S8 - Low-E cements. Pozzolans

Dr Sara PavíaDept of Civil Engineering

Trinity College Dublin

Energy efficiency of cement production

E-efficient machineryWaste materials as fuel (paint residues, used oil, scrap tyres)Modify product composition: • replace the cement with supplementary cementitious

materials such as FA [fly ash], GBS [granulated blastfurnace slag], manufactured pozzolans.

• modify clinker composition • to high belite cement -requires lower energy in

pyroprocessing but higher energy for grinding.• substitute the high T alite for the lower T Ca sulpho

aluminate phase.

Use pozzolansIndustrial and agricultural by-products with pozzolanic activity such as blast-furnace slag , fly ash, silica fume and rice husk ash have been used for the production of high performance PC concrete with a view to improve the durability and service life of concrete structures.

Partial binder replacement (up to 60%!!!)Economic and environmental benefits:

• recycling waste whose disposal poses a threat for the environment, • reducing cement content in concrete, with the subsequent drop in:

• energy consumption, • non-renewable natural raw material consumption and • CO2 emissions- climate change.

The replacement of PC by waste materials lowers the cost of construction simultaneously conserving energy and natural resources, thus reducing the negative impact of building on the environment.

Pozzolans• PFA- fly ash, silica fume, • CKD-cement kiln dust, • RHA-rice husk ash, • GGBS-granulated blastfurnace slag, • MS- microsilica, • CSF-condensed silica fume, • Ceramic dust e.g recycled fired-clay brick• thermally activated clays, • natural pozzolans.

Leca : Light Expanded Clay Aggregate.•lightweight, bloated particles of burnt clay•thousands of small, air-filled cavities •strength and thermal insulation properties.•a plastic clay is pretreated and then heated and expanded in a rotary kiln. •Finally, it is burned at 1100 °C.•Used to produce light weight concrete, blocks for wall construction, Lowers the dead load of a structure Lowers thermal transfer (better u-values)

Metakaolin-  dehydroxylated form of the clay mineral kaolinite.•china clay or kaolin, traditionally used in the manufacture of porcelain•thermally activated (530-630 °C)•admixture for concrete/cement applications. PC replacement e.g. 8–20% (by weight) •favorable engineering properties, including: filler effect, acceleration of hydration, pozzolanic reaction between 3 and14 days.

Ceramic dust- RBD (red brick dust); YBD (yellow brick dust); Tile (tile dust)•Variable composition•Recycled- control contamination

Microsilica (silica fume)-amorphous (non-crystalline) polymorph of SiO2-silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production.

Blastfurnace slag (BS) - by product of the steel industry. It results from the combination of iron ore with limestone flux. It is obtained in the manufacture of pig iron- an intermediate form of iron produced from iron ore subsequently worked into steel or wrought iron.When BS is quenched with water it forms a glassy material known as granulated blastfurnace slag (GBS). GGBS = Ground Granulated Blastfurnace Slag

RHA- agricultural by product

When rice is harvested and milled, roughly 78% of the paddy is rice and bran, the remaining 22% is the husk. In countries of large rice industries such as India or Brazil, the rice harvested is par-boiled in mills which are fuelled by burning the husks. On combustion, the cellulose-lignin matter in the husk burns away, leaving a porous silica skeleton which is grinded into fine particles with a large surface area known as rice husk ash.

Each tonne of rice harvested produces roughly 220 kg of husk, leaving 55 kg of ash after burning.

Rice husk is used as biomass, many power plants in Asia are fuelled with rice husk.

RHA is very siliceous, therefore its disposal poses a threat to the environment.

CKD - cement kiln dust• by-product of the manufacture of Portland cement, therefore an industrial

waste. • produced during the pyroprocessing process: when air moves into the kiln with

the fuel to provide oxygen for combustion, this gas flow picks up partly burnt raw material from the kiln.

CKD

• Over 30 million tons of CKD are produced worldwide per year.

• In some European countries such as Spain and Ireland, CKD is recycled as a raw feed substitute in cement manufacturing

• in other countries such as UK and the USA a significant proportion is land filled.

• The US cement industry generates approximately 4.1 million tons of CKD per year, 3.3 million of which is land filled.

• The UK cement industry claims that over 200,000 tonnes a year of landfill space could be saved if the surplus CKD is recycled back into the clinker-making process or if alternative uses are found.

PFA - pulverised fuel ash • a by product of (typically coal) fired power stations• around 500 million tons of coal ash (of which 80% is PFA) are generated per annum

world wide- approx. 35% reused.• Coal is pulverized into a fine powder, mixed with heated air and burned to produce

electricity. • The carbon in the coal burns off and the inorganic part of the coal –minerals such as

clays and feldspar- melt and form fused droplets that, on rapid cooling, solidify as the spherical glassy particles that comprise the coal ash.

• The lighter glass spheres are carried in the flue gasses and extracted by (electrical, mechanical or cyclone) precipitators

• The resultant material is used as engineering fill and as a component for concrete and concrete block production. The blocks are lightweight and have excellent thermal insulation properties.

PFA

When newly produced the dust is strongly alkaline; a pH as high as 11 is known, and >9 is normal.

Luke in Bernsted and Barnes eds 2002

Cement Type Embodied CO2

CEM III/A (36-65% GGBS) 610-360kg CO2/tonne

CEM IIA/L 750-880kg CO2/tonne

CEM I 930kg CO2/tonne

UK Concrete Centre, Specifying Sustainable Concrete, 2011Figures for the embodied CO2 of various cements:

Pozzolans

• materials with amorphous SiO2 or SiO2/ Al2O3 that react with Ca(OH)2 in the presence of water to form cementitious hydrates

• calcium silicate hydrates (CSH) and calcium silicate aluminate hydrates (CSAH) = hydration of PC clinker

• can result in faster setting times, higher mechanical strength, lower permeability, greater durability…

• Resultant properties depend on the reactivity of pozzolan

Pozzolanic reaction (similar to the hydration of PC clinker)

• Pozzolan + PC clinker + water = 1st- The clinker minerals quickly react with water (hydration) to form hydrates (cements) of which calcium silicate hydrate (C-S-H) and portlandite (lime=Ca(OH)2) are the most abundant.2nd – POZZOLANIC REACTION: THE POZZOLANS (SiO2 or SiO2/ Al2O3) REACT WITH THIS PORTLANDITE (Ca(OH)2) FORMING ADDITIONAL HYDRATES (CEMENTS).The hydrates formed are the same as those occurring on PC hydration.

• CSH - calcium silicate hydrate • CH – Portlandite= Ca(OH)2

• AFt – ettringite (calcium sulfoaluminate hydrate= CšAH)• AFm - hexagonal calcium aluminate hydrates= CAH

However in different %s (since the chemical composition of the pozzolan is different than that of the PC)

Depending on the level of pozzolan replacement, CH can be partially or entirely combined producing hydrates.

Preferably all CH should be combined to avoid flaws (large crystals-hexagonal plates) in the microstructure e.g. in the transition zone.

– pozzolans reduce the width of the interfacial zone between the paste and the aggregate (weakest area of concrete), reinforcing the microstructure of the transition zone Zhang et at 1996.

http://tcdlocalportal.tcd.ie/pls/public/staff.detail?p_unit=civil_engineering&p_name=pavias

Microstructure of the hardened pozzolan cement

• The main characteristic of the paste is the absence of large CH crystals and the presence of layers of hydrates covering residual pozzolan particles.

• The groundmass of the paste does not appreciably differ from that of PC

Effect of pozzolans in composites

• physical filler effect, increasing density by enhancing packing of the composite particles;

• they reduce the amount of pores and cause a refinement of the pore structure.

• The pozzolan particles act as nucleation points for the precipitation of hydrates: the CSH that forms by pozzolanic reaction fills the pores and strengthens the microstructure of the concrete, particularly around the coarse aggregate.

• this pore refinement transforms a composite from an opened to a closed pore system.

• the diameter of the mesopores is reduced (thinner and more segmented pores)- lower permeability.

• the overall porosity may increase (there may be a higher content of relatively large pores -macropores).

Research

Properties of pozzolan

• Particle Size• Specific Surface Area • Chemical Composition • Mineralogy• Amorphousness

Behaviour of composite

• Water Demand • Reactivity

Compressive StrengthChemical Conductivity

• Setting Time• Porosity

Methods - Physical characteristics pozzolans

• Particle Size – laser diffraction - Malvern Mastersizer 2000 • Particle Surface Area – BET method - Quantachrome Nova 4200e• Chemical Composition - % oxide - XRF• Mineralogy and amorphousness - XRD

Methods – Behaviour / properties of pozzolan pastes

• Lime: pozzolan pastes at 1:1 and 1:3 ratios and water content to a specific flow

• Water Demand – Initial flow • Setting Time – VICAT test• Reactivity –Mechanical method: Compressive strength; Chemical method: Conductivity.

Results – Specific Surface Area and Particle Size

Metakaolin, RHA, GGBS and PFA are the finest pozzolans (finer than lime).

MS particles flocculated -finer than determined by laser. (60% of the MS particles are sized under 1 µm, therefore the finest).

MS, Metakaolin and RHA - much greater specific surface area than any of the other pozzolans.

Results – Water Demand

• It depends on:• pozzolan’s particle size; • specific surface area; • lime:pozzolan ratio

• surface area has the greatest influence on water demand

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Water demand (amount of mixing water) affects workability, strength and shrinkage therefore, it impacts concrete/mortar quality and durability.

Results – Water demand of the pozzolans

• Water content determines initial flow of a paste/mortar.

• Measuring the amount of water required for a 1:1 (lime:pozzolan) paste to flow to a specific diameter provides the water demand of each specific pozzolan.

• Pozzolans/aggregate or any other components of mortars and concrete must not rise water demand as this can lead to strength, fracturing and other problems.

Flow table and conical mould.

Results –Reactivity

Both reactivity indices positioned the pozzolans in the same order of reactivity: Meta/GGBS/MS/RHA/Leca/PFA/YBD/Tile/RBD

Mechanical Index

• Increasing silica content results in a higher mechanical index

• The more abundant the hydrates [CSH] formed- the higher the strength.

• Strength depends on the amount, type and microstructure of hydrates formed: S hydrates contributing more to strength than Al hydrates.

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Reactivity and Strength Development

a relationship between increasing amorphous content and reactivity clearly evident; amorphous materials – •greater mobility and superficial location of their atoms•noncrystalline solids in which the atoms and molecules are not organized in a definite lattice pattern. •e.g. glass, plastic and gels [CSH].

• Metastar, GGBS, RHA and MS were found to be the most reactive pozzolans

high specific surface areahigh amorphounesssmall particle size

Setting time• The Vicat test determines the

rate of stiffening by dropping a needle from a fixed height and measuring its penetration.

• The stiffness is related to the formation of hydrates.

• The initial and final setting times (at 35mm and 0.5mm respectively) are standard references which provide comparative data between samples.

Results – Setting Timeall pozzolans speed up the initial set of the lime paste except for PFA

and MSall pozzolans reduced the final setting time of the lime paste by at

least 40%no clear relationship between reactivity and setting timea small increase in water content (5%) significantly slowed down the setting

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production parameters also determine reactivity• temperature, combustion environment, grinding.

E.g. RHA –Rice Husk Ash- Effect of production parameters on reactivity

Uncontrolled burning produces crystalline RHA, while controlled burning at lower temperatures produces RHA containing amorphous SiO2- greater reactivity.

Combustion time, temperature and environment affect both specific surface area (Nehdi et al. 2003) and carbon content.

RHA produced by uncontrolled burning is usually high in carbon, and this adversely affects the pozzolanic activity of the ash (Nair et al. 2006) and the rheology of the mortar or concrete (Chagas Cordeiro et al. 2009).

•RHA- agricultural by product

• The use of rice husk ash in concrete was patented in 1924.• As the silica content of RHA is so high (85 – 90%), it is considered

a ‘super-pozzolan’ and used in high performance concrete to enhance workability, strength and resistance to chemical attack and chloride corrosion of steel reinforcement.

• RHA increases compressive strength due to the capacity of the pozzolan of fixing the Ca (OH)2 generated during PC hydration to form CSH and CASH.

• RHA’s reactivity reaches a high IPA value (Index of Pozzolanic Activity), over the limits stipulated by the standards in order to qualify as a pozzolan.

Effect of cement replacement by RHA on the properties of concrete:

• increase of compressive strength with low level replacements: • a 5% cement replacement by RHA achieved a compressive strength 24% higher

than that of a PC control mix. • 8 and 10% level replacements achieved higher compressive strength at 28 and

91 days, that a PC control mix. • A 10% cement replacement has been reported to achieve excellent

performance leading to an increase in compressive strength, decrease in permeability, chloride penetration and decreased heat of cement hydration [Zhang et at 1996; Singhania N. P. 2004 [1] Nair et al. 2006 Ganesan et al. 2008].

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Effect of cement replacement by RHA on the properties of concrete:• Acceleration of rate of hydration of cement (Zhang et at 1996); • Reduction of alkali-aggregate reaction and • Reduction of hydration heat (Hasparyk et al 2004); • increase resistance to sulfate attack (Chindaprasirt 2007).

Compressive strength increases with RHA content up to 15% replacement

At 30% replacement, c.s. equivalent to that of control mix

Ganesan et al 2008

RHA

The water required for a standard consistency linearly increases with the %RHA

Up to 15%, increasing RHA content increases the initial setting time. At higher level replacement (20-25-30 and 35%) there is a decrease in the initial setting time

Ganesan et al 2008

PFA reacts with CH (Ca (OH)2) and water to form insoluble hydrates (CSH and CASH)•There is considerable variation on the rate and kinetics of reaction;

• Generally agreed that PFA’s pozz reaction becomes apparent from 3-14 after starting of hydration.

• Delay may be related to the dissolution of the spheres.

•In the pozzolanic reaction, CH formed during PC clinker hydration is consumed by the PFA.

•The consumption of CH is gradual over time, the rate differs depending on the composition of the PFA.

Luke in Bernsted and Barnes eds 2002

Luke in Bernsted and Barnes eds 2002

PFA fineness vs compressive strength of PC/PFA pastes

Luke in Bernsted and Barnes eds 2002

PC/PFA paste hydrated for 3 months showing ettringite.

Top-100%PFA;

Lower image- 80/20 (PFA/PC)

Chemical composition of CKD and typical composition of CKD and PC - standard content range in brackets

Chemical composition

CKD (%) typical composition CKD (%)

PC (%)

Maslehuddin et al. 2008

Peethamparan et al. 2008

CaO 42.0 38-50 37-55 63.7 (64)

S1O2 11.6 11-16 12-16 20.3 (22)

Al2O3 4.5 3.0-6.0 2.0-5.0 5.3 (5)

Fe2O3 2.9 1.0-4.0 1.7-2.3 3.9 (3)

MgO 1.3 0.0-2.0 1.2-2.7 2.1 (1)

K2O 0.6 3.0-13 1.4-7.0 0.1 (<1)

Na2O 0.1 0.0-2.0 0.1-0.8 0.4 (<1)

SO3 0.2 4.0-18 4.2-14.6 2.7 (3)

Cl-1 - 0.0-5.0 0.3-0.7 0.03 (<0.1)

Loss on ignition

35.5 5.0-25 4.0-29.6

The chemical and physical characteristics of the CKD are determined by the raw feed material, type of kiln operation, the dust collection system and fuel type.

In particular, the chemical composition of CKD depends on the raw materials used to produce the clinker and the type of kiln fuel.

CKD from dry-process kilns tends to be higher in calcium content than the dust arising from wet kilns

CKD contains alkalis and is a caustic material (the typical pH of CKD water mixtures is approximately 12).If used in concrete, corrosion of metal reinforcement may occur.

The high alkali content of CKD together with its sulphur and chlorine can enable crystallization of disruptive alkali sulphates and chlorides.

• CKD is finer than PC -further surface available for reaction.• 5% CKD replacement yielded the highest 90 days compressive

strength of 42 MPa - Al-Jabri et al. • compressive strength drops with increased CKD replacement,

however, 5%-10% replacement do not significantly reduced compressive strength - Ali et al.

• Siddique: comprehensive review of CKD in concrete concluding that, concrete containing low percentages of CKD replacement (5%) achieves almost equal compressive strength, flexural strength, toughness and freezing and thawing resistance than 100%PC mixes.

• blends containing as little as 70% PC can still exhibit adequate strength if only CKD is used as the blending waste material.

• Shoaib et al.: • optimum quantity of CKD which could be recycled in concrete types without

major strength loss. • the chloride in the CKD open the pore system of the hardened concrete

leading to a strength reduction.

Al Harty et al. 2003

Al Harty et al. 2003

Blastfurnace cements• Blastfurnace slag (BS) by product of the steel

industry. • GGBS = Ground Granulated Blastfurnace Slag • 1853 - early slag-lime cements• 1909 - 1st standard for BS cement• same constituents as PC but in different

amounts.

Chemical comp. % PC GGBS

SiO2 20.10 35.04

Al2O3 4.15 13.91

Fe2O3 2.50 0.29

CaO 61.30 39.43

MgO 3.13 6.13

K2O 0.39 0.39

Na2O 0.24 0.34

TiO2 0.24 0.42

P2O5 <0.90 <0.10

MnO - 0.43

SO3 4.04 2.43

Chemical composition of BS from different countries-ref Lea

GGBS contains clinkers so it is not strictly a pozzolan. However, the rate of reaction is slow and needs alkalis and sulphates to activate.

When mixed with PC, as PC hydrates, it releases alkalis and sulphates which serve as a activators for the BS.

Slag reaction products in the presence of different activators.

Microphotograph of SEM showing the morphology and size of the GGBS particles.

EDX spectrum of quantitative chemical analysis of GBS.

Bernsted and Barnes eds 2002

• The final properties of GBS concrete are determined by the reactivity of the slag (pozzolanicity- hydraulic activity) which control the amount of hydrates produced and the properties of the concrete.

• Reactivity depends on: • the reactive glass (amorphous) content: roughly linear

relation between strength and glass content. Increasing crystalline components reduce cementing properties.

• the chemical and mineralogical composition, • type of activator and • fineness of GBS (Ganesh Babu and Rama Kumar 2000).

• In general, the more basic slags are, the greater their hydraulic activity in the presence of alkalis.

• To ensure high alkalinity, without which slag would be hydraulically inactive, European Standard EN 197-1:1992, recommends that the ratio of CaO + MgO to SiO2 exceeds 1.

• at constant basicity, strength of concrete increases with the Al2O3 content.

• hydraulic activity is enhanced with an increase in Al2O3, CaO and MgO while an increase in SiO2 diminishes hydraulic activity- Frearson (1986)

Influence of Al2O3 in the development of strength- ref Lea

• GBS improves the general performance of PC composites • decreasing chloride diffusion and chloride ion permeability (Luo et al.

2003, Yun Yeau and Kyum Kim 2005); • reducing creep and drying shrinkage (Jianyong and Yan 2001);• increasing sulfate resistance (Higgins 2003, Binici and Aksogan 2006); • enhancing the ultimate compressive strength (Barnett et al. 2006) • reducing the heat of hydration and bleeding (Wainwright and Rey 2000).

• GGBS also improves concrete workability due to its high specific surface, marketed at 375-435 m3/kg with a fineness of approximately 460 Blaine (m2/kg.min).

• this makes GGBS finer than PC (typically, PC is 300 m3/kg). • this leads to increased workability and a better performance in bleeding,

setting times and heat evolution -Swamy (1986).

• Strength is determined by the specific surface area of the slag

Compressive strength of mortars made with CEM III/B, with 75% slag of specific surface areas 3000-4500 cm2/g – ref Lea

Bernsted and Barnes eds 2002

Effect of slag on concrete permeability

decrease chloride diffusion and chloride ion permeability

Increase sulfate resistance