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Geopolymer cement

This page is a development of the main article Geopolymer. From a terminological point of view, geopolymer cement is a 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 but rather geopolymer binder.

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 byproducts 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>1,65, M being Na or K) and water (See the definition for "user-friendly" reagent below). Room temperature hardening relies on the addition of calcium cations, essentially iron blast furnace slag.

Geopolymer cements cure more rapidly than Portland-based cements. They gain most of their strength within 24 hours. However, they set slowly enough that they can be mixed at a batch plant and delivered in a concrete mixer. Geopolymer cement also has the ability to form a strong chemical bond with all kind of rock-based aggregates. On March 2010, the US Department of Transportation Federal Highway Administration released a TechBrief titled "Geopolymer Concrete" that states: The production of versatile, cost-effective geopolymer cements that can be mixed and hardened essentially like Portland cement represents a “game changing” advancement, revolutionizing the construction of transportation infrastructure and the building industry."

There is often confusion between the meanings 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 or Geopolymer cement) and adds it to the concrete batch. On the contrary, geopolymer chemistry was from the start aimed at manufacturing binders and cements for various types of applications. For example the British company banah UK (www.banahuk.co.uk) sells its banah-Cem™ as geopolymer cement, whereas the Australian company Zeobond (www.zeobond.com) markets its E-crete™ as geopolymer concrete.

Portland cement chemistry vs Geopolymer cement chemistry

Left: hardening of Portland cement (P.C.) through simple hydration of Calcium Silicate into Calcium Di-Silicate hydrate and lime Ca(OH)2. Right: hardening (setting) of Geopolymer cement (GP) through poly-condensation of Potassium Oligo-(sialate-siloxo) into Potassium Poly(sialate-siloxo) cross linked network.

Alkali-activated materials vs Geopolymer cements. Geopolymerization chemistry requires appropriate terminologies and notions that are evidently different from those in use by Portland cement experts. The main article Geopolymer summarizes how geopolymer cements belong to the category of inorganic polymers. On this matter, the Australian Geopolymer Alliance [27] outlines on his web site the following statement: ' Davidovits developed the notion of a geopolymer (a Si/Al inorganic polymer) to better explain these chemical processes and the resultant material properties. To do so required a major shift in perspective, away from the classical crystalline hydration chemistry of conventional cement chemistry. To date this shift has not been well accepted by practitioners in the field of alkali activated cements who still tend to explain such reaction chemistry in Portland cement terminology.'

Indeed, geopolymer cement is sometimes mixed up with alkali-activated cement and concrete, developed more than 50 years ago by G.V. Glukhovsky in Ukraine, the former Soviet Union [10]. They were originally known under the names "soil silicate concretes" and "soil cements". Despite the negative impact of the wording "alkali-activation" on civil engineers because of the deleterious "alkali-aggregate-reaction", coined AAR or "alkali-silica-reaction" coined ASR (see for example the RILEM Committee 219-ACS Aggregate Reaction in Concrete Structures at http://www.rilem.org/gene/main.php?base=8750&gp_id=229), several cement scientists continue to promote the idea of "alkali-activated materials" or "alkali-activated geopolymers". These cements coined AAM encompass the specific fields of alkali-activated slags, alkali-activated coal fly ashes, blended cements (see RILEM Technical committee DTA) http://www.rilem.org/gene/main.php?base=8750&gp_id=290.

User-friendly alkaline-reagents Although geopolymerization does not rely on toxic organic solvents but only on water, it needs chemical ingredients that may be dangerous and therefore requires some safety procedures. Material Safety rules classify the alkaline products in two categories: corrosive products irritant products The two classes are recognizable through their respective logos displayed here.

The Table lists some alkaline chemicals and their corresponding safety label. The corrosive products must be handled with gloves, glasses and masks. They are User-hostile and cannot be implemented in mass applications without the appropriate safety procedures. In the second category one finds Portland cement or hydrated lime, typical mass products. Geopolymeric alkaline reagents belonging to this class may also be termed as User-friendly.

Unfortunately, the development of so-called alkali-activated-cements or alkali-activated "geopolymers" (the latter being a wrong terminology), and several recipes found in the literature and on the Internet, especially those based on fly ashes, comprise molar ratio in average below 1.0. Worse, looking only at low-costs consideration, not at safety and User-friendly issues, they propose systems based on pure NaOH (8M or 12M). These are User-hostile conditions and may not be used by the ordinary labor force employed in the field. Indeed, laws, regulations, and state directives push to enforce for more health protections and security protocols for workers’ safety.

On the opposite, Geopolymer cement recipes generally involve alkaline soluble silicates with starting molar ratio SiO2:M2O ranging from 1.45 to 1.95, essentially 1.60 to 1.85, i.e. user-friendly conditions.

Geopolymer cement categories

The emission depends on the category The categories comprise: - Slag-based geopolymer cement [32] - Rock-based geopolymer cement [33] - Fly ash-based geopolymer cement type 1: alkali-activated fly ash geopolymer [34] type 2: slag/fly ash-based geopolymer cement [35] - Ferro-sialate-based geopolymer cement [36]

Slag-based geopolymer cement Manufacture components: MK-750 + blast furnace slag + alkali silicate (user-friendly). Geopolymeric make-up: Si:Al = 2 in fact solid solution of Si:Al=1, Ca-poly(di-sialate) (anorthite type) + Si:Al =3, K-poly(sialate-disiloxo) (orthoclase type) and CSH Ca-disilicate hydrate.

The first geopolymer cement developed in the 1980s was of the type (K,Na,Ca)-poly(sialate) (or slag-based geopolymer cement) and resulted from the research developments carried out by J. Davidovits and J.L. Sawyer at Lone Star Industries, USA and yielded to the invention of the well known Pyrament® cement. The American patent application was filed in 1984 and the patent US 4,509,985 was granted on April 9, 1985 with the title 'Early high-strength mineral polymer'.

Rock-based geopolymer cement The replacement of a certain amount of MK-750 with selected volcanic tuffs yields geopolymer cement with better property and less CO2 emission than the simple slag-based geopolymer cement. Manufacture components: MK-750, blast furnace slag, volcanic tuffs (calcined or not calcined), mine tailings and alkali silicate (user-friendly). Geopolymeric make-up: Si:Al = 3, in fact solid solution of Si:Al=1 (anorthite type) + Si:Al =3-5 (Na,K)-poly(silate-multisiloxo) and CSH Ca-disilicate hydrate.

Fly ash-based geopolymer cements In 1997, building on the works conducted on slag-based geopolymeric cements, on the one hand and on the synthesis of zeolites from fly ashes on the other hand, Silverstrim et al. [37] and van Jaarsveld and van Deventer [38] developed geopolymeric fly ash-based cements. Silverstrim et al. US Patent 5,601,643 was titled 'Fly ash cementitious material and method of making a product'.

Their are presently two types: type 1: alkali-activated fly ash geopolymer [34] In general requires heat hardening at 60-80°C and is in fact part of the resulting fly-ash based concrete. NaOH+fly ash: fly ash particles surrounded with Si:Al= 1 to 2, zeolitic type (chabazite-Na and sodalite). type 2: slag/fly ash-based geopolymer cement: [35] Room-temperature cement hardening. User-friendly silicate solution + blast furnace slag + fly ash: fly ash particles surrounded with Si:Al= 2, (Ca,K)-poly(sialate-siloxo)

Ferro-sialate-based geopolymer cement [36]

CO2 emission during manufacture

Concrete (mixture of cement and aggregates) is the most commonly used construction material; its usage by communities across the globe is second only to water. Ever grander building and infrastructure projects require prodigious quantities of concrete with its binder of Portland cement whose manufacture is accompanied by large emissions of carbon dioxide CO2. This burgeoning worldwide demand for concrete is a great opportunity for the development of geopolymer cements of all types, with their much lower tally of carbon dioxide CO2.

CO2 emission during manufacture of Portland cement clinker Ordinary cement, often called by its formal name of Portland cement, is a serious atmospheric pollutant. Indeed, the manufacture of Portland cement clinker involves the calcination of calcium carbonate according to the reaction: 5CaCO3 + 2SiO2 ⇒ (3CaO,SiO2)(2CaO,SiO2) + 5CO2 The production of 1 tonne of cement directly generates 0.55 tonnes of chemical-CO2 and requires the combustion of carbon-fuel to yield an additional 0.40 tonnes of carbon dioxide. To simplify: 1 T of cement = 0.95 T of carbon dioxide The only exceptions are so-called ‘blended cements’, using such ingredients as coal fly ash, where the CO2 emissions are slightly suppressed, by a maximum of 10%-15%. There is no known technology to reduce carbon dioxide emissions of Portland cement any further.

On the opposite, Geopolymer cements do not rely on calcium carbonate and generate much less CO2 during manufacture, i.e. a reduction in the range of 40% to 80-90%. J. Davidovits delivered the first paper on this subject in 1993 at a symposium organized by the American Portland Cement Association [31]. 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. An article written in the scientific magazine New Scientist 1997 stated that: "...estimates for CO2 emissions from cement production have concentrated only on the former source [fuel combustion]. The UN’s Intergovernmental Panel on Climate Change puts 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 quotes 2.6 %. Now Joseph Davidovits of the Geopolymer Institute... has for the first time looked at both sources. He has calculated that world cement production of 1.4 billion tonnes a year produces 7 % of [world] current CO2 emissions (Pearce Fred, (1997), The concrete jungle overheats, New Scientist, issue 2091 (19 July 1997), page 14). Fifteen years later (2012), the situation has worsened with Portland cement CO2 emissions approaching 3 billion tonnes a year. See the video of the Keynote State of Geopolymer 2012, Section 3: Geopolymer Cements at: 32 min.

The fact that the dangers to the world’s ecological system from the manufacture of Portland cement is so little known by politicians and public makes the problem all the more urgent: when nothing is known, nothing is done. This situation clearly cannot continue if the world is going to survive.

Geopolymer Cements Energy Need and CO2 emission This section compares the energy needs and CO2 emissions for regular Portland cement and Rock-based Geopolymer Cements. The comparison proceeds between Portland cement and geopolymer cement with similar strength, i.e. average 40 MPa at 28 days. They have been several studies published on the subject that may be summarized in the following way:

Rock-based Geopolymer cement manufacture involves: - 70% by weight geological compounds (calcined) - blast furnace slag - alkali-silicate solution (industrial chemical). The presence of blast furnace slag provides room-temperature hardening and increases the mechanical strength.

Energy needs According to the US Portland Cement Association (2006), energy needs for Portland cement is in the range of 4700 MJ/tonne (average). The calculation for Rock- based geopolymer cement is performed with following parameters: - the blast furnace slag is available as by-product from the steel industry (no additional energy needed); - or must be manufactured (re-smelting from non granulated slag or from geological resources).

In the most favorable case — slag availability as by-product — there is a reduction of 71% of the energy needs in the manufacture of Rock-based geopolymer-cement in comparison with Portland cement. In the least favorable case —slag manufacture — the reduction reaches 43%.

CO2 emission during manufacture

In the most favorable case — slag availability as by-product — 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 — the reduction reaches 70%.

Fly ash-based cements Class F fly ashes do not require any further heat treatment. The calculation is therefore easier. One achieves emission in the range of 0,09 to 0,25 tonnes of CO2 / 1 tonne of fly ash- based cement, ie. CO2 emissions that are reduced in the range of 75 to 90%.

The need for standard geopolymer cement