Introduction

Industrial processes generate large quantities of by-products, many of which are treated as waste.  Disposing of industrial waste in landfills is costly and environmentally harmful, yet many waste types can be recycled or beneficially reused. 

Cement kiln dust, a fine-grained solid removed from cement kiln exhaust gas (USEPA 2022), is one such by-product with significant potential for reuse. The Canadian cement industry generates about 670,000 tonnes of cement kiln dust annually (Mahadi et al. 2021). It is alkaline in nature and contains nutrients like calcium (Ca), sulfur, and magnesium that are essential for crops. It has potential as a soil amendment, particularly for acidic soils (Rodd et al. 2004; 2010), where it can help mitigate acidity-related issues. While cement kiln dust has a potential to improve soil health, it lacks other essential nutrients such as nitrogen (N) and carbon (C). 

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While cement kiln dust effectively addresses soil acidity and supplies some mineral nutrients, its use alone is limited because it lacks other essential crop nutrients such as bioavailable nitrogen (N) and organic carbon (C). For cement kiln dust to be a successful and complete soil amendment, it must be paired with organic materials that can provide these missing components.To overcome the nutritional limitations of cement kiln dust and promote its beneficial reuse, this project explores its combined application with two organic by-products: digestate and humic acid.

 

• Digestate, another by-product from the biogas industry, typically has acidic pH and high levels of soluble organic C  and N . Its application often increases N₂O emissions (Dietrich et al. 2020), a potent greenhouse gas. Co-applying cement kiln dust with digestate may reduce soil acidity and N₂O emissions—partly through stabilization or solidification of the digestate (AMEC 2006)—and improve soil organic matter and crop yield compared to cement kiln dust alone (Ivezic et al. 2021). However, its effects are expected to vary depending on cement kiln dust type and soil properties.

 

• Humic acid, a naturally occurring organic compound from decomposed plant and microbial matter, also shows promise as a soil amendment. It improves soil structure, enhances nutrient retention, stimulates microbial activity, and buffers pH. Humic acid contributes to long-term carbon sequestration and may help mitigate greenhouse gas emissions when co-applied with industrial by-products, making it a valuable tool for sustainable land management.

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It is hypothesized that combined application of cement kiln dust, digestate, and humic acid will synergistically improve soil properties and reduce greenhouse gas emissions. Cement kiln dust’s alkalinity will neutralize digestate acidity, stabilizing nutrients and suppressing N₂O emissions, while digestate and humic acid provide essential nitrogen and carbon to enhance soil fertility and structure.

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Previous studies on forage land have shown cement kiln dust to increase calcium and potassium nutrition and forage yield more effectively than lime (Rodd et al. 2010). Current research extensively covers the use of cement kiln dust for soil stabilization and improvement (Mosa et al., 2017; Salih et al., 2025), as well as the roles of digestate and humic substances in soil fertility and GHG mitigation (Maffia et al., 2024; Valenzuela et al., 2021) . However, there is a significant gap in studies that investigate the synergistic effects and mechanisms of  combining cement kiln dust, digestate, and humic acid for GHG mitigation and soil improvement.

This project is in partnership with Norstar Industries Ltd., established in 1999 to promote sustainable resource use for environmental and economic benefit. 

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