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| Fig. 1: CO2 Emission Sources in Cement Production. [1] (Image Source: A. Wang, after Amran et al. [1]) |
Concrete remains a critical construction material globally, with cement production reaching 4.1 billion tons in 2023, up from 1.39 billion tons in 1995, reflecting growing demand driven by extensive infrastructure projects, particularly in developing economies. The majority of this production is concentrated in Asia, where China and India lead at 2 billion tons and 400 million tons per year, respectively, due to rapid urbanization. [1,2]
The environmental footprint of cement production is substantial. A key driver of this footprint is the energy-intensive process required to produce clinker, the active binding agent in Portland Cement. Producing each ton of clinker requires approximately 3.2 to 3.6 gigajoules (GJ) of energy, primarily to heat limestone to temperatures above 1400C, where it decomposes and releases CO2 as a byproduct. [1,2] Assuming the use of natural gas as the fuel source, at a current price of 2.6775 USD/MMBtu on November 5th 2024, the energy cost for producing one ton of clinker is estimated to be between 8.12 and 9.13 USD. [3] Given that each ton of cement produced emits nearly one ton of CO2, cement production is responsible for approximately 1.7 billion metric tons of CO2 emissions globally as of 2021, positioning it as one of the most significant industrial contributors to greenhouse gases, responsible for an estimated 7-8% of total global CO2 emissions. [4]
The industry has increasingly sought to mitigate these emissions by integrating Supplementary Cementitious Materials (SCMs) like fly ash and hemp, which allow a portion of traditional cement to be replaced, theoretically reducing both carbon output and energy use. However, since alternatives like Ashcrete and Hempcrete still rely on Portland Cement for structural integrity, the energy cost of 3.2 to 3.6 GJ per ton remains a central factor. While these materials offer promising reductions in environmental impact by supplementing rather than replacing cement, they do not eliminate the need for clinker and the associated emissions. Thus, any significant reduction in cements carbon footprint must contend with the high energy demand intrinsic to clinker production, underscoring the importance of innovating in both SCM usage and the core cement production process. [2]
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| Fig. 2: Energy Requirements for Clinker Production Stages. [1] (Image Source: A. Wang, after Amran et al. [1]) |
Ashcrete is a concrete alternative that uses fly ash, an industrial byproduct of coal combustion, as a key component. One of Ashcrete's main advantages is its cost-effectiveness, with fly ash often available at minimal cost due to its role as a waste product. Globally, around 544 million tons of fly ash are produced each year, yet only about 42% is reused, with the remainder disposed of in landfills. [4] Transportation costs for fly ash typically range from $10 to $20 per ton, depending on proximity to the source, which can further influence its feasibility as a supplementary material. [3] Ashcrete offers an environmentally preferable way to incorporate these coal byproducts into construction, supporting waste management efforts while reducing landfill reliance. However, while it repurposes fly ash, Ashcrete does not significantly reduce the carbon footprint of Portland cement itself, which emits approximately one ton of CO₂ per ton produced. Studies suggest that incorporating fly ash can partially replace the sand component in concrete mixes, which is already abundant and low-cost, without necessarily leading to direct energy savings. As a result, Ashcrete remains a practical option primarily in regions with ample fly ash supplies, aligning with local sustainability and waste reduction goals.
Hempcrete, a bio-based material made from hemp hurds combined with a lime-based binder, is increasingly considered a sustainable alternative in construction. Its low carbon footprint stems partly from the hemp plant's natural CO2 sequestration during growth. Estimates suggest that, under typical cultivation and processing conditions, hempcrete can sequester between 110 and 180 kg of CO2 per cubic meter, depending on agricultural practices and binder content. [5] Furthermore, the lime binder undergoes a gradual carbonation process, absorbing additional CO2 as it hardens over time, which may incrementally reduce the net carbon emissions associated with its use. Although hempcretes carbon sequestration capacity positions it as one of the few building materials with the potential for a net-negative carbon footprint, its environmental benefits can vary. For example, the CO2 savings can be modest if processing emissions or transport distances are substantial, underscoring the need for a localized supply chain for optimal sustainability impact. [6] In terms of structural properties, hempcrete's compressive strength is relatively low compared to traditional concrete, with typical values ranging from 0.2 to 1.2 MPa when using a 2:1 binder-to-hemp weight ratio. However, elevated compaction during casting can increase compressive strength up to 7.11 MPa, though this remains lower than the compressive strength of conventional concrete. [6] This limited strength makes hempcrete suitable primarily for non-load-bearing applications, such as insulation and infill, rather than for structural components.
When considering sustainable concrete alternatives, factors beyond cost and emissions play a significant role in material selection. Availability and regional access to resources like fly ash (for ashcrete) and hemp hurds (for hempcrete) heavily influence feasibility, as these materials are geographically dependent. Fly ash, for example, is a byproduct of coal combustion and only available near coal plantsa resource that is politically and environmentally contentious, as it often serves as a means to dispose of waste rather than provide significant performance benefits. Replacing sand with fly ash does little to reduce energy usage or emissions since sand itself is abundant and inexpensive. Similarly, hemp hurds depend on a stable supply chain of hemp processing infrastructure, which is limited in most regions and are tied to the messy politics of marijuana legalization. These dependencies mean that ashcrete and hempcrete are often not scalable or feasible in areas lacking these specific resources, which significantly restricts their applicability as large- scale, sustainable alternatives.
Long-term durability is another critical factor. While traditional concrete has a well-documented history of strength and resilience, alternative concretes like hempcrete lack comparable durability records. Hempcrete, in particular, is still undergoing testing, and its performance under varying climate conditions is uncertain. Its non-load-bearing nature further limits its use to insulation and non-structural applications, raising questions about whether its environmental benefits justify its adoption for limited-use cases. This lack of structural integrity makes hempcrete ill-suited for many applications where concrete's strength is essential, and it highlights that hempcrete is far from a true substitute for traditional concrete.
The lifecycle impact is also worth examining critically. While hempcretes biodegradability could theoretically reduce construction waste at the end of life, this benefit is often overstated. Biodegradability may be a minor factor in regions where recycling and repurposing of construction waste are already efficient. Furthermore, commercial scalability remains a major hurdle, as the infrastructure to process and supply hempcrete and ashcrete at scale is still underdeveloped. One could also questions the motivation of hempcrete supporters as an attempted support for legalization and mass growing of cannabis. Without significant advancements in processing capabilities and supply chains, these materials will struggle to meet large-scale construction needs and regulatory standards. Ultimately, these considerations emphasize that selecting a concrete alternative is not straightforward and depends on specific project goals, local availability, and environmental regulations. At present, sustainable alternatives like ashcrete and hempcrete might be best viewed as niche supplements rather than true replacements for conventional concrete, given their significant limitations and logistical challenges.
In summary, while Ashcrete and Hempcrete offer unique benefits, they are not full replacements for conventional concrete. Ashcrete manages coal byproducts but has limited energy savings, primarily acting as a sand substitute without reducing the core energy cost of clinker production, which requires around 3.2 to 3.6 GJ per ton. Hempcrete, valued for its carbon capture and insulation, remains costly and non-load-bearing, limiting its applications. These materials are best suited for niche, non-structural roles where regional availability and specific project needs align. For more impactful sustainability, refining traditional cement production, which emits nearly one ton of CO₂ per ton produced, may offer broader benefits.
© Alexander Wang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Amran et al., "Fly Ash-Based Eco-Efficient Concretes: A Comprehensive Review of the Short-Term Properties," Materials 14, 4264 (2021).
[2] Z. F. Akbulut et al., "Enhancing Concrete Performance through Sustainable Utilization of Class-C and Class-F Fly Ash: A Comprehensive Review," Sustainability 16, 4905 (2024).
[3] N. Asghari and A. M. Memari, "State of the Art Review of Attributes and Mechanical Properties of Hempcrete," Biomass 4, 65 (2024).
[4] R. M. Kalombe et al., "Fly Ash-Based Geopolymer Building Materials for Green and Sustainable Development," Materials 13, 5699 (2020).
[5] L. Essaghouri, R. Mao, and X. Li, "Environmental Benefits of Using Hempcrete Walls in Residential Construction: An LCA-Based Comparative Case Study in Morocco," Environ. Impact Assess. Rev. 100, 107085 (2023).
[6] T. Jami, S. R. Karade, and L. P. Singh, "A Review of the Properties of Hemp Concrete for Green Building Applications," J. Clean. Prod. 239, 117852 (2019).