Presentation on theme: "SESSION: Reuse and Recycling Environmentally Friendly Cementitious Binders Made from Coal Fly Ash Xianming Shi. Associate Director. Montana State University."— Presentation transcript:
SESSION: Reuse and Recycling Environmentally 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 Interests 1. 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 preservation 3. 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 ash Xianming Shi, PhD, PE, Research Professor Ning Xie, PhD, Assistant Research Professor A P RESENTATION AT 2014 TRB C OMMITTEE ADC60 S USTAINABLE & R ESILIENT I NFRASTRUCTURE W ORKSHOP N EW Y ORK C ITY, J UNE 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. 2008release of 5.4M cubic yards of coal fly ash
Environmental footprint of concrete industry Concrete: annual global production ~5.3B m 3 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!
XRD of the pastes after curing for 1/7/14/28 d A morphous structure w/ small amounts of crystals, mainly unreacted alumina (Al 2 O 3 ), periclase (MgO), hematite (Fe 2 O 3 ) + newly formed ettringite + some aluminum hydroxide (AH), C-S-H, M-A-S-H, C-A-S-H, nontronite
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
EDS of the pastes after curing for 1/7/14/28 d FA Spheres: dissolution of Ca-, Fe-, Al-rich phases + consumption of Ca 2+, Fe 3+, Al 3+, Mg 2+ by hydration Hydration Products: Al/Si, Fe/Si, Ca/Si, Mg/Si ↑↑ = uptake of Al 3+, Fe 3+, Ca 2+, Mg 2+
DSC/TGA of hardened pastes AH + ettringite (e.g., 4CaO·Al 2 O 3 ·SO 3 ·12H 2 O) Little CaCO 3 : due to low lime content Better heat resistance (less mass loss)
Hydration mechanism of the PFAP Reduce the formation of ettringite (e.g., 4CaO·Al 2 O 3 ·BO x ·12H 2 O) C-S-H C-A-(S)-H Ettringite + AH Ca 2+, Fe 3+, Al 3+, Mg 2+ + 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/cm 3 ) Very high electrical resistivity Outstanding micro-nano hardness & elastic modulus Low gas permeability coefficient (4.1×10 -17 m 2 /s) Reasonably low Cl - diffusion coefficient (1.9×10 -12 m 2 /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 Ca 2+, Fe 3+, Al 3+, Mg 2+ + 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 SO 3 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 Info Xianming 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/www.coe.montana.edu/me/faculty/Shi/ http://ine.uaf.edu/cesticc/ 406-994-6486 (Phone) 406-994-1697 (Fax)