Eligible technologies

Eligible activities

All projects certified under this methodology must cause carbon removals via accelerated and/or enhanced carbonation of alkaline minerals by CO₂.

Many different technological setups are eligible under the present methodology and widely described in the . 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

• , 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

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

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

• , 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 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 carbonation with biogenic, ambient and atmospheric CO₂,

• avoidance RCCs from carbonation with fossil and 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.

Carbonation 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.

Project scope

One project is defined as:

• the operation of one or more carbonation sites, where a carbonation site refers to one reactor or other instrument where carbonation 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 Eligibility criteria 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 carbonation 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 . 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:

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

• Expected carbonation timeline after reactor exit, based on the kinetics of the specific technology (e.g. full carbonation 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

Projects certified under this methodology shall have a maximum crediting period duration of 5 years, which can be renewed for a total of 20 years. See the Crediting Period Renewal section of the Procedures Manual for procedural details.

Carbonation batch

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

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

Measurements and reporting are performed at the carbonation 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 carbonation 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 carbonation 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 carbonation 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 carbonation 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.