Geopolymer ConcretesGeopolymer Concretes

www.liv.ac.uk/concrete

The aims of this projectNW Construction Knowledge Hub

All collaborating institutions have a role in disseminating knowledge to theAll collaborating institutions have a role in disseminating knowledge to the construction industries, but only Liverpool are involved in active research into low‐carbon construction materials.

Environmental considerations of OPC usage: CO2 emissions

Global usage of OPC 2 6 billion tonnes globallyGlobal usage of OPC 2.6 billion tonnes globally (2009 figures).

0.82 tonnes of CO2 for every tonne of OPC.

5‐8% of all human‐generated atmospheric CO2.

Geopolymer Concretes

Geopolymers refer to alkali‐activated binders (AAB’s).

Geopolymers studied for the last 40+ years.

Applications in replacing Portland cement‐based concrete materials.

Geopolymers are a cement‐free concrete.

Performance often exceeds that of Portland cement concretes.  

Increased• durability,• resistance to chemical attack,• fire protectionfire protection.

Geopolymer Concretes

Produced from natural and synthetic pozzolanic solids, activated with alkaline solutionsactivated with alkaline solutions.

Originally metakaolinite with siliceous solutions.

Use of other synthetic reactive aluminosilicate pozzolans and alkaline activators possible.

Waste stream or by product pozzolans readily availableWaste‐stream or by‐product pozzolans readily available , e.g. PFA, GGBS, etc.

Using PFA‐based geopolymers will have an impact on national and international targets on CO2 reduction.

Environmental Considerations of PFA: Landfill

Waste‐stream pozzolans such as PFA not recycled to their full capacity, and excesses are stockpiled or “l dfill d”“landfilled”.

Commercialisation of PFA in geopolymers will help g p y pto reduce landfill.

F t d d h l t tili i ti t kFuture demand may help to utilise existing stocks.

Geopolymer Concretes

Three basic steps  to reaction:

1. Dissolution of pozzolan with in alkaline solution,

2. Gel formation of aluminosilicate chains,

3. Polycondensation of chains to form interlocking network.

Our starting work on geopolymers

At Liverpool: Investigated mortars using PFA.

Effect of different lab‐grade alkali activators and curing temperatures on strength development.

Alkalis tested include:

• Sodium hydroxide, NaOH,• Potassium hydroxide, KOH,• Sodium silicate

The effect of curing temperature on strength

Example:

PFA ti t d ithPFA activated with Na-silicate SG 1.2

Increased curing temperature increases the strength ofIncreased curing temperature increases the strength of geopolymer by increasing reaction between PFA and alkali.

Effect of too high curing temperature!

Curing at 90oCCuring at 90oC:• development of a “muffin‐top” at the free surface.

Due to the expansive action of gases and vapours/steam developing in the paste.

Curing at <70oC:• eliminates swelling, • maintains good strength development.

The effect of activator concentration on strengthExample:PFA activated with sodium silicate of 3 different concentrations:

1. SG 1.52. SG 1.333. SG 1.2

The mass of activatorThe mass of activator solution added to mix was kept constant through these experiments

Using sodium silicate activator, increasing the concentration gives increase in strength.

Further experiments for hydroxide activation expected to be similar.

The effect of the activator on strengthExample:PFA activated with:

• sodium silicate SG1.5,KOH 10M

BS EN 771Class B Engineering brick

• KOH 10M,• NaOH 10M,

and for comparison:

• 2:1 sodium silicate and• 2:1 sodium silicate and KOH mix.

Curing temperature 70oC.Concrete masonry Alkali added to each mix to give a constant workability.

blocks

Type of activator, hydroxide or silicate, has an effect of the strength.

Alkali cation, K or Na, also has an effect.

What alkaline activators are in the waste-stream?

Current work uses lab‐grade alkaline activators.

Undertaking market research to identify:• What alkalis are currently in the waste‐stream in the UK?Q tit ?• Quantity?

• Quality?• Consistency through time?

Any further information that you can provide us with will be well received.

Porosity development

Geopolymer mixes continue to be affected by high porosity.  

b l dPorosity on a sub‐micron to mm scale, due to:

1. Porosity developed during mixing that cannot escape due to high viscosity of mixes.cannot escape due to high viscosity of mixes.

2. Porosity inherent to dissolution of hollow PFA cenospheres.  Porosity cannot escape d i lli d l d idue to viscous gelling and polycondensingpaste at elevated temperature.

Problems and benefits of porosity

P i i l d ll h bili i lPorosity is closed‐cell, hence permeability is low.

Potential problems:• Percolation of atmospheric acid solutions and e co at o o at osp e c ac d so ut o s a dsulphates.  Effects on paste not yet fully investigated.  Deterioration of re‐bar expected.

B fit f itBenefits of porosity:  • Resilience to freeze‐thaw weathering.• Thermal insulation.

Ways to manage porosity:• Use more suitable surfactants/ defoamers/ wetting agents/ superplasticisers.

Microstructural & microchemical characterisation

Important to understand why different materials have different strengthsdifferent strengths.

What does the material look like?

Is the material homogenous or heterogeneous?

Can initial ingredients be altered or processed to make a better product?a better product?

How does the reaction proceed in space and time?

Answers will help gain a fuller understanding of the products and help feed‐back into fine‐tuning the process.

Microstructural Characterisationwith Scanning Electron Microscopy Department of Earth & Ocean Science has with Scanning Electron Microscopy p

a state‐of‐the‐art electron optics facility.

Combining electron optic imaging capabilities with in situ chemical analysis 

CamScan X500 FEG‐SEM.

with EDS allows full microstructural characterisation of materials.

Philips XL30 W‐SEM.

Microstructural Characterisation

Reacted geopolymer matrix

PFA spherules

Fe nodules

Backscatter imaging can assess the microstructural characteristics:•quality and nature of fractionsQtz Sand •quality and nature of fractions,•porosity, •connectivity of structural elements/load bearing framework

Qtz Sand

elements/load bearing framework, •relationship between unreacted or partially reacted components,

•nature of interfaces between particlesBackscatter image of PFA – Na‐silicate geopolymer.

Porosity

nature of interfaces between particles and paste.

Microstructural Characterisation

Backscatter image of PFA – Na‐silicate geopolymer.Qtz Sand

Fe nodules

Reacted geopolymer matrix

Si Al

PorosityPFA spherules

Fe Mg

Chemical mapping provides means of assessing the distribution of elements in samples thus

Porosity Fe Mg

distribution of elements in samples, thus identifying constituent mineral components,  impurities, and spatial distribution of phases.

Ca K

Microstructural Characterisation

Graphite

Secondary electron imaging and chemical analysis of PFA provides:

• grain size distribution,• non‐aluminosilicate content,• micro‐chemistry.

Microstructural Characterisation

Qtz Sand Unreacted PFA Porosity

Secondary electron imaging and chemical analysis of PFA geopolymer identifies:identifies:

• nature of interface between aggregate particles and paste,

• unreacted PFA,• porosity,• desiccation cracks as lines of weakness and permeabilityweakness and permeability.

Other analytical techniquesco

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Quartz

X‐ray diffraction spectroscopy Assess the fraction of reactive amorphousmaterial in PFA

6000

8000Inte

nsity

(

Mullite

in PFA.

X‐ray fluorescence spectroscopyAnalysis of bulk chemistry of PFA or geopolymers.2000

4000

Significant non-crystaline component

Quartz

Mullite

Mullite

Coulter laser diffraction granulometryGrain size distribution of PFA.  Can compare this to the grain size distribution in geopolymer. Gives

5 10 15 20 25 30 35 40 45 50 55 60 652Theta (°)

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g y p

the grain size distribution in geopolymer.  Gives insight into grain‐size dependence of the reaction.

Environmental scanning electron microscopyb l dLower vacuum.  Observe real‐time curing and 

associated change in chemistry and elemental distribution.

Isothermal calorimeterDetermined rate and amount of reaction.

Future work

• Investigate effects of different alkali activators with a more detailed matrix of mixes.• The type of alkali

AluminatesHydroxidesHydroxidesSilicates

• Alkali concentration• Curing temperature

• Investigate other pozzolanic solids, particularly GGBS, Class C PFA.

• Study and refine geopolymer properties, e.g. management of porosity with admixtures.

Future thoughts and considerations: from the lab to the marketplace

Current limiting factor in commercialising PFA‐geopolymers is alkali activators.

To find widespread appeal, geopolymers need to be more user‐friendly:• Dry ready mixes• Dry ready‐mixes,• Minimising the corrosivity and irritation hazards.  This means using silicate activators preferentially to caustic ones (use of admixtures to manage rheology and porosity is necessary).  

Estimated usage of geopolymers in 2015 is 1000 million tonnes globally.  For geopolymers made from 100% recycled materials, work needs to be done now in order to establish maintain andwork needs to be done now in order to establish, maintain and secure the industry and infrastructure.

UK’s northwest industries have the opportunity to be innovative, pivotal and market‐leading.