Stefan Jacobsen 1 and Per Jahren 2 ABSTRACT. exposed hardened concrete surface, amount of recycled concrete (in which the carbonation

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1 Paper submitted to CANMET/ACI three day symposium on sustainable development and concrete technology, San Francisco, USA, September , ver. Nov Binding of CO 2 by Carbonation of Norwegian OPC Concrete by Stefan Jacobsen 1 and Per Jahren 2 ABSTRACT The amount of CO 2 emitted from OPC clinker production and later bound by carbonation was calculated by 6 parameters: CO 2 bound per unit of OPC fully hydrated and carbonated (found to be 0.3 g CO 2 /g OPC), concrete quality distribution, carbonation depth, exposed hardened concrete surface, amount of recycled concrete (in which the carbonation will be highly accelerated) and CO 2 emission per unit clinker produced. Our analysis shows that 11 % of the CO 2 emission from production of clinker is re-absorbed in Norwegian concrete. This is not included in environmental data on cement and concrete, although significant compared to proposals in the national follow up-plan of the Kyoto Protocol. The same data collection and analysis should be carried out for all countries. The practical lower limit of CO 2 -emission by calcination of limestone and use of 100 % CO 2 -neutral fuel in clinker production was found to be 0.4 g CO 2 emitted/g clinker produced. Fully carbonated cement based products can reduce this. Possible cement production with carbonated materials as raw material can mean further reduction. These are important areas of product development and research towards a sustainable development. Keywords: concrete, cement, carbonation, CO 2 -binding, recycling, product development 1 dr.ing., Norw. Building Res. Inst., P.O.box 123 Blindern, 0314 Oslo, Norway, stefan.jacobsen@byggforsk.no 2 P.J.Consult A/S, Vakåsveien 79, 1364 Hvalstad, Norway, pajahren@online.no

2 1 INTRODUCTION During production of cement and concrete, CO 2 is emitted due to the calcination of limestone and the fossil fuel used in the cement process, as well as some emissions in the further manufacturing and handling of cement and concrete. The binding of CO 2 by carbonation of hardened concrete, however, has so far not been included in figures on environmental burdens associated with production and use of cement and concrete. Estimates of this binding have been in the order 5-25 % of the emission during production of cement and concrete according to Jahren (1). The CO 2 -binding will also include the accelerated binding that occurs when concrete structures are demolished, crushed and used as recycled concrete aggregate (2). The calculation presented may serve as a tool in the further development to reduce CO 2 -emissions associated with concrete. It can also be used for calculation of binding of CO 2 in other countries by collecting and entering data in the same way as we have done. METHOD AND DATA 6 parameters are used for the analysis: CO 2 bound per unit of cement fully hydrated and carbonated, concrete quality distribution, carbonation depth, exposed hardened concrete surface, amount of recycled concrete (in which the carbonation will be highly accelerated) and CO 2 emission per unit clinker produced. These are the basic factors controlling the CO 2 - binding, although somewhat simplified compared to the multitude of effects that normally are associated with most chemical and transport related processes in concrete technology. CO 2 bound per unit of cement fully hydrated and carbonated Traditionally, CH (= Ca(OH) 2 ) is assumed to be the main carbonable hydration product. According to the reaction stoicheiometry of Ordinary Portland Cement (OPC)

3 2 hydration proposed by Herholdt et al. (3), and assuming that 75 % of OPC is C 2 S and C 3 S, 1 g OPC produces g of CH when fully hydrated by the following reactions (3): 2 C 3 S + 6 H = C 3 S 2 H CH 2 C 2 S + 4 H = C 3 S 2 H 3 + CH Conservatively we assume binding of g of CO 2 when fully carbonated (CH+CO 2 = H+CaCO 3 ). If we assume that OPC contains 65 % of CaO, then 0.165/(0.65 * 44/56) = 32.3 % of the CaO in OPC is carbonable (weights: CaO: 56, CO 2 : 44). Matala (4), however, identified 17 different carbonation reactions in a thorough literature survey. 60 % of the CaO in concrete was said to be carbonable. Assuming that the mean molar balance between CaO and CO 2 is 1:1 like in the traditional carbonation reaction, we get * (60/32.3) = grams of CO 2 bound per gram of OPC fully hydrated and carbonated. It is therefore realistic to use 0.3 gram CO 2 bound per gram OPC fully hydrated and carbonated. Concrete quality distribution Tables 1 and 2 show distribution of quality of consumed historical and present-day concrete, respectively, based on the authors knowledge about the Norwegian concrete market. Carbonation depth Figures 1 and 2 show time vs. carbonation depths for various concrete qualities to which the concrete is assumed fully carbonated. We assume that total carbonation reaches a maximum depth of mm depending on concrete quality. The average is 27 mm for historical concrete and 15.5 mm for present day concrete. These are given in both Tables 1 and 2, and in Figures 1 and 2. In present-day concrete we use the carbonation depth after 20 years of exposure. Using a finite maximum carbonation depth in structures is due to the

4 3 limitation of carbonation caused by maximum wetting and drying depths, as proposed by Ho et al (5). Total CO 2 -binding per year is the sum of the average maximum carbonation of the average concrete quality in structures and the carbonation in recycled (crushed) concrete. Exposed hardened concrete surface 90 % of all concrete structures are assumed to carbonate unidirectionally as an infinite slab with thickness 10 cm (or from both sides of a slab with thickness 20 cm). This means 90 % of 8 m 2 of exposed surface per cubic metre of concrete consumed. This figure is chosen due to uncertainties about the amounts of continuously too wet, too dry and coated concrete. Amount of recycled concrete According to Norwegian statistics, some million metric tonnes of demolished concrete are produced annually (6). Compared to an annual consumption of about 4,5 million m 3 concrete (approximately 10 million tonnes), recycled/crushed concrete at present makes up 10 % of the consumption of new concrete. We assume that all recycled concrete is fully carbonated within a year after the crushing since the cement paste of the concrete is fragmented and left in thin layers after demolition, as observed by Jacobsen et al. (7). CO 2 -emission per unit cement produced The Norwegian cement industry consists of two plants. The best one emitted 0.81 tonnes of CO 2 /tonne of clinker produced in The average for the two plants was 0.85 according to the environmental reports from the cement manufacturer NORCEM (8). Somewhat variable over years, Norwegian cement plants export 30 % of their production. Import of cement is less than 20 % of national consumption. Including this uncertainty, our calculation data are judged to be on the conservative side.

5 4 RESULTS AND DISCUSSION Based on the 6 parameters presented, the fraction of emitted CO 2 that is bound by carbonation, is calculated simply by multiplying binding with carbonated portion and dividing by emission per tonne of clinker produced. CO 2 -binding in historical concrete Structures: 0.3*0.27*0.9*(1/0.85) = g CO 2 bound/g CO 2 emitted Recycled : 0.3*0.73*0.1*(1/0.85) = Total: = g CO 2 bound/g CO 2 emitted This means that 11 % of the CO 2 emitted from clinker production in Norway is again absorbed by carbonation of our historical concrete. CO 2 -binding in present day concrete quality Structures: 0.3*0.155*0.9*(1/0.85) = g CO 2 bound/g CO 2 emitted Recycled : 0.3*0.1*0.845*(1/0.85) = Total: = g CO 2 bound/g CO 2 emitted Comparing historical and present-day concrete we see that in the latter case, the CO 2 - binding is lower, as expected due to increased use of dense concrete: 8 % of the emission from clinker production. Increasing the amount of recycled concrete will increase the binding in present day concrete. The 20 year-limit on the total carbonation depth of present day concrete has been chosen due to the probable reduced life cycle of concrete structures in the future. This is because of expectedly faster changes in industry, business, infrastructure, settlements/housing etc. This should be reflected in an increased portion of recycled concrete and thus increased binding. Keeping an estimate of 11 % binding of CO 2 -emission as in historical concrete is therefore realistic.

6 5 Significance compared to the Norwegian follow-up of the Kyoto-protocol. The annual cement consumption in Norway is approximately 1.5 million tonnes. This gives a binding by carbonation close to tonnes of CO 2. This figure is equal to the binding assumed in the Norwegian follow-up plan to the Kyoto protocol (9) in the period by establishing 300 km 2 of new forest. There are also other interesting actions and figures in (9) to compare with. The binding of CO 2 by carbonation of concrete is thus important, but has not yet been taken into account in figures on environmental burdens of cement and concrete, as far as we are concerned. Our point here is simply that the right figures should be used, and the presented method is available for calculation globally. Practical and theoretical lower limit for CO 2 -emission By exchanging all fossil cement fuel with CO 2 -neutral fuel, i.e. synergistic fuel based on organic waste that would have been burnt anyway, the CO 2 emission in clinker production based on calcination of limestone can be reduced to the calcination portion. This makes up approximately 0.5 g CO 2 emitted/g clinker produced. The practical lower limit will depend on the carbonation as shown in this paper. Also, the amount of concrete that needs protection against reinforcement corrosion must be taken into account. Using the mentioned distribution between structures and recycled concrete, the practical limit will be (0.112*0.85) = approximately 0.4 g CO 2 emitted/g clinker produced. The theoretical lower limit for CO 2 - emission will be between 0.4 and ( ) = 0.2 g CO 2 emitted/g clinker. How close we can get depends on how much the carbonation can be increased in those cement based materials that are not designed to protect black steel from corrosion. Product development and research There are several possibilities for product development towards increased CO 2 - binding in addition to increasing the crushing and recycling of concrete. One example was

7 6 presented by Bhanumatidas et al in (10). Jahren gave an overview of several other possibilities in (1,11). Fully carbonated cement based products should be developed to a larger scale than today since carbonation is well known to increase both strength and density of non-reinforced OPC concrete. This also contributes to reduce the CO 2 -emission further down towards the theoretical limit of 0.2 g CO 2 emitted/g clinker produced from limestone in a 100 % synergistically fuelled clinker production. If other raw materials than natural limestone are used in clinker manufacturing, e.g. recycled concrete in the raw feed, the emission during calcination can become even lower. The technical and environmental properties of such OPC are known to be excellent according to Sanchez et al (12). The CO 2 bound by carbonation, however, will then have to be regarded as part of a closed loop. The validity of the calculation of binding presented in this paper will consequently be reduced with increasing portion of recycled concrete in the raw feed. Product development integrated in the cement production comes in addition to the former kind of approaches that are related to development of concrete and cement based materials. Others (10-14) also discussed both types. There is also other research ahead. Some areas are: sensitivity studies of parameters and uncertainties in data, actual carbonation in existing concrete, and measurement of binding and gradients or profiles of binding for various cementitious materials by for example comparing surface- and core concrete. The use of better transport models for rate of carbonation can also be used to fit the data on carbonation profiles. Finally, a complete enduse analysis of concrete should be made.

8 7 CONCLUSION Calculation based on 6 parameters shows that 11 % of the emission from production of clinker in Norway is re-absorbed in our concrete by carbonation. This includes accelerated carbonation in recycled concrete. The binding used was 0.3 g CO 2 /g OPC fully hydrated and carbonated. The re-absorption is not included in environmental data on cement and concrete, although significant compared to proposals in the national follow up-plan of the Kyoto Protocol. The practical lower limit of CO 2 -emission from OPC-clinker production by calcination of limestone and use of 100 % CO 2 -neutral fuel is 0.4 g CO 2 emitted/g clinker produced. The theoretical lower limit is between 0.4 and 0.2, depending on, product development of fully carbonated cement based products and recycling. Introduction of carbonated material like recycled concrete in the raw feed could reduce the emission even further. However, the effect of carbonation will be reduced when going towards a closed clinker loop. Our calculation method can be used in all countries by collecting and entering national data on emission, concrete production etc. The right global figures for CO 2 -emission can then be established as aid in the further product and process development towards a sustainable development. REFERENCES 1. Jahren P.: Sustainable development for the cement and concrete industry, Betong Industrien no.4 (1998) pp (In Norwegian). 2. Jacobsen S.: Use of recycled concrete in building and construction, Norw. Road. Authorities conf. Oct. 19, Clarion Hotel Gardemoen (1999) 10 p. (In Norwegian) 3. Herholdt A.D. et al.: Beton-Bogen, Aalborg-Portland ISBN (1979) 719 p. (In Danish)

9 8 4. Matala S.: Effects of carbonation on the pore structure of granulated blast furnace slag concrete, Helsinki Univ. of Tech. ISBN (1995) 161 p./14 app. 5. Ho D. et al.: Carbonation of concrete incorporating fly ash or a chemical admixture. Proc. CANMET/ACI (Ed.V.M.Malhotra), ACI SP-79 (1983) p Norwegian Bureau of Statistics (SSB) Weekly statistics no.50 (1999) pp.5-6 (In Norwegian) 7. Jacobsen S., et al.: Properties and frost durability of recycled aggregates and -concrete from Oslo, Norway. Proc. Sustainable construction use of recycled concrete (Ed.R.K.Dhir), Thomas Telford, London, ISBN (1998) pp NORCEM environmental reports (1998) Brevik plant 9 p. and Kjøpsvik plant 10 p., (in Norwegian) 9. The Norwegian follow-up plan of the Kyoto protocol, Stortingsmelding nr.29, The Norwegian Ministry of The Environment, 90 p. (1998) (In Norwegian) 10. Bhanumathidas N., Kalidas N.: FaL-G as brick and cementitious material: an effective sink for CO 2, Three-Day CANMET /ACI Int. Symp. on Sustainable Development of the Cement and Concrete Industry (Ed.V.M.Malhotra) (1998) pp Jahren P.: Sustainable development of cement and concrete: Two practical examples of typical implications for the future, ref as (10), pp Sanchez J.A. et al: Reuse of building rubble in cement manufacturing, ref as (7), pp Damtoft J.S.: Use of fly ash and other waste materials as raw feed and energy source in the Danish cement Industry, ref. as (10), pp Glavind M., Munch-Petersen. C. et al.: Green concrete for the future, ref as (10), pp.31-42

10 9 Table 1. Distribution and total carbonation of historically consumed concrete qualities Concrete Portion (%) C45 15 C35 25 C25 25 C15 35 Mean carbonation: Total carb. (mm) weighed 1,5 4 7, mm Table 2. Distribution and carbonation of present day concrete qualities Concrete Portion (%) C45 15 C35 40 C25 25 C15 20 Mean carbonation, 20 years: Total carb. (mm) weighed 1,4 5,2 4,5 4,4 15,5 mm

11 10 Figure 1. Carbonation depths of different concrete qualities 100 C45 Total carbonation (mm) 10 C35 C25 C year 100 Figure 2. Annual carbonation. 6 Annual carbonation (mm) C45 C35 C25 C year

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