GHG quantification

General GHG quantification rules can be found in the Rainbow Standard Rules.

Calculations of GHG emissions for the baseline and project scenarios shall follow a robust, recognized method and good practice guidance. The overall methodological approach is a comparative life cycle assessment (LCA) at the project-scale, based on ISO 14064-2:2019.

Mineralization projects certified under this methodology may be eligible for removal and avoidance Rainbow Carbon Credits.

Avoidance RCCs from fossil or calcination CO₂ storage are calculated using the same approach as for removals from biogenic and atmospheric CO₂, and are simply assigned a different credit type (avoidance instead of removal).

Avoidance RCCs from reduced cement use are calculated and issued according to a separate accounting mechanism, described below. This conservative approach results in double counting the project's induced emissions, and avoids the need for allocation of emissions/removals.

GHG quantification shall be completed for each reporting period. The duration of the reporting period is chosen by the Project Developer and may be either each mineralization batch, each calendar year, another duration shorter than 1 year, or a maximum of 18 months.

This methodology shall be used in conjunction with the following Rainbow modules:

Processing and energy use

Transportation

Infrastructure and machinery

Functional unit

Two different functional units are used:

• 1 tonne of mineralized material produced (e.g. 1 tonne of carbonated concrete, 1 tonne of carbonated aggregate...)

• 1 tonne of captured CO₂

Credits are issued on the basis of mineralized material production, so this may be considered the main functional unit. Captured CO₂ is used as a secondary functional unit for comparability across CDR technologies.

System boundary

The project system boundary shall include only the activities that are additional to the business as usual (BAU) scenario.

The baseline scenario system boundary shall include the processes that would have occurred in the absence of the project.

For example,

• if the project performs direct carbonation of fresh concrete during hydration, the project boundary shall not include the upstream emissions from cement production.

• if the project captures biogenic CO₂ flue gas from an anaerobic digestion site, the project boundary shall not include the emissions from anaerobic digestion, or the embodied emissions from the digester, because these would have occurred anyway. The project boundary shall include the embodied emissions from the CO₂ capture machinery added to the digester, because this was installed and used specifically for the purpose of carbon capture.

• if the project uses a portion of a concrete facility's flue gas (diverting the CO₂ stream, using part for mineralization , and returning the unused portion to the main flue gas stream to be emitted), the project boundary shall not include the emission of unused CO₂ that returns to the flue gas stream.

A summary of the calculation approach is presented below, and detailed descriptions and equations for calculating GHGs are in the respective Project and Baseline scenario sections.

Summary: Carbon storage from mineralization

For removals and avoidance from mineralization:

• The project system boundary shall include at least the following elements where relevant and additional (i.e. beyond BAU):

  • production of any non-waste inputs and additives

  • transport of inputs to the project site (e.g. CO₂, recycled concrete, alkaline feedstock...)

  • onsite energy use (electricity, fuels, heat...) related to e.g. preparation of feedstock, and the mineralization process

  • fugitive CO₂ leaks during CO₂ transport

  • carbon storage

  • see Figure 1 below for the system boundary diagram.

• The baseline system boundary shall include:

  • any removals that would have occurred naturally from mineralization of the alkaline feedstock, and/or

  • any use-phase carbonation benefits that would have naturally occurred at greater rates had the project not altered the material or process.

Removals and avoidance from mineralization are calculated using the following high-level equations, detailed in their respective sections below.

Calculations: Removals and avoidance from mineralization

$\textbf{(Eq.1)}\ S_{net}= \Sigma{S}_{project} - \Sigma{S}_{baseline} - \Sigma{E}_{project}$

where,

• $S_{net}$ represents the net carbon storage from the project during the reporting period, in tonnes of CO$_2$eq. This storage may be counted as removals or avoidance, depending on the type of CO₂ stored, as calculated in Eq. 2 and 3.

• $S_{project}$ represents the project's gross GHG storage from mineralization, in tonnes of CO$_2$eq, calculated in Eq. 15 or 19.

• $S_{baseline}$ represents any baseline GHG removals from mineralization of alkaline minerals in their alternative use, representing permanent storage that would have occurred anyway in the absence of the project, in tonnes of CO$_2$eq, calculated in Eq. 7.

• $E_{project}$ represents the project's induced GHG emissions, in tonnes of CO$_2$eq, calculated in Eq. 8.

$\textbf{(Eq.2)}\ Removal\ RCCs = S_{net}* F_{bio,\ atm}$

Where,

• $Removal\ RCCs$ represents the amount of removal Rainbow Carbon Credits to be issued during the reporting period, from carbon storage from mineralization.

• $S_{net}$ was calculated in Eq. 1.

• $F_{bio,\ atm}$ represents the fraction of CO₂ that is biogenic or atmospheric, as described in the Allocation of captured carbon: removals vs. avoidance section

$\textbf{(Eq.3)}\ Avoidance\ RCCs = S_{net}* (1-F_{bio,\ atm})$

Where,

• $Avoidance\ RCCs$ represents the amount of avoidance Rainbow Carbon Credits to be issued during the reporting period, from carbon storage from mineralization.

• $S_{net}$ was calculated in Eq. 1.

• $F_{bio,\ atm}$ represents the fraction of CO₂ that is biogenic or atmospheric.

Summary: Reduced cement

Allocation

Allocation of captured carbon: removals vs. avoidance

Credits can be issued from mineralization processes that result in both

• CDR/removals, from using biogenic and atmospheric (e.g. DAC) CO₂, and

• CCS/avoidance, from using fossil and calcination CO₂.

The same calculation method applies to all CO₂ sources, they are simply assigned different credit types upon issuance (i.e. removal vs avoidance, see Eq. 2 and 3).

If the CO₂ stream used in the mineralization batch is 100% biogenic/atmospheric or 100% fossil/calcination, no allocation is needed. All carbon storage and project induced emissions are fully attributed to removal or avoidance, respectively.

If the CO₂ stream is mixed, Project Developers must determine the proportion of biogenic/atmospheric vs. fossil/calcination carbon, according to Article 39 of the EU ETS monitoring and reporting (even for non-EU based projects), summarized here for informative purposes only:

• conservatively assume all CO₂ is fossil/calcination CO₂, or

• use mass balance of material inputs by type, or

• use measurement method, e.g. C14 testing, or

• use other standards and analytical methods, subject to approval by Rainbow and the VVB.

The proportion of CO₂ types shall be used to allocate the following, which are accounted for in Eq. 2 and 3:

• Carbon storage: CO₂ flows shall assume a proportional repartition of the two CO₂ types in the different CO₂ fates (e.g. successfully carbonated CO₂, inflow and outflow CO₂, unsuccessfully carbonated CO₂ left in pore space...).

• Induced emissions: project emissions shall be proportionally assigned to removal or avoidance based on the share of CO₂ input from each source.

For example, if a project:

• has a mixed CO₂ stream of 50% fossil CO₂ and 50% biogenic CO₂

• measures total gross carbon storage of 100 tCO₂eq

• calculates project induced emissions of 10 tCO₂eq, plus 1 tonne of fugitive CO₂ leaked from transport

Gross carbon storage repartitioned proportionally:

• 50 tCO₂eq from fossil CO₂

• 50 tCO₂eq from biogenic CO₂

Project induced emissions repartitioned proportionally:

• 5 tCO₂eq from fossil CO₂

• 5 tCO₂eq from biogenic CO₂

Fugitive CO₂ leaked from transport, repartitioned proportionally:

• 0.5 tonnes fossil CO₂ leaked, counted as 0.5 tCO₂eq (fossil CO₂ has a GWP of 1)

• 0.5 tonnes biogenic CO₂ leaked, counted as 0 tCO₂eq (biogenic CO₂ has a GWP of 0)

This would result in

• Avoidance credits from fossil CO₂ mineralization = $50-5-0.5 = 44.5\ tCO_2eq$

• Removal credits from biogenic CO₂ mineralization = $50-5-0 = 45.0\ tCO_2eq$

Allocation between existing activities and baseline activities

When a process is shared between the project scope and BAU activities (e.g. total electricity use at a site performing both cement manufacturing and mineralization), only the portion attributable to the project and additional to the baseline should be included. This allocation should follow one of the approaches below:

• Subdivide the system and isolate measurements to collect only input/output data directly relevant for the project scope (e.g. install electricity meters at the entry point of the mineralization process).

• If subdivision is not feasible, allocate shared processes based on a relevant underlying characteristic of the shared systems (e.g. by mass for jointly transported materials, by economic value for co-products with distinct markets...).

This allocation shall be applied at the data collection stage. Project Developers shall do this allocation outside of the GHG quantification equations, and submit allocated data into the removal and avoidance calculations (with justification/proof of work for allocation).

Assumptions

1. By default, future uses (beyond the product's first life) or end-of-life treatment of the carbonated material will not lead to reversals. This assumes no significant changes in environmental conditions (e.g. pH or fire exposure) that would cause CO₂ release.

2. A standard transport distance of 50 km is assumed for final product (concrete and/or carbonated solid materials) delivery. Transport emissions for distances below this threshold are considered equivalent between the baseline and project scenarios, and can be excluded from the project system boundary. Transport emissions for distances above this threshold shall be included in project induced emissions calculations.

3. For directly carbonated cement (e.g. during curing or hydration/mixing), it is assumed that either no significant amount of unreacted CO₂ remains trapped in the pore space, or that any trapped CO₂ will eventually react fully with the cement matrix.

4. All carbonated material from the same mineralization batch has similar characteristics.

Baseline scenario

The baseline scenario is twofold, and is detailed in following sections:

• Removals from mineralization that would have occurred anyway, from alternative feedstock use/management (for mineralization of solid materials such as SCMs and aggregates) and in use-stage concrete carbonation (for all technology types).

• Avoidance from reduced cement shall be considered for projects issuing avoidance credits from reduced cement use, thanks to improved binder strength.

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.

Baseline CO₂ storage

The baseline scenario shall account for natural mineralization from both:

• the alternate fate of non-cement alkaline feedstock, and

• the use-phase natural mineralization of cement-based feedstock, that would have occurred anyway.

For alkaline feedstocks other than cement, Project Developers shall assess the natural mineralization of the feedstock upon exposure to atmospheric CO₂, if it hadn't been used by the project for accelerated carbonation. The extent of natural carbonation depends on the:

• alternative fate of alkaline feedstock

• type and duration of exposure to CO₂

• mineralogy

• particle size

Project Developers shall either:

• estimate baseline removals in alkaline feedstock using a description and proof of common practices for managing the alkaline material, mineralization models, scientific literature, or internal experiments, or

• only if the feedstock is recycled concrete aggregate, opt for a default assumed carbon removal rate in the baseline scenario of 6.67 kgCO₂eq/m³ of recycled concrete, in loose aggregate form.

For direct use of cement as a feedstock (e.g. carbonation curing), some natural mineralization of feedstock occurs during its use-phase in concrete. If the project technology only accelerates the rate of this carbonation, rather than causing additional net carbonation gains, then the portion of mineralization that would have occurred anyway (albeit at a slower pace) shall not be credited. Project Developers shall demonstrate that the CO₂ mineralized by the project would not have occurred naturally without the project intervention. This may be justified by:

• The fundamental design or operating principles of the technology, or

• Selecting an appropriate post-treatment measurement time that excludes mineralization likely to occur during early use-phase conditions, or

• Modeling the expected BAU mineralization, using recognized datasets or modeling tools, or

• Opting for a default assumed carbon removal rate in the baseline scenario of 125 kgCO₂eq/tonne of carbonated cement.

Any natural carbon unaccounted for, that would have occurred without the intervention, shall be counted as baseline removals.

If a first screening assessment based on conservative estimates demonstrates combined baseline removals (sum of alkaline feedstock and concrete use phase) are <1% of the net project removals, then baseline removals may be set to 1%, and the Project Developer does not need to collect more precise baseline removal information. Otherwise, the Project Developer may choose to collect/model baseline removals in detail in order to prove and apply a lower baseline removal rate.

Calculations: Baseline CO₂ stored

$\textbf{(Eq.7)}\ S_{baseline}= S_{mineralization feedstock}+S_{use\ phase\ carbonation}$

Where

• $S_{baseline}$ was described in Eq 1

• $S_{mineralization\ feedstock}$ represents baseline carbon storage from natural mineralization of alkaline feedstock, corresponding to the amount of feedstock used by the project in the reporting period, in tCO₂eq.

• $S_{use\ phase\ carbonation}$ represents baseline removals from mineralization of cement during the concrete use-phase that are larger than the use-phase carbonation for the project material, corresponding to the amount of concrete or cement produced by the project in the reporting period, in tCO₂eq.

Baseline induced emissions

Baseline induced emissions are only considered for avoidance credits from avoided cement production. In this case, baseline induced emissions shall include the emissions from producing an equivalent amount of cement to serve the same purpose (compressive strength, lifetime...) as the cement produced in the project scenario.

Emissions are calculated based on the quantity and emission intensity of cement used in the project scenario compared to the baseline. While the emission factor of cement production remains the same (since mineralization projects typically do not alter upstream or downstream cement/concrete production), the total amount of cement needed in the concrete design mix may differ. This is because the project may create a stronger binder, requiring less cement than conventional practices.

Quantity of cement

To determine the quantity of cement avoided, Project Developers shall provide the cement usage ratio between the project and baseline scenarios for each end use of the carbonated material.

This shall be proven using the stated concrete mix designs used by the client using the carbonated material, demonstrating a lower cement use than otherwise used, or similar project-specific estimates (i.e. not default global replacement rates).

Emission factor of cement

Calculations from this life cycle stage are presented above in Eq. 4-6 in the system boundary section above.

Project scenario

An example of a typical project design and system boundary is shown in Figure 1. Each life cycle stage is detailed in the following sections.

Figure 1 System diagram showing what is included and excluded in GHG quantification of the project scenario. Life cycle stages are color coded, and correspond to the sections below, where each stage is described in detail. The dashed line represents the system boundary, showing which processes' emissions are included in the GHG quantification, and which are excluded.

Calculations: Total project induced emissions

$\textbf{(Eq. 8)}\ E_{project}= E_{CO_2\ capture} +E_{feedstock}+E_{mineralization}$

Where,

• $E_{project}$ was described in Eq. 1.

• $E_{CO_2\ capture}$ is calculated in Eq. 9.

• $E_{feedstock}$ is calculated in Eq. 13.

• $E_{mineralization}$ is calculated in Eq. 14.

Project CO₂ capture

This stage includes process emissions from the CO₂ capture facility, CO₂ transport emissions, and CO₂ leakage during transport.

CO2 capture

Induced emissions from the CO₂ capture process shall only include emissions/activities that would not have occurred in the baseline.

Typically, CO₂ capture is done on industrial sites that are already operating and emitting CO₂. In this case, emissions from the industrial site operations and embodied emissions shall not be counted towards the CO₂ capture. Furthermore, the CO₂ itself is considered a waste product, and according to the waste cutoff LCA principle, is modeled as entering the project system boundary with no environmental burden/emissions.

Processes that may be considered in this stage may include but are not limited to:

• additional infrastructure/machinery/instruments that are required for carbon capture (note that only substantial pieces that contribute to at least 1% of the total project life cycle emissions should be included, according to the Riverse Standard Rules. This can be assessed with a screening LCA using estimates, and if deemed substantial, more precise data shall be provided).

• additional energy use required for carbon capture: Project Developers shall isolate the amount of energy used at the site that is only for carbon capture, that is not related to the site's BAU activities.

• energy or material use from purification and processing of CO₂ streams, for example through chemical (e.g. amine-based absorption) or physical treatment (cryogenic separation or membrane separation).

For infrastructure calculations and emission factors, see the Infrastructure and machinery module. For energy use calculations and emission factors, see the Processing and energy use module.

CO2 transport

CO2 leakage during transport

Calculations: Project CO₂ capture

$\textbf{(Eq. 9)}\ E_{CO_2\ capture}=E_{CO_2\ capture\ process}+E_{CO_2\ capture\ infra}+E_{CO_2\ transport}+E_{CO_2\ transport\ leakage}$

Where

• $E_{CO_2\ capture}$ represents the total project emissions from the CO₂ capture life cycle stage, in tCO₂eq.

• $E_{CO_2\ capture\ process}$ represents the emissions from any additional energy or consumable materials used in the CO₂ capture process, calculated using the Processing and energy use module, in tCO₂eq.

• $E_{CO_2\ capture\ infra}$ represents the emissions from any additional infrastructure or machinery used for CO₂ capture, calculated using the Infrastructure and machinery module, in tCO₂eq.

• $E_{CO_2\ transport}$ represents the emissions from transporting CO₂ to the mineralization site, calculated using the Transportation module, in tCO₂eq.

• $E_{CO_2\ transport\ leakage}$ represents the emissions from fugitive CO₂ leaked during CO₂ transport, in tCO₂eq. It may be calculated using Eq 10, 11, 12, or a different approach.

$\textbf{(Eq. 10)}\ E_{CO_2\ transport\ leakage}= Purchased_{CO_2}-Inflow_{CO_2}$

Where,

• $E_{CO_2\ transport\ leakage}$ was described in Eq. 9.

• $Purchased_{CO_2}$ represents the total mass of CO₂ leaving the CO₂ supplier and destined for the mineralization site, throughout the reporting period, in tCO₂eq.

• $Inflow_{CO_2}$ represents the mass of gaseous CO₂ entering the mineralization process (e.g. entering a reactor) throughout the reporting period, in tCO₂eq. It may be calculated using Eq. 20, or provided via other operations records.

$\textbf{(Eq. 11)}\ E_{CO_2\ transport\ leakage}= Purchased_{CO_2} \times R_{leakage}$

Where,

• $E_{CO_2\ transport\ leakage}$ was described in Eq. 9.

• $Purchased_{CO_2}$ was described in Eq. 10.

• $R_{leakage}$ represents the default leakage rate of the given transport mode (e.g. truck, pipeline...), for all transport modes used in project operations, in tCO₂ lost/tCO₂, or as a fraction.

$\textbf{(Eq. 12)}\ E_{CO_2\ transport\ leakage}= T_{CO_2} \times R_{leakage,\ T}$

Where,

• $E_{CO_2\ transport\ leakage}$ was described in Eq. 9.

• $T_{CO_2}$ represents the truck or ship transport segment considered, in tCO₂*km.

• $R_{leakage}$ represents the default leakage rate of the given transport mode (e.g. truck, pipeline...), in tCO₂ lost/tCO₂*km.

Project feedstock provisioning

This life cycle stage shall include the production, processing, and transport of alkaline feedstock to be carbonated.

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Requirements for modeling induced emissions from feedstock production are presented in Table 2.

Table 2 The approach for modeling GHG emissions from various types of feedstock are presented here.

Feedstock type	Example	GHG quantification
Waste, no value	Recycled concrete aggregate	Feedstock enters the project system boundary with no emissions. The system boundary starts with the transport step where feedstock is diverted from its BAU use and sent to the project site, or the first non-BAU treatment step, whichever comes first.
Produced for the sole purpose of mineralization	Olivine	All feedstock production/mining/sourcing emissions shall be counted towards project induced emissions.
Valuable product, but not produced for the purpose of mineralization	Ordinary Portland cement (OPC) for carbonation curing	Production emissions are excluded, because they would have happened anyway/would be the same in the baseline scenario. The system boundary shall only include any processing steps specifically to prepare the feedstock for mineralization.
Valuable co-product	Steel slag	A share of the production emissions shall be allocated to the co-product, preferably based on economic allocation

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Examples of feedstock processing/preparation for mineralization that may be considered in this stage may include but are not limited to:

• feedstock preparation/processing to increase mineral purity (e.g. magnetic separation of iron), to increase carbonation rates

• feedstock preparation/processing to increase surface area (e.g. grinding), to increase carbonation rates

• heating, drying, wetting, to obtain optimal feedstock moisture content and diffusivity

For calculations and emission factors, see the Processing and energy use module.

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Transportation of alkaline feedstock shall include the delivery transport from the alkaline material production source to the carbonation site. For calculations and emission factors, see the Transportation module.

Calculations: Project feedstock provisioning

$\textbf{(Eq. 13)}\ E_{feedstock}=E_{feedstock\ production}+E_{feedstock\ processing}+E_{feedstock\ transport}$

Where

• $E_{feedstock}$ represents the total project emissions from the feedstock provisioning life cycle stage, in tCO₂eq.

• $E_{feedstock\ production}$ represents the emissions from feedstock production, in tCO₂eq. This may be zero if the feedstock is a waste, may share emissions allocated between a coproduct, or may fully assume emissions if it is produced for the purpose of mineralization.

• $E_{feedstock\ processing}$ represents the emissions from any additional energy or mineralization materials used in feedstock processing (e.g. griding, heating...), calculated using the Processing and energy use module, in tCO₂eq.

• $E_{feedstock\ transport}$ represents the emissions from transporting feedstock from its production site to the mineralization site, calculated using the Transportation module, in tCO₂eq.

Project mineralization process

This stage includes induced emissions from the mineralization process, including energy use, input and machinery use, and transport/delivery of the carbonated material; plus any CO₂ leaked from the reactor. All induced emissions from the mineralization process shall be included and counted towards the project GHG quantification, because they are all by definition additional to baseline conditions and part of the mineralization project.

Energy and input use

This shall include energy use (electricity, heat and fuel) for heating, maintaining temperature, and compression/maintaining pressure, to be measured directly for each reported period. These may be provided by, for example:

• measurements for the whole site, and allocated to the project if needed (e.g. site-wide electricity bills), or

• measurements for specific machinery used by the project (e.g. energy meters), or

• calculated using machinery power requirements and operation hours.

Depending on the project-specific technology, this may also include but is not limited to:

• additives to increase dissolution rates (where dissolution of metal ions is the precursor to mineralization )

• water

For energy use calculations and emission factors, see the Processing and energy use module.

Infrastructure/machinery

Transport/delivery

CO2 leakage

Calculations: Project mineralization process

$\textbf{(Eq.14)}\ E_{mineralization}=E_{mineralization \ energy}+E_{mineralization \ infra}+E_{transport}+E_{CO_2\ leak}$

Where

• $E_{carbonation}$ represents the total project emissions from the onsite carbonation life cycle stage, in tCO₂eq per reporting period.

• $E_{mineralization\ energy}$ represents the emissions from energy or consumable inputs used in the mineralization process, calculated using the Processing and energy use module, in tCO₂eq.

• $E_{mineralization\ infra}$ represents the emissions from infrastructure or machinery used for mineralization (i.e. reactors), calculated using the Infrastructure and machinery module, in tCO₂eq per reporting period.

• $E_{transport}$ represents the emissions from transporting the carbonated material from its production site to the user, calculated using the Transportation module, in tCO₂eq per reporting period. It is only considered if the transport distance is greater than 50 km.

• $E_{CO_2\ leak}$ represents the emissions from direct CO₂ leakage from the reactor/mineralization site to the atmosphere, in tCO₂eq per reporting period. Project Developers shall calculate this in external files, using their preferred measurement setup, and report the final value per reporting period in the MRV.

Project CO₂ storage

Carbon storage shall be determined using project-specific measurements and CO₂ mass balance calculations, using either:

1. Solid sample: Periodic measurements on a representative sample of solid carbonated material, measuring its carbon content compared to a baseline material, using TGA or dry combustion/TCA

2. Gas inflow-outflow: continuous measurements of CO₂ gas inflow minus outflow.

Each method is described in detail below. Cross verification of carbon storage measurements with another method is encouraged but not required.

Solid sample

All solid sample carbon content measurement shall be conducted on:

• A carbonated sample from the project, and

• A non-carbonated control sample of the same material.

The difference in CO₂ content between the two, measured using TGA or dry combustion/TCA, shall be used to quantify the amount of CO₂ removed by the project activity.

To ensure consistency:

• Project and control samples must be collected at the same time interval after exiting the reactor (e.g. 24 hours, 1 week, 1 month).

• Both samples must be stored under identical conditions between mineralization and measurement to avoid variations due to natural ambient mineralization.

All measurements shall be performed on at least one representative sample at the following frequency:

• For each mineralization batch (with batch validity limited to 1 year), or

• At least once per quarter, or

• Every 500 tonnes CO₂ removed, whichever comes first.

Refer to the Sampling and measurements section for detailed procedures on sampling approach, frequency, and traceability.

Calculations: Project CO₂ storage, solid-sample

$\textbf{(Eq. 15)}\ S_{project}= \Delta CO_2eq*A_{P,\ material}$

Where,

• $S_{project}$ represents total carbon storage from the project in the reporting period, in tCO₂eq.

• $\Delta CO_2eq$ represents the increase in CO₂ storage in the carbonated material vs the baseline material, as an absolute increase of tCO₂eq/t carbonated material. Calculated in Eq. 16.

• $A_{P,\ material}$ represents the amount of carbonated material produced by the project in the reporting period, in tonnes of dry material.

$\textbf{(Eq. 16)}\ \Delta CO_2eq=CO_2eq_{project}-CO_2eq_{control}$

Where,

• $\Delta CO_2eq$ was described in Eq. 15.

• $CO_2eq_{project}$ represents the concentration of CO₂ equivalent in the carbonated project material, derived from measured carbonate content using an approved solid sample measurement (TGA or dry combustion), in tCO₂eq/t of dry material. It is calculated in equations below for each measurement method.

• $CO_2eq_{control}$ represents the concentration of CO₂ equivalent in the non-carbonated control material, derived from measured carbonate content using the same measurement approach as for $CO_2eq_{project}$ . It is calculated in equations below for each measurement method.

Project Developers shall use either Eq. 17 or Eq. 18 to measure project and control $CO_2eq$.

$\textbf{(Eq. 17)}\ CO_2eq_{project,\ TGA} = \%CO_{2\, loss} \div100$

Where,

• $CO_2eq_{project,\ TGA}$ represents the concentration of CO₂eq in the material, tCO₂eq/t of dry material, derived from measured carbonate content using TGA. The same equation shall be used for $CO_2eq_{control}$.

• $\%CO_{2\, loss}$ represents the mass loss percentage of CO₂, directly measured using TGA at 600–800 °C, in % mass loss or tCO₂ lost/100 t dry material. Divided by 100 to convert to t/t.

$\textbf{(Eq. 18)}\ CO_2eq_{project,\ dry\ combustion} = \%C_{mass} \div 100 \times C\ to\ CO_2$

Where,

• $CO_2eq_{project,\ dry\ combustion}$ represents the concentration of CO₂eq in the material, tCO₂eq/t of dry material, derived from measured carbonate content using dry combustion. The same equation shall be used for $CO_2eq_{control}$.

• $\%C_{mass}$ represents the measured inorganic or total carbon content of the material, in % mass of carbon or t C/100 t dry material. Divided by 100 to convert to t/t.

• $C\ to\ CO_2$ represents the molecular weight conversion factor between carbon and CO₂, and equals 3.67.

Gas inflow outflow

Gas inflow-outflow measurements shall be taken continuously (at least 1x per minute) and summarized and reported daily.

Gas measurements shall use a calibrated flow metering with ±1.0% accuracy or better. Project Developers shall provide equipment calibration certificates and QA/QC procedures.

Any projects using:

1. Technology type: carbonated solid materials to add to e.g. concrete or asphalt, and

2. Measurement type: gas inflow-outflow,

shall also account for unreacted CO₂ trapped in pore space of the carbonated material. This shall be calculated using conversions and subtracted from carbon storage measurements, according to Eq. 22.

Calculations: Project CO₂ storage, gas inflow-outflow

$\textbf{(Eq. 19)}\ S_{project}= \sum (Inflow_{CO_2}- Outflow_{CO_2}- Pore_{CO_2})$

Where,

• $S_{project}$ represents total carbon storage from the project, summed over the reporting period, in tCO₂eq.

• $Inflow_{CO_2}$ represents the daily recorded mass of gaseous CO₂ entering the carbonation process (e.g. entering a reactor), in tCO₂eq, calculated in Eq. 20.

• $Outflow_{CO_2}$ represents the daily recorded mass of gaseous CO₂ exiting the carbonation process (e.g. exiting a reactor), in tCO₂eq, calculated in Eq. 20.

• $Pore_{CO_2}$ represents the unreacted CO₂ stuck in pore space of the carbonated material, in tCO₂eq, calculated in Eq. 21. It shall only be included for projects that carbonate solid materials (e.g. carbonating SCMs to add to concrete).

$\textbf{(Eq. 20)}\ Flow_{CO_2,\ i}= V_{CO_2,\ i}*C_{CO_2,\ i}$

Where,

• $Flow_{CO_2\ i}$ represents the flow of CO₂ for $i$ types of CO₂, either inflow or outflow from the carbonation process, in tCO₂eq/day.

• $V_{CO_2,\ i}$ represents the volume of CO₂ inflow or outflow of the carbonation process, at standard temperature and pressure, in m³ of gas/day.

• $C_{CO_2,\ i}$ represents the weighted average daily concentration of CO₂ inflow or outflow of the carbonation process, at standard temperature and pressure, in tCO₂/m³ of gas.

$\textbf{(Eq. 21)}\ Pore_{CO_2} = \frac{p}{RT} \times \epsilon \times y_{CO_2} \times \frac{M_{CO_2}}{\rho_{bulk}}$

• $Pore_{CO_2}$ represents CO₂ trapped in pore space in the carbonated material, in tCO₂eq/t carbonated material. This term is only required for projects carbonating solid materials, to add to e.g. concrete or asphalt.

• $\frac{p}{RT}$ represents the molar concentration of an ideal gas (in this case, CO₂), in mol/m³. Under standard conditions, the terms would be total gas pressure ($p= 101325\ Pa$ ), temperature ( $T=298\ K$), and the ideal gas constant ( $R=8.3145J/ (mol*K)$), for a total term value of 40.89 mol/m³.

• $\epsilon$ represents the gas void fraction of the material (i.e. fraction of volume per m³ that is pore space), unitless. This value may be measured, or estimated using secondary literature for well-defined, common, homogeneous alkaline feedstocks.

• $y_{CO_2}$ represents the molar fraction of CO₂ in the pore gas, measured via gas analysis or conservatively assumed, unitless. It can conservatively be assumed to equal 1 (100% CO₂ atmosphere).

• $M_{CO_2}$ represents the molar mass of CO₂, which equals 0.000044 t/mol.

• $ho_{bulk}$ represents the bulk density of the dry carbonated material, measured or estimated using secondary sources, in kg/m³.

Data sources

The required primary data for GHG quantification from all projects, regardless of measurement approach, are presented in Table 3. Required primary data for projects using solid-sample carbon storage measurements are in Table 4, and for projects using gaseous inflow-outflow are in Table 5. These data shall be provided for each reporting period, unless specified otherwise, and made publicly available.

Table 3 Summary of primary data needed from all projects and their source. Asterisks (*) indicate which data are only required for initial project certification and validation, and do not need to be monitored and updated during verification. Two asterisks (**) indicate which data are only necessary if the project is eligible for avoidance credits from reduced cement use.

Parameter	Unit	Source
Amount of CO2 leaving CO2 capture facility	t CO2 per reporting period	Operations records, sales contracts, invoices
Repartition of CO2 types purchased, entering mineralization facility	fraction	Operations records, sales contracts, invoices
Transport distance or amount of fuel, and transport mode, for CO2 delivery	tonne*km, or kg fuel, or L fuel	Operations records
Amount and type of alkaline feedstock used	tonne/reporting period	Operations records
Amount and type of infrastructure/machinery used for CO2 capture*	kg, tonne, or m3; and material type	Technical design documents
Amount and type of infrastructure/machinery used for mineralization*	kg, tonne, or m3; and material type	Technical design documents
Transport distance or amount of fuel, and transport mode, for alkaline feedstock delivery	tonne*km, or kg fuel, or L fuel	Operations records
Baseline removal calculations from alkaline feedstock mineralization	kgCO2eq/tonne feedstock	Models, calculations
Baseline and project concrete use phase carbonation calculations	kgCO2eq/tonne concrete	Models, calculations
Amount cement needed in project scenario**	kg cement used/reporting period	Operations records
Amount cement needed in baseline scenario**	kg cement equivalent calculated/ reporting period	Cement mix designs, statements from clients, mandatory concrete mixes
Cement mix design and emission factor for avoided cement**	kgCO2eq/tonne cement	Project-specific sources (e.g. EPDs), low-carbon cement thresholds (e.g. provided by the Global Cement and Concrete Association Low Carbon Rating), or Ecoinvent
Energy and/or material use from CO2 capture	kg, liter, kWh, MWh, GWh, m3; and material type	Operations records
Energy and/or material use from CO2 purification	kg, liter, kWh, MWh, GWh, m3; and material type	Operations records
Energy and/or material use from alkaline feedstock processing	kg, liter, kWh, MWh, GWh, m3; and material type	Operations records
Energy and/or material use from mineralization	kg, liter, kWh, MWh, GWh, m3; and material type	Operations records
Transport distance or amount of fuel, and transport mode, for carbonated material delivery (if distance >50 km)	tonne*km, or kg fuel, or L fuel	Operations records
Amount of carbonated material produced	tonne	Operations records

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Table 4 Summary of primary data needed from projects using solid-sample CO₂ storage measurements, and their source. Project Developers shall provide only one of the two data sources listed.

Parameter	Unit	Source
TGA: Carbon storage in project and control materials	mass loss percentage of CO2	Laboratory measurements
Dry combustion: Carbon storage in project and control materials	% mass of carbon or t C/100 t dry material	Laboratory measurements

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Table 5 Summary of primary data needed from projects using gas inflow-outflow CO₂ storage measurements, and their source.

Parameter	Unit	Source
Volumetric flow of CO2 inflow and outflow	m3 of gas/day	Primary measurements, sensors
Concentration of CO2 inflow and outflow	t CO2/m3 gas	Primary measurements, sensors
$\epsilon$ gas void fraction of the material (if carbonating solid materials)	fraction of volume per m3	measured or estimated using secondary sources
$y_{CO_2}$ molar fraction of CO2 in the pore gas (if carbonating solid materials)	unitless	gas analysis or conservatively assumed to equal 1
$ho_{bulk}$ bulk density of the dry carbonated material (if carbonating solid materials)	kg/m3	measured or estimated using secondary sources

The ecoinvent database version 3.11 (hereafter referred to as ecoinvent) shall be the main source of emission factors unless otherwise specified. Ecoinvent is preferred because it is traceable, reliable, and well-recognized. The ecoinvent processes selected are detailed in Appendix 1.

Uncertainty assessment

See general instructions for uncertainty assessment in the Rainbow Standard Rules. The outcome of the assessment shall be used to determine the percent of RCCs to eliminate with the discount factor.

The following assumptions have low uncertainty:

• Baseline delivery of concrete or aggregates is 50 km.

• Directly carbonated cement will have no CO₂ trapped in pore space.

The following assumptions have moderate uncertainty:

• Future uses or end-of-life treatment of the carbonated material will not lead to reversals.

• All carbonated material from the same mineralization batch has similar characteristics.

The baseline scenario selection at the methodology level has low uncertainty, because it requires a project-specific assessment of the specific amount and type. The specific circumstances, amount and type of baseline material must be proven by the Project Developer, and their uncertainty shall be assessed at the project level. There is low uncertainty that the baseline scenario includes baseline removals and cement.

The equations have low uncertainty, because they consist of straightforward conversions. No models are used in this methodology. Secondary data include default baseline mineralization rates for a selection of alkaline feedstocks, which comes with low uncertainty due to its very small contribution and the low sensitivity of final results to changes in this value.

The uncertainty at the module level is estimated to be low. This translates to an expected discount factor of at least 3% for projects using this module.