Eligibility and scope

Eligible technologies

All projects certified under this methodology must perform accelerated and/or enhanced carbonation of alkaline minerals by CO₂, in an ex-situ environment, to:

• generate additional carbon removals and/or

• reduce cement usage.

Many different technological setups are eligible under the present methodology and widely described in the scientific literature. Eligible project setups may include but are not limited to the following technological processes, final products, CO₂ sources, and alkaline materials:

Eligible technologies

• carbonation curing of cementitious materials

• carbonation during fresh concrete mixing/hydration

• carbonation of solid materials, to add to e.g. concrete or asphalt

• carbonation of liquids such as concrete slurry water, to add to e.g. concrete or asphalt

Eligible products

• Ready mix concrete, mixed and installed

• Pre-cast concrete blocks

• SCMs, aggregates and other solids; destined for use in concrete, asphalt, road subbase or construction fill

• Landfilled solid materials, or "no use"

Eligible CO₂ sources

• atmospheric CO₂ from direct air capture (DAC) or ambient air

• waste/byproduct biogenic CO₂ from point-source capture (e.g. bioenergy production)

• fossil CO₂ from point-source capture (e.g. coal plants)

• calcination CO₂, or other non-fossil CO₂ from heavy industries

• mixed CO₂ from waste incineration

• pure or mixed CO₂ streams of any type listed above

Eligible alkaline sources

• Coal fly ash

• Cement residues (e.g. cement kiln dust)

• Concrete production waste (e.g. concrete wastewater)

• Cement in fresh mixed concrete

• Recycled concrete

• Paper mill residues

• Municipal incineration, biomass incineration and CHP waste (e.g., bottom ash, APC residues)

• Steel manufacturing waste

• Nickel tailings

• Red mud from aluminum processing

• Natural rock, mining waste

Projects that contain components not listed above, but that meet the requirements of the present methodology and the Rainbow Standard Rules, shall be considered on a case-by-case basis.

Projects under this methodology are eligible for

• removal Rainbow Carbon Credits (RCCs) from mineralization with biogenic, ambient and atmospheric CO₂,

• avoidance RCCs from mineralization with fossil and calcination CO₂, and

• avoidance RCCs from reduction of cement use.

Carbon removals under this methodology are estimated to have a permanence horizon of at least 1000 years. Reversal risks and baseline removals are assessed according to this duration.

Credits are issued on the basis of carbonated materials production.

Mineralization with fossil and calcination CO₂ counts as Carbon Capture and Storage (CCS) and is issued avoidance credits.

These do not count for carbon dioxide removal (CDR), and are referred to as carbon storage throughout the methodology.

Centralized vs distributed sites

Mineralization projects may be highly distributed, deploying the same mineralization technology and process to many different sites. For example, mineralization may occur directly in concrete mixing trucks, with small amounts of carbon storage taking place at each carbonation site.

To account for this, projects shall be classified as either centralized vs distributed. A distributed project is defined as one where:

• Each mineralization site is expected to issue fewer than 100 RRCs annually, based on carbon storage via mineralization (i.e. excluding reduced cement avoidance), and

• The following characteristics are consistent across sites in a group:

  • Mineralization reaction type, the metal(s) contributing to mineral formation, and expected carbonate compound

  • Expected mineralization timeline post-reactor, based on the kinetics of the specific technology (e.g. full mineralization occurs within 24 hours, 1 week, etc.)

  • Reactor operating temperature and pressure (within a target range of ±10%)

The main distinction is that distributed projects may use empirical data from a representative subset of sites, after meeting rigorous qualification conditions, whereas centralized projects require measurements from all sites.

See the Sampling and measurements sections for full details.

Certification requirements

Crediting period duration:

The maximum duration of the crediting period for projects certified under this methodology is 5 years. Upon reaching the maximum duration, a project's crediting period may be renewed, according to the Crediting Period Renewal procedure.

Monitoring period duration

The default monitoring period duration is one year, but may be shorter at the Project Developer's request. Project Developers shall submit a Monitoring Report at least once per 24 months. Failure to do so shall result in the project being deregistered.

Site audits

Project Developers may choose between remote or in-person site audits unless any of the following apply:

• If the project issues more than 5,000 RCCs/year, at least 3 sites must have in-person audits. If the project has fewer than 3 sites, all sites must be audited in-person. Sites selected for in-person audits must be representative of all sites. Remaining sites may choose their audit format.

• If any individual site issues more than 5,000 RCCs/year, that site must have an in-person audit, regardless of project-level volume.

The Rainbow team may require an in-person audit for any project at any time.

Versioning and project compliance

When this methodology is revised, projects are required to comply with the latest version for subsequent verifications of RCCs.

Project scope

One project is defined as:

• the operation of one or more mineralization sites, where a mineralization site refers to one reactor or other instrument where mineralization occurs,

• within a single country,

• using the same technology (e.g. carbonation curing of cementitious materials vs. carbonation during fresh concrete mixing/hydration, see Eligible technologies above),

• using the same monitoring approach (see Project CO₂ stored), and

• operated at sites that are under the oversight or data access of a single Project Developer, regardless of whether the developer directly owns or manages each site.

Additional sites may be added to the project on an ongoing basis, provided the sites meet the requirements listed above and in the Principles & requirements section. The addition of a site is considered a major project update, which will be assessed by a VVB during the project's next verification audit.

The project scope covers all additional removals and induced emissions, caused by the project activity, that would not have occurred in the baseline scenario/in the absence of the project.

For example, if the project is co-located with a concrete manufacturing site, the emissions from concrete production (i.e. energy use emissions, cement production, infrastructure embodied emissions...) that would have occurred anyway are not counted towards the project induced emissions.

Project Developers shall categorize the project's carbon capture and mineralization sites as either retrofits/additions on top of existing sites, or the installation of new sites. This helps distinguish between the project scope and the baseline. Project Developers shall prove the extent of existing operations of any connected activities using historical proof of operations of the existing site/s.

Project Developers shall summarize the following key descriptive meta-information for the project:

• Mineralization reaction type, type of metal/s contributing to mineral formation, and expected carbonate compound type

• Expected mineralization timeline after reactor exit, based on the kinetics of the specific technology (e.g. full mineralization occurs within 24 hours, 1 week, etc.)

• Temperature and pressure of the reactor (within a target range of ±10%)

• Carbon storage measurement approach: gas inflow-outflow or solid-sample

• Centralized or distributed project

Baseline scope

The baseline represents the processes that would have occurred in the absence of the project. Project Developers shall evaluate the baseline according to the following three outcomes, detailed in the sections below:

• Feedstock management: baseline removals

• Material production: Reduced cement use

• Product use phase

Feedstock management: baseline removals

This includes removals from feedstock mineralization that would have occurred anyway, from business-as-usual (BAU):

• feedstock use or management, for projects mineralizing solid materials such as SCMs and aggregates, and

• use-stage concrete carbonation, for all technology types.

Project Developers shall justify the estimated baseline removals using transparent sources, project-specific estimates, and conservative assumptions, accounting for the specific feedstock type used and its source. Quantification details are in the GHG quantification section.

Material production: Reduced cement use

If projects issue avoidance credits from reduced cement use, due to improved binder strength from mineralization, the baseline scenario shall include the BAU production of the reduced cement, ensuring that:

• the equivalent amount replaced is conservatively calculated and

• the type of cement modeled in the baseline is an accurate and representative substitute.

Quantification details are in the GHG quantification section. Steps shall be taken to adjust the representative baseline downward to result in below-BAU baseline emissions in the baseline.

Product use phase

Project Developers shall identify the baseline material and function that the project carbonated material replaces. They shall justify that the project material meets the same quality and performance standards as the baseline material it replaces. Project Developers shall prove this using representative test results from pilot testing, R&D laboratories, or full-scale operations.

Project Developers shall justify that using their carbonated product in concrete (or other final product) leads to the same quality and performance as the baseline material, considering:

• durability

• service lifetime

• compressive strength (where relevant)

• use-phase mineralization (where relevant)

• other performance characteristics related to non-concrete end uses.

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The baseline scenario structure remains valid for the entire crediting period but may be significantly revised earlier if:

• The Project Developer notifies Rainbow of a substantial change in project operations or baseline conditions, and/or

• The methodology is revised, affecting the baseline scenario.

The specific values within the baseline scenario will be updated during each crediting period, using project data to accurately reflect the equivalent of the project’s operations.

Mineralization batch

A mineralization batch is the quantity of mineralized/carbonated material produced using the same input materials (alkaline feedstock and CO₂) and operating conditions, at one site. Details on the delineation of mineralization batches are in Table 1. It is assumed that all carbonated material from the same mineralization batch has similar characteristics relevant to mineralization performance and measurement accuracy (i.e. carbonation rate, bulk density…). This facilitates mineralized material solid sampling and measurements, where a representative sample should taken for each mineralization batch.

The project-specific definition of the mineralization batch shall be clearly described in the PDD.

Measurements and reporting are performed at the mineralization batch level. Verification and credit issuance may be done per carbonation batch, or annually on the cumulative carbonation batches from that year.

It is in the Project Developer's best interest to define mineralization batches at a granular level, and resample and remeasure accordingly. Doing so helps reduce measurement variability and improves confidence that reported CO₂ storage variability falls within an acceptable range. Solid sample measurements are only accepted if the relative standard deviation (RSD) is less than 10%, and the measurement showing the lowest carbon content will be used for the Project and the highest is used for the Baseline.

Table 1 Specific examples of how to delineate a mineralization batch are summarized.

Characteristic	Conditions for changing carbonation batch
Alkaline feedstock: singular or homogeneous material	A change in material category, based on recognized standards (e.g. ASTM, EN), A substantial change in material properties (recommended ±3%), even within the same product category, such as mineralogical composition (e.g. proportion of CaO, MgO, silicates, aluminates), bulk density or pH A change in supplier or geographic source of the feedstock*
Alkaline feedstock: composite or heterogeneous material	A change in material category, based on recognized standards (e.g. ASTM, EN), A substantial change in material properties (recommended ±3%), even within the same product category, such as mineralogical composition (e.g. proportion of CaO, MgO, silicates, aluminates), bulk density or pH A substantial change in the fraction of reactive material in the design mix (recommended ±10%) A change in supplier or geographic source of the feedstock*
CO2 input	The biogenic vs. fossil fraction of the CO2 stream changes substantially (recommended ±3%) The CO2 supplier or geographic source changes*
Carbonation site	The physical location and/or machinery in which the reaction is occurring.
Duration	A mineralization batch has a maximum validity of 365 days. After this period, a new batch must be defined and monitored, even if feedstock and CO2 conditions remain unchanged. A mineralization batch may be non-contiguous. For example, if Feedstock 1 is carbonated on Day 1 and again on Day 3 (with a different feedstock used on Day 2), the operations on Days 1 and 3 can be treated as the same batch.

*If the Project Developer can prove that changes in the supplier or geographic source do not lead to material changes in the product used, in ways that affect the parameters used in GHG quantification, then a change in change in supplier or geographic source may be ignored for purpose of defining carbonation batch.