Ecological and economic analyses of geopolymer concrete mixes for external structural elements

Background

The development of alternative binders is currently experiencing a renaissance. The reasons for this are partly the rising costs of primary raw materials (and their regional scarcity) and partly also a growing awareness of environmental problems. Widely varying pressures can be observed on the environmental side. Because of emissions trading there is much emphasis in the leading industrial nations on the reduction of CO₂ emissions during the production of cements. In addition to this the emergent industrial nations, such as India and China, have not yet developed any adequate ways of utilizing industrial wastes, resulting in a search for possible ways of utilizing the large amounts of ash and slag produced. It is hoped that binders based on geopolymers (GP) will prevent the large-scale landfilling of ash and slag, with the corresponding conservation of primary raw materials, and also cause the emission of significantly smaller quantities of greenhouse gases when compared with cement-based binders.

2 Introduction

Geopolymers represent an aluminosilicate binder system
[1-3] that has been known since the late 60s [4]. So far geopolymers have never been produced in large quantities or over long periods for industrial applications. A few industrial applications, for example as railway sleepers, are known from Eastern ­Europe and Russia. This has also been reported more recently from Spain [5], but the production was stopped after a short time. A mixture of cement and geopolymer found a significant ­application under the trade name of Pyrament in the 90s in the USA. The cement producer Lone Star Industries used Pyrament in civil engineering and road building, for producing rapidly usable pavements (e.g. aircraft landing strips for military purposes) and also in hydraulic engineering [6]. However, these applications of Pyrament were stopped by Lone Star in 1996. Geopolymers are currently being used in the USA for repairing wastewater pipes [7]. In Germany a plant belonging to HT Troplast for the industrial production of “Trolit” foamed geopolymer moulded parts [8] was closed down again shortly after commissioning. The following niche applications of alkali-activated binders or geopolymers produced in very small quantities have been identified in Europe¹: 

Renotech
(Finland)refractory geopolymer-based
adhesive  HQ Camfil
(Sweden)air filter probably based on
geopolymers Pyromeral
(France)various special geopolymer-based products Technology of
geopolymers
(Czech Republic) special customer-specific

products  F. Willich GmbH (Germany)exhaust gas pipes, insulation products probably based on geopolymers BPS-Zwickau
(Germany)consolidation and immo

bilization of toxic or

radioactive residues by

geopolymersKeraguss
(Germany)products for thermal insulation or fire protection, probably based on geopolymers MC Bauchemie
(Germany)acid-resistant surface coating and repair mortar for the wastewater sector in sewage plants, probably based on geopolymers

The use of geopolymers as bulk building materials has been attributed with an appreciable potential for reducing CO₂ emissions [9]. Numerous investigations have shown that because of their technical properties geopolymers appear suitable for unreinforced and reinforced concrete applications and also comply with international cement concrete regulations [10-11] but at present there are no known applications of geopolymers as bulk building materials anywhere in the world.

 Only from Australia is there a report of an industrial application as a building material. The company Zeobond with the trade mark E-Crete that was formed recently in Australia intend to offer various geopolymer-based building products in the next few years [12]. Currently in Australia an intensive research into geopolymer applications is carried out.

Development of geopolymer-based
mix ­formulations for concrete applications

The development of basic mix formulations for geopolymers as bulk building materials in concrete applications was preceded by intensive raw material screening. The suitability of 58 primary and secondary raw materials was investigated [13]. The raw material screening covered various mineral wastes, ashes, slags, clays and volcanic deposits. Economic and ecological (including health) aspects were also included in addition to the technical suitability in order to identify the most promising raw materials [14-15]. Exploratory trials were carried out with all the raw materials to establish their technical suitability. These exploratory trials included, for example, the dissolving of silicate and aluminate monomers in alkaline solutions and measuring the compressive and flexural tensile strengths [16].  

A requirement profile was drawn up for the specific application of geopolymers as bulk building material mixes for external wall elements. With the aid of multi-criteria decision analysis hard coal fly ash and finely ground granulated blast furnace slag were identified as the most promising solid components for this application [15,17]. Although the use of, for example, metakao-lin, was interesting from the technical perspective it was not suitable as a main binder component for producing bulk building materials because of economic and ecological deficiencies.  

Three basic mix formulations containing fly ash and ground granulated blast furnace slag as main components (Table 1) were developed for the use of geopolymer concrete in external wall elements. In each case the technical objectives for the mix formulations were high strength and rapid strength development as well as high resistance to freeze-thaw with and without de-icing salt (durability). The liquid activator used was a mixture of water glass solution (Woellner sodium silicate 37/38, Na₂O approx. 8%) and a NaOH/water solution (Table 1). 

The geopolymer concretes (gp-concretes) were compared with a Portland cement concrete. With regard to ecological and economic analyses and evaluations it is necessary to make sure that the two types of concrete fulfil the same functions. Only in this way is it possible to make an adequate comparison of the different systems and increase the level of knowledge. The functional comparison unit was therefore a concrete that was resistant to freeze-thaw with and without de-icing salt and fulfilled at least the requirements for exposure classes XF2 and XF4 as defined in DIN EN 206-1/DIN 1045-2. According to these standards the cement concrete must have a minimum cement content of 320 kg/m³ and a water-cement-ratio of 0.5. On the basis of experience a 20 kg higher cement content was used for the cement concrete mix. The following parameters were also kept the same to improve the comparability of the two types of concrete:  

» No use of concrete admixtures²

»  Same quantity and grading curve for the aggregate

» Same volumetric ratio of binder paste and aggregate

Technical properties of the basic mixes

Various investigations were carried out on the fresh and solid concretes. The detailed results have already been published [18]. The essential findings can be summarized as follows:

» the strengths increase from MI-1 to MI-3 and then to MI-5 with increasing content of ground granulated blast furnace slag and decreasing content of hard coal fly ash

» MI-3 with an ash:slag ratio of 50:50 fulfils the strength requirements for XF4 with C 30/37 after 28 days, as does the reference mix (CEM I concrete)

» the geopolymer mix MI-5 that is rich in ground granulated blast furnace slag even meets the requirements of the C 35/45 strength class (cf. Fig. 7)

» the geopolymer mix MI-1 that is rich in ash falls significantly below the strength requirements and can only be assigned to the C 12/15 strength class

» MI-1 also fails both with respect to freeze-thaw attack (CF) and to freeze-thaw attack with de-icing salt (CDF)

» MI-3 passes the CF test at the same level as the CEM I reference concrete

» the CDF test was not passed by either by MI-3 or by the CEM I reference concrete³, i.e. the mass loss due to scaling was more than 1500 g/m² after 28 freeze-thaw-cycles

» MI-5 passed both the CF and the CDF tests and therefore exhibited a greater resistance than the reference concrete (Portland cement concrete) in both tests.

Because of the technical superiority of MI-5 this mix formulation was examined further for the additional optimization steps.

Economic analysis of the basic mixes

A raw material cost comparison was carried out for the production of geopolymer concrete and CEM I concrete on the assumption that the concrete producer has to buy in all raw materials from outside. The price for the reference year 2007 was requested at the terms for a major purchaser in Germany. The significant differences between the minimum and maximum production costs for concretes in Fig 2 arise from the widely varying information for the raw material costs and the uncertainty of the data (e.g. for ground granulated blast furnace slag).

It is apparent that the (minimum and maximum) costs rise with decreasing fly ash content and increasing ground granulated blast furnace slag content as well as with decreasing sodium hydroxide and increasing sodium silicate solution (from MI-1 to MI-5). The lowest production costs for geopolymers in the best case are less than the minimum costs for CEM I concrete and in the worst case the maximum costs are more than the maximum costs for CEM I concrete.

In spite of the wide range of costs Fig. 2 shows that in principle geopolymer-based concretes can be economically competitive with Portland-cement-based systems. It should also be borne in mind that, in contrast to comparable cement systems, the geopolymer systems still have significant potential for optimization, especially as the gp-systems are still at the start of a specific learning curve (e.g. [19]).

On the other hand, the cost of cement concretes could be reduced by single-digit percentage points by the use of blended cements (CEM II or CEM III) provided that their use is permitted in accordance with DIN EN 206-1/DIN 1045-2.

 The raw material costs for the two types of concrete are itemized in Fig. 3 to identify the cost drivers. In addition to the aggregate (gravel, sand) as the main component (approx. 80 mass %)⁴ in both mixes, the cement is the main cost driver in the cement concrete. On the other hand, in the geopolymer concrete all components contribute to a different extent to the total costs, while i.e. the solid component ground granulated blast furnace slag and the activator sodium hydroxide contribute noteworthy to the total costs, the solid component hard coal fly ash contributes only to a lesser extent. The activator sodium silicate solution contributes with the highest specific costs considerably to the total costs.

Ecological evaluation of the basic mixes

A Life Cycle Assessment (LCA) as specified in DIN ISO 14040 [20] was carried out for an ecological evaluation of the geopolymer concretes. This investigation examined the phases from extraction of the resources to production of the product (cradle to gate), but without transport costs⁵. The system limits are shown in Fig. 4. The functional unit is 1 m³ concrete of exposure classes XF2 and XF4 as defined in DIN EN 206-1/DIN 1045-2 [18]. The Life Cycle Inventory (LCI) data for the individual raw materials and processes were taken from various sources (Table 2).

 Most of the data for the raw materials were taken from the Ecoinvent database [21]⁶. For fly ash and ground granulated blast furnace slag the data were taken from industry and from the literature. Fly ash and ground granulated blast furnace slag are wastes so no ecological footprints of the pre chain processes were considered⁷. Only the additional expenses for obtaining raw materials suitable for concrete applications are taken into account. In the case of fly ash these are low (intermediate storage, decompaction) but for ground granulated blast furnace slag they are relatively high (fine grinding). 

The CML method [22] was used for the life cycle impact assessment. The method takes different environmental effects into account by using appropriate environmental impact indicators. Two environment indicators that are important for this comparison (GWP, ADP) are singled out:

» GWP ­(global warming potential, [kg CO₂ equivalent]) takes account of all gas emissions (e.g. CO₂, CH₄, N₂O and fluoro-chlorohydrocarbons)

» ADP ­(abiotic resource depletion potential, [ kg Sb equivalent]) is used as the indicator for the consumption of natural, inanimate, not regenerable, resources (e.g. metal ores, crude oil)

The cumulative energy demand (CED) is also reported as the total indicator for primary energy consumption.

 Notification of an indicator for human toxicity is also extremely appropriate and interesting for evaluating concrete production. However, this type of assessment and evaluation is not supported because of gaps in the data at the life cycle inventory level. This means that the use of evaluation procedures with only one ecological indicator (single score indicator) such as the Eco-indicator 99 [23] would be unrewarding in this instance.

Comparison of unoptimized basic mixes
with CEM I concrete

The results of the environmental impact are shown in
Fig. 5. As with the economic analysis, the values of the environmental indicators for the geopolymer concretes increase with increasing levels of ground granulated blast furnace slag and sodium silicate solution (and with decreasing levels of fly ash and NaOH) for all three of the environmental indicators examined (GWP, ADP and CED). A bivalent result can be gathered from the comparison of geopolymer concretes with cement concrete. On the one hand it is apparent that the cement concrete causes up to about 40% less consumption of resources (ADP) than unoptimized gp-concretes. For the gp-concretes the production of sodium silicate solution and sodium hydroxide are the main drivers of the consumption of resources, alongside the use of sand and gravel and expenditure for grinding the granulated blast furnace slag. For the cement concrete, in addition to the aggregate, the production of the cement is by far the most dominant factor in the consumption of resources. 

The energy consumption (CED) of the cement concrete is also up to about 40 % less than for the gp-concretes. Once again the production of sodium silicate and hydroxide are the main drivers of energy consumption for the geopolymers alongside the use of sand and gravel and expenditure for grinding the ground granulated blast furnace slag. Cement is again by far the most dominant factor in the energy consumption for the cement concrete. 

On the other hand, there are clear advantages for the gp-concrete with respect to the greenhouse potential indicator GWP. The gp-concretes cause up to more than 50 % less greenhouse potential than the cement concrete. Fig. 6 shows that the sodium silicate solution and sodium hydroxide activators contribute substantially to the GWP although the quantities used are relatively small (cf. Table 1).  

For the cement concrete the greenhouse potential is dominated almost entirely (97%) by the cement production. The greenhouse gases produced, in this case mainly CO₂, come both from the use of energy (especially fuels) in the burning process and from the calcining reaction of the limestone in which CO₂ is released [24].

Optimization of the geopolymer concretes

The results obtained from the economic and ecological analyses can now be used for optimizing geopolymer concrete mix formulations. However, the optimization of gp-concretes solely from the economic and ecological aspects cannot be considered appropriate, as otherwise the technical performance of the geopolymers would be unintentionally diminished again.  

The optimization stage had the following objectives:

» stabilization of the technical properties

» improvement of the ecological properties

» reduction of the production costs

The starting point for the optimization was the basic mix MI-5 (with 80% ground granulated blast furnace slag and 20% fly ash as the solid materials, Table 1) that was appreciably superior to the reference cement concrete on the technical side with respect to compressive strength and resistance to freeze-thaw with de-icing salt [18]. The economic and ecological analyses have both shown
(Figs. 3 and 6) that the sodium silicate and sodium hydroxide activators contribute substantially to the production costs and environmental impact. Various approaches were therefore followed for reducing the amount of activator solution. Further investigations [25] have also shown that heat curing of the geopolymer concretes can, depending on the extent and configuration, have a strong effect on the cost and environmental profiles⁸. Mix formulations should therefore, for preference, be developed that do not require any heat curing to achieve the technical properties. 

From the technical, economic and ecological aspects the mix formulation MI-6IS/20°C (Table 3) emerged as particularly promising at the optimization stage. This is a geopolymer mix in which the levels of sodium silicate solution and the sodium hydroxide were both reduced by 50% compared with MI-5. The reduced amount of activator solution was offset by the use of a reactive liquid waste material⁹. The exact composition of the mix formulation is shown in Table 3. No active heat treatment was employed during the production of this geopolymer concrete, i.e. the concrete was stored at room temperature ­(approx. 20° C [18]).

Effect of optimization on the technical performance

The optimized geopolymer concrete MI-6IS20°C exhibits improved technical performance in spite of the lack of heat curing and in spite of the reduced quantity of primary activator solution (due to replacement by reactive liquid waste). The MI-5 geopolymer concrete had already reached compressive strength values of about 60 MPa after 28 days, while the reference concrete only reached about 46 MPa [18]. The MI-6IS geopolymer concrete exceeded the compressive strength of the MI-5 and after only 28 days reached values > 70 MPa (Fig. 7). 

The durability of the concretes were tested by the resistance to freeze-thaw without and with de-icing salt using the CF and CDF tests [18]. The investigations that were carried out showed that in principle all the geopolymer mixes based on ground granulated blast furnace slag and fly ash exhibited very good resistance to freeze-thaw cycles. The mass losses due to scaling of all the geopolymer concretes lay significantly below the limit of 1500 g/m² after 28 freeze-thaw cycles (Fig. 8). The best durability characteristics were exhibited by the optimized mix MI-6/IS20°C (replacement of the activator by reactive waste material, reduced sodium silicate content, no heat curing). Mass losses due to scaling of less than 300 g/m² were measured for the resistance to freeze-thaw both without and with de-icing salt.

8.2 Effect of the optimization on the ecological performance

The environmental profiles of the concretes under investigation are shown in Fig. 9. The results of the analysis show that in the case of MI-6 replacement of the activator by a reactive waste material has significantly improved the environmental profile when compared with the original MI-5 geopolymer. When the geopolymer concrete is compared with the reference ­cement concrete the optimization achieved comparable values in the case of the cumulative (primary) energy expenditure (CED) and in the case of the abiotic resource depletion potential (ADP). However, the optimization achieved a 70% lower contribution to the global warning potential (GWP), which can be regarded as a substantial environmental advantage of the geopolymer concrete. 

The environmentally relevant results should also be borne in mind in the technical investigation (Figs. 7
and 8), especially as the optimized gp-concrete MI-6 also exhibits significant advantages in compressive strength and in the investigations into the resistance to freeze-thaw without and with de-icing salt. These advantages on the technical side can be used for additional optimization, e.g. through further decrease of the active materials or reduction of the dimensions of a structural element, with a corresponding improvement in the environmental profile. An alternative is to evaluate the extent to which the increased technical performance has a beneficial effect on increased durability and therefore an extended utilization phase.

9 Summary and discussion

Geopolymers are a technologically interesting group of materials that are already being used in various niche applications. The test results shown here also demonstrate the technical capabilities of this group of materials in potential bulk applications, such as concrete. When compared with cement-based concrete systems it can comply with technical and economic requirements (or even exceed them) and also reduce the environmental impact [27]. The potential for reducing environmental impact results primarily from the use of secondary raw materials such as ground granulated blast furnace slag and fly ash but also possibly as a result of a longer service life in the various applications. However, any wide implementation in the developed industrial nations seems doubtful because of the limited availability of the secondary raw materials. There are already established uses for blast furnace slag and hard coal fly ash, especially for the production of building materials. On the other hand, countries with strong economic growth, such as China or India, are searching for possible ways of using the slag and ash that are produced in large quantities. Geopolymer-based binder systems are ideal here as a recycling option. In every case the challenge on the technical side lies in also making lignite fly ash usable for the production of geopolymer products so as not to be in competition (e.g. with hard coal fly ash) for its use in other systems. However, in the industrialized countries the lignite fly ash that is produced is regarded by the lignite-fired power station operators as of very minor importance from the aspect of utilization. For this reason the power station operators pay little attention to the problems of disruptive materials and fluctuating quality in lignite fly ash – important parameters for high-grade recycling in binder systems – although there is clearly an overall economic potential for saving resources.

 In geopolymer systems the primary and/or secondary solid materials are activated by strongly alkaline liquids. Sodium/potassium silicate solution and sodium/potassium hydroxide are traditionally used for this purpose. Both chemicals come from processes that make intensive use of resources and energy.

 This explains the strong influence of the activators on the economic and ecological profiles of geopolymer concretes, even when moderate quantities are used. Reduction of the amount of activator in an attempt to optimize the geopolymer concrete mix formulations would by itself lead to loss of performance on the technical side. For this reason there has been a search for a secondary reactive replacement material that would make it possible to reduce the amount of primary activator liquid used. The results of this optimization have confirmed the aspired improved environmental performance but have also indicated other improvements on the technical side. In particular, further environmental advantages due to an increased service life can be inferred from the superior durability results of the optimized geopolymer concrete mix formulation. However, in this case there is again the question about the availability of these reactive waste materials for bulk applications. For this reason the choice of the location for producing geopolymer concretes should ideally be guided by the availability of resources (sufficient quality and quantity).