Concrete Without Cement ??- Geopolymer

History

In the year of 1950 Viktor Glukovsky of USSR developed concrete material without Portland cement, originally known under the names “Soil Silicate Concrete” or “Soil cement”.

The geopolymer concept was first introduced by Joseph Davidovits, a French Scientist in 1976 for alkali aluminosilicate binder and thereafter he further updated his work and established clarity about geopolymer in 1991.

It would be worth knowing that some years back a controversy had been raised whether Pyramid at Egypt made out of geopolymers, since the nearest source of such stone was far off. However, later on it was established that pyramids were made out of special stone but not geopolymer.

What is Geopolymer

Geopolymers are chains of network of mineral molecules linked with covalent bonds. They are made out of a reaction of aluminosilicates and active alkalis either at an ambient temperature in presence of some pozzolanic materials like Cao or calcium hydroxide or at an elevated temperature from 50 to 80 deg C, which polymerizes the materials into a molecular chains and networks to form a hardening binder.

Most of the aluminosilicates in use are mostly byproducts like Fly Ash, Slag based material, clay, etc. and activated alkali source could be alkali from the plant or even Industrial waste rich in alkali. These materials contain high silica and alumina. Sometimes geopolymers are called Alkali Activated Cement or Inorganic Cement.

What is the Need of Geopolymers?

Concrete is the second largest material in use after water by human being. It may sound unrealistic but it’s a fact and use is increasing each and every day. The most important ingredient in concrete is cement. Name anything in modern world -concrete would appear along with cement.

Cement, a single industry accounts for over 5% of global carbon dioxide emissions.

Every year the production of cement is going up @ 2.5 % and with the production level of 3.27 billion MT in 2010, the total cement production would go up close to 5 billion MT in 2030.

To manufacture one MT of cement, 4.7 million Btu of energy is required and which is equivalent to burning of 400 pounds of coal, resulting the emissions of nearly one MT (535 cubic meter) of carbon dioxide gas. One can well imagine how dangerous it would be with 5 billion MT of cement in 2030, i.e., 2675 billion cum of carbon di oxide emission only from cement plants.

Can we stop the use of concrete, the answer is — certainly No.

What is the choice left — There are two alternatives,

i) to reduce energy consumption in cement plant on a war footing to drop carbon foot prints. This may not be possible overnight as involves a lot of operational issues.

ii) Looking for suitable alternative material, which can be used efficiently in concrete making and with no compromise in functional properties and long-term durability aspects.

Already the use of a large number of alternative cementitious materials like Fly Ash, GGBS, Clay of different compositions, silica fumes, either mostly in combination or singularly (except silica fumes) have been established through research and as well as practice. These materials may be used to the extent of replacing cement up to 30–50% or even little more.

Unfortunately, the use could not go up to the desired high level in view many so called constraints, though, these materials can perform even better than conventional concrete in terms of long-term durability and having mechanical properties as good as conventional concrete.

Large use of alternative cementitious materials would have contributed a lot reduction in carbon di oxide emissions, even then, it would not have helped to produce a concrete totally free from cement, which means drastic reduction in carbon foot print or green gas effect.

In search towards the same, scientist invented geopolymers with the objective of use of alternative material, again at no compromise on any parameters against regular concrete.

It is the need of the hour to use materials like geopolymers resulting minimum or nil carbon di oxide emission.

Raw Materials for Geopolymers and Process

Raw Materials

Any material rich in aluminium and silicone can be used as raw material. There are many materials that can be used to create geopolymers. Kaolinite was the first material widely used in geopolymer synthesis, after successfully uses of this new material, the scientists start developing new raw materials, such as calcinated clays, or industrial waste (e.g., slag, ashes — Fly ash, Rice husk ash, waste glass, aluminium mine tailings, etc.) and natural silico -aluminates (e.g., pure Al2O3–2SiO2 powder) zeolites. Alkali Activators are commercially available Sodium or Potassium Silicates, Or Industrial Waste — alkaline effluent by products. The minimum ratio between silicon and aluminium must be 2:1.

Process

The geopolymer results after the exothermic process carried out through oligomers, process known as repolymerization. It is a polymer with very long reticular network, where the specific tetragonal network of aluminates and groups of silicates are, also, present. The bond between these tetrahedrons is equilibrated by alkali ions of: K+ , Na+ or Li+ .

In general, repolymerization can be divided into three stages, as shown below.

First of them is dissolution, when the solid alumino-silicates material is dissolved because of the water and alkali activator presence. After eliminating a small amount of water, the reorientation starts, now the group atoms take their place in the structure. During the solidification (at 20 Celsius degree or higher approximate 1000 0 C) the water is almost totally eliminated and the material shows its final form.

Process Flow Chart

Geopolymer Concrete

In addition to the following items Concrete Admixture may be used to have a finer. control

Characteristics of Geopolymer Concrete

It can achieve compressive strength of 70 MPa at a short time. Alkali activated slag based geopolymers can achieve compressive strength 80 to 100 MPa within 7 days and it can further grow with passage of time since hydration continues. After heat curing of other geopolymers, there is only limited increase in compressive strength. It also has higher resistance to Freeze thaw cycle.

Low drying shrinkage in comparison to other concrete.

It is well suited for thick and well restrained concrete structural members.

Low heat of hydration in comparison to conventional concrete.

Low Thermal Conductivity value and comparable with refractory Bricks. Geopolymer concrete has k value of lower than 0.3 w/m K.

Higher Durability in absence of nil or low presence of lime. Has very low permeability to chloride ion, sulphates and good resistance to Sulphuric acid, alkali and salts.

Geopolymer (heat cured) has a very high Fire resistance and there is no change in performance at 1000 -1200 deg C.

Slag Activated geopolymers do have similar fire resistance property like conventional concrete.

Comparison- Conventional Portland cement Based Concrete & Geopolymer Conc.

Uses & Application

There is a limited use of geopolymer concrete so far. Primary reason could be many like awareness, adequate details of raw materials sourcing , specifications and references. Also, absence of standards could be another important reason.

Geopolymer concrete may be used for all kinds of constructions like conventional concrete barring few limitations,

Geopolymers so far has been used for Pavements, Retaining Walls, Water Tanks and Pre cast Bridge Deck.

A complete four storied building including the structural members have been constructed at the university of Queensland for their Global Change Institute.

In Australia & USA some Taxi ways at an Airport have been built with geopolymer (slag & fly ash combination) by a company named Wagner.

How Eco Friendly Is Geopolymer

Limitation of Geopolymers

The workability with geopolymer concrete is limited. Activated slag based geopolymer concrete has workability of 15 -30 minutes. Addition of Fly Ash would increase the workability to 45–60 minutes.

This limitation may become a constrained for use for in situ concreting. However, Precast constructions can be well managed with above workability time.

Part of Raw materials are bit hazardous; hence, it requires careful handling.

Precast constructions can be well managed with above workability time.

There is acute limitation on source and availability. There is one source for raw materials for Geo cement at Chennai.

So far, in India it has been mostly tried only at a laboratory scale, therefore, no cost indication is possible for bulk uses.

Conclusion

In today’s context when we are all concerned to reduce the carbon foot print, I feel geopolymer cement could be a unique tool to achieve a significant drop in carbon di oxide emissions.

However, to make the use of geopolymer cement successful, immediate strong steps needs to be taken.

1. Increase of Awareness

2. Availability / source of Materials

3. Preparation of Standard and Specifications.

4. Building up References

5. Bodies like IITs/ BIS/ IRC/CBRI/ ICI/ ACI and several may initiate Activities towards developing the use of geopolymer cement.

The above are not easy and would be time taking. Therefore, appeal all concerned Bodies associated with cement and concrete to maximize the use of Blended Cement which also may make a sizeable drop carbon footprint.

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