the use of fly ash in completely recyclable...

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 35

THE USE OF FLY ASH IN COMPLETELY RECYCLABLE CONCRETE

M. De Schepper, P. Van den Heede, N. De Belie, Magnel Laboratory for Concrete Research, Ghent University, Belgium

ABSTRACT: The concept of completely recyclable concrete (CRC) has been developed to close the loop. By adequately incorporating limestone aggregates and industrial by-products, the chemical composition should resemble the composition of Portland clinker which enables the use of CRC as a raw material in Portland cement production. This paper investigates whether the chemical composition can be adjusted by partially replacing cement with pozzolanic fly ash. CRC mixtures were produced with 50% fly ash and 50% ordinary Portland cement or blast-furnace cement. At the age of one month laboratory clinker and cement were produced from the CRCs. The resulting cement had a low compressive strength and hydraulic reactivity caused by a high belite content, so corrections were necessary. It is concluded from this paper that cement with hydraulic properties can be produced from CRC, but the chemical composition of all used concrete materials needs to be known for recycling without modification.

1 INTRODUCTION

According to Lauritzen and Hahn [Lau92] approximately 500-1000 kg/habitant Construction and Demolition Waste (CDW) is produced annually in developed countries, and these numbers are continuously growing. As the construction sector uses 50% of the earth’s raw material and produces 50% of its waste [Ani96], the development of more durable and sustainable building products is crucial. To reduce the impact of building products on the environment, the recycling of CDW is of great importance and offers three benefits [Edw99]: reduce the demand upon new resources, cut down on transport as well as production energy costs and recycle waste which would otherwise be lost as landfill. An approximation of the average composition of CDW is given in Fig. 1.1. In this figure is shown that a great part of CDW (roughly 40%) consists of concrete, which indicates the importance of research towards concrete recycling.

Fig. 1.1. Approximation of the average CDW composition (excl. earth, sand, dirt). [DeB07]

36 DE SCHEPPER, VAN DEN HEEDE , DE BELIE: The Use of Fly Ash in Completely Recyclable Concrete

Today, CDW is mainly (for about 90% [Tam06]) used in low level applications, namely as unbound material for foundations, e.g. in road construction. However, all mineral demolition waste could be recycled as crushed aggregates for concrete, but these reduce the compressive strength and affect the workability due to higher values of water absorption [Top04]. It has been intensively studied which quantities of recycled aggregates can be applied in concrete without negatively affecting the properties, and an amount of 20% is acceptable [Bon07]. Furthermore the use of recycled aggregates in concrete may increase the risk of durability problems, such as frost-thaw damage [Gok04], alkali-silica reaction [Des99], or chloride induced reinforcement corrosion [Mas01].

Nowadays, the cement industries tend to use alternative raw materials for the production of cement and clinker. For instance, the use of CDW within the cement production was investigated by Galbenis et al. [Gal06]. To advance this use of CDW, Completely Recyclable Concrete (CRC) is designed for reincarnation within the cement production, following the Cradle-to-cradle (C2C) principle [McD02]. In C2C production, all material inputs and outputs are seen either as technical resources or as biological nutrients. Biological nutrients can be composted or consumed and technical resources can be recycled or reused without loss of quality. CRC is an example of the latter and becomes a technical resource for cement production because the chemical composition of CRC will be similar to that of cement raw materials. In the current study, fly ash was used to achieve an optimal chemical composition of CRC. If CRC is used on a regular basis, a closed concrete-cement-concrete material cycle will arise, which is completely different from the current life cycle of traditional concrete.

2 METHODS AND MATERIALS

2.1 Design for reincarnation

Completely Recyclable Concrete is intended to be recycled as raw material within the cement production from a predesign stage. Therefore the chemical composition of CRC is aimed to be that of traditional cement raw meal, of which the requirements are well known. When this raw meal with its accurate chemical composition, is heated up to about 1450 °C, traditional Portland clinker with hydraulic properties is formed. This Portland clinker exists for about two thirds of calcium oxide (CaO), which makes it necessary to incorporate limestone aggregates into CRC. The second most important oxide is siliciumdioxide (SiO2), which is found in sand, fly ash and clay. The other components, Al2O3 and Fe2O3, are often present in the materials providing CaO and SiO2.

CRC is designed after determining the chemical composition of concrete materials. The designed CRC mixtures were evaluated by parameters normally used for the assessment of traditional cement raw meals. These parameters are the lime saturation factor (LSF), the silica modulus (SM), the alumina modulus (AM) and the hydraulic modulus (HM). The LSF is the ratio of the lime available in the raw meal, to the lime chemically necessary to react with the present SiO2, Al2O3 and Fe2O3 [Tay97]. Unlike the LSF, which has a theoretical basis, the other parameters are empirical. The SM has a major influence on the formation of the melt. The AM only has a significant effect on clinker formation at low temperatures. The HM is used to evaluate the hydraulic activity regarding the strength development of the cement. The boundaries for these parameters are given in Table 3.1.


In this study the accurate chemical composition of CRC is achieved by using limestone aggregates and fly ash as cement replacement. The mixture proportions of the produced CRCs are presented in Table 2.1. Two types of cement were used: ordinary Portland cement (CEM I 52.5 N; CRC I) and blast-furnace cement (CEM III/A 42.5 N LA; CRC III). The influence of the cement replacement on the properties of concrete will not be discussed within this paper.

Table 2.1. Concrete mixture proportions (W/B = water to binder ratio) Concrete materials CRC I CRC III River sand [kg/m³] 135.6 118.7 Limestone sand [kg/m³] 526.8 547.1 Limestone aggregate 2/6 [kg/m³] 353.7 350.0 Limestone aggregate 6/20 [kg/m³] 677.8 678.2 CEM I 52.5 N [kg/m³] 221.7 - CEM III/A 42.5 N LA [kg/m³] - 221.7 Fly ash [kg/m³] 221.7 221.7 Water [kg/m³] 177.3 177.3 W/B [-] 0.40 0.40

2.2 Regeneration of cement

The designed CRC was produced and cured according to NBN B 15-001 (2004). The produced cubes were tested for their compressive strength at the age of one month. These tested cubes were then crushed by hand and ground in a laboratory mill. After mixing the resulting powders with water, small tablets (d = 5 mm and h = 5 mm) were formed in a perforated PVC-plate which is then dried at room temperature for 24 hours.

A small amount of each cement raw meal was burned at 1350 °C, 1400 °C and 1450 °C in a laboratory furnace for 30 minutes. The clinker formed within this heating process was used for microscopic analysis and measuring the free lime content. For the production of cement, a bigger amount was burned in a laboratory furnace by raising the temperature at a constant rate (15 °C/min) to the maximum temperature and maintaining this temperature for 30 minutes. After burning, all clinkers were cooled in air.

The regenerated cements of CRC I and III were produced by grinding the regenerated clinker burned at 1350 °C with calcium sulphate, aiming for a SO3 to Al2O3 ratio of 0.6 [Tay97]. Calcium sulphate hemihydrate and anhydrite were respectively added to the clinker regenerated from CRC I and III. Small amounts of cement were produced from all raw meals by grinding the clinker with calcium sulphate anhydrite to measure the hydration heat by isothermal calorimetry. The cements produced from CRC I LF (see section 2.3) were tested after grinding for 10 s, 20 s or 30 s to evaluate the influence of the fineness on the cement reactivity.

2.3 Raw meal correction

Belite clinker and cement was identified by testing the clinkers and cements produced from the CRCs (see section 3.1). Belite cement has many advantages for the environment [Mor07], [Kac09], such as a lower CO2 emission and a lower energy consumption. However, the main disadvantage is the lack of early strength. Where alite delivers its final strength after 28 days, belite delivers it only after one year.


In order to produce a regenerated clinker with a high hydraulic reactivity limestone filler was added to the milled CRC I to obtain a CaO/SiO2-ratio of 2.9 [Kac09] and an accurate value for the LSF (CRC I LF). After calculations it was seen that the addition of 42.7 w/w% limestone filler was needed. This correction should make it possible for alite to be formed (see section 3.1), resulting in a reactive clinker.

2.4 Testing of clinker and cement properties

The quality of the produced clinkers was evaluated by determining the free lime content. The ethylene-glycol method based on BS EN 196-2 and EN 196-2 was used to determine this parameter. On the clinkers from CRC I and III also a microscopic analysis was performed. The clinker sections were etched with HF and then observed using a light microscope. An X-ray diffraction analysis was executed, using a BRUKER D8 ADVANCE apparatus (CRC I and III) or a SIEMENS D5000 DIFFRACTOMETER (CRC I LF).

Cements produced from CRC I and III were tested on their properties by determining the compressive strength according to NBN EN 196-1 (2005). The hydration heat was measured for all cements with an isothermal calorimeter (TAM AIR).

3 RESULTS AND DISCUSSION

3.1 Production of belite clinker and cement

Within the design of the CRC, the assumption was made that the chemical compositions of the fine aggregates (limestone aggregate 2/6) and the coarse aggregates (limestone aggregate 6/20) were similar to that of the limestone sand. Afterwards, a chemical analysis showed that the CaO and SiO2 content of the limestone aggregates were respectively over- and underestimated. The latter has considerable consequences for the parameters LSF, SM, AM and HM and the mineral composition (see Table 3.1). The significant lack of lime prevents the formation of alite, and belite cement will be regenerated. An explanation for the absence of alite is found within the heating process. In this heating process, belite (2CaO.SiO2) is the first formed calcium silicate. Starting from a temperature of 1300 °C, alite (3CaO.SiO2) will be formed with the present belite and free lime. Thus, if a temperature of 1300 °C is reached in the heating process and all free lime is consumed for the formation of belite, alite can not be formed.

3.2 Quality of the regenerated clinker

In Table 3.2 the free lime content of all clinkers is presented. To prevent the expansion of free lime in cement, the free lime content should be less than 3% [Bel06], which is the case for all clinkers, except CRC I LF burned at 1350 °C.

The conclusion that the burning temperature was satisfactory if the free lime content is below 3% can’t be drawn. The low free lime content of the clinker regenerated from CRC I and III is due to the low lime content of the raw meal, causing a lot of belite and no alite was formed. For the clinker regenerated from CRC I LF a clear decrease of the free lime is seen between 1350 °C and 1400 °C, which is due to the consumption of CaO by the alite formation between


both temperatures. The small increase between 1400 and 1450 °C is negligible as the reproducibility of this test is ± 0.2%.

Table 3.1. Values of LSF, SM, AM and HM and mineral composition of CRC Designed mixture Real mixture CRC I CRC III CRC I CRC III

Limits (Desired limits)

Parameters (limits and formulas according to [Gal06]) LSF [-] 0.96 0.95 0.62 0.63 0.66-1.02 (0.92-0.96) SM [-] 3.26 3.19 4.15 3.98 1.9-3.2 (2.3-2.7) AM [-] 2.75 3.41 2.80 3.20 1.3-2.5 (1.3-1.7) HM [-] 2.24 2.21 1.49 1.50 1.7-2.3 (~2)

Mineral composition using Bogue calculations (limits according to [Bel06]) C3S [w/w%] 74.12 69.75 -26.32* -24.99* 40-75 C2S [w/w%] 10.10 13.18 109.87* 107.15* 10-35 C3A [w/w%] 10.53 11.98 11.40 12.50 0-15

C4AF [w/w%] 5.73 4.95 6.06 5.60 1-20 * Due to a high SiO2 and low CaO content the formulas of Bogue do not seem applicable

Table 3.2. Free lime content [w/w%] of regenerated cements burned at different temperatures fCaO [w/w%] after burning at … 1350 °C 1400 °C 1450 °C

CRC I 0.14 0.11 0.11 CRC III 0.15 0.11 0.18

CRC I LF 4.85 1.79 2.15

The micrographs of the clinkers produced from CRC I and III turned out to be very similar. In Fig. 3.1, the micrographs of the clinker regenerated from CRC I show a less dispersed porosity and a more homogeneous crystallization by increasing the burning temperature, which results in a better clinker. Looking at the crystals more in detail (Fig. 3.2), a non-traditional clinker is observed. These main round brown crystals are difficult to identify. The round form suggests belite, however the brown color suggests alite [Hew98]. In addition, the interstitial material is not traditional and difficult to identify.

Also, the X-ray diffraction analyses were clearly similar for both clinkers regenerated from CRC I and III. The X-ray diffraction pattern of the clinker regenerated from CRC I is shown in Fig. 3.3. In this figure only belite (2) and no alite, aluminate (3CaO.Al2O3) or ferrite (4CaO.Al2O3.Fe2O3) could be observed, the phases that are found in traditional clinkers. From the latter we can indeed conclude that the round brown crystals observed in the micrographs are belite. The observed akermanite (10), which disappears at 1450 °C, and bredigite (11) are phases which can be found in blast-furnace slag and clinker with a high Al2O3 and MgO content. Furthermore, akermanite can occasionally be found in fly ash [Tay97], [Haw98], [Odl00]. In contrast with the clinker from CRC I and III, the formation of alite and to a lesser degree belite was confirmed by the X-ray diffraction analysis of the clinker regenerated from CRC I LF. The presence of this alite should result in a more reactive clinker and cement.


Fig. 3.1. Micrographs of regenerated clinker from CRC I burned at 1350 °C (left), 1400 °C (middle)

and 1450 °C (right). The clinker section is etched with HF-vapour and observed with light microscopy. The white arrows indicate the pores.

Fig. 3.2. Micrographs of regenerated clinker from CRC I burned at 1350 °C (left), 1400 °C (middle) and 1450 °C (right). The clinker section is etched with HF-vapour and observed with light microscopy.The white and black arrows respectively indicate the (belite) crystals and the interstitial material.

Fig. 3.3. XRD-pattern of regenerated clinker from CRC I burned at 1350 °C, 1400 °C and 1450 °C.

The absence of akermanite after burning at 1450 °C


3.3 Hydraulic properties and compressive strength of the regenerated cement

For the regenerated cement from CRC I and III, the compressive strength at 28 days was respectively found to be 3.96 MPa and 7.13 MPa. Compared to traditional Portland cement, the compressive strength is low due to the high belite content of the clinker and the low specific surface area (Blaine, according to NBN EN 196-6 (1991)): 3228 cm²/g and 2616 cm²/g respectively, which should reach 4800 cm²/g for a cement with strength class 52.5. The difference between the regenerated cement from CRC I and III is probably due to the addition of different types of calcium sulphate (hemihydrate or anhydrate).

The heat production rate of the regenerated cements is displayed in Fig. 4.1. For the cements produced from CRC I and III no peaks are observed and the reactivity is negligible as only a total heat of respectively 57.86 J/g and 69.83 J/g is produced after 7 days. A reactive cement was produced from CRC I LF, as the heat production rate summits once or twice within the same range of traditional Portland cement (CEM I 52.5 N). For these cements the accelerating effect of a higher fineness is clearly seen. Also the total heat release, and thus the hydration degree, is higher with a higher fineness. Comparing the total heat releases, the cements burned at 1400 °C are the most reactive. The cement of CRC I LF burned at 1400 °C and ground for 30s is the most reactive and reaches a total heat release of 312.78 J/g after 7 days, which is 90% of the heat release of traditional Portland cement (CEM I 52.5 N).

The very first hydration peak caused by the first hydration reactions at the moment of mixing the samples, will be neglected within this paper. Then, the first peak observed for all cements regenerated from CRC I LF, is likely caused by the hydration of alite, similar to the hydration of CEM I 52.5 N. The source of the second peak, which is observed for the cement burned at 1400 °C, has not been identified yet. Maybe the heat production rate summits after the first C3S peak due to the formation of monosulphate out of ettringite and hydrated aluminate. But on the other hand, this explanation is overruled by the absence of this peak for the cement regenerated from the same raw meal and burned at 1450 °C. A higher burning temperature should not have an effect on the aluminate content, influencing this peak. Also, performing the hydration tests, the glass ampoules containing the cement pastes all cracked if originating from CRC I LF, due to an expansive reaction. It is probable that these cracks go hand in hand with the second peak, as the cracks were more serious for the cement burned at 1400 °C. The hydration of free lime seemed to be avoided by limiting the free lime content to 3%. Therefore, the limit of 3% is perhaps not strict enough, or the expansion was caused by another reaction, both making further research necessary.

4 CONCLUSIONS

In order to produce a CRC with good recycling prospects, the main conclusion for this study is that it is very important to know the chemical composition of all used cement materials, as the limits for LSF, SM, AM and HM are very strict. Besides the quality of the clinker, the specific surface area (Blaine) and the type of the added calcium sulphate are also important parameters for the hydraulic reactivity and the compressive strength of the cement.

Regarding further research, the properties of CRC regarding compressive strength and durability need to be verified. It can also be noted that the use of industrial by-products like fly ash and blast furnace slag is recommended. With a lower Portland clinker content, the emissions of CO2 and the use of energy, resulting from the clinker manufacturing process can be reduced significantly.


Thinking ahead, there is a need for high level recycling applications for concrete rubble. From this and other [McD02], [Tam04] studies, it seems that the development of Completely Recyclable Concrete could be a solution. This CRC does not lead to CDW and instead becomes a useful material for the cement production. Finally, by the design for reincarnation, the valuable earth’s natural resources are also given a second life.

0 12 24 36 48 60 72 0

5

10

15

20

Cement paste age (h)

q (J

/g/h

)

CRC ICRC IIICRC I LF 1400 °C 10sCRC I LF 1400 °C 20sCRC I LF 1400 °C 30sCEM I 52.5 N

0 12 24 36 48 60 72

0

5

10

15

20

Cement paste age (h)

q (J

/g/h

)

CRC I LF 1450 °C 10sCRC I LF 1450 °C 20sCRC I LF 1450 °C 30sCEM I 52.5 N

Fig. 4.1. The heat production rate q (Joule per g binder per h) of cement pastes with W/C = 0.4

under isothermal conditions (20 °C)

ACKNOWLEDGMENTS

The author wants to thank the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for financial support.

REFERENCES

[Lau92] Lauritzen E. K., and Hahn N. J. (1992). Building Waste-Generation and Recycling, ISWA.

[Ani96] Anink D., Boonstra C., and Mak J. (1996). Handbook of sustainable building, James & James, London.

[Edw99] Edwards B. (1999). Sustainable architecture: European directives and building design, Architectural Press, Oxford.

[DeB07] De Belie N. and Robeyst N. (2007). Recycling of construction materials. In: Kashino N., Van Gemert D., Imamoto K. (eds.) Environment-conscious construction materials and systems. State of the art report of TC 192-ECM. RILEM Report Nr. 37, RILEM Publications S.A.R.L., Bagneux, 11-23, ISBN: 978-2-35158-053-0.

[Tam06] Tam V. W. Y. and Tam C. M. (2006) “A review on the viable technology for construction waste recycling.” Resources, Conservation and Recycling, 47(3), 209-221.

[Top04] Topcu I.B. and Sengel S. (2004). “Properties of concretes produced with waste concrete aggregate.” Cement and Concrete Research, 34(8), 1307-1312.

[Bon07] Bonte K. and Van Laethem K. (2007). Puingranulaten in stortbeton klassen C12/15 en C16/20. Vergelijkende prestatiestudie. Eindwerk KHBO, Opleiding industrieel ingenieur bouwkunde.


[Gok04] Gokce A., Nagataki S., Saeki T. and Hisada M. (2004). “Freezing and thawing resistance of air-entrained concrete incorporating recycled coarse aggregate: The role of air content in demolished concrete.” Cement and Concrete Research, 34(5), 799-806.

[Des99] Desmyter J., Blockmans S., and De Pauw P. (1999). “Puingranulaten en gerecycleerd beton: nieuwe resultaten en ontwikkelingen (Rubble aggregates and recycled concrete: new results and developments).” WTCB Tijdschrift, Herfst 1999, 11-19.

[Mas01] Masters N. (2001). Sustainable use of new and recycled materials in coastal and fluvial construction: a guidance manual, Thomas Telford, London.

[Gal06] Galbenis C.-T. & Tsimas S. (2006). Use of construction and demolition wastes as raw materials in cement clinker production. China Particuology, 4(2), 83-85.

[McD02] McDonough W. & Braungart M. (2002). Cradle-to-cradle. Remaking the way we make things. North Point Press, ISBN 0-86547-587-3.

[Mor07] Morsli K., de la Torre A.G., Zahir M. & Aranda M. A. G. (2007). “Mineralogical phase analysis of alkali and sulphate bearing belite rich laboratory clinkers.” Cement and Concrete Research, 37, 639-646.

[Kac09] Kacimi L., Simon-Masseron A., Salem S., Ghomari A. and Derriche Z. (2009). “Synthesis of belite cement clinker of high hydraulic reactivity, Cement and Concrete Research, 39, 559-565.

[Tay97] Taylor H. F. W. (1997). Cement chemistry. Thomas Telford, London, second edition. [Bel06] Belgische BetonGroepering (2006). Betontechnologie. Belgische BetonGroepering. [Hew98] Hewlett P. C. (1998). Lea’s chemistry of cement and concrete. Arnold, London, fourth

edition. [Odl00] Odler I. (2000). Special Inorganic Cements, Taylor & Francis. [Tam04] Tamura M., Noguchi T. & Tomosawa F. (2004). Cementitious waste-free-type

completely recyclable concrete. In Kashino N. & Ohama Y. (eds.), Proceedings of the RILEM International Symposium on Environment-Conscious Materials and Systems for Sustainable Development, 61-71.

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