Greener Foundations: Enhancing Geopolymer Concrete with Wollastonite Carbonation and Photocatalysis

The Problem:

The manufacturing of Ordinary Portland Cement (OPC), a key component in concrete, currently accounts for 8% of global anthropogenic carbon dioxide (CO2) emissions. In the process of making OPC, limestone must undergo calcination, in which it is heated to extreme temperatures to release CO2 from its composition. This translates in 0.5 to 1 tonne of CO2 released for every tonne of OPC made. With a dire need of extensive emission cutbacks to mitigate the climate crisis, it is crucial that we invest more research into developing sustainable building materials that can significantly lower emissions from this sector.

The Solution:

Geopolymer concrete enhanced with wollastonite aggregates and a biochar-supported photocatalyst reacts with and effectively stores high concentrations of CO2 during carbonation curing. Due to the formation of stable carbonate phases during curing, this geopolymer prototype is more durable and stronger than commonly used residential concrete. With further research and development, this prototype could evolve into a product capable of significantly reducing emissions from the construction industry, and from areas with high concentrations of CO2, such as cities.

Objectives of Greener Foundations:

1.     Develop a novel geopolymer-based prototype that can sequester more carbon dioxide than common residential concrete.

2.     Determine what applications the prototype could be used in through compressive strength testing.

3.     Analyze the performance, cost, and carbon footprint of the prototype to determine its eligibility and efficiency in structural applications.

*Steps are explained more clearly in the images above.

1.     Developing the geopolymer mix design:

The Kriven formula, a generalized expression that uses oxide ratios to determine geopolymer composition, was used to develop the mix design of this geopolymer prototype. The mix design was calculated in the generalized following steps:

1)     The required volume of the mix was calculated.

2)     The density and mix fraction of the geopolymer, wollastonite aggregates and activator solution was calculated.

3)     Molar ratios of the metakaolin aluminosilicate precursor were compared to the Kriven formula to determine the required composition of the activator solution.

4)     Molar quantities were converted to kilograms for final weight.

5)     Aggregate component was divided into coarse (45%) and fine (55%) fractions.

2.      Materials:

·        Metakaolin (aluminosilicate precursor)

·        Activator solution (sodium silicate, sodium hydroxide)

·        Wollastonite aggregate (coarse and fine fractions)

·        Biochar-supported zinc oxide (photocatalyst)

Toxic chemicals defined by WHMIS were used in this project under adult supervision with personal protective equipment.

3.     Experimental groups:

3 experimental groups were observed in this project:

1)     Geopolymer enhanced with wollastonite aggregates and biochar-supported zinc oxide (prototype group)

2)     Geopolymer enhanced with wollastonite aggregates (secondary control group)

3)     OPC concrete (primary control group)

4.     Curing:

Samples were cured at 80% RH in moderate temperatures for 30 days, including a 3-week carbonation curing phase in which samples were cured at 5,000 ppm CO2 concentration. The CO2 uptake of each experimental group was observed during this phase to determine if the prototype group sequesters more CO2 than the control groups.

5.     Strength testing:

Samples were transferred to the University of Ottawa, faculty of engineering to undergo compressive strength testing. This step determined the prototype’s applicability in various structures.

6.     Statistical analysis:

The statistical significance of the prototype’s performance and strength was determined using ANOVA and t-test.

*More data provided in the images above.

How the Prototype Works:

The prototype made in this project sequesters CO2 in 3 principal ways:

1.     Carbonation reaction in the alkaline geopolymer pore solution:

·        Unbonded alkalis (Na+) in the pore solution of the geopolymer form stable carbonate phases with the CO2.

2.     Carbonation/mineralization of wollastonite aggregates:

·        Wollastonite (CaSiO3) undergoes a carbonation reaction with the CO2 to form calcium carbonate and amorphous silica.

3.     Photocatalytic reduction/activation of carbon dioxide:

·        Photogenerated electrons in the biochar-supported photocatalytic matrix weaken the linear structure of CO2 molecules to allow for accelerated bonding with alkalis in the geopolymer network. CO2 molecules can additionally be reduced into organic molecules and radicals.

During the carbonation curing phase, the prototype is exposed to high concentrations of CO2. This results in accelerated carbonation, a process in which chemical compounds interact with the CO2 to inhibit rapid CO2 uptake. The durability of the carbonates formed during accelerated carbonation help support the strength and structure of the prototype.

CO2 Sequestration of the Prototype:

The prototype group absorbed the most CO2 during the carbonation process, with 16.27% of injected CO2 having been absorbed. The primary control group (OPC) absorbed 9.18% and the secondary control group (geopolymer and wollastonite) absorbed 11.27%. An analysis of variants (ANOVA) test was used to determine if the difference between all 3 groups was significant, with a resulting p-value of 0.29 (not significant).

A t-test was used to determine the significance of the difference between the primary control group (OPC) and the prototype group, with a resulting p value of 0.08 (not significant).

While the prototype did not absorb significantly more CO2 than the other groups, more experimental replication could provide more accurate statistical results. Furthermore, a p-value of 0.08 is adequate for this proof-of-concept study as it is approaching a significant value (0.05).

Compressive Strength Testing Results:

The prototype group sustained the required strength to be used in construction projects of commercial slabs, curbs, and sidewalks, with one sample sustaining 29 megapascals and the other sustaining 31 megapascals. The strength of this group tested below the strength of the geopolymer and wollastonite control group, which had one sample sustain 30 megapascals and the other 36. The OPC group demonstrated the lowest strength, with one sample sustaining 21 megapascals and the other 24. These results dictate the prototype’s potential to be used in various applications as a more sustainable, potentially stronger alternative to OPC concrete.

Cost Analysis:

Using standard prices of geopolymer materials ordered in bulk, the cost of the prototype was determined to be $30.00 per square metre. This is 40-60% more expensive than OPC concrete, which ranges from $12.00-$18.00 per square metre. 33% of the total cost is attributed to the chemicals required in the activator solution (sodium silicate and sodium hydroxide). The metakaolin precursor used as the binder is also more expensive than cement clinker per kilogram, further increasing cost.

Conclusion:

Over the three-week carbonation curing period, observations showed an increased sequestration rate of 40% from the OPC group to the prototype group. These findings support the initial hypothesis that enhancing geopolymer concrete by adding wollastonite aggregates and incorporating biochar-based photocatalysts into the matrix would result in higher CO₂ sequestration rates, though the difference between the experimental groups was not deemed significant according to ANOVA and t-test.

The prototype samples demonstrated an adequate loading capacity, which express the prototype’s potential to be used in construction projects of infrastructure such as sidewalks, curbs, and commercial slabs.

With this data, it can be concluded that the experimental prototype containing wollastonite aggregate and biochar-based photocatalysts shows larger CO₂ sequestration capacity through accelerated wollastonite carbonation and photocatalytic reduction/activation of CO₂ molecules during curing. The compressive strength test showed that this prototype sustains less force than that of the group without the Bio-BPs, but more than the standard OPC control group.

While this proof-of-concept study primarily focused on observing the efficacy of the additives to sequester CO2 into the concrete prototype, the cost and carbon footprint was additionally analysed to determine how applicable/beneficial mass production of this product would be compared to OPC concrete. The cost is about twice that of OPC concrete, and the carbon footprint was roughly calculated to be also nearly twice as much (see explanation for this in next steps section). Further research will work on making this product more cost-efficient and sustainable.

Next Steps:

Due to the limited accessibility to cheap, sustainable geopolymer materials for this project, the current prototype has a large carbon footprint and is not cost-efficient. As a solution, I implemented the experimental additives into a more sustainable, lower-cost mix design that uses fly ash as the aluminosilicate precursor and rice husk ash as the source of silicate. These substitutions would lower the cost and carbon footprint significantly. The next step would be to mix, cure, and observe this new geopolymer design to create a highly sustainable, cost-effective, and carbon-sequestering concrete prototype.

Thank You:

This project would not have been possible without the resources and expertise provided by Dr. Muslim Majeed, a materials engineer in the engineering faculty at the University of Ottawa, who provided the metakaolin, concrete cylinders, and strength testing equipment used in this project. I am also thankful to Bob Vasily, the president of Canadian Wollastonite, who provided the wollastonite aggregates and essential expertise on the characteristics of the mineral. I would additionally like to thank my family, friends, educators and community for their support in my scientific journey and aspirations. I would not have the resources nor the mindset to complete my science projects without your help and encouragement.

The following is a list of the sources used to develop this project:

·        Aaron, Mr, and Paul Wilson. 2015. “Establishing a Mix Design Procedure for Geopolymer Concrete in Fulfilment of the Requirements of Bachelor of Engineering (Civil).” https://sear.unisq.edu.au/29250/1/Wilson_A_Lokuge.pdf.

·        Kolade, Anuoluwapo Sola, Bolanle Deborah Ikotun, and Damilola Oyewumi Oyejobi. 2025. “Composition and Performance Driven Mix Design Methodology for Geopolymer Mortars.” Discover Civil Engineering 2 (1). https://doi.org/10.1007/s44290-025-00327-4.

·        Parveen, Parveen, and Dhirendra Singhal. 2017. “Development of Mix Design Method for Geopolymer Concrete.” Advances in Concrete Construction 5 (4): 377–90. https://doi.org/10.12989/acc.2017.5.4.377.

·        Professor, Asst. n.d. “‘A Characteristic Study of Light Weight Geopolymer Concrete.’” International Research Journal of Engineering and Technology. Accessed March 17, 2026. https://www.irjet.net/archives/V3/i2/IRJET-V3I2271.pdf.

·        Recent progress in biochar-based photocatalysts for environmental remediation - ScienceDirect

·        “Wollastonite: An Agricultural Overview.” n.d. Accessed March 17, 2026. https://www.canadianwollastonite.com/wp-content/uploads/2023/08/a-Wollastonite-An-Agricultural-Overview-20230817.pdf.

·        Yang, Xin, Qingwen Yang, Huixing Dai, Lizi Chen, Miao Deng, Runlong Feng, Jun Li, Zhouyueyang Cheng, and Huan Gao. 2025. “Preparation, Characterization, and Analysis of Wollastonite-Enhanced Kaolinite-Based Porous Geopolymer and Its Reinforcing Mechanism.” Construction and Building Materials 503 (November): 144536. https://doi.org/10.1016/j.conbuildmat.2025.144536.

·        Mian, Md Manik, and Guijian Liu. 2018. “Recent Progress in Biochar-Supported Photocatalysts: Synthesis, Role of Biochar, and Applications.” RSC Advances 8 (26): 14237–48. https://doi.org/10.1039/c8ra02258e.

·        Hendy, Cecilia M., Gavin C. Smith, Zihao Xu, Tianquan Lian, and Nathan T. Jui. 2021. “Radical Chain Reduction via Carbon Dioxide Radical Anion (CO2•–).” Journal of the American Chemical Society 143 (24): 8987–92. https://doi.org/10.1021/jacs.1c04427.

·        Saini, Pratibha, Krishan Kumar, Surendra Saini, Mukul Sethi, Priyanka Meena, Aditya Gurjar, Wolfgang Weigand, and Vijay Parewa. 2025. “Photocatalytic Single Electron Reduction of CO2 into Carbon Dioxide Radical Anion (CO2·−): Generation, Detection and Chemical Utilization.” Journal of Energy Chemistry 105 (March): 525–59. https://doi.org/10.1016/j.jechem.2025.02.013.

·        Chen, Shikun, Shengqian Ruan, Qiang Zeng, Yi Liu, Mingzhong Zhang, Ye Tian, and Dongming Yan. 2022. “Pore Structure of Geopolymer Materials and Its Correlations to Engineering Properties: A Review.” Construction and Building Materials 328 (April): 127064–64. https://doi.org/10.1016/j.conbuildmat.2022.127064.

·        Han, Weiwei, Yigang Lv, Shiyu Wang, Jie Qiao, Chaosheng Zou, Miao Su, and Hui Peng. 2023. “Effects of Al/Na and Si/Na Molar Ratios on the Alkalinity of Metakaolin-Based Geopolymer Pore Solutions.” Materials 16 (5): 1929. https://doi.org/10.3390/ma16051929.

·        Mazzotti, Marco, Switzerland Lead, Juan Carlos, Abanades Spain, Rodney Allam, Francis Meunier, Edward Rubin, Juan Carlos Sanchez, and Ron Zevenhoven. n.d. “Mineral Carbonation and Industrial Uses of Carbon Dioxide Coordinating Lead Author.” Accessed March 17, 2026. https://canadianwollastonite.com/wp-content/uploads/2018/02/Mineral-carbonation-and-industrial-uses-of-CO2.pdf.

·        Low, Norman M.P., and J.J. Beaudoin. 1992. “Mechanical Properties of High Performance Cement Binders Reinforced with Wollastonite Micro-Fibres.” Cement and Concrete Research 22 (5): 981–89. https://doi.org/10.1016/0008-8846(92)90122-c.

·        Wang, Yingbin, Mengyuan Lv, Ying Su, Xingyang He, Bohumir Strnadel, and Xiong Qin. 2025. “The Carbon Mineralization Behavior of Wollastonite and Its Impact on Pozzolanic Reactivity.” Construction and Building Materials 495 (September): 143738. https://doi.org/10.1016/j.conbuildmat.2025.143738.

·        Li, Zhuguo, and Ko Ikeda. 2024. “Compositions and Microstructures of Carbonated Geopolymers with Different Precursors.” Materials 17 (7): 1491.

·        Zhao, Cheng, Ziqing Li, Shuangdi Peng, Jie Liu, Qiaoyun Wu, and Xiong Xu. 2024. “State-of-The-Art Review of Geopolymer Concrete Carbonation: From Impact Analysis to Model Establishment.” Case Studies in Construction Materials 20 (April): e03124. https://doi.org/10.1016/j.cscm.2024.e03124.

·        Msomba, Catherine Y., Makungu M. Madirisha, and Kessy F. Kilulya. 2025. “Formulation of Geopolymer with Assisted Carbonation and High-Salinity Brine Curing to Strengthen Wellbore Integrity in Deep Oil and Gas Wells.” Arabian Journal for Science and Engineering, November. https://doi.org/10.1007/s13369-025-10693-x.

·        “Metakaolin - an Overview | ScienceDirect Topics.” n.d. Www.sciencedirect.com. https://www.sciencedirect.com/topics/engineering/metakaolin.

·        jonnyhough. 2025. “What Is Metakaolin? | Concrete Society.” Concrete Society. July 8, 2025. https://www.concrete.org.uk/fingertips/what-is-metakaolin/.

·        “Step-By-Step Guide to Making Sodium Hydroxide Solutions.” 2023. Practical Science. September 18, 2023. https://practical-science.com/2023/09/18/standard-solution-sodium-hydroxide/.

·        Helmenstine, Anne Marie. 2010. “How to Prepare a Sodium Hydroxide Solution.” ThoughtCo. 2010. https://www.thoughtco.com/prepare-sodium-hydroxide-or-naoh-solution-608150.

·        PubChem. 2020. “Sodium Silicate.” @Pubchem. PubChem. 2020. https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-silicate.

·        “Properties of Sodium Silicate.” 2026. Webqc.org. 2026. https://www.webqc.org/compound-Sodium+silicate-Na2SiO3.html#google_vignette.

·        “Basic Principles of Biochar Production - Biochar for Sustainable Soils.” 2019. Biochar for Sustainable Soils. 2019. https://biochar.international/guides/basic-principles-of-biochar-production/.

·        “Biochar Basics: An A-To-Z Guide to Biochar Production, Use, and Benefits a Little Background.” n.d. https://www.fs.usda.gov/rm/pubs_journals/rmrs/sycu/2022/sycu_54_2022_05_biochar_basics.pdf.

·        Mousa, Sahar A, D A Wissa, H H Hassan, A A Ebnalwaled, and S A Khairy. 2024. “Enhanced Photocatalytic Activity of Green Synthesized Zinc Oxide Nanoparticles Using Low-Cost Plant Extracts.” Scientific Reports 14 (1). https://doi.org/10.1038/s41598-024-66975-1.

·        “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.” 2023. Astm.org. 2023. https://store.astm.org/c0039_c0039m-21.html.

·        “Concrete Compressive Strength - an Overview | ScienceDirect Topics.” n.d. Www.sciencedirect.com. https://www.sciencedirect.com/topics/engineering/concrete-compressive-strength.

·        Balogh, Anne . 2023. “Concrete Compressive Strength Explained (What Is PSI?) - Concrete Network.” ConcreteNetwork.com. May 26, 2023. https://www.concretenetwork.com/concrete/compressive-strength-psi.html.

·        Bob. 2025. “Understanding Concrete Compression Test Fracture Types: A Guide to Tension, Shear, and Flexural Failures - ConcreteCaptain.com.” ConcreteCaptain.com. July 4, 2025. https://concretecaptain.com/concrete-compression-test-fracture-types/.

·        McLellan, Benjamin C., Ross P. Williams, Janine Lay, Arie van Riessen, and Glen D. Corder. 2011. “Costs and Carbon Emissions for Geopolymer Pastes in Comparison to Ordinary Portland Cement.” Journal of Cleaner Production 19 (9): 1080–90. https://doi.org/10.1016/j.jclepro.2011.02.010.

·        Adnan, and Muhammad Anas. 2025. “Geopolymer Concrete as a Sustainable Alternative to OPC.” Journal of Umm Al-Qura University for Engineering and Architecture, June. https://doi.org/10.1007/s43995-025-00155-8.

·        “Geopolymer Compressive Strength - Bing.” 2025. Bing. 2025. https://www.bing.com/search?q=geopolymer%20compressive%20strength&qs=n&form=QBRE&sp=-1&ghc=1&lq=0&pq=geopolymer%20compressive%20strength&sc=12-31&sk=&cvid=46A4AA99DAE940C084548C9BD669FAEB.

·        Castillo, Hengels, Humberto Collado, Thomas Droguett, Sebastián Sánchez, Mario Vesely, Pamela Garrido, and Sergio Palma. 2021. “Factors Affecting the Compressive Strength of Geopolymers: A Review.” Minerals 11 (12): 1317. https://doi.org/10.3390/min11121317.