Environmental implications of Geopolymers

How should we consider geopolymers? Many scientists and civil engineers are mistaking alkali-activation for geopolymers, fuelling confusion, using them as synonyms without understanding what they really are. We find in the literature either LCAs of geopolymer cements/concretes or LCA of alkali-activated-materials. The latter encompass the specific fields of alkali-activated slags, alkali-activated coal fly ashes, alkali-activated blended Portland cement.

A dedicated Geopolymer Institute video deals with the major differences prevailing between alkali-activated-materials and geopolymer cements: Why Alkali-Activated Materials are NOT Geopolymers? http://www.geopolymer.org/faq/alkali-activated-materials-geopolymers. First, we explain the main differences between alkali-activated-concrete, alkali-activated-slag, alkali-activated-fly ash on one hand and Slag-based Geopolymer cement on the other hand, in terms of chemistry, molecular structure, long-term durability. In a second part, we comment the industrialization of Slag/fly ash-based geopolymer cement/concrete by the company Wagners, Australia, and we focus on the results provided by the carbonation testing data obtained for ordinary Portland cement, alkali-activated-slag and Slag/fly ash-based geopolymer concrete EFC. The tests were carried out at the Royal Melbourne Institute of Technology RMIT in Australia. Geopolymer cement behaves like regular Portland cement, whereas alkali-activated-slag gets very bad carbonation results.

Are geopolymers a new material, a new binder or new cement for concrete? Geopolymers are all of these. They are new binders and resins for coatings and adhesives, fiber composites, waste encapsulation and new cement for concrete. The wide variety of applications includes: fire resistant materials, decorative stone artefacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock refractories, bio-technologies (materials for medicinal applications), foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin systems, radioactive and toxic waste containment, arts and decoration, cultural heritage, archaeology and history of sciences.

Geopolymers are not only cement/concrete. We have geopolymer resins and binders as well as geopolymer cements. From a terminological point of view, cement is a geopolymer binding system that hardens at room (ambient) temperature, like regular Portland cement. If a geopolymer compound requires heat setting it may not be called geopolymer cement. Therefore, a review of the environmental implications and Life Cycle Assessment LCA of geopolymers must be split into two subthemes in order to cover the variety of applications, namely:

• Environmental implications of geopolymer resins/binders;
• Environmental implications of geopolymer cements/concretes.

1. Environmental implications of geopolymer resins and binders: benefits and impact (LCA)

The invention of geopolymer resins and binders goes back to 1972, when, in the aftermath of various catastrophic fires in France causing hundreds of casualties in public buildings which involved common organic plastic, research on non-flammable and non-combustible plastic materials became our priority.

We founded a private research company in 1972, Cordi SA (called later Cordi-Géopolymère), to develop new inorganic fire-resistant polymer materials which we called «geopolymers» (mineral polymers resulting from geochemistry or geosynthesis). Still today, the environmental impact LCA for organic polymers is limited in the data gathered during manufacture, with very few or no data on service life and behaviour during fire.

We knew that we would not reach fire resistance and zero toxicity with organic chemistry. We started to look at chemical reactions implying non-burnable materials, more specially clays. We learned from a ceramicist team that it was possible to manufacture ceramic tiles at low temperatures (< 200°C). One component of clay, kaolinite, reacted with caustic soda NaOH at 150°C. The industrial application of this kaolinite reaction with alkali began in the ceramic industry with Olsen in 1934 and was, later on, reinvented in 1970 by the Russian team Berg et al. for the manufacture of ceramic wall tiles.

A scientific approach on the environmental benefits, different to modern environmental impact LCA, was assessed in the 1990’s when we started the development of fire resistant geopolymer-fiber-composite panels for aircraft cabin interiors. The American Federal Aviation Administration (FAA) with R. Lyon initiated this R&D at the head of the Fire Section of FAA (1999-2000). According to FAA, all major accidents involving cabin/cargo fires would have been avoided with fireproof geopolymer composites. Indeed, all materials presently used in aircraft constructions (aluminium and plastics) burn as evidenced by the remaining hull of a Saudi Arabian Lockheed L-1011 in 1990. Although the plane landed safely, all 301 aboard died before rescue crews could reach them. The fire started in the aft cargo compartment and the crew failed to take immediate steps to evacuate the plane after landing. Generally, when a plane crash-lands and catches fire, half the people who survive the impact may not get out in time. That is because the plastics in the cabin—the seat cushions, carpeting, walls and luggage bins—are combustible. And when they burn, they give off flammable gases that, in two minutes, can explode into a fireball. The main objective of the FAA project was summarized in a short sentence: " Giving Survivors More Time to Escape". It involved three partners:  the Civil Engineering Department of Rutgers University, USA. (P. Balaguru) in charge of the making of the geopolymer/carbon fiber composite, the Fire Section of FAA, Atlantic City, USA (R. Lyon, 1997), where the testing where performed, and our company Cordi-Géopolymère, Saint-Quentin, France, which supplied the geopolymer, resins. The performances of the geopolymer/carbon fiber composite were compared with those of the organic polymer matrices currently in use, such as: thermoset (vinyl ester, epoxy), advanced thermoset (BMI, PI), phenolic, and engineering thermoplastic (PPS, PEEK).

The present special issue, acknowledging the scarcity of publication on the problem, tries to collect the best of the scientific production on the environmental implications of geopolymer resins and binders used in industry applications.

LCA for organic polymers, with emphasis on the danger resulting from the emission of toxic fumes during fire did not exist at that time, and even today. The Human Toxicity Potential values found in the LCA for organic products (resins or plastics) do not include any measurement of fire hazard. The human toxicity potential (HTP), a calculated index that reflects the potential harm of a unit of chemical released into the environment, is based on both the inherent toxicity of a compound and its potential dose. Total emissions can be evaluated in terms of dichlorobenzene equivalence (carcinogens) and toluene equivalents (noncarcinogens). The potential dose is calculated using a generic fate and exposure model which determines the distribution of a chemical in a model environment and accounts for a number of exposure routes, including inhalation, ingestion of produce, fish, and meat, and dermal contact with water and soil. No mention of fire hazards and toxic fumes emissions.

Geopolymers are used commercially in foundries for the fabrication of sand cores for aluminium casting. However, very little data are available in the literature. The main focus is here dedicated to the elimination of the emission sources of organic resins during all stages of the sand cores manufacture in aluminium foundries. There are highly volatile components (amine, formaldehyde, furan, isocyanate, etc.) emitted during sand mixing, core making, core storage, oven drying and cast of aluminium (with pyrolysis and recombination products).

On the opposite, any geopolymer binder types are emission free. This topic was addressed in two separated presentations at the Geopolymer Camp. One by Walllenhorst (2010) on the very successful implementation of the Inotec® Inorganic Moulding Material in the automobile industry, involving Na-poly(siloxonate) type geopolymer (Na)-(Si-O-Si-O-Si-O). The second, in 2014 by Krahula on the technology of mold and core production with inorganic binder system Geopol®, involving Na-poly(sialate-siloxo) type geopolymer (Na)-(Si-O-Al-O-Si-O-).

The present selection of papers dedicated to the Environmental implications of geopolymer resins and binders used in industry applications begins with the previously cited R. Lyon et al. (1997) Fire resistant aluminosilicate composites, Technical Paper # 1 at http://www.geopolymer.org/category/library/technical-papers/

It compares Geopolymer/Carbon-fiber composite to organic matrix composites being used for infrastructure and transportation applications. At irradiance levels of 50 kW/m² typical of the heat flux in a well developed fire, glass- or carbon-reinforced polyester, vinyl ester, epoxy, bismaleimide, cyanate ester, polyimide, phenolic, and engineering thermoplastic laminates ignited readily and released appreciable heat and smoke, while carbon-fiber reinforced geopolymer composites did not ignite, burn, or release any smoke even after extended heat flux exposure. The geopolymer matrix is of the K-poly(sialate-multisiloxo) type with a ratio Si:Al in the range of 18 to 35. The carbon fiber composite retains sixty-three percent of its original flexural strength after a simulated large fire exposure. This paper was written almost 20 years ago.  Though not specific as a modern LCA per se, it is probably the most relevant paper of that period, even as it only scratches the surface of this still largely unexplored research topic.

Another paper by the same group was published in 2000, (Hammel et al., 2000) in an Elsevier publication. It is titled Strength retention of fire resistant aluminosilicate–carbon composites under wet–dry conditions, Composites: Part B 31 (2000) 107–111.

A more recent paper by the well known racing cars manufacture McLaren, UK, The development of a high temperature tensile testing rig for composite laminates, Composites: Part A 52 (2013) 99–105, by J. Mills-Brown et al. stresses the exceptional properties of a geopolymer matrix of the polysialate type. This study aimed to develop a high temperature tensile test capable of testing fiber reinforced composites up to 1000 °C, in order to understand the behavior of certain composites at these temperatures and produce data suitable in the design of high temperature structures (mechanical strength as well as emission and toxicity).

There exists only one LCA paper published in 2009 comparing a geopolymer mortar with an organic lining. It is found at the end of one LCA for geopolymer concrete established by a team at Bauhaus University, Weimar, Germany (Weil et al. 2009) on the protective coatings of Portland cement based sewage pipes. Titled Life-cycle analysis of geopolymers, by Weil et al. (2009), it is the Chapter 10 of the book Geopolymer, Structure, processing, properties and industrial applications, edited by J. Provis and J. van Deventer, Woodhead Publishing Limited. When increased resistance against acid is requested and/or a longer life cycle is desired, concrete pipes are often lined with high-density polyethylene (HDPE). A metakaolin/fly ash-based geopolymer grout reaches much lower values for all environmental impact indicators considered, with and without consideration of raw material transportation. This study did not address the issue of fire and toxic fume emission.

However, it is possible to compare the LCA of geopolymer resins with specific organic molecules. In the following Table 1, we put the environmental impact of a petroleum based epoxy resin taken from a paper published by La Rosa et al. (2014) Bio-based versus traditional polymer composites. A life cycle assessment perspective, Journal of Cleaner Production 74 (2014) 135-144, and the value for geopolymer resin constituents of the type Na-PSS, poly(sialate-siloxo) involving NaOH powder, Na-silicate and metakaolin MK-750. The values for geopolymer were taken from the paper by Habert et al. (2011), An environmental evaluation of geopolymer based concrete production: reviewing current research trends, Journal of Cleaner Production 19 (2011) 1229-1238, except for the Na-silicate value obtained from Fawer et al. (1999) Life Cycle Inventories for the Production of Sodium Silicates, Int. J. LCA 4 (4) 207-212 (1999).  (See the discussion below in LCA for geopolymer cements/concretes).

Table 1: environmental impact of a petroleum based epoxy resin compared with geopolymer resin constituents of the type Na-PSS, Na-poly(sialate-siloxo), comprising MK = 353 kg,  Na-Silicate =588 kg and NaOH=59 kg, for 1000 kg.

Impact category	Units	Petroleum based-epoxy resin	NaOH powder	Na-silicate 3.3  37%% solid	Metakaolin MK-750	Geopolymer resin (Na-PSS)
Abiotic Depletion (ADP)	Kg Sb eq.	56.4	16.40	7.22	0.168	5.27
Acidification Potential (AP)	Kg SO2 eq.	40.3	10.7	5.22	0.32	3.81
Eutrophication Potencial (EP)	Kg PO4 eq.	6.6	0.810	0.495	0.049	0.35
Global Warming Potential (GWP)	Kg CO2 eq.	6,663	2,024	425	92.4	282.6
Ozone Layer Depletion Potential (ODP)	Kg CFC-11 eq.	1.26E-6	13.8E-5	8.82E-5	1.52E-6	6.05E-5
Human Toxicity Potential (HTP)	Kg 1.4-DB eq.	490.44	957	803	23	536.75
Freschwater Aquatic Ecotoxicity Potential (FAETP)	Kg 1.4-DB eq.	246.5	240	212	32	150.11
Territorial Ecotoxicity Potential (TETP)	Kg 1.4-DB eq.	29.1	46.6	8.96	0.323	8.13

The geopolymer resin outperforms the epoxy resin value with respect to Global Warming Potential by a factor of 23, as well as all others, except for Human Toxicity Potential, which is a surprise. This is due to the high value given for this Na-silicate solution.

Several published LCAs of soluble silicates used in detergents are providing contradictory results on Human Toxicity Potential values. According to some authors (Warne et al. 1999) , the solution form of sodium silicate was markedly more toxic than the solid form. The difference in toxicity of these two forms is possibly due to the presence of NaOH in the solution (14% mass/mass). We find this reasoning strange because there is no free NaOH in Na-silicate 3.3 (37%%) solid solution, only Na-silicate oligomers. In addition, it is well known for decades that it is harmless for human, according to all MSDS (Materials Safety Data Sheets) available for commercial soluble silicates. May be, one database made a mistake, and we hope to find it in the next future.

2. Environmental implications of geopolymer cements/concretes: environmental benefits and impact (LCA)

There is often confusion between the meaning of the two terms cement and concrete. A cement is a binder whereas concrete is the composite material resulting from the addition of cement to stone aggregates. In other words, to produce concrete one purchases cement (generally Portland cement) and adds stone aggregates to the concrete batch with water. From a terminological point of view, cement is a geopolymer binding system that hardens at room temperature, like regular Portland cement. If a geopolymer compound requires heat setting it may not be called geopolymer cement. Geopolymer cement is an innovative material and a real alternative to conventional Portland cement for use in transportation infrastructure, construction and offshore applications. It relies on minimally processed natural materials or industrial by-products to significantly reduce its carbon footprint, while also being very resistant to many of the durability issues that can plague conventional concretes.

Creating geopolymer cement requires an alumina silicate material, a user-friendly alkaline reagent (sodium or potassium soluble silicates with a molar ratio MR SiO₂:M₂O>1.50, M being Na or K) and water. Room temperature hardening relies on the addition of calcium cations, essentially iron blast furnace slag. The raw material MK-750 is a kaolinitic clay calcined at 750°C, hence the designation MK-750 in this paper.

Geopolymer cement is sometimes mixed up with alkali-activated cement and concrete, or alkali-activated-material. Despite more than 50 years of trials in Eastern Europe (Glukhovsky, 1965; Tailing and Brandstetr, 1989) and China (Shi et al., 2006), alkali-activated materials are not manufactured separately and not sold to third parties as commercial cement. The chemistry is used only in the making of alkali-activated concretes, either with alkali-salts (Na₂SO₄, Na₂CO₃, etc.), alkali hydroxides (NaOH, KOH) and alkali-silicates, essentially on blast-furnace slag.

The first geopolymer cement was developed in the 1980s and was of the type (K,Na,Ca)- poly(sialate) or metakaolin MK-750/slag/(Na,K) silicate -based geopolymer cement. It resulted from the research developments carried out by Davidovits and Sawyer at Lone Star Industries, USA and yielded to the invention of the well-known Pyrament® cement. The development of this new cement did not result from any ecological or environmental concerns but focused on niche applications such as early high strength. It was discovered that the addition of ground blast furnace slag, which is a latent hydraulic cementitious product, to the poly(sialate) type of geopolymer resin described above in Section 1) for geopolymer resin/binder, accelerates the setting time and significantly improves compressive and flexural strength. The resulting Davidovits/Sawyer US patent was filed in Feb. 22, 1984, and the patent US 4,509,985 was granted on April 9, 1985, titled 'Early High-Strength Mineral Polymer'. The corresponding European Patent, filed in 1985, is titled 'Early High-Strength Concrete Composition'. James Sawyer's team adapted the geopolymeric cement formulations for use in the production of precast and prestressed concrete (heat cured Pyrament), while also developing ultra-rapid high ultimate strength cement (ambient temperature cured Pyrament). The latter enables pavement to be placed so that heavy traffic can traverse in four hours.

At Lone Star, Heitzmann and Sawyer (1989) likewise blended Portland cement with geopolymer. Their purpose was to take advantage of the good properties of geopolymer cement along with the low manufacturing cost of Portland cement. The resulting Pyrament Blended Cement (PBC) is very close to alkali-activated pozzolanic cement (US Patent 4,842,649). Pyrament PBC cement comprises 80% ordinary Portland cement and 20% of geopolymeric raw materials, that is, calcium silicate and alumino-silicates MK-750. The early-high-strength cement comprises an alkaline salt (K₂CO₃) and a retarder, citric acid. As of fall 1993, Pyrament PBC concrete was listed for over 50 industrial facilities in the USA, 57 military installations in the USA, and 7 in other countries, and for non-military airports. The US Army Corps of Engineers released a well-documented study on the properties of Pyrament Blended Cements based concretes, which were performing better than had ever been expected for high-quality concretes. Pyrament PBC manufacture stopped in 1997, when Lone Star Industries was sold to a European cement company. After twenty five years of heavy service at airports, the Pyrament PBC concretes are in excellent conditions contrary to Portland cement concretes which had to be replaced or repaired (see Geopolymer Camp 2011 Keynote Video State of Geopolymer R&D 2011 http://www.geopolymer.org/conference/gpcamp/gpcamp-2011 (at time 27:30).

In the 1980s, environmental concerns were only focusing on pollution produced by toxic mining wastes and radioactive wastes, i.e. on the advantages of using geopolymer cement. They were not dealing with the environmental impact of its production (LCA). The Canadian governmental agency CANMET funded a research project into the solidification of radioactive residues, jointly carried out by the companies Cordi- Géopolymère (France) and Comrie (Toronto, Canada) (1987-1988). It was discovered that the MK-750/Slag/(Na,K) silicate-based geopolymer cement had a very high potential for toxic and radioactive waste management. Indeed, geopolymerization is a geosynthesis (a reaction that chemically integrates minerals) that involves naturally occurring silico-aluminates. Silicon and aluminium atoms react to form molecules that are chemically and structurally comparable to those binding natural rock (cited in the paper by Davidovits, 1991).

Later on, the European research project GEOCISTEM (1994-1997) successfully tested this technology in the context of the East-German mining and milling remediation project, carried out by the German company Wismut. In other investigations carried out in 1998, special emphasis had been laid on the solidification of sludge contaminated by decay products of the U-238 and U-235 series, arsenic, and a variety of hydrocarbons. This sludge stem from existing sedimentation ponds  and, more importantly, from water treatment plants  which remove radioactive and heavy metals from mine effluents prior to their discharge into rivers.

The disposal of radioactive and toxic sludge must meet at least two conditions:

1. Safe chemical encapsulation of the contaminants, i.e., prevention of their release into ground and seepage water in order to minimize the health risks via the water path. Contaminant release is controlled by the leaching properties of the immobilization matrix.
2. Structural stability with respect to adverse environmental conditions such as rapid changes of temperature and humidity, microbial and chemical aggression and mechanical stress, in order to guarantee safe handling during operation time and minimize the risk of uncontrolled spread of contaminated matter over the next several hundred years.

These conditions cannot be reached with conventional Portland cement essentially because of its very weak resistance to acids. Alkali-activated-materials are also providing bad results due to high leachates of toxic cations. In fact, any safe heavy metals or radioactive containment requires the implementation of a technology involving MK-750/K-silicate-based geopolymers, exclusively. The geopolymer tested was a commercial geopolymer of the (K,Ca)–poly(sialate-siloxo) (K,Ca)–PSS, type, called Geopolymite 50 manufactured by Cordi-Géopolymère, France. This is a MK-750 Ca/based geopolymer involving two components: ?

• 100 parts A: powder (metakaolin MK-750  50, slag  50, mineral filler  20);
• 100 parts B: liquid (K-silicate with MR = 1.30) ?

This property of MK750/slag/(Na,K) silicate-based geopolymer cement was confirmed later on by Perera et al. (2006) who went further and compared the stabilization efficiency of fly ash-based and MK-750 based geopolymeric systems. 1 and 5 wt. % Cs and Sr were added as the nitrates or hydroxides separately. The geopolymers were characterized by X-ray diffraction analysis, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). PCT tests were carried out on selected materials, which were heated to temperatures as high as 900°C. They found that MK-750-based geopolymers have much lower leach rates for Na and K than fly ash-based materials. Also the leach rates for Cs and Sr are much lower than those for the fly ash based geopolymers.

Perera et al. (2005) had already confirmed the superiority of the metakaolin MK-750-based geopolymer for the safe immobilization of Pb. In a geopolymer matrix prepared by mixing a MK-750 precursor with a solution of silicate, 1 wt. % Pb as the nitrate was immobilized. Under the United States Environmental Protection Agency test protocol the Pb release was less than 5 ppm, the acceptable limit for landfills in the U.S.A. Electron microscopy showed Pb was present in the major amorphous phase.

Consequently, any safe heavy metals or radioactive containment requires the implementation of a technology involving MK-750-based geopolymers, exclusively. It is precisely this method that was proposed initially by Davidovits at the beginning of his study on toxic waste management in 1987.

After the first successful testing performed in the CANMET (1988) project, extensive laboratory investigations were carried out later in 1996–1999 in Europe. The German company Wismut and the Saxon State Office for Environment set up an intensive program (Hermann et al. 1999). The Soviet-German company Wismut heavily extracted uranium from 1945 until 1990, to the exclusive use of the former Soviet Union USSR for its nuclear weapons. Wismut was the third world uranium producer during this period. The pilot-scale experimentation fully reproduced the laboratory results. The pilot-scale experiment, involving 20 tonnes of low level radioactive mining waste, impressively demonstrated the technological maturity of the geopolymer technology presented at the Geopolymer Conference 1999.

The first study on the environmental impact (LCA), with respect to Global Warming Potential (GWP) related to the CO₂ emission comparison between Portland cement manufacture and geopolymer cement started as early as 1990, at PennState Materials Research Laboratory, Pennstate University, USA. Unfortunately, American Agencies (DOE and EPA) stated that this was not an important issue and both institutions declined to support research proposals. The theoretical studies were presented at several conferences, Davidovits (1993, 1994). Ordinary Portland cement is a serious atmospheric pollutant. Studies have shown that approximately 0.85 to 1.0 tons of carbon dioxide gas is released into the atmosphere for every ton of Portland cement which is made anywhere in the world.

The Portland cement industry reacted strongly by lobbying the legal institutions so that they delivered CO₂ emission numbers, which did not include the part related to calcium carbonate decomposition, focusing only on combustion emission. In 1997, UN’s Intergovernmental Panel on Climate Change put the industry’s total contribution to CO₂ emissions at 2.4%; the Carbon Dioxide Information Analysis Center at the Oak Ridge National Laboratory in Tennessee quoted 2.6 %, instead of including both sources: energy (representing 45% of the CO₂ emission) and decarbonation of calcium carbonate (the main raw material) for 55%, totalizing 5,80 %  of [world] 1997 CO₂ emission. Eighteen years later, the situation has worsened with Portland cement CO₂ emissions approaching 3 billion tonnes a year (Hasanbeigi et al. 2012).

In 2002 at the World Climate Congress in Rio de Janeiro, statistics integrated the actual values and the Portland cement industry started introducing so-called "blended cements", involving the addition of mineral ingredients such as coal fly ash, so that today, 2015, the reduction may reach a maximum of 30%-50%.

Geopolymer cements are manufactured in a different manner than that of Portland cement. They do not require extreme high temperature kilns, with large expenditure of fuel, nor do they require such a large capital investment in plant and equipment. Thermal processing of naturally occurring alkali- silico-aluminates and alumino-silicates (geological resources available on all continents) is providing suitable geopolymeric raw materials.

We mentioned above that the applications dealing with toxic/radioactive waste management were carried out with commercial geopolymer cement called Geopolymite 50 (Davidovits, 1991) with the ratios MK-750/slag/K-silicate being 1/1/2 respectively. Due to the high amount of K-silicate, this first geopolymer cement cannot be proposed as a valuable replacement or competitor for Portland cement because it is too expensive. In addition, as will be discussed below, the high amount of alkali-silicate is not appropriate with respect to the Global Warning Potential in the environmental impact assessment LCA.

Therefore, as early as 1993, we set our effort in R&D projects dedicated to the strong reduction of the alkali-silicate amount, for concrete and building applications. We already mentioned the European R&D project GEOCISTEM. With the help of the geological team involved in the project, this second generation of geopolymer cements was based on the replacement of the alkali (Na,K)-silicates  with a selection of cheap high alkali volcanic tuffs. The project sought to manufacture cost-effectively geopolymer cements for applications dealing with the long-term containment of hazardous and toxic wastes on one hand, and in construction and civil engineering on the other hand, with strong CO₂ emission, up to 80% when compared with Portland cement.

This geopolymer cement of the second generation is coined "Rock-based geopolymer cement". The manufacture includes the components with the ratios MK-750/slag/volcanic tuffs/ alkali silicate being 1/1/2/1. Compared with the first generation, the amount of alkali-silicate solution is reduced from 50% by weight to 20% by weight. A more competitive geopolymer cement with lower CO₂ is obtained, according to the Davidovits patent (2003) when instead of making a mixture of MK-750 and feldspathic-volcanic rock, one uses naturally occurring geological products containing these two elements in-situ. Indeed, kaolinite is the result of the weathering of feldspars and it is naturally found in weathered granitic residual rocks. The weathered granitic residual rock consists of 20 to 40 percent by weight of kaolinite and 80 to 60 percent by weight of feldspathic, and quartzitic residual sand containing reactive silica. In order to have a maximum reactivity, the weathered granitic residual rock in which kaolinization is very advanced, is calcined at a temperature ranging between 650°C and 800°C and, on one hand, ground at an average grain size of 15-25 microns for the feldspathic and quartzitic parts, the kaolinitic part, on the other hand, having naturally a quite lower particle size. In that case, the make up of this rock-based geopolymer cement comprised the ratios slag/weathered granite/ alkali silicate being 1.5/3.5/1. Compared to the first generation, the amount of alkali-silicate solution is reduced from 50% by weight to 17% by weight.

It must be noted that these rock-based cements have very high mechanical strength, in the range of 100-125 MPa compressive strength at 28 days. It seems obvious that a reduction of the most expensive element, namely alkali-silicate solution, may go down to 10 % for a regular cement/concrete of the type 30-35 MPa.

The energy needs and CO₂ emissions calculations for this second generation of geopolymer cements (Davidovits, 2013) are significantly reduced. In the most favourable case — slag availability as waste (no allocation) — there is a reduction of 59% of the energy needs in the manufacture of Rock-based geopolymer cement in comparison with Portland cement. ?In the least favorable case —slag manufacture (allocation) — the reduction reaches 43%. ?

As for CO₂ emission, in the most favorable case — slag availability as waste (no allocation) — there is a reduction of 80% of the CO₂ emission during manufacture of Rock-based geopolymer cement in comparison with Portland cement. ?In the least favorable case —slag manufacture (allocation) — the reduction reaches 70%. ?

It was recently discovered that weathered basaltic rocks, lateritic type, also contain kaolinitic species similarly to weathered granites. This geological resource comprises up to 40% by weight of kaolinitic clay and up to 40% iron oxides. This is a valuable raw material for Rock-based geopolymer cements.

In other words, geological resources for Rock-based geopolymer cements are available throughout all the five continents. When limestone, the main raw material in the production of Portland, is not available, geopolymer cement has the potential to receive the same traction in the market place as has Portland cement. We have seen that main categories of materials from which geopolymer cements may be derived are MK-750/slag-based, rock/slag-based. These geopolymer cements require blast furnace slag for their manufacture and thus the geopolymer cement industry will be intrinsically linked with the availability of slag by-products from the iron and steel industry. Or it must be manufactured as mentioned above in the CO₂ emissions calculations.

Concerning the environmental implications of Rock-based geopolymer cement, we must rely on the fact that it contains MK-750 and, therefore, follow the rules developed above on MK-750-based geopolymer cement.

There is a third category of geopolymer cement based on another industrial waste, coal fly ash, essentially low calcium fly ash of class F. This material has been extensively studied since  year 2008. Looking in the database Science Direct on 14/04/2015, we found 1,102 occurrences with the keywords 'geopolymer + fly ash'. The majority of the scientific articles were published in Construction and Building Materials (232), Cement and Concrete Composites (82), Cement and Concrete Research (81), Ceramics International (58) and Journal of Hazardous Materials (48).

As early as 1994, Davidovits mentioned the potential for this fly ash-based geopolymer cement. Here is the excerpt of the paper available in the Geopolymer Institute Library, Technical paper #5 Global Warming Impact on the Cement and Aggregates Industries: " ... Development means implementing the use of electricity on one hand and building infrastructures and houses on the other hand; in short, electricity and concrete. The by-product of electricity production with coal firing is fly ash. The innovative step would be to produce electricity and low- CO₂ cement (geopolymeric cement), in the same plant, by adapting and implementing fly-ash production into Geopolymeric raw material, without any supplementary chemical- CO₂ emission. .... , this would allow electricity utilities to produce  million of tonnes of low- CO₂ fly ash-based Geopolymer cement. .... In other words, implementing such a new technology would give a wide potential for any further development of electricity production with coal firing plants."

Presently, we have two types based on Class F fly ashes:

- Type 1: alkali-activated fly ash material:? it uses high caustic NaOH (user-hostile) + fly ash; after dissolution in the caustic slurry, the fly ash particles become embedded in an alumino-silicate gel with Si:Al= 1 to 2, building a zeolitic type (chabazite-Na and sodalite) matrix. In general, it requires heat hardening at 60-80°C and is not manufactured separately as a cement but used to directly manufacture fly-ash based concrete.

            This alkali-activation of fly ash is very often qualified with the term "geopolymer" which is totally wrong. These studies are interesting from a scientific point of view but are generating confusion in the geopolymer manufacturing community. Despite the proven fact of their dangerousness, their causticity, they are recommended as the "Current State of the Art" in several review papers. Although, during the Geopolymer 2005 Conference in Saint-Quentin, end users representatives complained about this situation, several scientists do not take this situation into account and continue to promote highly corrosive systems in their alkali-activated-materials AAM. This could explain why their cement technology does not achieve any applications at all.

- Type 2: slag/fly ash-based geopolymer cement (user-friendly): it uses the incongruent covalent bonding concept developed in Davidovits' book Geopolymer Chemistry and Applications, which allows the fabrication of fly ash-based geopolymer with non corrosive conditions in a user-friendly system. One obtains a room-temperature cement hardening with user-friendly silicate solution + blast furnace slag + fly ash. The fly ash particles are embedded in a geopolymeric matrix with Si:Al= 2, (Ca,K)-poly(sialate-siloxo). This material resulted from the EU sponsored R&D project "Understanding and mastering coal fired ash geopolymerization process in order to turn potential into profit", known under the acronym GEOASH (2004-2007). In this project, since the idea is to use the geopolymer as a cement, the curing is taking place at ambient temperature, with a modified (Ca,K)–based system that does not include MK-750. The Final Technical and Scientific Report was presented mid 2008, and information were published, for example Nugteren et al. (2005), Davidovits (2005), Álvarez-Ayuso (2008), Davidovits et al. (2008), Izquierdo et al. (2009). One finds a dedicated paper in the Geopolymer Institute Library, Technical paper #22: GEOASH, ambient temp. hardening of fly ash-based geopolymer cements.

In the case of the slag/fly ash-based geopolymer cement (Type 2) (Davidovits et al. 2006), the make up comprises the ratios slag/fly ash/ alkali silicate being 1/5/1. This is for cement developing a compressive strength in the range of 100 MPa at 28 days. Compared with the first generation, the amount of alkali-silicate solution is reduced from 50% by weight to 14% by weight. For a lower strength, in the range of 40 MPa, the ratios are 1/8/1, i.e. a reduction of the amount of alkali-silicate solution down to 10 % by weight.

With respect to the environmental implications, according to Izquierdo et al. (2009), Type 1: alkali-activated fly ash material leads to products in which the mobility of oxyanionic species is 5 to 50 times higher than the Type 2 slag/fly ash-based geopolymer cement (user-friendly). For example, Vanadium values are 10 mg/kg for geopolymer to be compared with 500 mg/kg for alkali-activated-materials. In the geopolymeric matrices, the cations are fixed or trapped inside the synthesized poly(sialate) frameworks. Taking into account other relevant properties (higher compressive strengths and lower conversion costs), it can be concluded that the geopolymerization method is more attractive and safer than the simple alkali-activation process.

Notwithstanding this fact, the majority of published Environmental Impact LCA studies are dealing with alkali-activated materials. They ignore the evolution of the formulations for geopolymer cement since its invention in 1983-85 (see Table 2), simply because it pertains to industrial and commercial implementation, not to "regular" scientific studies. It is a fact that the scientific community continues to neglect the patent literature.

The first LCA was presented at the Geopolymer Camp 2010 by Habert et al. (2010) (see at http://www.geopolymer.org/conference/gpcamp/gpcamp-2010) and it was a shock for the attendance. They claimed that, in terms of CO₂ emission, geopolymer cement was not better than Portland cement, and worse for other parameters. One of their studies involved a mix design containing metakaolin MK-750 and Na-silicate and, because of the high amount of alkali silicate needed in the formulation, they claimed that geopolymer cement emitted twice the amount of Portland cement. This statement was taken for granted by other scientists without any further consideration.

Habert et al. did not recognize that this formulation was not geopolymer cement but rather a geopolymer resin/binder, because it does not harden at ambient temperature. This geopolymer material was discussed in Section 1) above and its environmental impact presented in comparison with organic resins/binders, not as a replacement of Portland cement. Its Global Warming Potential was calculated to be 282.6  kg CO₂ / 1000 kg, compared to 6,663 kg CO₂ for epoxy. We already mentioned the strong reductions in the amount of alkali-silicate (K-silicate) that occurred in the various developments of the geopolymer cement types, since 1985 until 2006. They are summarized in Table 2.

Table 2: Evolution of the amount in potassium-silicate % by weight of geopolymeric formulation for room temperature hardening geopolymer cements since 1983-85 (see explanation in the text).

Pyrament (1985)	Geopolymite 50 (1987)	Rock-based (1997) 100 MPa	Rock-based (2002) 50 MPa	Fly Ash-based (2006) 100 MPa	Fly Ash-based (2006) 40 MPa
50 %	50 %	20 %	17 %	14 %	10 %

LCA of commercialized geopolymer cement/concretes are seldom. This is due to proprietary reasons. One is dedicated to the geopolymer concrete named E-Crete, developed by the Australian company Zeobond and written on behalf of one of its licensees (see at http://www.start2see.com.au/site/index.php/component/zj_fileseller/?view=tag&tag=E-Crete&Itemid=). It is a slag/fly ash/alkali-silicate-based geopolymer cement/concrete (Type 2). The reduction in greenhouse gas emissions for E-Crete compared to standard concrete is primarily attributed to savings achieved through the use of a geopolymer binder. As E-Crete and standard concrete are similar in non-binder materials used and behavior after production, there is some dilution of the benefits when measured over the full life cycle (LCA). The greenhouse gas emissions during the life cycle of E-Crete are approximately 62%-66% lower than emissions from the reference concrete. The E-Crete geopolymer cement has ca. 80% lower embodied greenhouse gas intensity than an equivalent amount of ordinary Portland cement binder used in reference concrete of a similar strength, confirming the data published by the Geopolymer Institute.

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The present selection of papers dedicated to the Environmental benefits of geopolymer cements begins with the previously cited Davidovits (1991) paper Geopolymers: Inorganic Polymeric New Materials originally published in J. Thermal Analysis, Vol. 37, 1633-1656 (1991). It summarizes the work carried out for CANMET, Ottawa, Canada on toxic mining wastes safe containment. Though not specific to Environmental benefits, this paper, written almost 26 years ago is the most cited scientific paper on geopolymer science.

The applications in the foundry industry are the papers by Wallenhorst (2010), Industrial application in Foundry Business   and Krahula (2014)The technology of mould and core production with inorganic binder system - Practical using in foundry industry Keynote video here.

A review paper presented at the Geopolymer Conference 2002, Melbourne, Australia, Environmentally Driven Geopolymer Cement Applications discusses the results obtained either during the CANMET (1987-1988) project or during the GEOCISTEM and Wismut (1998) pilot experimentations on Uranium mine tailings and radioactive sludge.

The paper Solidification of Various Radioactive Residues by Géopolymère With Special Emphasis On Long-Term-Stability http://www.geopolymer.org/library/technical-papers/13-solidifaction-of-various-radioactive-waste-geopolymer-long-term-stability by Herman et al. (1999) describes with more details the pilot experimentation carried out at Wismut with the MK-750/slag/K-silicate geopolymer cement type on radioactive waste safe containment.

The major role played by metakaolin MK-750-based geopolymer in radioactive waste management is confirmed in a recent paper by Xu et al. (2014), DuraLith geopolymer waste form for Hanford secondary waste: Correlating setting behaviour to hydration heat evolution, Journal of Hazardous Materials 278 (2014) 34–39.

The Environmental benefits of Type 2 fly ash-based geopolymer cement are presented in Izquierdo et al. (2009) paper Coal fly ash-based geopolymers: microstructure and metal leaching, Journal of Hazardous Materials, 166, 561–566). The results are also summarized in the Technical paper #22 by Davidovits et al. (2014) GEOASH: ambient temp. hardening of fly ash-based geopolymer cements, http://www.geopolymer.org/library/technical-papers/geoash-fly-ash-geopolymer.

Another paper by the same group (2010) The role of open and closed curing conditions on the leaching properties of fly ash-slag-based geopolymers, Journal of Hazardous Materials 176 (2010) 623–628, provides more details on the best  experimental ambient temperature curing method. It indicates that the discussed curing condition induces a significant effect on several properties such as the leaching values of toxic metals.

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The present selection of papers dedicated to the Environmental impact LCA of geopolymer cements begins with the previously cited Davidovits (1994) paper Global Warming Impact on the Cement and Aggregates Industry, http://www.geopolymer.org/library/technical-papers/5-global-warming-impact-on-the-cement-and-aggregates-industry dedicated to the low CO₂ emission potential of geopolymer cements, when compared to Portland cement.

Although this first paper was written 21 years ago, all the calculation discussed in terms of CO₂ emission have been verified and actualized, for example in the paper by Hasanbeigi et al. (2012) Emerging energy-efficiency and CO₂ emission-reduction technologies for cement and concrete production: A technical review, Renewable and Sustainable Energy Reviews 16 (2012) 6220–6238.

Before starting the critical analysis of the LCA papers published in Elsevier's journals, the paper Geopolymer cement review 2013, http://www.geopolymer.org/library/technical-papers/21-geopolymer-cement-review-2013 , confirms the good CO₂ emissions values estimated for Rock-based geopolymer cements, as well as for Type 2 slag/fly ash-based geopolymer cement. Compared to Portland cement, the reductions are in the range of 70 % to 90 %. These values do not include any additional external constraints like transport from or to the utility. They reflect the actual potential as soon as industrialization starts in full swing.

We mentioned that the presentation at the Geopolymer Camp 2010 by Habert (2010) was a shock for the attendance. We could get precise details in Habert et al. (2011) paper An environmental evaluation of geopolymer based concrete production: reviewing current research trends, Journal of Cleaner Production 19 (2011) 1229-1238. It is a well-documented survey, which lists numerous mix-designs, the majority of them pertaining to alkali-activated-materials, not to genuine geopolymer cements/concretes. In fact they took each mineral ingredient separately, blast-furnace slag, fly ash, metakaolin, each of them being "activated" by a Na-silicate solution. They studied their LCA implications and stated that current alkali-activated mix designs made from fly ash alone or blast-furnace slag alone emit less CO₂ than Portland cement. However, this reduction is not sufficient to achieve the objectives. This study also highlights that the environmental impact of alkali-activated-materials stems from the use of the sodium silicate solution. In this case, the sodium silicate solution would lead to a pollution transfer within all of the other environmental impact categories.

The authors concluded that the ideal solution would be to strongly diminish the amount of alkali-silicate and, consequently, to follow the genuine geopolymer methods discovered and implemented by Davidovits. We quote: "...the solution, proposed by Davidovits, has the advantage of using less [silicate] and slag than pure GBFS geopolymer concrete. This is beneficial from an environmental point of view..." In other words: away from alkali-activated-materials.

In fact, another paper by Ouellet-Plamondon and Habert (2014) confirms that their study was dedicated to alkali-activated materials, not to geopolymer cements at all. It is found as Chapter 25, of the Handbook of Alkali Activated Cements, Mortars and Concretes, and is titled: Life Cycle Assessment (LCA) of alkali-activated cements and concretes.

Actually, the most important document is the LCA paper by Fawer et al. (1999) Life Cycle Inventories for the Production of Sodium Silicates, Int. J. LCA 4 (4) 207-212 (1999). Life Cycle Inventories were compiled by EMPA St. Gallen / Switzerland from 12 West European silicate producers covering about 93% of the total alkaline silicate production in Western Europe. The production routes for five typical commercial sodium silicate products were traced back to the extraction of the relevant raw materials from the earth.

The CO₂ emission for the glass sodium silicate Na-silicate 3.3 (WR) furnace lumps (100%) is 1,066 kg /tonne, and for the solution Na-silicate 3.3 (WR) furnace route, 37% solid, 424 kg/tonne. This solution is precisely the major raw ingredient in all mix designs listed in Habert's paper. Although Habert et al. write in their paper, quotation "data for sodium silicate solution come from Fawer et al. (1999)", the value given in their Table 2 for the solution (37% solid) is 1.14 kg CO₂ eq. This is in the range of the value for the solid glass (100%), not diluted in water, in Fawer's paper, instead of the expected 0.424 kg CO₂ eq for the solution (we mentioned this value in our Table 1 above). This is a methodological flagrant error. We may therefore conclude that all the CO₂ emissions and environmental impacts calculated in Habert et al. paper are wrong and must be roughly divided by 2. 

We have the same situation in another paper by Turner and Collins (2013), Carbon dioxide equivalent (CO₂-e) emissions: A comparison between geopolymer and OPC cement concrete, Construction and Building Materials 43 (2013) 125–130. The Australian team could not get any data from silicate producers. They calculated theoretically the CO₂ emissions for the Na-silicate glass (100%), got 1.222 kg CO₂ eq/kg for the emission arising during manufacturing (i.e. a value higher than Fawer and Habert), added 30% more for transport, and ended with a total emission estimate of 1.514 kg CO₂ eq per kg sodium silicate solution. The problem is that, like in Habert et al. paper, they used this value, estimated for the 100% solid lumps, on place of the actual value of the diluted silicate solution (45% solid). Here too, the CO₂ emissions calculated are wrong and must be roughly divided by 2.

The paper by McLellan et al. (2011) Costs and carbon emissions for geopolymer pastes in comparison to ordinary Portland cement, Journal of Cleaner Production 19 (2011) 1080-1090, provides accurate values for the silicate solution. The value found for the solid form, 100% concentration, was multiplied by 0.37 to get the value for the solution at 37% concentration. Yet, the final conclusions are highly affected by the enormous transport distances found in Australia. The source locations for the sodium silicate are China, India, UK, and USA. They conclude that compared with emissions from Portland cement concrete, emissions from geopolymer concrete can be 97% lower up to 14% higher. Each application for geopolymers therefore needs to be assessed for its specific location, given that the impact of location on overall sustainability is one of the determining factors.

Another paper providing the actual numbers for the sodium silicate solution, namely 0.445 CO₂ kg-eq, was published by Heath et al. (2014), Minimising the global warming potential of clay-based geopolymers, Journal of Cleaner Production 78 (2014) 75-83. This value is in the range of the number given in Fawer et al. paper. Their target was to replace the expensive metakaolin MK-750 by cheaper calcined meta-clays.

All LCAs published are also focusing on the amount of CO₂ that must be added to the original manufacture emissions in order to reflect the long distances that the raw ingredients and chemicals (metakaolin, slag, alkali-silicates) have to go all over before reaching their destinations. Sometimes, these distances are enormous: 6000 km for metakaolin or Na-silicate. This could contribute to a doubling of the Global Warming Potential numbers.

We feel, there is something unfair in these calculations. Local special environmental impact assessments are generalized to serve as references for the entire world. But the most striking element is that each paper compares a well-established, 170 years old industry involving hundreds of cement plants and terminals, with a start-up situation. Thinking in terms of innovation and R&D results implementation, the authors would have been better inspired in calculating at least 2 cases: first, their present laboratory situation, second, the one that will prevail in 5-10 years from now when industrialisation starts in full swing. There is a lack in the methodology as well as in standard procedures.

For people involved in R&D and innovation, the logic would have been to consider the market forces. As a matter of fact, business will foster the manufacturing of the chemicals and ingredients to take place as close as possible to the market. We know for example that a global major alkali-silicate manufacturing company has launched the marketing of a slag/fly ash-based geopolymer cement/concrete of Type 2 (see the geopolymer concrete Alacrete at http://www.alacrete.in). It is logical to understand why their target is to cover the emerging countries, India, Africa, and others, with alkali-silicates production sites located close to the market and to the geopolymer cement manufacturing sites.

The editors hope this selection will inspire additional, and much-needed, research on the environmental implications of genuine geopolymer cement mix designs, bearing in mind that industrialization and commercialization already started with the production of structural geopolymer concretes for public buildings and infrastructures (airport). See Wagners' geopolymer concrete EFC at http://www.wagnerscft.com.au/products/efc and at Geopolymer Institute News pages http://www.geopolymer.org/news.

References

Hammell J.A, Balaguru P.N, Lyon R.E, Strength retention of fire resistant aluminosilicate–carbon composites under wet–dry conditions, Composites: Part B 31 (2000) 107–111.

Mills-Brown J., Potter K., Foster S., Batho T., (2013) The development of a high temperature tensile testing rig for composite laminates, Composites: Part A 52 (2013) 99–105.

Weil M., Dombrowski K., Buchwald A., (2009), Life-cycle analysis of geopolymers, in Geopolymer, Structure, processing, properties and industrial applications, edited by J. Provis and J. van Deventer, Woodhead Publishing Limited.

La Rosa A.D., Recca G., Summerscales J., Latteri A., Cozzo G., Cicala G., (2014), Bio-based versus traditional polymer composites: A life cycle assessment perspective, Journal of Cleaner Production 74 (2014) 135-144.

Habert G., d’Espinose de Lacaillerie J.B., Roussel N., (2011), An environmental evaluation of geopolymer based concrete production: reviewing current research trends, Journal of Cleaner Production 19 (2011) 1229-1238.

Warne M. St. J., Schifko A. D., Toxicity of laundry detergent components to a freshwater cladoceran and their contribution to detergent toxicity, Ecotoxicology and Environmental Safety 44, 196-206 (1999).

Hasanbeigi Ali, Price Lynn, Lin Elina , Emerging energy-efficiency and CO2 emission-reduction technologies for cement and concrete production: A technical review, Renewable and Sustainable Energy Reviews 16 (2012) 6220–6238.

Álvarez-Ayuso E., Querol X., Alastuey A., Moreno N., Izquierdo M., Font O., Moreno T., Diez S., Ramonich E.V. and Barra M., (2008), Environmental, physical and structural characterization of geopolymer matrices from coal (Co-) combustion fly ashes, Journal of Hazardous Materials, 154, 175–183.

Izquierdo M., Querol X., Davidovits J., Antenucci D., Nugteren H. and Fernández- Pereira C., (2009), Coal fly ash-based geopolymers: microstructure and metal leaching, Journal of Hazardous Materials, 166, 561–566.

Xu H., Gong W., Syltebo L., Lutze W., Pegg I. L., (2014), DuraLith geopolymer waste form for Hanford secondary waste: Correlating setting behavior to hydration heat evolution, Journal of Hazardous Materials 278 (2014) 34–39.

Izquierdo M., Querol X., Phillipart C., Antenucci D., Towler M., (2010) The role of open and closed curing conditions on the leaching properties of fly ash-slag-based geopolymers, Journal of Hazardous Materials 176 (2010) 623–628.

Ouellet-Plamondon C., Habert G., (2014), Life Cycle Assessment (LCA) of alkali-activated cements and concretes, Handbook of Alkali Activated Cements, Mortars and Concretes, ed. by Pacheco-Torgal et al., Woodhead Publishing, 2014.

Turner L. K., Collins F. G., (2013), Carbon dioxide equivalent (CO2-e) emissions: A comparison between geopolymer and OPC cement concrete, Construction and Building Materials 43 (2013) 125–130.

McLellan B. C., Williams R. P., Lay J., van Riessen A., Corder G. D., (2011), Costs and carbon emissions for geopolymer pastes in comparison to ordinary Portland cement, Journal of Cleaner Production 19 (2011) 1080-1090.

Heath A., Paine K., McManus M., (2014), Minimising the global warming potential of clay based geopolymers, Journal of Cleaner Production 78 (2014)  75-83.

Further reading:

Lyon R.E, Foden A.J., Balaguru P.N., Davidovits J. and Davidovics M., (1997), Properties of Geopolymer Matrix-Carbon Fiber Composites, Fire and Materials, 21, 67–73.

Fawer M., Concannon M., Rieber W., (1999), Life Cycle Inventories for the Production of Sodium Silicates, Int. J. LCA 4 (4) 207-212 (1999).

Glukhovsky V.D., (1965), Soil silicates, Their Properties, Technology and Manufacturing and Fields of Application, Doct Tech Sc. Degree thesis, Civil Engineering Institute. Kiev.

Talling B. and Brandstetr J., (1989), Present state and future of alkali-activated slag concrete, 3rd International Conference on the Use of Fly Ash, Silica Fume, Slag & Natural Pozzolans in Concrete, ACI SP-114, Trondheim, Norway, vol. 2, 1519–1545.

Shi C., Krivenko P.V. and Roy D.M., (2006), Alkali-Activated Cements and Concretes, Taylor & Francis ed.,
London.

Davidovits J., (1991), Geopolymers: Inorganic Polymeric New Materials, J. Thermal Analysis, Vol. 37, 1633-1656 (1991).

Perera D.S., Vance E.R., Aly Z., Davis J. and Nicholson C.L., (2006), Immobilization of Cs and Sr in geopolymers with Si:Al molar ratio of ≈ 2, Ceramic Transactions, 176, 91–96.?

Perera D.S., Aly Z., Vance E.R. and Mizumo M., (2005), Immobilization of Pb in a geopolymer matrix, Journal of the American Ceramic Society, 88 (9), 2586–2588.?

Hermann E., Kunze C., Gatzweiler R., Kiessig G. and Davidovits J., (1999), Solidification of various radioactive residues by Geopolymere with special emphasis on long-term stability, Geopolymer ’99 Proceedings, 211–228.

Davidovits J., (1993), Geopolymer Cements to minimize Carbon-Dioxide Green- house Warming, Ceramic Transactions, 37, Cement-Based Materials: Present, Future and Environmental Aspects, pp. 165–182.

Davidovits J., (1994), Global Warming Impact on the Cement and Aggregates Industries, World Resource Review, 6, No. 2, 263–278.

Davidovits J. and Davidovits R., (2003), Poly(sialate-disiloxo)-based geopolymeric cement and production method thereof, PCT Patent WO03/099738, US Patent US 7,229,491.

Davidovits J., Geopolymer cement review 2013, Geopolymer Institute Library, Technical-papers #21.

Davidovits J., Geopolymer Chemistry and Applications, (2008-2011), 3rd ed. Institut Géopolymère, Saint-Quentin, France.

Nugteren H., Davidovits J., Antenucci D., Fernández Pereira C. and Querol X., (2005), Geopolymerization of fly ash, WOCA 2005 Proceedings, World of Coal Ash Conference.

Davidovits J., (2005), Geopolymer chemistry and sustainable Development, The poly(sialate) terminology: a very useful and simple model for the promotion and understanding of green-chemistry, Geopolymer 2005 Proceedings, 9–15.

Davidovits J., Izquierdo M., Querol X., Antennuci D., Nugteren H., Butselaar- Orthlieb V., Fernández-Pereira C. and Luna Y., (2008), The European Re- search Project GEOASH: geopolymeric cement based on European fly-ashes, Proceedings 2nd Intern. Conf. Ceramics ICC2, Verona, Italy.

Davidovits J., Izquierdo M., Querol X., Antennuci D. , Nugteren H., Butselaar-Orthlieb V., Fernández-Pereira C. and Luna Y., (2014), The European Research Project GEOASH: Geopolymer Cement Based On European Coal Fly Ashes, Technical Paper #22, Geopolymer Institute Library, www.geopolymer.org (2014).

Davidovits J., Davidovits R. and Davidovits M., (2006), Ciment géopolymèrique à base de cendres volantes et à grande innocuité d’emploi, French Patent Application 06/06923; International Patent Publication WO 2008/012438.