete production. Accordingly, this will culminate in the need to recycle aggregate into aggregate for concrete, and it is no exaggeration to say that these recycled materials can account for the greatest part of future aggregate for concrete. It is therefore vital to convert recycling from quantity-oriented to quality-oriented recycling. In other words, it is necessary to find optimum recycling methods with due consideration to the material balance, while promoting the production and supply of high-quality recycled aggregate. 2. BACKGROUNDS AND CONCEPT OF THE NEW TECHNOLOGY The uses for concrete lumps to be recycled are determined by the qualities of the recycled material, such as density and water absorption, which vary depending on the percentage of cement paste contained within or adhering to the surfaces of the original aggregate, and the quality of recycled aggregate depending on the production method. Figure 1 shows general methods for producing recycled road bottoming, recycled aggregate for leveling concrete (low-quality recycled aggregate), and recycled aggregate having qualities comparable to those of natural aggregate and used for structural concrete (high-quality recycled aggregate). Japan Industrial Standards were established for recycled aggregate for concrete to promote its utilization in 2005 through However, a dilemma in producing the recycled aggregate has discouraged its utilization; i.e. high quality recycled aggregate requires much energy and generates by-product powder [2], while low quality recycled aggregate results in decreased concrete performance [3]. 334

3 Demolished Concrete Lumps Jaw Crusher Impact Crusher Vibratory Sieves Heating Tower Vibratory Sieves Cone Crusher Coarse Aggregate Scrubber Road Subbase, Backfill Vibratory Sieves Fine Aggregate Scrubber Low Quality Recycled Coarse Aggregates Low Quality Recycled Fine Aggregates High Quality Recycled Coarse Aggregates Vibratory Sieves High Quality Recycled Fine Aggregates Powder (a) Road Subbase (b) Low Quality Recycled Aggregates (c) High Quality Recycled Aggregates Figure 1. Recycling process of concrete lumps Since a resource-recycling society needs to be established, development of technology to produce a high quality recycled aggregate with low energy and minimum waste is important. The objective of our study is to achieve full recycling of aggregate which would lead to reduction of concrete waste and contribute to radically improving the situation of the resource depletion problem. Noguchi et al. [4] proposed the aggregate recovery-type completely recyclable concrete technology aiming at low energy closed cycle. This technology is a proactive recycling technique that involves the coating of a surface modification agent such as repellent on the aggregate and will allow a recovery of high quality aggregate at low energy. The authors demonstrated its feasibility through a series of experiments [5]. However this technology is based on reduced bonding force between the aggregate and cement matrix, and therefore compatibility of performances that are in a trade-off relationship; mechanical properties and recycling performance was very difficult to ensure and new technologies have been sought. The proposed new technology consists of two techniques as shown in Figure 2; concrete strength enhancement technique and aggregate recovery technique. The former involves, differing from conventional techniques, surface modification to increase the bonding force between the coarse aggregate and the cement matrix by coating a binder evenly on the surface of the coarse aggregate. Silica fume was used as the binder expecting to enhance chemical bonding force due to pozzolanic reaction. The latter aims at recycling aggregate with low energy, which involves inclusion of dielectric material in the binder. When applied with microwave radiation, the dielectric material on the surface of the aggregate is heated and the interface between the aggregate and cement matrix is weakened locally and thus the separability of the aggregate and cement matrix is improved. This study aims at developing an aggregate recovery technology based on the above two techniques to ensure compatibility of the performances in a trade-off relationship; mechanical properties of the concrete and aggregate recycling performance. 335

4 2nd International RILEM Conference on Progress of Recycling in the Built Environment C o n crete stren g th en h a n cem en t tech n o lo g y Interface of aggregate and binder Interface of binder and mortar matrix To enhance chemical bonding Silica fume Mortar matrix Binder Aggregate A t u se E n h a n cem en t o f co n crete stren g th b y su rfa ce m o d ifica tio n A t u se D ielectric m a teria l m ixin g w ith b in d er M icro w a v e h ea tin g S elective h ea tin g Interface of aggregate and binder A fter m icro w a v e h ea tin g W ea ken in g o f a g g reg ate su rfa ce Interface of binder and mortar matrix Crush To enhance chemical bonding Silica fume Aggregate Mortar matrix Dielectric material coated modification Dielectric material binder Dielectric material C o n stru ctio n o f d ielectric m a teria l C ru sh in g p ro cessin g H ig h -q u a lity a g g reg a te w ith lo w en erg y A g g reg a te recyclin g tech n o lo g y Figure 2. Microwave-based aggregate recovering completely recyclable concrete technology 3. STRENGTH ENHANCEMENT BY MODIFYING AGGREGATE SURFACE 3.1 Experimental program In the experiment, crushed sandstone (surface-dry density: 2.66g/cm3, water absorption: 0.70%) was used as a coarse aggregate to apply the modification, low viscosity epoxy resin as a binder to contain bonding force enhancement material between the aggregate and the cement matrix, and silica fume and by-product powder as the bonding enhancement material to improve chemical bonding and mechanical strength by a pozzolanic reaction. The byproduct powder (absolute density: 2.35g/cm3, specific surface area: 1,877cm2/g) was generated at the intermediate treatment plant for road bottoming materials. The mixing ratios of the silica fume and the by-product powder are shown in Table 1. Each powder was applied to the binder after the binder was coated on the aggregate and before it hardened. The surfacedry density of the modified aggregate was in the range of g/cm3 and the water absorption less than 0.5%. The mix proportion of the concrete is shown in Table 2. The modified aggregate was used directly without surface-drying. The fine aggregate used was river sand (surface-dry density: 2.58g/cm3, water absorption: 2.21% and fineness modulus: 2.67). 336

5 Symbol Table 1. Test samples Weight ratio Silica fume By-product powder O 0 0 No N 0 0 Yes S Yes SP Yes SP Yes SP Yes P Yes Surface modification Table 2. Mix proportion of concrete Absolute volume (L/m3) W/C (%) Target air (%) s/a (%) Water Cement Fine aggregate Coarse aggregate Results and discussions Sections of hardened concrete are shown in Figure 3 for the normal and modified aggregates. The modified layers can be clearly seen on the surface of the aggregate which are about 1mm or less in thickness, generally suggesting even modification. Results of the compressive strength tests for specimens at the ages of 7 and 28 days and modulus of elasticity at the age of 28 days are shown in Figure 4. The strength of the specimen N was about half of that of the specimen O, which is assumed due to insufficient mechanical friction force because of the resin specific gloss surface of the aggregate as is observed in Figure 2 because no powder was attached to the surface. Comparing compressive strengths for specimens N and P at the age of 28 days, the strength was increased by a factor of two because of mechanical friction force at the surface of the aggregate in the specimen P, though unhydrated powder needs to be taken into account. It should be noted that the mechanical friction force allowed the specimen P to achieve almost the same level of strength as that of specimen O prepared using normal aggregates. Another noteworthy point are the cases of specimen SP80 and SP90, whose strengths are larger than the specimen N by a factor of about 2.5 or the specimen O by a factor of 1.2 or larger. This means that, assuming that the increase in the strength of the specimen P is only due to the increase in the mechanical friction force, the increase in the chemical bonding force due to pozzolanic reaction served the increase in the strength by 30%. This suggests that the combination of the increase in the mechanical friction force on the surface of the aggregate and increase in the chemical bonding force results in drastic increase in strength of concrete. The reason why there was no difference in the strength between specimens S and AP in which chemical bonding force can be expected and specimen P which contains by-product powder only is assumed as follows; the particle size of the silica fume was smaller than that of the by-product powder by a factor of 100, generating an insufficient fitting effect for generating mechanical friction force or a large quantity of non reactive silica fume when much silica fume is used. Thus powders with 337

6 a particle size of around 10μ such as by-product powders need to be used to increase the mechanical friction force and silica fume should be added to the by-product powder by 10-20% of the by-product powder to generate a chemical bonding force. The modulus of elasticity was smaller than that of the normal aggregate concrete by 10% due to the formation of the epoxy layer by applying this technology. This is assumed due to a smaller modulus of elasticity of epoxy resin than that of concrete by a factor of 10. Crushed sandstone (Normal aggregate): O Modified aggregate: SP90 Figure 3. Surface of normal and modified aggregates in the hardened concrete 60 7days 28days Young s m odulus at 28days (N /m m ) 2 Compressive strength Young's m odulus (kn /m m ) 2 0 O N S SP50 SP80 SP90 P 0 Figure 4. Compressive strength and modulus of elasticity 338

7 4. RECOVERY OF AGGREGATE UTILIZING MICROWAVE HEATING 4.1 Experimental program A study was made on the recoverability of aggregate from concrete using aggregate coated with not only silica fume and by-product powder but also dielectric material, ferrous oxide as shown in Figure 5. Preferential heating of dielectric material coated modification layer by irradiating microwave to the concrete causes decrease of the bonding force between the aggregate and the cement matrix and facilitate the recovery of aggregate accompanied with a small quantity of cement paste. If high quality aggregates could be obtained, trade-off relationship between mechanical properties of the concrete and recoverability of aggregates could be overcome, which could not be achieved with conventional technologies. It takes 60 seconds to heat the dielectric material coated aggregate to around 300ºC with microwave radiation at the power of 1,800W. Considering attenuation of the microwave within the concrete, the concrete was heated for 90 seconds with microwave radiation at the power of 1,800W. The specimens used were those cured for 28 days in water, with a dimension of φ50mm 100mm. Since an explosive fracture was observed during the heating in the preliminary test, the specimens were dried in air for several days and in the dryer at 40ºC for safety before conducting heating tests. After heated with microwave radiation, the specimens were roughly crushed with a jaw crusher, and subjected to a rubbing treatment with the Los Angeles Abrasion Machine to remove cement mortar. The rubbing medium used was 12 steel balls with average diameter of 46.8mm (total weight of about 5kg) and the number of rotation was 333. The recycled coarse aggregate with a size of 5mm or larger obtained through the above process was subjected to hydrochloric treatment to remove cement paste attached on the aggregate and finally original aggregates were recovered. Silica fume + By-product powder Dielectric material (FeO) Figure 5. Aggregate coated with silica fume, by-product powder and ferrous oxide 339

8 Table 3. Test conditions of coarse aggregate recovery Specimen Dielectric material Coated modification Microwave heating Electric oven heating O1 - - O2 No 2.45GHz, 1800W, 90 sec - O3-300ºC (60 min) SP SP80 Yes 2.45GHz, 1800W, 90 sec - SP GHz, 1800W, 90 sec - The aggregate recovery tests were conducted for the specimen O prepared using normal aggregate and the specimen SP80 that showed high strength. Test conditions are shown in Table 3. For the specimen prepared using normal aggregate, both microwave heating and electric oven heating were included in the test cases assuming the application of the heat rubbing treatment. 4.2 Results and discussions The recovery rate of the recycled coarse aggregate is shown in Figure 6. The specimen O1 (not heated) contains much cement paste in spite of undergoing rubbing treatment. With respect to the specimen O2 heated by microwave radiation and O3 heated in the electric oven, more cement paste is removed than that without heating but it is not so much compared to that of specimen undergone heat rubbing treatment as reported in the past papers [6]. This means that the rubbing machine used in this study could not provide sufficient grinding effect. For the specimen SP-1 that was not heated, the recovery rate was the same as that of the specimen O1 but there was a significant difference in the fraction of original aggregate. It is assumed that, in the specimen SP90-1, the original aggregate has not been separated from but attached with mortar firmly. No surface of original aggregates was observed by visual observation. This indicates that the surface modification enhanced the bonding between the surface of the aggregate and the cement matrix as previously described. Since it contains less cement paste than the specimen O1, recycled aggregate with better quality could be recovered by providing surface modification without utilizing microwave heating. Looking into the breakdown of the recycled coarse aggregate recovery rate of the specimens of SP80 and SP90-2 that were heated by microwave radiation, the recovery rate of the paste and fine aggregate were around 7%, indicating high recovery rate of original coarse aggregate and a high quality of the recycled aggregate. Visual observation also identified many original aggregate with little cement paste. That is, the microwave heating weakened the strength of the surface of the aggregate including the dielectric material coated modification layer, which facilitated the effective recovery of the original aggregate. Considering less performance of the rubbing machine used in this study, higher quality aggregate attached with less cement paste may be possible to be recovered in practice. From the above discussion, it can be concluded that the dielectric material coated modification followed by heating by microwave radiation could allow recovery of high quality original aggregate from concrete with the aggregate fraction of 90% or higher. 340

9 R ecovery rate of recycled coarse aggregate (%) O1 N o heating O riginalaggregate Fine aggregate Paste O2 M icrow ave heating O3 Electric oven heating SP90-1 N o heating SP80 M icrow ave heating Figure 6. Recovery rate of recycled coarse aggregate SP90-2 M icrow ave heating 5. EMISSION OF CO 2 DURING THE AGGREGATE RECYCLING Eenergy consumption when the microwave heating was applied to the aggregate recycling process was compared with conventional technologies to examine its superiority. The energy consumption was assessed in terms of CO 2 emission from the treatment of 1 ton of concrete lumps. Beside the energy consumed for microwave heating, the energy required for roughly crushing process before the heating and that for scrubbing process after the heating were added. The energy consumption calculated for each case was converted to CO 2 emission based on JEMAI-LCA [6]. The quantity of CO 2 emitted during the treatment of 1 ton of concrete lumps is shown in Figure 7. The CO 2 emitted using the microwave heating method is extremely smaller than that from the furnace heating and scrubbing method that could produce extremely high quality aggregate, even accounting the emissions during the roughy crushing and scrubbing process and almost the same as that from the eccentric rotor method that could produce moderately high quality aggregate. Considering the extremely high quality of recycled aggregate and the small CO 2 emission, the new technology seems advantageous than conventional ones. 341

10 Q uantity of C O 2 em itted (kg/ton :A ggregate) Heat treatm ent and the rubbing m ethod Quantity of C O 2 em itted in M icrow ave-based A ggregate R ecovering Heating process :Rated power of Microw ave oven by using test (The am ount of one processing is 10kg.) C rushing process :Anamnestic cas e (R ubbing m ethod ) Eccentric rotor m ethod (M echanicalrubbing m ethod) M anufacturing process of aggregate Rubbing m ethod Microw ave heating 30sec. 60sec. 90sec. 120sec. Microw ave heating (H eating time) Figure 7. Quantity of CO 2 emitted during the treatment of 1 ton of concrete lumps 6. CONCLUSIONS A combination of surface modification using silica fume and by-product powder to enhance the strength of concrete and microwave heating of dielectric material will allow efficient recovery of high quality aggregate, achieving compatibility of mechanical properties and recycling of aggregate that are in a trade-off relationship. The new technology has achieved smaller CO 2 emission compared to conventional technologies as well as high performance of concrete, energy saving in recycling of concrete and full recovery of recycled aggregate at the same time. Conclusions in this study are summarized as follows: Coarse aggregate coated with silica fume and by-product powder at the silica fume-byproduct powder ratio of % will increase the strength of concrete by 20% compared to that of the concrete with the same mix and using normal aggregate. Ferrous oxide is appropriate as a dielectric material to be used for the microwave-based aggregate recovering full concrete recycling technology. Coupling dielectric material coated modification and microwave heating will allow the recovery of high quality original aggregate on the recovery rate of 90 % or higher. Energy consumption of the aggregate recycling utilizing microwave heating is very small compared to conventional technologies. Consequently the proposed microwave heating recycling technology for concrete is expected to play an important role to establish resource-recycling construction society in the future. 342

11 REFERENCES [1] Ministry of the Environment, 'Annual Report on the Environment and the Sound Material-Cycle Society in Japan 2008' (2009). [2] Shima, H., Tateyashiki, H., Matsuhashi, R. and Yoshida, Y., 'An Advanced Concrete Recycling Technology and its Applicability Assessment through Input-Output Analysis', Journal of Advanced Concrete Technology 3 (1) (2005) [3] Recycled Aggregate Standardization Committee, 'Report on Current Status and Prospect of Concrete using Recycled Aggregate - Outline of the JIS and for the Promotion of Utilization' (Japan Concrete Institute, November 2006) (In Japanese). [4] Noguchi, T. and Tamura, M., 'Concrete design towards complete recycling', Structural Concrete 2 (3) (2001) [5] Tsujino, M., Noguchi, T., Tamura, M., Kanematsu, M., and Maruyama, I., 'Application of Conventionally Recycled Coarse Aggregate to Concrete Structure by Surface Modification Treatment', Journal of Advanced Concrete Technology 5 (1) (2007) [6] Software for Life Cycle Assessment (JEMAI-LCA Pro) (Japan Environmental management Association for Industry, JEMAI) (In Japanese). 343