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Sustainable applications for utilization the construction waste of aerated concrete
Journal of Cleaner Production 230 (2019) 430e444
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Sustainable applications for utilization the construction waste of aerated concrete n Gyurko *, Bence Jankus, Olive r Fenyvesi, Rita Nemes Zolta } egyetem Rkp. 3, Hungary Department of Construction Materials and Technologies, Budapest University of Technology and Economics, 1111, Budapest, Mu
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2018 Received in revised form 25 April 2019 Accepted 26 April 2019 Available online 30 April 2019
In the next decades many buildings and structures, constructed from autoclaved aerated concrete (AAC) will reach their end of lifetime, therefore solutions should be developed to recycle the resulting waste material. AAC blocks cannot be reused in the form of building blocks as traditional clay bricks. The typical form of AAC waste is crushed stone or powder. Present study is dealing with the possibilities of the recycling of AAC waste as concrete aggregate, prefabricated concrete tiles, concrete blocks, shuttering blocks and cement supplementary material. The concrete mixes were designed in a way to apply as much AAC waste as possible and to require the least further processing (crushing, pulverizing) of the waste material at the same time. Mechanical (compressive strength), thermal (heat conductivity) and durability (frost and freeze-thaw resistance) properties of the samples containing AAC waste as aggregate or supplementary material were tested. The results indicated that opportunities in applying aerated concrete waste in granular form as aggregate for load bearing purposes are limited, but in powdered form can be highly advantageous. However, if the load bearing capacity is not a strong requirement there are many feasible modes to recycle AAC waste as vertical wall coverings, stumped concrete wall, design exposed concrete or lightweight aggregate concrete. In advance to granular form, adding powdered AAC to normal concrete, the durability attributes were improved. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Aerated (cellular) concrete Waste material Durability Recycling Supplementary material Secondary raw material
1. Introduction and research objectives Autoclaved aerated concrete (AAC) is a world-wide well-known construction material, however, it is a very young compared to the ancient ones e.g. timber, natural stone or even traditional concrete (Kausay, 2002). The wide application of AAC in construction industry have been started between 1940e50 in Europe (see Fig. 2). The first buildings, made of its earlier version (aerated concrete, AC) are about to reach their design lifetime. In the EU many industrial and residential buildings were constructed from AAC blocks mostly designed for 50 years of lifetime. Therefore, large amount of AC and AAC waste can be expected to appear in building industry in the next decade(s). At the demolition of these buildings the potential in recycling the waste material arises. The application of recycled materials in new concrete structures is inviting (Mohammadhosseini et al., 2017). It is not only economically beneficial (lower cost related to waste
} egyetem rkp. 3, Hungary. * Corresponding author. 1111, Budapest, Mu ), bence.jankus@ E-mail addresses: [email protected] (Z. Gyurko gmail.com (B. Jankus), [email protected] (O. Fenyvesi), nemes.rita@ epito.bme.hu (R. Nemes). https://doi.org/10.1016/j.jclepro.2019.04.357 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
management), but a greener and more sustainable building industrial processes could be developed. Waste handling is getting higher and higher importance in our consumption based society. Therefore, both jurisprudence and technical science have to pay more attention to this topic. The first European (EU) regulation, which has dealt with the topic in-detail, was published in 2000 (European Commission, 2000). This was the first comprehensive edict in the EU dealing with building industrial waste in deep details. The operative national regulations in connection with the EU regulation contain both compulsions and concessions. According to these regulations above a defined limit in investment it is compulsory to use recycled building materials (see the regulations for the prescribed rate). However, many extra terms and financial support can be received in procurement processes according to green considerations. In 2008 an EC regulation (European Commission, 2008) was declared which requires the member states to specify how they plan to develop the recycling rate in the different fields of waste. Due to this, several research targeted different recycling methods of communal and industrial zsa and Nemes, waste (Hoffmann et al., 2003; Nemes, 2015; Jo 2002). During the last decade several high capacity waste
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handling sites have been founded in the EU to handle building industrial waste. Thanks to these sites, the rate of building industrial waste dumped in communal landfills was strongly reduced. According to the latest European waste production statistics by Eurostat (refreshed in May 2017) the total amount of waste produced in the EU have not significantly changed from 2004 to 2014 (the change is within 2.5%), as it can be seen in Fig. 1. Besides this, the amount of building industrial waste increased by 12%. According to statistics it is obvious that building industrial waste has a great role in European waste management politics and the recycling rate in Europe is still not high enough. Nowadays approximately 50% of all waste is getting recycled. To use the huge amount of demolition waste, new recycling options should be developed, supported by laboratory tests and their results. This is the first step to develop a new product that can be applied in the construction industry. In the literature only a few studies can be found dealing with the recycling of AAC and those studies only dealt with the application of AAC as coarse aggregate (Topcu and Seridemir (2007); Sinica et al., 2009), however, those studies showed that it is possible to apply AAC waste as aggregate in concrete. Present paper introduces different possibilities of reuse the AAC waste, coming from its production (aerated concrete powder) and its demolition (crushed recycled AAC aggregate), as it can be seen in Fig. 2. There are three major forms in which AAC can be recycled: in block, crushed or powder form. In crushed form AAC can be recycled in lightweight aggregate concrete (crushed AAC waste is a lightweight aggregate; rT < 2000 kg/m3) or in no-fines concrete (here only applied as coarse aggregate). Based on that the research consist five phases (Fig. 2): Phase 0: Block form. To reuse AAC in block form requires its demolition to be performed in a very professional and careful manner. This process (if it is even possible) requires extraordinary economical effort and time, making it not feasible for practical applications. Phase 1: Load bearing lightweight aggregate. Concrete mixes were designed to produce load bearing lightweight aggregate concrete. Based on literature recommendations paste saturated mixtures were designed with high cement dosage. Here AAC was applied as fine and coarse aggregate (Section 3). Phase 2: Non-load bearing lightweight aggregate. Applications introduced to use AAC as fine and coarse aggregate in non-load bearing applications (Section 3). Phase 3: No-fines concrete aggregate. AAC waste was applied in no-fines concrete as aggregate. Here the aim was to decrease the
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amount of cement and thus present a cost-effective solution for the recycle of AAC (Section 4). Phase 4: Powder form. During the cutting process and production of AAC blocks or rather the demolition of AAC buildings large amount of AAC powder waste arises. This powder could be applied in concrete as filling material or as supplementary material (Section 5). The aim of the study was to introduce applications not only with the coarse aggregate of concrete but the complete aggregate matrix is built up from recycled AAC. The further processing (crushing, pulverizing) of the waste aggregate is expensive, thus it is aimed to find applications where the most AAC waste is used with the least further processing. Besides that, it was intended to reuse all the form of AAC waste, because recently AAC waste powder is not recycled. The recycling of the powder fraction of normal concrete waste is problem as well, which has no satisfactory solution. 2. Characteristics of AAC in different forms During the production of AAC blocks as well as during the construction and demolition of AAC buildings, waste material is generated in various sizes. In Fig. 3 it is sieved into six fractions. In the present research, one type of AAC with the body density of 440 kg/m3 was applied to produce the aggregate (in all form). The investigated type of AAC was chosen based on business sales statistics that were received from the manufacturer company. It was taken into consideration to test the other available types, but this parameter was kept constant during the present research. It is planned to be tested in further research phases with demolished AAC waste used as aggregate as well. The most important properties of the original AAC block (the base material of the study), based on its declaration of performance: Mean compressive strength: 3.0 N/mm2, Thermal conductivity: 0.125 W/mK, Water vapour permeability: 5/10. In a mix design point of view there are some important properties of the aggregate depending on the grain size. Therefore, many differences were found in the lightweight aggregate performance compared with earlier researches, where AAC waste was used only as coarse aggregate (Sinica et al., 2009; Shui et al., 2014). These main properties were particle body density and total porosity of the different grain sizes. It is because during the crush process porosity is decreasing, while the particle body density as well as the
Fig. 1. Waste production in the EU between 2004 and 2014.
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Fig. 2. Possibilities in recycling of AAC as construction material and research phases.
Fig. 3. Different particle sizes (0/0.25 mm; 0.25/1.0 mm; 1/2 mm; 2/4 mm; 4/8 mm; 8/16) of crushed recycled aggregate from the same AAC waste material.
bulk density (in different rate) are increasing, as it is shown in Fig. 4. These parameters are extremely important, because they are input data of the concrete mix design process. This effect is not obviously true in case of the water demand of the lightweight aggregate concrete mixture, because as the inner open porosity (apparent water absorption capacity) of the aggregate is decreasing during the crush process, the specific surface of the smaller grain sizes is increasing, which can be seen in Table 1 and Fig. 5. These parameters are determinative in the point of view of consistency, especially if the concrete is going to be pumped, because a lower consistency mixture can stick in the pipe. This change in the water
demand of the concrete mixture should be calculated during the mix design (water dosage and super plasticiser admixtures). The open porosity causes internal curing of the cement matrix, which is advantageous for concrete parameters (e.g. strength, water tightness, early age shrinkage, durability, etc.), therefore it is also advantageous in the point of view of cost efficiency. Water absorption in case of the 0/0.25 and 0.25/1 fractions cannot be measured, because the water amount on the surface of the grains is higher than the water amount inside the grains. Water absorption of the coarse aggregate was determined by using pycnometric method. The 1 h water absorption value was used to
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Fig. 4. Change of the particle body (apparent) density and bulk density due to the aggregate particle size.
Table 1 Aggregate material properties due to the grain size. Grain size [mm]
Water absorption capacity [m%]
Water absorption capacity [V%]
Particle body (apparent) density [g/cm3]
0/0.25 0.25/1 1/2 2/4 4/8 8/16
e e 14.7% 13.0% 17.1% 15.3%
e e 24.2% 19.3% 19.4% 13.0%
2.041 1.939 1.645 1.481 1.136 0.848
Fig. 5. Change of water absorption capacity of aggregate due to the aggregate particle size.
design the water demand of the mixes in the different phases. 3. Application of recycled crushed AAC waste as LWA in LWAC (phase 1 and 2) In the literature it was demonstrated that there is a potential to use waste materials (economically viable way) to produce lightweight concrete (Makul and Sua-iam, 2016; Kashani et al., 2017). In
the last years, two different research phases were completed with AAC waste as lightweight concrete aggregate (LWA), used in lightweight aggregate concretes (LWACs) for different purposes. In both research phases 100% of the aggregate was substituted by waste material. In the first research phase LWAC mixtures were designed containing crushed AAC aggregate for load bearing and insulating purposes. 30 different mixes were designed for the research, where the aggregate to cement ratio was different (see
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the value ranges in Table 2). Here the mixtures were tested according to the European standards for concrete (MSZ, 2009c). In the second research phase mixtures were designed for vertical covering application (e.g. tiles). In this research phase the material was treated and tested according to the European standards for artificial stone (MSZ, 2016; MSZ, 2005; MSZ, 2013). Detailed information about the design parameters, the applied standards and the test results are presented in Table 2 (Fenyvesi and Jankus, 2015; Fenyvesi, 2014). The type and dosage of cement were chosen based on the purpose of application. For a load bearing function high strength mortar is needed with high cement amount. In the interests of ecological effectiveness and cost reduction the load bearing mixtures were designed with CEM II type cement. CEM II BeS type of cement has a significant proportion of blast furnace slag used for its production therefore fewer amount of clinker is needed which makes this type ecologically advantageous against other e.g. CEM I types of cement. Based on literature recommendations the mix design of LWAC targeted to provide paste saturated mixtures (Ujhelyi, 1960). Based on the greater gap ratio of the AAC aggregate compared to the commonly used quartz gravel and sand a larger cement dosage should be applied in order to achieve the wished high saturation. This is the reason to use 500e650 kg/m3 cement in the mixtures of the 1st research phase. In case of other types of LWAC usually oversaturated mixtures are the optimal for strength parameters, because the hardened cement stone gives the load bearing part of the composite material. In our research strength parameters were not primary important, therefore paste saturated concrete was investigated, but other (no fines and oversaturated) mixtures have also been tested. In the second research phase vertical wall coverings were targeted. Therefore the main aim was to form aesthetic appearance, thus a CEM I type (white cement) was chosen and colouring pigments were added to the mixtures as well. The dosage was planned k and Magyari, according to reference recommendations (Koza k et al., 2011). In the surface appear2013; Magyari, 2005; Koza ance the colour of the crushed AAC waste aggregate was also applied differ from the cement matrix coloured with different kind of oxide pigments, as it can be seen in Fig. 6. Here the strength and density of the mortar matrix could be lower, because only a selfsupport load bearing capacity of the resulting concrete was the requirement. Thus lower cement amount can be applied and a more economical concrete can be designed. AAC waste was prepared for suitable aggregate particles to lightweight concrete by a jaw crusher. After one crushing period
particle size distribution was tested by standard sieve test. The results showed that the produced particle size distribution is in the II. quality class according the recent Hungarian regulations (Fig. 7), therefore it is suitable for concrete aggregate without any addition of normal aggregate which was tested in the first research phase (MSZ, 2004). This is a major progress against previous researches, where only recycled coarse (d 4.0 mm) AAC aggregate was used and quartz sand was added as fine aggregate (Topcu and Seridemir (2007); Sinica et al., 2009). In our first research phase pure one-phasecrushed AAC bulk was used as lightweight aggregate. The applied maximum particle size was 16 mm. In the second research phase, to improve strength parameters and reduce cement dosage, the bulk was sieved into six, single fractions that can be seen in Fig. 7. The fractions were added to the mixtures according to the limit grading curve B given in recent Hungarian standard (see Fig. 7) (MSZ, 2004). After a half year of storage, map surface cracking (mapping) was observed on the specimens made in the first research phase, shown in Fig. 8. Therefore, PP fibre were added to the mixtures in the second phase, to prevent the formation of early age deformation cracks on the surface of the specimens, and the possible causes k and Magyari, 2013; Magyari, 2005; Koza k were investigated (Koza et al., 2011). In the crushing process high amount of fine aggregate (under 2 mm grain size) is formed for which other use/application should be founded (see Phase 5). In the first research phase the aim was to produce LWAC mixtures for combined structural and thermal insulating purposes. This means that the body density of the lightweight concrete shall be between 600 and 1600 kg/m3. The common applications of this kind of LWACs are usually prefabricated masonry blocks, monolith concrete walls and slabs, acoustic insulations. Their compressive strength is usually between 10 ÷ 20 N/mm2 (MEASZ, 1995). According to the purpose of this kind of LWAC main material parameters (density, compressive strength, thermal conductivity and water tightness) were investigated. The usual compressive strength and density range was reached, as it can be seen in Table 2. Thermal conductivity of the material is better (lower), than the commonly applied clay brick or natural stone masonry elements, but not as good as thermal insulations and AC or AAC (the base material of the aggregate). Water tightness can be classified in the middle classes according to the MSZ EN 206e1 European concrete standard, which means that the material can be applied in undersoil wall structures too (MSZ, 2000). In the first research phase it was proven, that AAC waste can be handled as secondary raw material as lightweight aggregate. It can
Table 2 Design parameters and test results (n.t.: not tested property). Research plan
1st Research phase
2nd Research phase 1st Mixing stage
2nd Mixing stage
Grain size distribution
Class II
Curve-B
Curve-B
Cement type Cement dosage Water-cement ratio Aggregate (AAC) dosage Fresh concrete density PP fibers Colouring Body density Compressive strength Thermal conductivity Water tightness Flexural-tensile strength Impact resistance Freeze-thaw resistance
CEM II BeS 42.5 N (V) 500e650 kg/m3 0.55 550e750 kg/m3 1680e1690 kg/m3 e e 1380e1450 kg/m3 12e16 N/mm2 0.39e0.45 W/mK XV1(H) e XV2(H) n.t. n.t. n.t.
CEM I 52.5 N (grey) 350e450 kg/m3 0.4 940e1030 kg/m3 1540e1580 kg/m3 1.5 kg/m3 5% by mass of cement 1250e1350 kg/m3 6e9 N/mm2 0.28e0.36 W/mK n.t. 1.5e2,5 N/mm2 45e55 cm non-resistant
CEM I 52.5 N (white) 350e450 kg/m3 0.4 940e1030 kg/m3 1540e1580 kg/m3 1.5 kg/m3 5% by mass of cement 1250e1350 kg/m3 2.5 N/mm2 0.28e0.36 W/mK n.t. 0.6e0.8 N/mm2 25e30 cm non-resistant
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Fig. 6. Different kind of coloured LWAC specimens made with AAC aggregate (polished surface).
Fig. 7. Aggregate size distribution in the first research phase (MSZ, 2004).
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Fig. 8. Surface cracks (mapping) on the surface of the specimen from the first research phase.
be applied as fine and coarse aggregate of LWAC with combined (load bearing and thermal insulating) purpose, e.g. for prefabricated or monolith masonry elements, walls or columns. In the second research phase, the application of the recycled crushed waste AAC aggregate LWAC was investigated as vertical, indoor concrete covering (e.g. tiles or monolithic form). For this purpose artificial stone standard EN 15286:2013 was applied (MSZ, 2013). According to this standard compressive, flexural tensile and flexural impact strength, frost resistance and thermal conductivity were tested. DTG (differential thermal analysis) and XRD (X-ray diffraction analysis) were applied to investigate the early age deformation crack formation observed in first research phase. As the results (Table 2) show the compressive and flexural tensile strength of the tested LWAC is high enough for covering purposes, but not enough for structural elements. Despite that the strength parameters were not outstanding, it was also found, that the flexural impact strength was appreciably higher, than in case of normal concrete. This means, that the ductility (strain capacity) of the material is higher, than it is usually measured on normal concretes which is advantageous in case of covering applications (the material should be more ductile, than the supporting structural elements). This parameter could be enhanced if the early age deformation crack formation can be avoided. To explore this reaction, differential thermal analyses (derivatograhpy) have been
performed on the AAC waste, on the colouring pigments, and on the hardened concrete as well. The TG-DTG-DTA curves of the AAC showed that the raw material contains approximately 10e12 m% gypsum, and in the TG-DTG-DTA curves of the LWAC no gypsum content can be found, as it can be seen on Fig. 9 and Fig. 10. This means that the high amount of gypsum content formed other crystal structure in the LWAC after setting. In the pigment no gypsum or other harmful material content was found. The map-like cracks on the surface of the specimens (Fig. 8.) indicated some kind of swelling reaction that can be caused by the ettringite formation of gypsum content. The ettringite content cannot be seen on DTG curves, therefore XRD tests were made on LWAC specimens to investigate this possible cause for the crack formation. The permissible maximum acid-soluble sulphate content of recycled concrete aggregates according to the DIN 4226e100:2002 standard (DIN, 2002) and the DAfStb-Richtlinie (2010) technical specification (DAfStb, 2010) is 0.8 m%. In the recycled aggregate, investigated by this research, this value was higher due to the gypsum content. The aluminate modulus (AM) of the used cement was higher than 17 (AM > 17). AM is one indicator for the predisposition of the cement to sulphate corrosion. The AM of normal Portland cements is usually between 1 and 2.5 (AM ¼ 1.0e2.5). In case of sulphate resistant Portland cements AM < 0.64, for moderate sulphate resistant Portland cements AM < 1.0.
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Fig. 9. DTG curve of the tested LWAC (Notation: 1.: water content; 2.: goethite; 3.: structural water).
zs et al., 1989). Fig. 10. DTG curve of the AAC (Notation: 1.: gypsum; 2.: CaCO3; 3.: tobermorite. (Bala
If AM is above 1.0, it increases the intensity of a clinker mineral formation, called tricalcium-aluminate (3CaO.Al2O3, in silicate chemical nomenclature: C3A) which form calcium-aluminatehydrate: 3CaO$Al2O3 þ Ca(OH)2 þ 12H2O ¼ 4CaO$Al2O3$13H2O Moreover, the formed calcium-aluminate-hydrate has very low strength, so this reaction may completely ruins microstructure of concrete. This is the reason why cement factories add gypsum to the clinker, since C3A reacts with the gypsum, and a new phase, ettringite (or trisulphate) is formed: 3CaO$Al2O3 þ 3CaSO4$2H2O þ 26H2O ¼ 3CaO$Al2O3$3CaSO4$32H2O Ettringite is a mineral of large size due to the 32 mol crystallization water in its structure, and the crystallization pressure during formation can reach 100 N/mm2. This is not a problem, in the young
age of concrete, when it is still plastic and it is able to be deformed. Unfortunately in our case, the production of ettringite was shifted in time, and still formed, when the strength holding tricalcium-silicate (3CaO.SiO2 commonly known as alite; silicate chemical nomenclature: C3S) minerals started to generate the calcium-silicate-hydrate products in the hydration process. The crystallization pressure of ettringite crystals destroys the bond between the C3S hydration products, and this way microcracks are formed in the cement matrix. On the XRD images of the 1-day-old samples both anhydride (Fig. 11) and ettringite (Fig. 12) were detected. Comparing the XRD images of ettringite, taken on 1-day-old and 7-day-old samples, it was proved that the amount of ettringite has been significantly increased, and gypsum content has been decreased (Fig. 11 e solid line is the 1-day-old sample, dotted line is the 7-day-old sample). For further proofing, swelling has also been measured on fresh samples from 1 to 7 days of age (Fig. 13). Samples JT1, JT2, JT3 were
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Fig. 11. XRD images with the lines of andydrite (around 22 and 30 on the abscissa) at the age of 1 (solid line) and 7 days (dotted line).
Fig. 12. XRD images diffraktogram with the lines of ettringite (around 27 on the abscissa).
cured on air under permanent climatic conditions (20 ± 2 C, and 65 ± 10% relative humidity). Samples JT4, JT5 were cured under water. Another finding is that under water the samples have swollen more that of the samples cured on air. This is due to the curing water supply for hydrate genesis (from anhydride), which maintains the formation of ettringite. The more amount of ettringite can formed in the pore structure of the concrete, the higher value of swelling of concrete will induced, which cause crack forming in the material. This harmful chemical reaction can be avoided if low AM cement is used for LWAC mixing with AAC waste aggregate. 4. Application as aggregate of no-fines concrete (phase 3) During the 1st research phase some difficulties (high water absorption) and disadvantages (high mortar and cement demand, durability problems) were found, when AAC was used as lightweight aggregate. If saturated paste is not a requirement, as it was in case of Phase 1, then these problems can be eliminated. In case of
no-fines concrete the paste is not saturated and it is one of the most effective application of waste aggregate. No-fines concrete contains only a minimal amount of cement, which is the most costly constituent of concrete. No-fines concrete is a type of lightweight concrete, which has extremely low tensile strength, thus it has resistance only against compressive loads (Carsana et al., 2013). However, if this requirement is met then a cost-efficient concrete can be produced. In the first step no-fines concrete cubes were casted using aerated concrete (AC) waste as aggregate. The mix contained extremely low amount of cement (100 kg/m3), relative low water-to-cement ratio (0.5) and gap ratio (25%). The amount of aggregate is 20% by volume. The used crushed AC aggregate has its diameter between 4 and 16 mm. As the result of the crushing process, the particles larger the 16 mm were laminar and hard to mix, thus they were not applied in the concrete mix. The water absorption of this aggregate is extremely high, therefore in the first step of the mixing the aggregate was mixed only with water (100% by its mass) and only after that were the cement and mixing water amount added. The
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Fig. 13. Deformation of young age samples cured in climate chamber (JT 1e3) and under water (JT 4e5).
water content of the aggregate evaporates slowly from the concrete during the hardening, which behaves as an internal curing. This is highly beneficial in practice, because the curing of no-fines concrete is complicated and the only ideal solution is to keep the concrete in the formwork at least for 7 days, which is not always feasible. The body density and compressive strength of the samples were measured: body density of concrete: 1100e1300 kg/m3, compressive strength: 4e8 MPa. The possibilities of application in aspect of these results: material of self-supporting wall or decorative function stumped concrete (which is a special type of exposed concrete), shuttering element. The so-called “Stumped concrete” is popular among architects used as part of a design facade or fence (Fig. 14). Stumped concrete was casted in laboratory using crushed AAC as aggregate. After mixing the samples were only stumped (manually compacted with a rod) to reach the desired aesthetic effect. Compacting with vibration is not allowed in this case, because it leads to the segregation of the aggregate particles. In this case the requirements of strength are low, it shall be only self-supporting (no load bearing function), thus the measured strength fulfils this requirement. It can be used as the design element of a two layers wall (one load bearing wall e one design wall) or without load bearing supporting wall (e.g. fence). The above mentioned are both outdoor applications, where the question of frost resistance arises. In case of frost resistance of a normal concrete, the critical points are the porosity of cement mortar and the porosity of the aggregate. In no-fines concrete the ratio of the cement mortar is minimal and the gap ratio of the aggregate is high, thus in the gaps water has enough space for freezing-induced thermal expansion. The sample in Fig. 14 was exposed to environmental effects for two years. After two years, there were no failure observed neither about frost resistance nor about sulphate swelling. However, it is recommended to separate the stumped concrete wall from the ground to avoid water absorption from the soil, which can lead to durability failures. Another possible application of no-fines AAC aggregate concrete can be as shuttering blocks. Samples were casted from the mixture designed by the authors, described in Table 3. The maximum
Fig. 14. Stumped concrete wall containing various no-fines aggregates (AAC is the 2nd (with Portland cement) and the 4th (with white cement) from the top).
diameter (dmax) of the aggregate was 10 mm, to be able to produce elements with minimum wall thickness of 2.5 cm. A popular production technology of manufacturing prefabricated units was simulated during which the shuttering blocks are produced in a vibration press. The fresh concrete, casted from the mix detailed in Table 3, is stabile right after compacting, thus the formwork can be
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removed, and it becomes re-useable for the next sample. This is highly advantageous in pre-fabrication, where the concrete blocks are made on a conveyor belt. It is recommended to store the blocks in a high humidity storage if they are movable (after 1 day the blocks are movable). In our laboratory there was no storage room with high humidity available, unlike in a prefabricating factory. Where it is possible, even higher compressive strength could be achieved. The size of the blocks was 20 40 20 cm with two locks (Fig. 15.). The average net density of concrete (on 150 mm edge cube) was 1255 kg/m3, while the average gross density of the block was 840 kg/m3. The compressive strength of concrete (measured on a 150 mm edge cube) was 5.32 MPa, the element strength was 1.43 MPa, as it can be seen in Table 4. This block can be used as a self-supporting element or as shuttering element filled with normal concrete as well. In addition to the strength and the density, the thermal conductivity was measured on 20 20 10 cm elements. The measured thermal conductivity was 0.34 W/mK that is 1/6 of a normal weight concrete, meaning a more beneficial thermal insulation capacity. Based on the measurements both no-fines concrete applications (shuttering elements and stumped concrete) are feasible solutions for the usage of AAC waste, however compared to the previous applications (Phase 1 and 2), they apply less amount of AAC waste. 5. Application in powdered form (phase 4) The cutting, chiseling and crushing process of cellular concrete blocks produces a significant amount of fine powder, which cannot be utilized in the later part of the construction. Besides that, during the production of AAC blocks large amount of industrial waste is generated in the form of AAC powder. This powder cannot be fully utilized in the previously described recycling solutions (Phase 1e3). Powder wastes typically applied in concrete as filler material, which in most cases decreases the strength of concrete. Until the magnitude of this decrease is not significant this is a feasible utilization of waste powder. If the grain size of powder is sufficiently small and its other properties are suitable as well, then it is possible that it will work as a supplementary material. In the present research phase fine powder was added to the concrete mix. It was shown in Section 3 that cellular concrete in crushed form is unfavorable as outdoor structural concrete aggregate. However, in the literature several researches can be found, which are proving that concrete produced from waste materials can be advantageous € (Ozalp et al., 2016; Fenyvesi, 2014). More significant performance improvement (in strength and durability) can be reached, if the waste material is added to the concrete mix in powdered form (Gonzalez et al., 2016; Aprianti S, 2017). Besides that, applying waste materials in powdered form in the concrete mix can have the following positive effects: waste material is recycled. In this case it is advantageous if as much added to the mix as possible. It reduces the cost of the concrete mix (e.g. cement is partly substituted by waste material). It can increase the performance of concrete (used as additive or as supplementary cementitious material - SCM).
Table 3 Constituents of used no-fines concrete. Constituents/dosages
liter/m3
kg/m3
cement water aggregate (crushed aerated concrete) 4e10 mm air
300 150 500 250
100 150 250 e
Therefore, it was intended to add the aerated concrete powder (ACP) to concrete mixes and investigate its performance. Normal structural concrete is mostly subjected to compressive stresses, thus compressive strength test was performed on the produced specimens. Besides that, the effect of freezing (which is the most common durability issue with outdoor concrete structures) was tested. 5.1. Concrete mixes and experiment description In the concrete mix design phase it was aimed to design a concrete, which has relatively low freeze-thaw resistance (to highlight the effect of the additive) and its strength class is widely used in the industry (C25/30). It was also intended to apply low amount of cement, to be cost effective. Based on these requirements the following mixture was designed as reference mix as it can be seen in Table 5. As it can be seen in Table 5 the maximum aggregate size was 16 mm. In this research phase CEM I cement was applied, because it does not contain any supplementary material (e.g.: slag). Thus the effect of the relatively small amount of ACP is easier to monitor. The mixture with ACP was the same as detailed in Table 5, but 10% (proportionally to the cement mass e 27 kg/m3) ACP was added to it. Note that waste material powder was added to the mix and it is not substituting cement and the value of dosage was chosen based i, 2016). The aggregate on literature recommendations (Borosnyo size of the ACP is belonged to the 0/0.25 fraction based on sieve tests (its maximum diameter was 0.09 mm). ACP is very beneficial from a production point of view, because it can be added to the concrete mix without any preparation. Using the two above defined mixtures test samples (100 100 100 mm cubes) were produced, at least three for every type of test. The samples were subjected to three laboratory tests: uniaxial compressive strength test, frost resistance test, and freeze-thaw resistance test. The uniaxial compressive strength test was performed at 28 days of age of the samples after 14 days of wet curing, as it is defined in the Eurocode standard (MSZ, 2009a). The frost resistance test is aiming to investigate the effect of frost on the compressive strength of concrete and it was carried out based on the MSZ EN 4715/3e72 standard (MSZ, 1972). After the given number of freezing cycles, the specimens were tested for compressive strength. The freeze-thaw test is similar to the frost test but in this case the amount of removed material (which scaled due to the freeze-thaw cycles) was measured (the less the better the freezethaw resistance) based on the recommendations of the standard CEN/TS 12390e9:2007 (CEN, 2006). 5.2. Test results and discussion The compressive strength test results showed that ACP highly increases the strength of concrete. Compared to the reference mixture it increased the compressive strength by 37% (reference mix used as basis), as it can be seen on Fig. 16. The increase in strength can be explained by the effect of the ACP, which introduces small particles in the mix that decreases the porosity of concrete, resulting a stronger material. Other specimen from the same mixing were subjected to freezing cycles (150 in total) and their compressive strength was measured after 50, 100 and 150 cycles. It was observed in case of the reference mix, that until 100 cycles only minor decrease in strength was measured, however after reaching a critical number of freezing
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441
Fig. 15. Shuttering blocks from AAC after fabrication.
Table 4 Test results measured on the shuttering blocks and on its material. Block density [kg/m3] Concrete density [kg/m3] Compressive strength of the block [MPa] Compressive strength of the concrete [MPa] Thermal conductivity (l) [W/mK] 840
1255
1.43
5.32
0.34
Table 5 Reference concrete mix design. Component Cement Water Aggregate
CEM I 42.5 N w/c 0/4 [mm] 4/8 [mm] 8/16 [mm]
Sum
cycles the compressive strength of the specimens highly decreased (with 40%). In case of the mix containing ACP only 3.5% of strength drop was measured, which is more beneficial, than the performance of the reference mix (Fig. 17). ACP increases the resistance of concrete against frost, which indicates that ACP has a positive effect on the durability of concrete and can increase the lifetime of an outdoor concrete structure. Simultaneously with the other test, the freeze-thaw resistance test was carried out as well. The weight loss of the samples was measured and the deterioration of the concrete surface was determined. As it can be seen on Fig. 18, after 56 freezing cycles the concrete with ACP lost much less weight (45%), than the reference mix. The values measured in both cases are quite high, as it was aimed during the mix design (see Section 5.1). It is also important to
Distribution [-]
Mass [kg/m3]
e 0.57 0.47 0.25 0.28 1
270 154 936 498 557 2415
note, that with air-entraining agents an even higher freeze-thaw resistance can be reached, but air-entraining agents strongly decrease the compressive strength of concrete. Altogether, in both durability tests the mix with ACP performed advantageously and it also increased the compressive strength of the material, without changing the production cost of concrete. In Phase 4 it was shown that AAC waste in fine powder form (dmax < 0.09 mm) with correct dosage (þ10%) can increase mechanical (þ37% compressive strength) and durability (65% weight loss) characteristics of normal strength concrete. Based on that, application of AAC powder as filling material is a suitable way for recycling. The results indicate that AAC powder works similarly to other supplementary cementitious materials (SCMs). AAC powder has many similarities to other SCMs, like it has high
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6. Summary and conclusion 6.1. Summary Present study is dealing with the possible recycling methods of aerated concrete construction (building and prefabrication) waste material. Besides the most obvious application (load bearing aggregate) many other utilization methods were shown (covering tiles, shuttering elements, stumped concrete) and investigated. A novel method was introduced in which AAC waste was applied in powder form in concrete. The following forms of application (see Fig. 2) was introduced applying aerated concrete waste:
Fig. 16. Average compressive strength of the specimens.
specific surface area and contains CSH crystals. These crystals can integrate with the concrete's crystal skeleton and strengthen it. It was still not proven that ACP takes an active role in the hydraulic processes during hardening of concrete, but it was seen that its positive effect depends on the particle size of ACP. If the particle size is larger than 0.09 mm the positive effect of ACP cannot be seen. Particles under this size could only contain nano pores, which are related to CSH phases (Schober, 2011). It is planned by the authors to investigate the effect of AAC powder applied as SCM and compare it to other SCMs. Besides that, the amount of ACP could be optimized as well.
Phase 0: in block form: not feasible in practice. Phase 1: in crushed form: as load bearing lightweight aggregate (LWA) in lightweight aggregate concrete (LWAC). It is possible to produce load bearing LWAC using AAC, but high cement amount is needed. The results show that it has relatively low strength characteristics and has frost resistance issues. This material can be used as masonry blocks, monolithic or prefabricated walls. Phase 2: in crushed form: as non-load bearing LWA, used for covering tiles and for architectural applications. In this application lower cement amount can be applied. Esthetical covering tiles can be fabricated. Phase 3: in crushed form: as aggregate of no-fines concrete (shuttering elements and stumped concrete). This mix has very low cement content, less chemical problems as in case of Phase 1 and proportionally more waste material can be applied. It has to be mentioned here that due to the containing of sulphate, the stockpiling of AAC is normally not allowed due to possible leaching problems. Therefore, the application of the crushed AAC aggregate in new products has to be free of leaching.
Fig. 17. Results of the frost resistance test.
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Fig. 18. Results of the freeze-thaw resistance test, measured on the sawn surface of the specimens.
Phase 4: in powdered form: as filling or supplementary material. A feasible solution was found for the recycling of AAC powder. When fine AAC powder is applied it can increase the strength and durability characteristics of concrete and works similarly to SCMs. In the next subsections the results and findings of the research Phases are summarized. 6.2. AAC as lightweight aggregate (phase 1 and 2) During the present researches several interesting physical and chemical effect were found, which affect directly the application of crushed AAC waste as lightweight concrete aggregate: crushed AAC waste can be used as not only for coarse, but also as fine lightweight aggregate in LWAC, in the crushing process high amount of fine aggregate (under 2 mm grain size) is formed for which other use/application should be founded, from recycled crushed AAC as LWA concrete can fabricated for vertical structural (prefabricated masonry block, and reinforced monolithic LWAC) elements (wall and columns) of middle height buildings due to its compressive strength performance. The elements made from this material have higher compressive strength, than other elements which are commonly applied in the building industry, undersoil elements can be also made due to the water tightness of the tested material, thinner outer walling masonry units can be fabricated from this material compared with other normal concrete or LWAC materials due to its low thermal conductivity, the dead load of this elements is half of the dead load of the normal concrete, and reinforcing steel bars can also be applied in this structural elements, the material has bad frost resistance, and high porosity, so that it cannot be applied as outer or horizontal coverings in its natural form (e.g. without coating or hydrophobisation),
it can be applied as internal vertical covering as prefabricated elements or monolithic wall, due to its load bearing capacity, ductility, good strain capacity, feeling handwarm to touch (low thermal conductivity), knowing the ettringite formation in concrete or other cement based material only sulphate resistant cements can be mixed in this LWACs! The main utility of this material is that it (as fine and coarse aggregates too) can be fabricated from 100% of construction waste only by crushing the demolished masonry elements, can manufactured in prefabrication and also on the building sites and save high amount (the aggregate gives approximately the 60 ÷ 70 vol percent of the concrete) of commonly used mined aggregates. Due to this mine rehabilitation works and costs can be also reduced. Additional utility is that if it can be applied on the building site many energy and cost can be saved of the transporting of aggregates. This solution can be economical if high amount of AAC structures are demolished in a building site, and also high amount of masonry block elements (or landfilling) are needed for constructing a new building or structure. In this case additional equipment is needed on the building site for fabricating the new elements. Fabrication cost and energy demand of the investigated material is lower than in case of commonly applied normal concrete and approximately the same as the commonly used masonry elements. The most important development in this material is that the 60e70 V% (100% of the aggregate) of it made from secondary raw material, which amount do not need to be delivered, deposited and stored from the place of origin will save a lot of resources, both in cost, time and energy, and also mean significant environmental benefit. In the next decades many AAC building and structure will reach their end of life, therefore applications should be developed to recycle this huge amount of waste material, some solutions were described according to our investigations.
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6.3. AAC as no-fines concrete aggregate (phase 3) The water absorption of AAC aggregate is extremely high therefore, prior to mixing it is pre-mixed with water. This water content of the aggregate evaporates slowly from the concrete during its hardening, which is highly beneficial in practice, because it behaves as an internal curing. 100% of the aggregate was substituted by waste material. Possibilities to apply AAC as no-fines aggregate: o stumped concrete: cannot be used as structural concrete (low compressive strength), but it can be applied as outdoor design element (no issues with frost resistance), o shuttering element: possible to utilize as structural element (e.g.: filled with reinforced concrete) and it has beneficial thermal conductivity (1/6 of normal concrete).
6.4. AAC in powdered form (phase 4) The aerated concrete powder (ACP) is the result of the cutting or crushing process of cellular concrete blocks and it can be applied in concrete mixes without any preparation. ACP added to the concrete mix with a proper dosage (10% compared to cement amount) can increase the compressive strength of concrete with 37%. It means that 10% of the amount of cement (the most expensive part of the concrete mix) can be substituted by a waste material. ACP can also increase the frost and freeze-thaw resistance of normal concrete, which is advantageous in case of outdoor concrete structures and can increase their lifetime. During the frost resistance test, after 150 freezing cycles, it lost less than 5% of its compressive strength, which is only slightly higher than the measurement error limit. 7. Future work Continuing the presented research, authors are intending to investigate ACP applied in concretes with higher cement amount. It is also planned to use different cement types and different density AACs to generalize the results. Besides that, the long-term effects of ACP is planned to be investigated. Acknowledgments Authors are grateful to the Hungarian Scientific Research Fund (OTKA) for the financial support of the OTKA K 109233 research project. Special thanks to Bengineer House Kft., Duna-Dr ava Cement Kft., ria Kft., MC-Bauchemie Kft. and BASF Hunga ria Kft. for SIKA Hunga providing the materials used in the experiments. References Aprianti S, E., 2017. A huge number of artificial waste material can be supplementary cementitious material (SCM) for concrete production e a review part II. J. Clean. Prod. 142, 4178e4194. zsa, Z., Csa nyi, E., Tomor, K., 1989. A Ga zbeton Zsugoroda s Balazs, G., Jo €kkente se nek Leheto } se ge Homok Adagola ssal (Possibilities to Reduce Cso Shrinkage of Gas Concrete with Sand Dispensing, in Hungarian), Test Report. Budapest University of Technology, p. 64. i, A., 2016. Long term durability performance and mechanical properties of Borosnyo high performance concretes with combined use of supplementary cementing materials. Constr. Build. Mater. 112, 307e324. https://doi.org/10.1016/ j.conbuildmat.2016.02.224. Carsana, M., Tittarelli, F., Bertolini, L., 2013. Use of no-fines concrete as a building material: strength, durability properties and corrosion protection of embedded steel. Cement Concr. Res. 48, 64e73 https://doi.org/10.1016/ j.cemconres.2013.02.006.
European Committee for Standardization (CEN), 2006. CEN/TS 12390-9 Testing Hardened Concrete - Part 9: Freeze-Thaw Resistance - Scaling, p. 24. DAfStb (Deutscher Ausschuss für Stahlbeton), 2010. DAfStb-Richtlinie Beton nach € rnungen nach DIN DIN EN 206-1 und DIN 1045-2 mit rezyklierten Gesteinsko EN 12620 (Concrete technical specification for EN 206-1 and DIN 1045-2 with recycled aggregates for DIN EN 12620). DIN, 2002. DIN 4226-100:2002-02 Aggregates for Concrete and Mortar - Part 100: Recycled Aggregates. European Commission, 2000. 2000/532/EC Commission Decision on Waste List. European Commission, 2008. 2008/98/EC of the European Parliament and of the Council on Waste and Repealing Certain Directives. Fenyvesi, O., 2014. Recycling of Different Industrial Wastes as Concrete Aggregate }? Hullade k? Nem! Haszonanyag! Conference, (In Hungarian). Meddo pp. 111e118. Budapest, Hungary (2014). Fenyvesi, O., Jankus, B., 2015. Opportunities in recycling AAC waste as aggregate for lightweight concrete. Epa. J. Silic. Based Compos. Mater. 67, 66e70. Gonzalez, M., Tighe, S.L., Hui, K., Rahman, S., de Oliveira Lima, A., 2016. Evaluation of freeze/thaw and scaling response of nanoconcrete for Portland Cement Concrete (PCC) pavements. Constr. Build. Mater. 120, 465e472. zsa, Zs, Nemes, R., 2003. Lightweight aggregate produced from glass Hoffmann, L., Jo waste (in Hungarian). Epa. J. Silic. Based Compos. Mater. 55, 13e17. https://doi. org/10.14382/epitoanyag-jsbcm.2003.3. zsa, Zs, Nemes, R., 2002. Recycled glass aggregate for lightweight concrete. Concr. Jo Struct. 3, 41e46. Kashani, A., Ngo, T.D., Mendis, P., Black, J.R., 2017. A sustainable application of recycled tyre crumbs as insulator in lightweight cellular concrete. J. Clean. Prod. 149, 925e935. zbeton-po rusbeton (Gasbeton-Porenbeton e in Hungarian). Kausay, Tibor, 2002. Ga vf., 7-8. sz BETON, X.e am, 25. oldal, ISSN: 1218-4837. Koz ak, J., Magyari, B., 2013. Construction of Budapest Metro line four, Part 4: FRC píte s 15 (2), 53e56. coverage, applied at three stations; in Hungarian. Vasbetone Koz ak, J., Magyari, B., Tassi, G., 2011. PPFRC cornice and wall covering. In: Proceedings Of the Innovative Materials and Technologies for Concrete Structures. Central European Congress on Concrete Engineering, pp. 393e396. Magyari, B., 2005. Fibre reinforced concrete elements applied on facades. In: Proceedings of the Keep Concrete Attractive, Fib Symposium, vol. 1, pp. 86e91. Makul, N., Sua-iam, G., 2016. Characteristics and utilization of sugarcane filter cake waste in the production of lightweight foamed concrete. J. Clean. Prod. 126, 118e133. MEASZ, 1995. ME-04.19:1995 Construction of Concrete and Reinforced Concrete (in Hungarian). Mohammadhosseini, H., Yatim, J.M., Sam, A.R.M., Awal, A.S.M.A., 2017. Durability performance of green concrete composites containing waste carpet fibers and palm oil fuel ash. J. Clean. Prod. 144, 448e458. rdult Beton Vizsga lata. Hidrotechnikai MSZ, 1972. MSZ 4715/3-72 Standard, Megszila gok. (Testing Hardened Concrete. Hydrotechnical Properties. e in Tulajdonsa Hungarian). MSZ, 2000. EN 206-1. Concrete - Part 1: Specification, Performance, Production and Conformity. MSZ, 2004. MSZ 4798-1:2004 Concrete d. Part 1: Specification, Performance, Production and Conformity, and the Apply Conditions in Hungary. MSZ, 2005. EN 14617-9:2005 Agglomerated Stone. Test Methods. Determination of Impact Resistance. MSZ, 2009a. EN 12390-7:2009, Testing Hardened Concrete. Part 7: Density of Hardened Concrete, Appendix to Eurocode Standard. MSZ, 2009c. EN 12390-3:2009. Testing Hardened Concrete. Compressive Strength of Test Specimens. MSZ, 2013. EN 15286:2013. Agglomerated Stone. Slabs and Tiles for Wall Finishes (Internal and External). MSZ, 2016. EN 14617-2:2016. Agglomerated Stone. Test Methods. Determination of Flexural Strength (Bending). Nemes, R., 2015. Surface properties of lightweight aggregate concrete and its correlation with durability. Mater. Sci. Forum 812, 207e212. https://doi.org/10. 4028/www.scientific.net/MSF.812.207. € € Sahin, A., 2016. Effects of recycled agOzalp, F., Yilmaz, H.D., Kara, M., Kaya, O., gregates from construction and demolition wastes on mechanical and permeability properties of paving stone, kerb and concrete pipes. Constr. Build. Mater. 110, 17e23. Schober, G., 2011. Porosity in autoclaved aerated concrete (AAC): a review on pore structure, types of porosity, measurement methods and effects of porosity on properties. In: 5th International Conference On Autoclaved Aerated Concrete (Bydgoszcz, Poland: Cement-Wapno-Beton), pp. 351e359. Special Issue: 39e43. Shui, Z., Lu, J., Tian, S., Shen, P.,S., Ding, S., 2014. Preparation of new cementitious system using fly ash and dehydrated autoclaved aerated concrete. J. Wuhan Univ. Technol.-Materials Sci. Ed. 29 (4), 726e732. https://doi.org/10.1007/ s11595-014-0987-3. Sinica, M., Sezamanas, G., Mikulskis, D., Kligys, M., Cesnauskas, V., Zacharcenko, P., Kuprijenko, P., Scerbina, N., Pivenj, N., 2009. Investigation of the composite material with inclusions of autoclaved aerated concrete chips. Mater. Sci. 15 (4), 356e362. ISSN 1392-1320. Topcu, I.B., Seridemir, M., 2007. Prediction of properties of waste AAC aggregate concrete using artificial neural network. Comput. Mater. Sci. 41 (1), 117e125. ISSN: 0927-0256. Ujhelyi, J., 1960. Types, Design of Composition, and Mixing of Lightweight Aggre}oktata si Jegyzetell Va llalat, Budapest. gate Concrete (In Hungarian). Felso ato
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Sustainable applications for utilization the construction waste of aerated concrete n Gyurko *, Bence Jankus, Olive r Fenyvesi, Rita Nemes Zolta } egyetem Rkp. 3, Hungary Department of Construction Materials and Technologies, Budapest University of Technology and Economics, 1111, Budapest, Mu
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2018 Received in revised form 25 April 2019 Accepted 26 April 2019 Available online 30 April 2019
In the next decades many buildings and structures, constructed from autoclaved aerated concrete (AAC) will reach their end of lifetime, therefore solutions should be developed to recycle the resulting waste material. AAC blocks cannot be reused in the form of building blocks as traditional clay bricks. The typical form of AAC waste is crushed stone or powder. Present study is dealing with the possibilities of the recycling of AAC waste as concrete aggregate, prefabricated concrete tiles, concrete blocks, shuttering blocks and cement supplementary material. The concrete mixes were designed in a way to apply as much AAC waste as possible and to require the least further processing (crushing, pulverizing) of the waste material at the same time. Mechanical (compressive strength), thermal (heat conductivity) and durability (frost and freeze-thaw resistance) properties of the samples containing AAC waste as aggregate or supplementary material were tested. The results indicated that opportunities in applying aerated concrete waste in granular form as aggregate for load bearing purposes are limited, but in powdered form can be highly advantageous. However, if the load bearing capacity is not a strong requirement there are many feasible modes to recycle AAC waste as vertical wall coverings, stumped concrete wall, design exposed concrete or lightweight aggregate concrete. In advance to granular form, adding powdered AAC to normal concrete, the durability attributes were improved. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Aerated (cellular) concrete Waste material Durability Recycling Supplementary material Secondary raw material
1. Introduction and research objectives Autoclaved aerated concrete (AAC) is a world-wide well-known construction material, however, it is a very young compared to the ancient ones e.g. timber, natural stone or even traditional concrete (Kausay, 2002). The wide application of AAC in construction industry have been started between 1940e50 in Europe (see Fig. 2). The first buildings, made of its earlier version (aerated concrete, AC) are about to reach their design lifetime. In the EU many industrial and residential buildings were constructed from AAC blocks mostly designed for 50 years of lifetime. Therefore, large amount of AC and AAC waste can be expected to appear in building industry in the next decade(s). At the demolition of these buildings the potential in recycling the waste material arises. The application of recycled materials in new concrete structures is inviting (Mohammadhosseini et al., 2017). It is not only economically beneficial (lower cost related to waste
} egyetem rkp. 3, Hungary. * Corresponding author. 1111, Budapest, Mu ), bence.jankus@ E-mail addresses: [email protected] (Z. Gyurko gmail.com (B. Jankus), [email protected] (O. Fenyvesi), nemes.rita@ epito.bme.hu (R. Nemes). https://doi.org/10.1016/j.jclepro.2019.04.357 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
management), but a greener and more sustainable building industrial processes could be developed. Waste handling is getting higher and higher importance in our consumption based society. Therefore, both jurisprudence and technical science have to pay more attention to this topic. The first European (EU) regulation, which has dealt with the topic in-detail, was published in 2000 (European Commission, 2000). This was the first comprehensive edict in the EU dealing with building industrial waste in deep details. The operative national regulations in connection with the EU regulation contain both compulsions and concessions. According to these regulations above a defined limit in investment it is compulsory to use recycled building materials (see the regulations for the prescribed rate). However, many extra terms and financial support can be received in procurement processes according to green considerations. In 2008 an EC regulation (European Commission, 2008) was declared which requires the member states to specify how they plan to develop the recycling rate in the different fields of waste. Due to this, several research targeted different recycling methods of communal and industrial zsa and Nemes, waste (Hoffmann et al., 2003; Nemes, 2015; Jo 2002). During the last decade several high capacity waste
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handling sites have been founded in the EU to handle building industrial waste. Thanks to these sites, the rate of building industrial waste dumped in communal landfills was strongly reduced. According to the latest European waste production statistics by Eurostat (refreshed in May 2017) the total amount of waste produced in the EU have not significantly changed from 2004 to 2014 (the change is within 2.5%), as it can be seen in Fig. 1. Besides this, the amount of building industrial waste increased by 12%. According to statistics it is obvious that building industrial waste has a great role in European waste management politics and the recycling rate in Europe is still not high enough. Nowadays approximately 50% of all waste is getting recycled. To use the huge amount of demolition waste, new recycling options should be developed, supported by laboratory tests and their results. This is the first step to develop a new product that can be applied in the construction industry. In the literature only a few studies can be found dealing with the recycling of AAC and those studies only dealt with the application of AAC as coarse aggregate (Topcu and Seridemir (2007); Sinica et al., 2009), however, those studies showed that it is possible to apply AAC waste as aggregate in concrete. Present paper introduces different possibilities of reuse the AAC waste, coming from its production (aerated concrete powder) and its demolition (crushed recycled AAC aggregate), as it can be seen in Fig. 2. There are three major forms in which AAC can be recycled: in block, crushed or powder form. In crushed form AAC can be recycled in lightweight aggregate concrete (crushed AAC waste is a lightweight aggregate; rT < 2000 kg/m3) or in no-fines concrete (here only applied as coarse aggregate). Based on that the research consist five phases (Fig. 2): Phase 0: Block form. To reuse AAC in block form requires its demolition to be performed in a very professional and careful manner. This process (if it is even possible) requires extraordinary economical effort and time, making it not feasible for practical applications. Phase 1: Load bearing lightweight aggregate. Concrete mixes were designed to produce load bearing lightweight aggregate concrete. Based on literature recommendations paste saturated mixtures were designed with high cement dosage. Here AAC was applied as fine and coarse aggregate (Section 3). Phase 2: Non-load bearing lightweight aggregate. Applications introduced to use AAC as fine and coarse aggregate in non-load bearing applications (Section 3). Phase 3: No-fines concrete aggregate. AAC waste was applied in no-fines concrete as aggregate. Here the aim was to decrease the
431
amount of cement and thus present a cost-effective solution for the recycle of AAC (Section 4). Phase 4: Powder form. During the cutting process and production of AAC blocks or rather the demolition of AAC buildings large amount of AAC powder waste arises. This powder could be applied in concrete as filling material or as supplementary material (Section 5). The aim of the study was to introduce applications not only with the coarse aggregate of concrete but the complete aggregate matrix is built up from recycled AAC. The further processing (crushing, pulverizing) of the waste aggregate is expensive, thus it is aimed to find applications where the most AAC waste is used with the least further processing. Besides that, it was intended to reuse all the form of AAC waste, because recently AAC waste powder is not recycled. The recycling of the powder fraction of normal concrete waste is problem as well, which has no satisfactory solution. 2. Characteristics of AAC in different forms During the production of AAC blocks as well as during the construction and demolition of AAC buildings, waste material is generated in various sizes. In Fig. 3 it is sieved into six fractions. In the present research, one type of AAC with the body density of 440 kg/m3 was applied to produce the aggregate (in all form). The investigated type of AAC was chosen based on business sales statistics that were received from the manufacturer company. It was taken into consideration to test the other available types, but this parameter was kept constant during the present research. It is planned to be tested in further research phases with demolished AAC waste used as aggregate as well. The most important properties of the original AAC block (the base material of the study), based on its declaration of performance: Mean compressive strength: 3.0 N/mm2, Thermal conductivity: 0.125 W/mK, Water vapour permeability: 5/10. In a mix design point of view there are some important properties of the aggregate depending on the grain size. Therefore, many differences were found in the lightweight aggregate performance compared with earlier researches, where AAC waste was used only as coarse aggregate (Sinica et al., 2009; Shui et al., 2014). These main properties were particle body density and total porosity of the different grain sizes. It is because during the crush process porosity is decreasing, while the particle body density as well as the
Fig. 1. Waste production in the EU between 2004 and 2014.
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Fig. 2. Possibilities in recycling of AAC as construction material and research phases.
Fig. 3. Different particle sizes (0/0.25 mm; 0.25/1.0 mm; 1/2 mm; 2/4 mm; 4/8 mm; 8/16) of crushed recycled aggregate from the same AAC waste material.
bulk density (in different rate) are increasing, as it is shown in Fig. 4. These parameters are extremely important, because they are input data of the concrete mix design process. This effect is not obviously true in case of the water demand of the lightweight aggregate concrete mixture, because as the inner open porosity (apparent water absorption capacity) of the aggregate is decreasing during the crush process, the specific surface of the smaller grain sizes is increasing, which can be seen in Table 1 and Fig. 5. These parameters are determinative in the point of view of consistency, especially if the concrete is going to be pumped, because a lower consistency mixture can stick in the pipe. This change in the water
demand of the concrete mixture should be calculated during the mix design (water dosage and super plasticiser admixtures). The open porosity causes internal curing of the cement matrix, which is advantageous for concrete parameters (e.g. strength, water tightness, early age shrinkage, durability, etc.), therefore it is also advantageous in the point of view of cost efficiency. Water absorption in case of the 0/0.25 and 0.25/1 fractions cannot be measured, because the water amount on the surface of the grains is higher than the water amount inside the grains. Water absorption of the coarse aggregate was determined by using pycnometric method. The 1 h water absorption value was used to
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Fig. 4. Change of the particle body (apparent) density and bulk density due to the aggregate particle size.
Table 1 Aggregate material properties due to the grain size. Grain size [mm]
Water absorption capacity [m%]
Water absorption capacity [V%]
Particle body (apparent) density [g/cm3]
0/0.25 0.25/1 1/2 2/4 4/8 8/16
e e 14.7% 13.0% 17.1% 15.3%
e e 24.2% 19.3% 19.4% 13.0%
2.041 1.939 1.645 1.481 1.136 0.848
Fig. 5. Change of water absorption capacity of aggregate due to the aggregate particle size.
design the water demand of the mixes in the different phases. 3. Application of recycled crushed AAC waste as LWA in LWAC (phase 1 and 2) In the literature it was demonstrated that there is a potential to use waste materials (economically viable way) to produce lightweight concrete (Makul and Sua-iam, 2016; Kashani et al., 2017). In
the last years, two different research phases were completed with AAC waste as lightweight concrete aggregate (LWA), used in lightweight aggregate concretes (LWACs) for different purposes. In both research phases 100% of the aggregate was substituted by waste material. In the first research phase LWAC mixtures were designed containing crushed AAC aggregate for load bearing and insulating purposes. 30 different mixes were designed for the research, where the aggregate to cement ratio was different (see
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the value ranges in Table 2). Here the mixtures were tested according to the European standards for concrete (MSZ, 2009c). In the second research phase mixtures were designed for vertical covering application (e.g. tiles). In this research phase the material was treated and tested according to the European standards for artificial stone (MSZ, 2016; MSZ, 2005; MSZ, 2013). Detailed information about the design parameters, the applied standards and the test results are presented in Table 2 (Fenyvesi and Jankus, 2015; Fenyvesi, 2014). The type and dosage of cement were chosen based on the purpose of application. For a load bearing function high strength mortar is needed with high cement amount. In the interests of ecological effectiveness and cost reduction the load bearing mixtures were designed with CEM II type cement. CEM II BeS type of cement has a significant proportion of blast furnace slag used for its production therefore fewer amount of clinker is needed which makes this type ecologically advantageous against other e.g. CEM I types of cement. Based on literature recommendations the mix design of LWAC targeted to provide paste saturated mixtures (Ujhelyi, 1960). Based on the greater gap ratio of the AAC aggregate compared to the commonly used quartz gravel and sand a larger cement dosage should be applied in order to achieve the wished high saturation. This is the reason to use 500e650 kg/m3 cement in the mixtures of the 1st research phase. In case of other types of LWAC usually oversaturated mixtures are the optimal for strength parameters, because the hardened cement stone gives the load bearing part of the composite material. In our research strength parameters were not primary important, therefore paste saturated concrete was investigated, but other (no fines and oversaturated) mixtures have also been tested. In the second research phase vertical wall coverings were targeted. Therefore the main aim was to form aesthetic appearance, thus a CEM I type (white cement) was chosen and colouring pigments were added to the mixtures as well. The dosage was planned k and Magyari, according to reference recommendations (Koza k et al., 2011). In the surface appear2013; Magyari, 2005; Koza ance the colour of the crushed AAC waste aggregate was also applied differ from the cement matrix coloured with different kind of oxide pigments, as it can be seen in Fig. 6. Here the strength and density of the mortar matrix could be lower, because only a selfsupport load bearing capacity of the resulting concrete was the requirement. Thus lower cement amount can be applied and a more economical concrete can be designed. AAC waste was prepared for suitable aggregate particles to lightweight concrete by a jaw crusher. After one crushing period
particle size distribution was tested by standard sieve test. The results showed that the produced particle size distribution is in the II. quality class according the recent Hungarian regulations (Fig. 7), therefore it is suitable for concrete aggregate without any addition of normal aggregate which was tested in the first research phase (MSZ, 2004). This is a major progress against previous researches, where only recycled coarse (d 4.0 mm) AAC aggregate was used and quartz sand was added as fine aggregate (Topcu and Seridemir (2007); Sinica et al., 2009). In our first research phase pure one-phasecrushed AAC bulk was used as lightweight aggregate. The applied maximum particle size was 16 mm. In the second research phase, to improve strength parameters and reduce cement dosage, the bulk was sieved into six, single fractions that can be seen in Fig. 7. The fractions were added to the mixtures according to the limit grading curve B given in recent Hungarian standard (see Fig. 7) (MSZ, 2004). After a half year of storage, map surface cracking (mapping) was observed on the specimens made in the first research phase, shown in Fig. 8. Therefore, PP fibre were added to the mixtures in the second phase, to prevent the formation of early age deformation cracks on the surface of the specimens, and the possible causes k and Magyari, 2013; Magyari, 2005; Koza k were investigated (Koza et al., 2011). In the crushing process high amount of fine aggregate (under 2 mm grain size) is formed for which other use/application should be founded (see Phase 5). In the first research phase the aim was to produce LWAC mixtures for combined structural and thermal insulating purposes. This means that the body density of the lightweight concrete shall be between 600 and 1600 kg/m3. The common applications of this kind of LWACs are usually prefabricated masonry blocks, monolith concrete walls and slabs, acoustic insulations. Their compressive strength is usually between 10 ÷ 20 N/mm2 (MEASZ, 1995). According to the purpose of this kind of LWAC main material parameters (density, compressive strength, thermal conductivity and water tightness) were investigated. The usual compressive strength and density range was reached, as it can be seen in Table 2. Thermal conductivity of the material is better (lower), than the commonly applied clay brick or natural stone masonry elements, but not as good as thermal insulations and AC or AAC (the base material of the aggregate). Water tightness can be classified in the middle classes according to the MSZ EN 206e1 European concrete standard, which means that the material can be applied in undersoil wall structures too (MSZ, 2000). In the first research phase it was proven, that AAC waste can be handled as secondary raw material as lightweight aggregate. It can
Table 2 Design parameters and test results (n.t.: not tested property). Research plan
1st Research phase
2nd Research phase 1st Mixing stage
2nd Mixing stage
Grain size distribution
Class II
Curve-B
Curve-B
Cement type Cement dosage Water-cement ratio Aggregate (AAC) dosage Fresh concrete density PP fibers Colouring Body density Compressive strength Thermal conductivity Water tightness Flexural-tensile strength Impact resistance Freeze-thaw resistance
CEM II BeS 42.5 N (V) 500e650 kg/m3 0.55 550e750 kg/m3 1680e1690 kg/m3 e e 1380e1450 kg/m3 12e16 N/mm2 0.39e0.45 W/mK XV1(H) e XV2(H) n.t. n.t. n.t.
CEM I 52.5 N (grey) 350e450 kg/m3 0.4 940e1030 kg/m3 1540e1580 kg/m3 1.5 kg/m3 5% by mass of cement 1250e1350 kg/m3 6e9 N/mm2 0.28e0.36 W/mK n.t. 1.5e2,5 N/mm2 45e55 cm non-resistant
CEM I 52.5 N (white) 350e450 kg/m3 0.4 940e1030 kg/m3 1540e1580 kg/m3 1.5 kg/m3 5% by mass of cement 1250e1350 kg/m3 2.5 N/mm2 0.28e0.36 W/mK n.t. 0.6e0.8 N/mm2 25e30 cm non-resistant
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Fig. 6. Different kind of coloured LWAC specimens made with AAC aggregate (polished surface).
Fig. 7. Aggregate size distribution in the first research phase (MSZ, 2004).
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Fig. 8. Surface cracks (mapping) on the surface of the specimen from the first research phase.
be applied as fine and coarse aggregate of LWAC with combined (load bearing and thermal insulating) purpose, e.g. for prefabricated or monolith masonry elements, walls or columns. In the second research phase, the application of the recycled crushed waste AAC aggregate LWAC was investigated as vertical, indoor concrete covering (e.g. tiles or monolithic form). For this purpose artificial stone standard EN 15286:2013 was applied (MSZ, 2013). According to this standard compressive, flexural tensile and flexural impact strength, frost resistance and thermal conductivity were tested. DTG (differential thermal analysis) and XRD (X-ray diffraction analysis) were applied to investigate the early age deformation crack formation observed in first research phase. As the results (Table 2) show the compressive and flexural tensile strength of the tested LWAC is high enough for covering purposes, but not enough for structural elements. Despite that the strength parameters were not outstanding, it was also found, that the flexural impact strength was appreciably higher, than in case of normal concrete. This means, that the ductility (strain capacity) of the material is higher, than it is usually measured on normal concretes which is advantageous in case of covering applications (the material should be more ductile, than the supporting structural elements). This parameter could be enhanced if the early age deformation crack formation can be avoided. To explore this reaction, differential thermal analyses (derivatograhpy) have been
performed on the AAC waste, on the colouring pigments, and on the hardened concrete as well. The TG-DTG-DTA curves of the AAC showed that the raw material contains approximately 10e12 m% gypsum, and in the TG-DTG-DTA curves of the LWAC no gypsum content can be found, as it can be seen on Fig. 9 and Fig. 10. This means that the high amount of gypsum content formed other crystal structure in the LWAC after setting. In the pigment no gypsum or other harmful material content was found. The map-like cracks on the surface of the specimens (Fig. 8.) indicated some kind of swelling reaction that can be caused by the ettringite formation of gypsum content. The ettringite content cannot be seen on DTG curves, therefore XRD tests were made on LWAC specimens to investigate this possible cause for the crack formation. The permissible maximum acid-soluble sulphate content of recycled concrete aggregates according to the DIN 4226e100:2002 standard (DIN, 2002) and the DAfStb-Richtlinie (2010) technical specification (DAfStb, 2010) is 0.8 m%. In the recycled aggregate, investigated by this research, this value was higher due to the gypsum content. The aluminate modulus (AM) of the used cement was higher than 17 (AM > 17). AM is one indicator for the predisposition of the cement to sulphate corrosion. The AM of normal Portland cements is usually between 1 and 2.5 (AM ¼ 1.0e2.5). In case of sulphate resistant Portland cements AM < 0.64, for moderate sulphate resistant Portland cements AM < 1.0.
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Fig. 9. DTG curve of the tested LWAC (Notation: 1.: water content; 2.: goethite; 3.: structural water).
zs et al., 1989). Fig. 10. DTG curve of the AAC (Notation: 1.: gypsum; 2.: CaCO3; 3.: tobermorite. (Bala
If AM is above 1.0, it increases the intensity of a clinker mineral formation, called tricalcium-aluminate (3CaO.Al2O3, in silicate chemical nomenclature: C3A) which form calcium-aluminatehydrate: 3CaO$Al2O3 þ Ca(OH)2 þ 12H2O ¼ 4CaO$Al2O3$13H2O Moreover, the formed calcium-aluminate-hydrate has very low strength, so this reaction may completely ruins microstructure of concrete. This is the reason why cement factories add gypsum to the clinker, since C3A reacts with the gypsum, and a new phase, ettringite (or trisulphate) is formed: 3CaO$Al2O3 þ 3CaSO4$2H2O þ 26H2O ¼ 3CaO$Al2O3$3CaSO4$32H2O Ettringite is a mineral of large size due to the 32 mol crystallization water in its structure, and the crystallization pressure during formation can reach 100 N/mm2. This is not a problem, in the young
age of concrete, when it is still plastic and it is able to be deformed. Unfortunately in our case, the production of ettringite was shifted in time, and still formed, when the strength holding tricalcium-silicate (3CaO.SiO2 commonly known as alite; silicate chemical nomenclature: C3S) minerals started to generate the calcium-silicate-hydrate products in the hydration process. The crystallization pressure of ettringite crystals destroys the bond between the C3S hydration products, and this way microcracks are formed in the cement matrix. On the XRD images of the 1-day-old samples both anhydride (Fig. 11) and ettringite (Fig. 12) were detected. Comparing the XRD images of ettringite, taken on 1-day-old and 7-day-old samples, it was proved that the amount of ettringite has been significantly increased, and gypsum content has been decreased (Fig. 11 e solid line is the 1-day-old sample, dotted line is the 7-day-old sample). For further proofing, swelling has also been measured on fresh samples from 1 to 7 days of age (Fig. 13). Samples JT1, JT2, JT3 were
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Fig. 11. XRD images with the lines of andydrite (around 22 and 30 on the abscissa) at the age of 1 (solid line) and 7 days (dotted line).
Fig. 12. XRD images diffraktogram with the lines of ettringite (around 27 on the abscissa).
cured on air under permanent climatic conditions (20 ± 2 C, and 65 ± 10% relative humidity). Samples JT4, JT5 were cured under water. Another finding is that under water the samples have swollen more that of the samples cured on air. This is due to the curing water supply for hydrate genesis (from anhydride), which maintains the formation of ettringite. The more amount of ettringite can formed in the pore structure of the concrete, the higher value of swelling of concrete will induced, which cause crack forming in the material. This harmful chemical reaction can be avoided if low AM cement is used for LWAC mixing with AAC waste aggregate. 4. Application as aggregate of no-fines concrete (phase 3) During the 1st research phase some difficulties (high water absorption) and disadvantages (high mortar and cement demand, durability problems) were found, when AAC was used as lightweight aggregate. If saturated paste is not a requirement, as it was in case of Phase 1, then these problems can be eliminated. In case of
no-fines concrete the paste is not saturated and it is one of the most effective application of waste aggregate. No-fines concrete contains only a minimal amount of cement, which is the most costly constituent of concrete. No-fines concrete is a type of lightweight concrete, which has extremely low tensile strength, thus it has resistance only against compressive loads (Carsana et al., 2013). However, if this requirement is met then a cost-efficient concrete can be produced. In the first step no-fines concrete cubes were casted using aerated concrete (AC) waste as aggregate. The mix contained extremely low amount of cement (100 kg/m3), relative low water-to-cement ratio (0.5) and gap ratio (25%). The amount of aggregate is 20% by volume. The used crushed AC aggregate has its diameter between 4 and 16 mm. As the result of the crushing process, the particles larger the 16 mm were laminar and hard to mix, thus they were not applied in the concrete mix. The water absorption of this aggregate is extremely high, therefore in the first step of the mixing the aggregate was mixed only with water (100% by its mass) and only after that were the cement and mixing water amount added. The
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Fig. 13. Deformation of young age samples cured in climate chamber (JT 1e3) and under water (JT 4e5).
water content of the aggregate evaporates slowly from the concrete during the hardening, which behaves as an internal curing. This is highly beneficial in practice, because the curing of no-fines concrete is complicated and the only ideal solution is to keep the concrete in the formwork at least for 7 days, which is not always feasible. The body density and compressive strength of the samples were measured: body density of concrete: 1100e1300 kg/m3, compressive strength: 4e8 MPa. The possibilities of application in aspect of these results: material of self-supporting wall or decorative function stumped concrete (which is a special type of exposed concrete), shuttering element. The so-called “Stumped concrete” is popular among architects used as part of a design facade or fence (Fig. 14). Stumped concrete was casted in laboratory using crushed AAC as aggregate. After mixing the samples were only stumped (manually compacted with a rod) to reach the desired aesthetic effect. Compacting with vibration is not allowed in this case, because it leads to the segregation of the aggregate particles. In this case the requirements of strength are low, it shall be only self-supporting (no load bearing function), thus the measured strength fulfils this requirement. It can be used as the design element of a two layers wall (one load bearing wall e one design wall) or without load bearing supporting wall (e.g. fence). The above mentioned are both outdoor applications, where the question of frost resistance arises. In case of frost resistance of a normal concrete, the critical points are the porosity of cement mortar and the porosity of the aggregate. In no-fines concrete the ratio of the cement mortar is minimal and the gap ratio of the aggregate is high, thus in the gaps water has enough space for freezing-induced thermal expansion. The sample in Fig. 14 was exposed to environmental effects for two years. After two years, there were no failure observed neither about frost resistance nor about sulphate swelling. However, it is recommended to separate the stumped concrete wall from the ground to avoid water absorption from the soil, which can lead to durability failures. Another possible application of no-fines AAC aggregate concrete can be as shuttering blocks. Samples were casted from the mixture designed by the authors, described in Table 3. The maximum
Fig. 14. Stumped concrete wall containing various no-fines aggregates (AAC is the 2nd (with Portland cement) and the 4th (with white cement) from the top).
diameter (dmax) of the aggregate was 10 mm, to be able to produce elements with minimum wall thickness of 2.5 cm. A popular production technology of manufacturing prefabricated units was simulated during which the shuttering blocks are produced in a vibration press. The fresh concrete, casted from the mix detailed in Table 3, is stabile right after compacting, thus the formwork can be
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removed, and it becomes re-useable for the next sample. This is highly advantageous in pre-fabrication, where the concrete blocks are made on a conveyor belt. It is recommended to store the blocks in a high humidity storage if they are movable (after 1 day the blocks are movable). In our laboratory there was no storage room with high humidity available, unlike in a prefabricating factory. Where it is possible, even higher compressive strength could be achieved. The size of the blocks was 20 40 20 cm with two locks (Fig. 15.). The average net density of concrete (on 150 mm edge cube) was 1255 kg/m3, while the average gross density of the block was 840 kg/m3. The compressive strength of concrete (measured on a 150 mm edge cube) was 5.32 MPa, the element strength was 1.43 MPa, as it can be seen in Table 4. This block can be used as a self-supporting element or as shuttering element filled with normal concrete as well. In addition to the strength and the density, the thermal conductivity was measured on 20 20 10 cm elements. The measured thermal conductivity was 0.34 W/mK that is 1/6 of a normal weight concrete, meaning a more beneficial thermal insulation capacity. Based on the measurements both no-fines concrete applications (shuttering elements and stumped concrete) are feasible solutions for the usage of AAC waste, however compared to the previous applications (Phase 1 and 2), they apply less amount of AAC waste. 5. Application in powdered form (phase 4) The cutting, chiseling and crushing process of cellular concrete blocks produces a significant amount of fine powder, which cannot be utilized in the later part of the construction. Besides that, during the production of AAC blocks large amount of industrial waste is generated in the form of AAC powder. This powder cannot be fully utilized in the previously described recycling solutions (Phase 1e3). Powder wastes typically applied in concrete as filler material, which in most cases decreases the strength of concrete. Until the magnitude of this decrease is not significant this is a feasible utilization of waste powder. If the grain size of powder is sufficiently small and its other properties are suitable as well, then it is possible that it will work as a supplementary material. In the present research phase fine powder was added to the concrete mix. It was shown in Section 3 that cellular concrete in crushed form is unfavorable as outdoor structural concrete aggregate. However, in the literature several researches can be found, which are proving that concrete produced from waste materials can be advantageous € (Ozalp et al., 2016; Fenyvesi, 2014). More significant performance improvement (in strength and durability) can be reached, if the waste material is added to the concrete mix in powdered form (Gonzalez et al., 2016; Aprianti S, 2017). Besides that, applying waste materials in powdered form in the concrete mix can have the following positive effects: waste material is recycled. In this case it is advantageous if as much added to the mix as possible. It reduces the cost of the concrete mix (e.g. cement is partly substituted by waste material). It can increase the performance of concrete (used as additive or as supplementary cementitious material - SCM).
Table 3 Constituents of used no-fines concrete. Constituents/dosages
liter/m3
kg/m3
cement water aggregate (crushed aerated concrete) 4e10 mm air
300 150 500 250
100 150 250 e
Therefore, it was intended to add the aerated concrete powder (ACP) to concrete mixes and investigate its performance. Normal structural concrete is mostly subjected to compressive stresses, thus compressive strength test was performed on the produced specimens. Besides that, the effect of freezing (which is the most common durability issue with outdoor concrete structures) was tested. 5.1. Concrete mixes and experiment description In the concrete mix design phase it was aimed to design a concrete, which has relatively low freeze-thaw resistance (to highlight the effect of the additive) and its strength class is widely used in the industry (C25/30). It was also intended to apply low amount of cement, to be cost effective. Based on these requirements the following mixture was designed as reference mix as it can be seen in Table 5. As it can be seen in Table 5 the maximum aggregate size was 16 mm. In this research phase CEM I cement was applied, because it does not contain any supplementary material (e.g.: slag). Thus the effect of the relatively small amount of ACP is easier to monitor. The mixture with ACP was the same as detailed in Table 5, but 10% (proportionally to the cement mass e 27 kg/m3) ACP was added to it. Note that waste material powder was added to the mix and it is not substituting cement and the value of dosage was chosen based i, 2016). The aggregate on literature recommendations (Borosnyo size of the ACP is belonged to the 0/0.25 fraction based on sieve tests (its maximum diameter was 0.09 mm). ACP is very beneficial from a production point of view, because it can be added to the concrete mix without any preparation. Using the two above defined mixtures test samples (100 100 100 mm cubes) were produced, at least three for every type of test. The samples were subjected to three laboratory tests: uniaxial compressive strength test, frost resistance test, and freeze-thaw resistance test. The uniaxial compressive strength test was performed at 28 days of age of the samples after 14 days of wet curing, as it is defined in the Eurocode standard (MSZ, 2009a). The frost resistance test is aiming to investigate the effect of frost on the compressive strength of concrete and it was carried out based on the MSZ EN 4715/3e72 standard (MSZ, 1972). After the given number of freezing cycles, the specimens were tested for compressive strength. The freeze-thaw test is similar to the frost test but in this case the amount of removed material (which scaled due to the freeze-thaw cycles) was measured (the less the better the freezethaw resistance) based on the recommendations of the standard CEN/TS 12390e9:2007 (CEN, 2006). 5.2. Test results and discussion The compressive strength test results showed that ACP highly increases the strength of concrete. Compared to the reference mixture it increased the compressive strength by 37% (reference mix used as basis), as it can be seen on Fig. 16. The increase in strength can be explained by the effect of the ACP, which introduces small particles in the mix that decreases the porosity of concrete, resulting a stronger material. Other specimen from the same mixing were subjected to freezing cycles (150 in total) and their compressive strength was measured after 50, 100 and 150 cycles. It was observed in case of the reference mix, that until 100 cycles only minor decrease in strength was measured, however after reaching a critical number of freezing
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Fig. 15. Shuttering blocks from AAC after fabrication.
Table 4 Test results measured on the shuttering blocks and on its material. Block density [kg/m3] Concrete density [kg/m3] Compressive strength of the block [MPa] Compressive strength of the concrete [MPa] Thermal conductivity (l) [W/mK] 840
1255
1.43
5.32
0.34
Table 5 Reference concrete mix design. Component Cement Water Aggregate
CEM I 42.5 N w/c 0/4 [mm] 4/8 [mm] 8/16 [mm]
Sum
cycles the compressive strength of the specimens highly decreased (with 40%). In case of the mix containing ACP only 3.5% of strength drop was measured, which is more beneficial, than the performance of the reference mix (Fig. 17). ACP increases the resistance of concrete against frost, which indicates that ACP has a positive effect on the durability of concrete and can increase the lifetime of an outdoor concrete structure. Simultaneously with the other test, the freeze-thaw resistance test was carried out as well. The weight loss of the samples was measured and the deterioration of the concrete surface was determined. As it can be seen on Fig. 18, after 56 freezing cycles the concrete with ACP lost much less weight (45%), than the reference mix. The values measured in both cases are quite high, as it was aimed during the mix design (see Section 5.1). It is also important to
Distribution [-]
Mass [kg/m3]
e 0.57 0.47 0.25 0.28 1
270 154 936 498 557 2415
note, that with air-entraining agents an even higher freeze-thaw resistance can be reached, but air-entraining agents strongly decrease the compressive strength of concrete. Altogether, in both durability tests the mix with ACP performed advantageously and it also increased the compressive strength of the material, without changing the production cost of concrete. In Phase 4 it was shown that AAC waste in fine powder form (dmax < 0.09 mm) with correct dosage (þ10%) can increase mechanical (þ37% compressive strength) and durability (65% weight loss) characteristics of normal strength concrete. Based on that, application of AAC powder as filling material is a suitable way for recycling. The results indicate that AAC powder works similarly to other supplementary cementitious materials (SCMs). AAC powder has many similarities to other SCMs, like it has high
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6. Summary and conclusion 6.1. Summary Present study is dealing with the possible recycling methods of aerated concrete construction (building and prefabrication) waste material. Besides the most obvious application (load bearing aggregate) many other utilization methods were shown (covering tiles, shuttering elements, stumped concrete) and investigated. A novel method was introduced in which AAC waste was applied in powder form in concrete. The following forms of application (see Fig. 2) was introduced applying aerated concrete waste:
Fig. 16. Average compressive strength of the specimens.
specific surface area and contains CSH crystals. These crystals can integrate with the concrete's crystal skeleton and strengthen it. It was still not proven that ACP takes an active role in the hydraulic processes during hardening of concrete, but it was seen that its positive effect depends on the particle size of ACP. If the particle size is larger than 0.09 mm the positive effect of ACP cannot be seen. Particles under this size could only contain nano pores, which are related to CSH phases (Schober, 2011). It is planned by the authors to investigate the effect of AAC powder applied as SCM and compare it to other SCMs. Besides that, the amount of ACP could be optimized as well.
Phase 0: in block form: not feasible in practice. Phase 1: in crushed form: as load bearing lightweight aggregate (LWA) in lightweight aggregate concrete (LWAC). It is possible to produce load bearing LWAC using AAC, but high cement amount is needed. The results show that it has relatively low strength characteristics and has frost resistance issues. This material can be used as masonry blocks, monolithic or prefabricated walls. Phase 2: in crushed form: as non-load bearing LWA, used for covering tiles and for architectural applications. In this application lower cement amount can be applied. Esthetical covering tiles can be fabricated. Phase 3: in crushed form: as aggregate of no-fines concrete (shuttering elements and stumped concrete). This mix has very low cement content, less chemical problems as in case of Phase 1 and proportionally more waste material can be applied. It has to be mentioned here that due to the containing of sulphate, the stockpiling of AAC is normally not allowed due to possible leaching problems. Therefore, the application of the crushed AAC aggregate in new products has to be free of leaching.
Fig. 17. Results of the frost resistance test.
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Fig. 18. Results of the freeze-thaw resistance test, measured on the sawn surface of the specimens.
Phase 4: in powdered form: as filling or supplementary material. A feasible solution was found for the recycling of AAC powder. When fine AAC powder is applied it can increase the strength and durability characteristics of concrete and works similarly to SCMs. In the next subsections the results and findings of the research Phases are summarized. 6.2. AAC as lightweight aggregate (phase 1 and 2) During the present researches several interesting physical and chemical effect were found, which affect directly the application of crushed AAC waste as lightweight concrete aggregate: crushed AAC waste can be used as not only for coarse, but also as fine lightweight aggregate in LWAC, in the crushing process high amount of fine aggregate (under 2 mm grain size) is formed for which other use/application should be founded, from recycled crushed AAC as LWA concrete can fabricated for vertical structural (prefabricated masonry block, and reinforced monolithic LWAC) elements (wall and columns) of middle height buildings due to its compressive strength performance. The elements made from this material have higher compressive strength, than other elements which are commonly applied in the building industry, undersoil elements can be also made due to the water tightness of the tested material, thinner outer walling masonry units can be fabricated from this material compared with other normal concrete or LWAC materials due to its low thermal conductivity, the dead load of this elements is half of the dead load of the normal concrete, and reinforcing steel bars can also be applied in this structural elements, the material has bad frost resistance, and high porosity, so that it cannot be applied as outer or horizontal coverings in its natural form (e.g. without coating or hydrophobisation),
it can be applied as internal vertical covering as prefabricated elements or monolithic wall, due to its load bearing capacity, ductility, good strain capacity, feeling handwarm to touch (low thermal conductivity), knowing the ettringite formation in concrete or other cement based material only sulphate resistant cements can be mixed in this LWACs! The main utility of this material is that it (as fine and coarse aggregates too) can be fabricated from 100% of construction waste only by crushing the demolished masonry elements, can manufactured in prefabrication and also on the building sites and save high amount (the aggregate gives approximately the 60 ÷ 70 vol percent of the concrete) of commonly used mined aggregates. Due to this mine rehabilitation works and costs can be also reduced. Additional utility is that if it can be applied on the building site many energy and cost can be saved of the transporting of aggregates. This solution can be economical if high amount of AAC structures are demolished in a building site, and also high amount of masonry block elements (or landfilling) are needed for constructing a new building or structure. In this case additional equipment is needed on the building site for fabricating the new elements. Fabrication cost and energy demand of the investigated material is lower than in case of commonly applied normal concrete and approximately the same as the commonly used masonry elements. The most important development in this material is that the 60e70 V% (100% of the aggregate) of it made from secondary raw material, which amount do not need to be delivered, deposited and stored from the place of origin will save a lot of resources, both in cost, time and energy, and also mean significant environmental benefit. In the next decades many AAC building and structure will reach their end of life, therefore applications should be developed to recycle this huge amount of waste material, some solutions were described according to our investigations.
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6.3. AAC as no-fines concrete aggregate (phase 3) The water absorption of AAC aggregate is extremely high therefore, prior to mixing it is pre-mixed with water. This water content of the aggregate evaporates slowly from the concrete during its hardening, which is highly beneficial in practice, because it behaves as an internal curing. 100% of the aggregate was substituted by waste material. Possibilities to apply AAC as no-fines aggregate: o stumped concrete: cannot be used as structural concrete (low compressive strength), but it can be applied as outdoor design element (no issues with frost resistance), o shuttering element: possible to utilize as structural element (e.g.: filled with reinforced concrete) and it has beneficial thermal conductivity (1/6 of normal concrete).
6.4. AAC in powdered form (phase 4) The aerated concrete powder (ACP) is the result of the cutting or crushing process of cellular concrete blocks and it can be applied in concrete mixes without any preparation. ACP added to the concrete mix with a proper dosage (10% compared to cement amount) can increase the compressive strength of concrete with 37%. It means that 10% of the amount of cement (the most expensive part of the concrete mix) can be substituted by a waste material. ACP can also increase the frost and freeze-thaw resistance of normal concrete, which is advantageous in case of outdoor concrete structures and can increase their lifetime. During the frost resistance test, after 150 freezing cycles, it lost less than 5% of its compressive strength, which is only slightly higher than the measurement error limit. 7. Future work Continuing the presented research, authors are intending to investigate ACP applied in concretes with higher cement amount. It is also planned to use different cement types and different density AACs to generalize the results. Besides that, the long-term effects of ACP is planned to be investigated. Acknowledgments Authors are grateful to the Hungarian Scientific Research Fund (OTKA) for the financial support of the OTKA K 109233 research project. Special thanks to Bengineer House Kft., Duna-Dr ava Cement Kft., ria Kft., MC-Bauchemie Kft. and BASF Hunga ria Kft. for SIKA Hunga providing the materials used in the experiments. References Aprianti S, E., 2017. A huge number of artificial waste material can be supplementary cementitious material (SCM) for concrete production e a review part II. J. Clean. Prod. 142, 4178e4194. zsa, Z., Csa nyi, E., Tomor, K., 1989. A Ga zbeton Zsugoroda s Balazs, G., Jo €kkente se nek Leheto } se ge Homok Adagola ssal (Possibilities to Reduce Cso Shrinkage of Gas Concrete with Sand Dispensing, in Hungarian), Test Report. Budapest University of Technology, p. 64. i, A., 2016. Long term durability performance and mechanical properties of Borosnyo high performance concretes with combined use of supplementary cementing materials. Constr. Build. Mater. 112, 307e324. https://doi.org/10.1016/ j.conbuildmat.2016.02.224. Carsana, M., Tittarelli, F., Bertolini, L., 2013. Use of no-fines concrete as a building material: strength, durability properties and corrosion protection of embedded steel. Cement Concr. Res. 48, 64e73 https://doi.org/10.1016/ j.cemconres.2013.02.006.
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