Strength related global warming potential of fly ash (+ silica fume) concrete with(out) mass/economic allocation of the by-products impact

Transcription

1 Strength related global warming potential of fly ash (+ silica fume) concrete with(out) mass/economic allocation of the by-products impact P. Van den Heede (1) and N. De Belie (1) (1) Magnel Laboratory for Concrete Research, Ghent University, Belgium P. Van den Heede and N. De Belie Abstract From European Directive 2008/98/EC, it is clear that secondary cementitious materials such as fly ash (FA) and silica fume (SF) can no longer be seen as mere waste products. Indeed, a certain environmental impact needs to be allocated to them when performing a life cycle assessment (LCA). This paper presents the results of a probabilistic LCA of concrete with various FA contents (15%, 40%, 50%). To evaluate the influence of strength differences between concrete mixes, a centric loaded concrete column was chosen as functional unit in a cradle-to-gate LCA. To obtain a higher early strength compared to the mix with 50% FA, 10% SF was added to the concrete mix with 40% FA. Partial environmental burdens were assigned to FA and SF using mass and economic allocation. When comparing 1 m³ of each concrete mix with the reference without FA or SF, the global warming potentials (GWPs) of the mixes with 15%, 40% and 50% FA increase above the reference when applying mass (GWP %, %, %) instead of economic allocation (GWP 1.9%, 23.9%, 11.8% when compared to the reference). LCA scores for concrete columns show a substantial benefit of the 10% FA addition due to the strength increase: GWP % in case of economic allocation. 1. INTRODUCTION Since its development in the late 80s of the previous century, we find that there is still insufficient quantitative proof for the so-called green label of high-volume fly ash (HVFA) concrete. Over the years, its environmental benefit in comparison with traditional concrete has only been roughly estimated based on the cement replacement level (at least 50%). This approach is usually being justified by the fact that the concrete constituent with the highest environmental impact at least with respect to greenhouse gas emissions (GHGs) is cement. Usually, around 800 kg of CO 2 are emitted per ton cement produced [1]. However, this simple estimation method does not take into account some important differences between the latter concrete type and mixes with ordinary Portland cement (OPC) as the only binder material. First of all, HVFA concrete has a slower strength development. A sufficient amount of the cement hydration product Ca(OH) 2 must be present first, before the pozzolanic FA reaction can kick in. Evidently, this property may lead to an increase in the 336

2 dimensioning of the structure and the total amount of concrete needed for manufacturing it. Secondly, depending on the environment, the durability and service life of HVFA concrete can be different from that of OPC concrete. Attempts have been made by Van den Heede et al. [2, 3, 4] to include both the strength and durability aspect in quantitative life cycle assessment studies conducted on simple structural elements (column, beam, ). However, within these analyses FA was always considered as a mere waste product without any environmental burden attributed to it. Chen et al. [5] pointed out that a recycled waste such as FA should be regarded as a by-product since all four byproduct criteria of the recent European Directive 2008/98/EC [6] are met: (i) the further use of FA is certain, (ii) it is produced as an integral part of the coal fired electricity production process, (iii) it can be used directly in concrete without any further processing other than normal industrial practise, and (iv) its further use is lawful (when conforming to NBN EN [7]). As a consequence, the environmental impact of the main product (electricity) should be partially assigned to the FA in LCA. In order to do so, Chen et al. calculated mass and economic allocation coefficients for FA. The same was done for other industrial byproducts used in concrete, e.g. blast furnace slag and SF [5, 8]. This paper presents the results of a comparative LCA of different concrete compositions containing various amounts of FA (0%, 15%, 40%, 50%) and silica fume (0%, 10%). In a first research step, the impact of 1 m³ of each concrete composition on the greenhouse effect and climate change was calculated while assuming three different allocation principles (no, mass and economic allocation) for the industrial by-products present in the concrete mixes. In a second research step, a centric loaded column was chosen as functional unit for LCA to evaluate the influence of strength differences on the environmental benefit of FA concrete. Again, this was done for the three allocation principles. 2. MATERIALS AND METHODS 2.1 Concrete mixtures In total, four concrete mixtures were manufactured (Table 1). Mix T(0.45) has a cement content and water-to-cement (W/C) ratio of 340 kg/m³ and 0.45, respectively. It is seen as the appropriate OPC reference concrete for exposure class XS2 conforming to NBN B [9]. This exposure class corresponds with an environment where steel reinforced concrete is permanently submerged in sea water. As a consequence, the concrete is considered to be susceptible to corrosion induced by chlorides. Apart from the OPC mix, a FA containing concrete composition (F15) conforming to the k value concept of NBN B [9] was made. By using the k value concept, the maximum fly ash-to-binder (F/B) ratio for a minimum total binder content equals only 15 %. With 50% of the binder material consisting of FA, mix F50 is obviously a HVFA concrete composition. Note that the HVFA mix design is characterized by a rather high total binder content (450 kg/m³) and low water-to-binder (W/B) ratio (0.35). The authors made this particular choice to ensure a strength class derived from the characteristic value of the 28 day compressive strength at least equal to the indicative value (C35/45) mentioned in NBN B [9] for exposure class XS2. Given the slow pozzolanic reaction of the FA, only an increase of the binder content and a decrease of the W/B ratio, can contribute to an acceptable strength after 28 days. 337

3 Evidently, 450 kg binder per m³ of F50 concrete is much higher than 340 kg cement per m³ of the T(0.45) reference. When comparing F50 with T(0.45), the reduced cement content of the former due to the 50% FA replacement is only 115 kg/m³. As a consequence, the environmental benefit of this HVFA mix with a strength class of at least C35/45, will be considerably less than 50% when subjected to a comparative LCA study. Therefore, we also developed a concrete mix with the same binder content (340 kg/m³) as the OPC reference, and a high strength at 28 days (C55/67). Its binder content consists of 50% Portland cement, 40% FA and 10% SF. To obtain a strength class equal to the one of T(0.45) (C45/55), it would be possible to use an even lower SF content in future research. The latter strategy can be beneficial for economical reasons. Table 1: Mix proportions and properties of the tested concrete mixtures T(0.45) F15 F50 F40SF10 1 Sand 0/4 (kg/m³) Aggregate 2/8 (kg/m³) Aggregate 8/16 (kg/m³) CEM I 52.5 N (kg/m³) Fly ash (kg/m³) Silica fume (kg/m³) Water (kg/m³) SP (ml/kg B) W/B (-) F/B (%) Slump S4 S4 S4 S4 Strength class C45/55 C40/50 C35/45 C55/ Strength and durability assessment Per concrete mix, 3 cubes with a 150 mm side were cast. After casting, the cubes were kept at a constant temperature and relative humidity (RH) of 20 C and 95 %, respectively. Demoulding took place the next day whereupon the specimens were stored again under the same conditions until the age of testing. After 28 days, all cubes were subjected to a compressive strength test conforming to NBN B [10]. From the resulting mean strength values and standard deviations on the individual values, the characteristic compressive strength of each mix was calculated to determine the applicable strength class conforming to NBN EN [11]. The chloride resistance of all concrete mixes, except for composition F40SF10, has been previously assessed in Van den Heede et al. [4] by means of a probabilistic service life prediction. For the OPC reference, an acceptable reliability index β ( 1.3) and probability of failure ( 0.10) could be guaranteed for a service life of around 60 years. Based on the same criteria, the estimated service lives of F15 and F50 exceeded the 100 year time frame by far. The durability of composition F40S10, containing both FA and SF, is currently still being investigated. Obviously, differences in service life between OPC and FA concrete can influence the LCA outcome significantly. A structure or structural element constructed with T(0.45) concrete will require repair or replacement after 60 years to ensure fulfillment of its function for at least another 40 years. The additional concrete manufacturing needed for this, should be incorporated within the LCA. However, since the durability assessment has not yet 338

4 been completed for mix F40SF10, the functional unit used in this LCA study is merely strength related and does not consider repair or replacement of the structure over time. 2.3 Life cycle assessment In correspondence with ISO [12], the LCA consisted of four major steps: the definition of goal and scope, the inventory analysis, the impact analysis and the interpretation Definition of goal and scope This LCA was conducted to quantify the reductions in global warming potential (GWP) associated with partially replacing cement (as produced from traditional Portland clinker burned in a rotary kiln with traditional fuels) with FA (and SF). This was done while assuming three different allocation principles (no, mass and economic allocation) for the industrial by-products incorporated in each concrete mix. Special attention was paid to their influence on the reduction of GHG emissions expressed in GWP, since this is the reason why concrete mixes with considerable amounts of FA were developed in the first place. As mentioned earlier in Section 2.2, only differences in strength were included in this LCA study. Therefore, an axially loaded column (height: 2.5 m, cross-section: rectangular) carrying a design load of 1500 kn was chosen as the functional unit for LCA (Figure 1). With omission of the durability aspect, this methodology is equivalent to a common cradle-to-gate approach. The experimental strength classes given in Table 1 were used for the column design. All calculations regarding concrete and steel reinforcement dimensioning were done conform NBN EN 1992 Eurocode 2 [13]. N d = 1500 kn 300 mm 300 mm h = 2.5 m 197 mm T(0.45) 253 mm F mm 300 mm 222 mm F mm F40S10 Figure 1: Column dimensions based on their experimental strength classes Inventory analysis Per concrete constituent, the necessary inventory data were collected from the Ecoinvent database [14]. Their proper short descriptions as mentioned in the database together with their probabilistic distributions and parameter values, are shown in Table 2. The mean values and standard deviations for the sand and aggregates, were calculated from the amounts of each material needed according to Fuller s optimal particle size distribution curve for three deliveries of sand and aggregates to our laboratory. Although based on only three Fuller calculations, the applicable distributions were assumed to be normal. 339

5 Table 2: Overview of the life cycle inventory data used per concrete mix Material data (kg) Distribution T(0.45) F15 F50 F40SF10 1 Sand, at mine/ch U Normal 766 ± ± ± ± 45 2 Gravel, round, at mine/ch U Normal 1135 ± ± ± ± Portland cement, strength class Z Constant , at plant/ch U 4 Fly ash a Constant Silica fume b Constant Tap water, at user/ch U Constant Superplasticizer (EFCA 2006) Constant Transport data (tkm) 1 Transport, barge/rer U Constant Transport, van <3.5t/RER U Normal 37.1 ± ± ± ± Transport, barge/rer U Constant Transport, van <3.5t/RER U Normal 54.9 ± ± ± ± Transport, van <3.5t/RER U Normal 34.4 ± ± ± ± Transport, van <3.5t/RER U Normal ± ± ± Transport, transoceanic freight Constant ship/oce U Transport, van <3.5t/RER U Normal ± Transport, van <3.5t/RER U Normal 0.18 ± ± ± ± 0.10 Processing data (kwh) Electricity, low voltage, production BE, at grid/be U Constant a patially contains following Ecoinvent data: Electricity, hard coal, at power plant/be U, through allocation b partially contains following Ecoinvent data: MG-silicon, at plant/no U, through allocation The amounts of OPC cement, FA, SF, water and SP were assumed to be accurately weighed and therefore considered as constants. For the allocation of impacts attributed to the industrial by-products FA and SF, the mass and economic allocation coefficients as proposed by Chen et al. [5, 8] were applied (Table 3). Table 3: Mass and economic allocation coefficients for FA and SF cf. Chen et al. [5, 8] Product Mass produced Market price Allocation by mass value Allocation by economic value Electricity 1 kwh * 0.1 /kwh 87.6% 99.0% FA kg 20 /t 12.4% a 1.0% a Si metal 1 kg 1200 /t 87.0% 95.2% SF 0.15 kg 400 /t 13.0% b 4.8% b * Equivalent to kg of hard coal used to produce electricity a allocation percentages applied to the following Ecoinvent data: Electricity, hard coal, at power plant/be U b allocation percentages applied to the following Ecoinvent data: MG-silicon, at plant/no U Inventory data regarding SPs were obtained from an environmental declaration published by the European Federation of Concrete Admixture Associations (EFCA) [15]. 340

6 Since the same amount of reinforcement steel was used for each concrete column, the environmental impact of the steel was not included in this case study. The transport of each constituent to the concrete mixing plant was also incorporated in the LCA study by including multiplication of their masses with the corresponding transport distances by van, barge or freight ship (Table 2). Only one supplier for each concrete constituent was considered in this study. A constant transport distance was assigned to the materials that were transported by barge (sands and aggregates) or ship (SF). In total, 33 ready mix concrete plants in Flanders were assumed to be the manufacturer of the mixes that were studied in this paper to estimate the (normal) distribution of the road transport distances on a local scale. For all Ecoinvent data, unit processes (U) were used in the modelling of each concrete mix. This was done to enable a full probabilistic uncertainty analysis of the calculated environmental scores using Monte Carlo (number of runs: 500) Impact analysis and interpretation The IPCC 2007 GWP 100a impact method was adopted to calculate the concrete s Global Warming Potential (GWP) expressed in CO2 equivalents for a time frame of 100 years. All calculations were done in the LCA software SimaPro RESULTS AND DISCUSSION 3.1 Influence of allocation With no impact of the main process (electricity and silicon metal production) allocated to the industrial by-product (FA and SF), GWP values for 1 m³ of F15, F50 and F40SF10 are lower than the amount of CO2 eq associated with 1 m³ of OPC reference T(0.45) (Figure 2a). However, for mix F15 this environmental benefit (GWP 4.5%) is very limited, mainly due to the small reduction in total cement content (317.6 kg C/m³ versus 340 kg C/m³). In case of HVFA mix F50 (containing only 225 kg C/m³), the reduction in global warming potential (GWP 22.1%) is more pronounced, but still much lower than the F/B ratio (50%) due to the high total binder content (450 kg B/m³). With a total binder content of 340 kg B/m³ consisting of 40% FA, 10% SF and 50% cement, mix F40SF10 emits 29.7% CO2 eq less than T(0.45). When applying the mass allocation principle, the GWPs of all FA (and SF) containing concrete mixes are higher (F15: %, F50: %, F40SF10: %) than the value calculated for T(0.45). The strong impact of using mass allocation for secondary cementing materials, was also found by Chen et al. [5, 8]. Although constant over time, the procedure induces large impacts on by-products and may discourage the cement industry to continue using them. With economic allocation on the other hand, the impacts attributed to FA and SF are much lower. Despite its susceptibility to price fluctuations, the economic allocation procedure more or less maintains the status of secondary cementing materials as waste that needs recycling [5, 8]. Given the scatter on the GWPs for each concrete mix in Figure 2a, it can be concluded that there is no significant difference between the no allocation and the economic allocation strategy. Compared to the former approach, the latter strategy results in a somewhat lower reduction in GWP: 1.9% (F15), 11.8% (F50), 23.9% (F40SF10). 341

7 GWP 1 m³ concrete (kg CO 2 eq ) GWP 1 column (kg CO 2 eq ) International Symposium on Life Cycle Assessment and Construction No Mass Economic (a) No Mass Economic (b) T(0.45) F15 F50 F40SF10 0 T(0.45) F15 F50 F40SF10 Figure 2: (a) Influence of allocation principle (no, mass, economic allocation) on the GWP of 1 m³ of each concrete mix, (b) Influence of the concrete strength class on the GWP of 1 centric loaded column manufactured with each concrete composition Note that only around 60% of the GWP can be attributed to the production of concrete constituents. Mainly transport is accountable for the rest of the impact. Thus, a limitation of transport is of great importance to reduce the overall burden even more. 3.2 Influence of strength When studying the influence of strength differences (Figure 2b), the previously observed environmental benefits for mixes F15 and F50 have completely disappeared for the no allocation strategy. Indeed, with a centric loaded column as functional unit, the lower strength classes of F15 (C40/50) and F50 (C35/45) compared to T(0.45) (C45/55) result in larger column dimensions and require more concrete (0.17 m³ and 0.19 m³, respectively, versus 0.15 m³). As a consequence, the corresponding GWP values have increased. In addition to the mass allocation approach, economic allocation now also results in extra environmental burdens for F15 and F50 (GWP % and %, respectively). However, note that in contrast with mass allocation, the extra burdens due to economic allocation are not significant. Incorporation of 10% SF resulted in a strength that is two classes higher than the strength class of T(0.45) (Table 1: C55/67 versus C45/55). As a result, the concrete volume needed to manufacture the column with mix F40SF10 could be reduced even more to 0.12 m³. With no impacts allocated to FA and SF, the GWP of mix F40S10 is 43.5% lower than the value recorded for reference T(0.45). When applying economic allocation, the GWP is 39.4% lower Under the assumption of mass allocation, the GWP exceeds the reference value with 13.7%. 4. CONCLUSIONS In conclusion, the GWP of FA concrete highly depends on its mixture properties (FA content, total binder content, strength class) and the allocation principle (no, mass and economic allocation) applied to the industrial by-products present in the mix. To stimulate further use of by-products in concrete, it is advised to implement economic allocation (or no allocation). A comparative LCA study considering the environmental impact of 1 m³ of concrete with economic allocation shows that the benefit of a 15 % FA concrete mix conforming to NBN 342

8 B is rather limited (GWP 1.9%), while a 50% HVFA concrete mix and a 40% FA concrete mix with 10% SF as ternary binder result in 11.9% and 23.9% less GHGs. By choosing a centric loaded column as functional unit, the high strength (C55/67) of the FA-SF concrete mix can be fully valorised. When adopting the economic allocation procedure, the GWP decreased with 39.4%. It shows the greater potential of combined FA-SF compositions in sustainable development of the concrete industry. REFERENCES [1] Josa, A., Aguado, A., Heino, A., Byars, E., Cardim, A., 'Comparative analysis of available life cycle inventories of cement in the EU', Cem. Concr. Res. 34 (8) (2004) [2] Van den Heede, P., Gruyaert, E., Robeyst, N., De Belie, N., 'Life cycle assessment of a column supported isostatic beam in high-volume fly ash concrete (HVFA concrete)', in 'Proceedings of the 2nd International Symposium on Service Life Design for Infrastructure, Volume I', Delft, October, 2010 (RILEM Publications, Bagneux, 2010) [3] Van den Heede, P., De Belie, N., 'The greenness of high-volume fly ash concrete when exposed to carbonation', in 'Advances in Construction Materials through Science and Engineering', Proceedings of the International RILEM Conference, Hong Kong, September, 2011 (RILEM Publications, Bagneux, 2011) p 205 in book of abstracts, paper 029 (8 p) on CD-rom. [4] Van den Heede, P., Maes, M., Gruyaert, E., De Belie, N., 'Full probabilistic service life prediction and life cycle assessment of concrete with industrial by-products in a submerged marine environment: a parameter study', in 'Sustainable Development of Energy, Water and Environment Systems', Proceedings of the 6 th Dubrovnik Conference, Session: 'Utilization of industrial byproducts towards sustainability', Dubrovnik, September, 2011 (Zagreb, Faculty of Mechanical Engineering and Naval Architecture, 2011) p 134 in book of abstracts, paper 0279 (11 p) on USB. [5] Chen, C., Habert, G., Bouzidi, Y., Jullien, A., Ventura, A, 'LCA allocation procedure used as an incitative method for waste recycling : An application to mineral additions in concrete', Resour. Conserv. Recy. 54 (12) (2010) [6] European Union, 'Directive 2008/98/EC of the European parliament and of the council on waste and repealing certain directives', Off. J. Eur. Union. L312 (2008) [7] NBN EN A1, 'Fly ash for concrete Part 1: Definitions, specifications and conformity criteria' (CEN, Brussels, 2008). [8] Chen, C., 'A study of traditional and alternative structural concretes by means of the life cycle assessment method' (in French), PhD thesis (Troyes, University of Technology of Troyes, 2009). [9] NBN B15-001, 'Supplement to NBN EN concrete specification, performance production and confirmity' (BIN, Brussels, 2004). [10] NBN B15-220, 'Concrete testing Determination of compressive strength' (in dutch) (BIN, Brussels, 1990). [11] NBN EN 206-1, 'Concrete Part 1: specification, performance, production and conformity' (European Committee for Standardization, Brussels, 2000). [12] ISO 14040, 'Environmental management Life cycle assessment Principles and framework' (ISO, Geneva, 2006). [13] NBN EN 1992, 'Eurocode 2: Design of concrete structures Part 1-1: General rules and rules for buildings (+ AC: 2008)' (BIN, Brussels, 2005). [14] Frischknecht, R., Jungbluth, N., eds, 'Overview and methodology. Final Report ecoinvent v2.0 No. 1' (Swiss Centre for Life Cycle Inventories, St-Gallen, 2007). [15] EFCA, 'EFCA Environmental Declaration Superplasticizing Admixures (2006), EFCA doc 325 ETG', available online: (last accessed 08/11). 343