Utilizing High-Alkali and High-Sulfur Supplementary Cementitious Materials for Sustainable Concrete Production
Open Access
- Author:
- Sharbaf, Mohammadreza
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 10, 2025
- Committee Members:
- Farshad Rajabipour, Chair & Dissertation Advisor
Christopher Gorski, Major Field Member
Juan Pablo Gevaudan Burgos, Outside Unit & Field Member
Aleksandra Radli¿ska, Major Field Member
Farshad Rajabipour, Program Head/Chair - Keywords:
- ASR
Volcanic ash
Ground bottom ash
Calcined clay
FBC fly ash
Miniature concrete prism test (MCPT)
Soluble Alkalis
Pore Solution
Alkali Binding
Low carbon concrete
Off-specification fly ash
Volume instability
Calcium sulfite
Low-carbon concrete
Sustainable concrete
Clinker substitution
Supplementary cementitious materials
Pozzolan
High-alkali SCMs
Akali-silica reaction
Available alkalis
Soluble alkalis
Pore solution
Alkali binding
High-sulfur fly ash
Internal sulfate attack
Hannebachite
High-SO3 coal ash products
Marginal coal ash products - Abstract:
- Clinker substitution with supplementary cementitious materials (SCMs) is a cornerstone of efforts to decarbonize cement and concrete production. SCMs not only reduce concrete's carbon footprint but also enhance its hardened properties and durability, including mitigating alkali-silica reaction (ASR) in reactive aggregate-containing concretes. However, supply shortages of traditional SCMs, such as coal fly ash and slag cement, have driven the exploration of alternative SCMs, including natural pozzolans, marginal and off-spec coal ashes, and ground glass. While promising, these alternatives present challenges: high alkali content, which may elevate pore solution alkalinity and raise questions about ASR mitigation, and high sulfur content, which may increase the risk of internal sulfate attack. This dissertation addresses these challenges through four studies: the first three focus on high-alkali SCMs, while the fourth examines high-sulfur SCMs. The first study introduces a new test method for quantifying the soluble fraction of alkalis in SCMs, crucial for understanding their influence on cement pore solution chemistry. Applied to 14 SCMs—including natural pozzolans, coal ashes, and ground glass—the test measured soluble sodium in a 1M KOH host solution and soluble potassium in a 1M NaOH solution over 180 days. Thermodynamic modeling was employed to validate the test results and ensure minimal solid-phase precipitation. The findings revealed that a significant portion of SCM alkalis is soluble, often exceeding levels measured by the ASTM C311 Available Alkali Test. The second study evaluates the impact of the 14 SCMs on cement paste pore solution alkalinity and pH in pastes where 20% of the cement was replaced by SCMs. Cement paste pore solutions were extracted and analyzed over one year. While the first study showed that most SCM alkalis are soluble, their pozzolanic reactions enhanced alkali binding capacity, outweighing the effects of soluble alkali content for most SCMs. Nine SCMs acted as net alkali sinks, one showed minimal impact, and four increased pore solution alkali content. Notably, all 14 SCMs reduced [OH⁻] levels below those of 100% cement pastes. Regression analysis identified that an SCM's ability to alter [OH⁻] in the pore solution depends on its soluble alkali content (measured by the proposed method in the first study), pozzolanic reactivity, and CaO/(SiO₂ + Al₂O₃) ratio based on bulk chemistry. The third study examines the effectiveness of these SCMs in mitigating ASR in concrete mixtures using the AASHTO T 380 standard test method. Seven aggregate groups with varying reactivity levels were tested. Results showed SCM effectiveness in ASR mitigation varied across aggregate combinations. Electrical surface resistivity measurements on the same prisms strongly correlated with 56-day ASR expansion, suggesting resistivity as a reliable early indicator of ASR mitigation potential. Regression analysis identified the control mix's 56-day expansion, pozzolanic reactivity, and SCM soluble alkali content as significant predictors of ASR mitigation. The study recommends performance testing using the AASHTO T 380 method for a more accurate assessment of ASR mitigation. The fourth study assesses the risk of internal sulfate attack in concrete containing high-sulfur coal ashes, which often exceed the 5.0% SO₃ limit set by ASTM C618. Findings show that fly ashes with up to 12.0% SO₃ at 20% replacement levels and binders with up to 5.0% SO₃ can be safely used without internal sulfate attack risks. Fly ash exceeding 5.0% SO₃ is recommended for use if it passes the ASTM C1038 lime water expansion test at the intended dosage. Quantitative X-ray diffraction and pore solution analyses revealed that ettringite formation, the primary driver of expansion, continues until pore solution sulfate is depleted, even as solid gypsum depletes within 24 hours. Sulfate availability in the pore solution was found to be the limiting factor for expansion. Collectively, these studies demonstrate the feasibility of sustainably using high-alkali and high-sulfur SCMs, providing pathways to durable, sustainable concrete production.