SESSION: Reuse and RecyclingEnvironmentally Friendly Cementitious Binders Made from Coal Fly Ash

Xianming Shi. Associate Director. Montana State University

The last decade has seen the complete replacement of cement by coal fly ashes in mortars or concretes to garner considerable interest. The vast majority of studies in this field have focused on alkali activated binder materials such as geopolymer and alkali activated fly ash. Novel uses of CFAs as cementitious binder can produce cost and energy savings and reduce greenhouse gas emissions and landfill waste. This work reports an environmentally friendly cementitious binders made from only the as-received class C coal fly ash, water, and a very small amount of borax (Na2B4O7), at room temperature and without direct alkali activation. Such implementation of CFAs as the sole binder in concretes and mortars would translate to even greater environmental and economic benefits, relative to the use of CFAs as supplementary cementitious material or their use in geopolymer and alkali activated fly ash. Previous studies at the Montana State University have demonstrated the feasibility of using selected CFAs as the sole binder for structural concrete and reported macro-scale engineering properties of this type of “green” concrete material. Nonetheless, mechanisms underlying the properties of this unconventional cementitious binder remain unclear, and this lack of understanding makes it difficult to transfer such technology to CFAs with similar or different physico-chemical characteristics. In this context, this work elucidates the hydration process and hydration mechanisms of this “green” cementitious binder, pure fly ash paste (PFAP), using ordinary Portland cement paste (OPCP) as the control.Furthermore, this work reports other “green” cementitious binders made from CFAs, such as the use of class C CFA to replace Portland cement at 80% by weight and the combined use of CFA and gypsum. All of these efforts are highly desirable as they contribute to waste utilization and the production of environmentally friendly concretes (EFCs).

Research Interests1. Durability of civil infrastructure: understanding, preventing or mitigating the impact of service environment on metals, concrete, asphalt and structures (e.g., corrosion monitoring system, high performance coatings, preservation and maintenance techniques for pavements, and rehabilitation techniques for bridge decks).2. Environmental sustainability: with a focus on the use of nanotechnology, green technology, and beneficial microorganisms for: environmentally friendly concrete, advanced cementitious materials, eco-friendly asphalt, green buildings, and environmental preservation3. Sustainable transportation systems engineering, especially products, technologies, and systems to facilitate environmentally responsible best practices in road weather management, snow and ice control, dust suppression, and other maintenance activities.4. New energy technologies: microbial fuel cells, energy harvesting, advanced functional materials, etc. 

Environmentally friendly cementitious binders made from

coal fly ashXianming Shi, PhD, PE, Research Professor

Ning Xie, PhD, Assistant Research Professor

A PRESENTATION AT 2014 TRB COMMITTEE ADC60SUSTAINABLE & RESILIENT INFRASTRUCTURE WORKSHOP

NEW YORK CITY, JUNE 18, 2014

A 25-ft wall of heavy metal contaminated coal fly ash, resulting from the release of 5.4M cubic yards of coal fly ash slurry into the Emory River, Tennessee, & nearby

land & water features, in Dec. 2008

Environmental footprint of concrete industry

• Concrete: annual global production ~5.3B m3

• Cement: 2.8 to 4B t/yr

CFAs: main by-products of coal combustion for electrical energy production

• Environmental risk unless being solidified

• U.S.: ~70M t/yr, 27% (~19M t) recycled, ~12M t utilized in concretes & mortars

Beneficial use of CFAs as partial replacement of cement

Recently: geopolymer, alkali activated fly ash

The Bigger Picture!

Chemical composition by XRF

Analysis of raw materials by SEM/EDS/XRD

Amorphous Al-rich and Ca-rich phases + some crystalline phases of quartz (SiO2), hematite (Fe2O3), free lime, periclase (MgO), alumina (Al2O3)

Analysis of raw materials

SEM morphologies of the non-spherical fly ash particles

Properties of hardened pastes

Properties of hardened pastes

XRD of the pastes after curing for 1/7/14/28 d

Amorphous structure w/ small amounts of crystals, mainly unreacted alumina (Al2O3), periclase (MgO), hematite (Fe2O3) + newly formed ettringite + some aluminum hydroxide (AH), C-S-H, M-A-S-H, C-A-S-H, nontronite

XRD of the pastes after curing for 1/7/14/28 d

SEM of the pastes after curing for 1/7/14/28 d

micro-cracks/ micro-pores

more hydration products w/ higher densities

unreacted small particles + some craters

Different fracture surface

Interfaces: main defects

few unreacted particles

SEM of the pastes after curing for 1/7/14/28 d

SEM of the cement pastes after curing

EDS of the pastes after curing for 1/7/14/28 d

FA Spheres: dissolution of Ca-, Fe-, Al-rich phases + consumption of Ca2+, Fe3+, Al3+, Mg2+ by hydration

Hydration Products: Al/Si, Fe/Si, Ca/Si, Mg/Si ↑↑ = uptake of Al3+, Fe3+, Ca2+, Mg2+

DSC/TGA of hardened pastes

AH + ettringite (e.g., 4CaO·Al2O3·SO3·12H2O)

Little CaCO3: due to low lime content

Better heat resistance (less mass loss)

Hydration mechanism of the PFAP

Reduce the formation of ettringite (e.g., 4CaO·Al2O3·BOx·12H2O)

C-S-H

C-A-(S)-H

Ettringite + AH

Ca2+, Fe3+, Al3+, Mg2+ + silicates to form amorphous Al-rich and Fe-rich binder phases + crystalline binder phases

Conclusions

A novel pure fly ash paste (in place of cement paste)

• Reasonable 28-d compressive strength (36 MPa)

• Rapid strength gain (19MPa/1d & 31 MPa/3d)

• Low bulk dry density (1.6 g/cm3)

• Very high electrical resistivity

• Outstanding micro-nano hardness & elastic modulus

• Low gas permeability coefficient (4.1×10-17 m2/s)

• Reasonably low Cl- diffusion coefficient (1.9×10-12 m2/s)

• A more refined microstructure

• Better heat resistance

A viable “green” construction binder suitable for a host of structural & non-structural applications

Conclusions

Advanced characterization (XRF/SEM/EDS/XRD/DSC/TGA)

• Complex hydration mechanisms: free Ca2+, Fe3+, Al3+, Mg2+ + silicates to form amorphous Al-rich & Fe-rich binder phases + crystalline binder phases (ettringite, AH, C-S-H, M-A-S-H, C-A-S-H, etc.)

• Hinges on the CFA’s low SO3 content + relatively high alkali content, coupled w/ the appropriate borax dosage and the relatively high lime, hematite, & alumina contents

The obtained knowledge sheds light on the role of class C CFA in the hydration process & may benefit the expanded use of various CFAs in paste/mortar/concrete…

Other Studies of Fly Ash Concrete

1. Use of class C CFA to replace Portland cement at 80 wt.%

2. Combined use of CFA + gypsum as binder

3. All contribute to waste utilization and the production of environmentally friendly concretes

4. Wang, X., Chen, J., Kong, Y., Shi, X.* Sequestration of Phosphorus from Wastewater by Cement-Based or Alternative Cementitious Materials. Water Research, 2014, DOI: 10.1016/j.watres.2014.05.021.

Contact InfoXianming Shi, Ph.D., P.E., Research Professor

Founding Director, Corrosion & Sustainable Infrastructure Lab

Western Transportation Institute, PO Box 174250, Bozeman, MT 59717-4250

Xianming_s@coe.montana.edu

Web: www.coe.montana.edu/me/faculty/Shi/

http://ine.uaf.edu/cesticc/

406-994-6486 (Phone)

406-994-1697 (Fax)

Q & A