Sulphation and carbonation properties of hydrated sorbents from a fluidized bed CO2 looping cycle reactor

Sulphation and carbonation have been performed on hydrated spent residues from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant operating as a CO₂ looping cycle unit. The sulphation and carbonation tests were done in an atmospheric pressure thermogravimetric analyzer (TGA), with the sulphation performed using synthetic flue gas (0.45% SO₂, 3% O₂, 15% CO₂ and N₂ balance). Additional tests were carried out in a tube furnace (TF) with a higher SO₂ concentration (1%) and conversions were determined by quantitative X-ray diffraction (QXRD) analyses. The morphology of the sulphated samples from the TF was examined by scanning electron microscopy (SEM). Sulphation tests were performed at 850 °C for 150 min and carbonation tests at 750 °C, 10 cycles for 15 min (7.5 min calcination + 7.5 min carbonation). Sulphation conversions obtained for the hydrated samples depended on sample type: in the TGA, they were ～75–85% (higher values were obtained for samples from the carbonator); and in the TF, values around 90% and 70% for sample from carbonator and calciner, respectively, were achieved, in comparison to the 40% conversion seen with the original sample. The SEM analyses showed significant residual porosity that can increase total conversion with longer sulphation time. The carbonation tests showed a smaller influence of the sample type and typical conversions after 10 cycles were 50% – about 10% higher than that for the original sample. The influence of hydration duration, in the range of 15–60 min, is not apparent, indicating that samples are ready for use for either SO₂ retention, or further CO₂ capture after at most 15 min using saturated steam. The present results show that, upon hydration, spent residues from FBC CO₂ capture cycles are good sorbents for both SO₂ retention and additional CO₂ capture.     

Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor

Results are presented on steam hydration of spent residues obtained from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant unit operating in a CO₂ looping cycle mode. The samples were collected from the unit under various conditions, which included electrical heating of the reactor, as well as firing with coal, and biomass under oxy-fuel combustion conditions. In addition, different operating times, i.e., number of cycles (25 min–455 min/1–25 cycles) were examined, with the carbonator operating at temperatures of 600–700 °C and the calciner at 850–900 °C. The samples collected came from the calciner, carbonator and cyclone. Steam hydration itself was done under atmospheric pressure in saturated steam at 100 °C for periods of 15, 30 and 60 min. The original limestone sample, as well as the spent samples from the pilot plant and the hydrated samples were examined to determine their hydration and carbonation levels, as well as their unreacted CaO content using TGA and XRD analysis. In addition, samples were characterized for pore distribution (nitrogen adsorption/desorption: BET and BJH), skeleton characterization, with density by He pycnometry and particle surface area morphology (SEM/EDX), as well as changes in sample volume during hydration (sample swelling). The results obtained showed successful hydration (typically only ～10% unreacted CaO) even for hydration periods as short as 15 min, and very favorable sample properties. Their pore surface area, pore volume distribution and swelling during hydration are promising with regard to their use in additional CO₂ capture cycles or SO₂ retention. However, their predisposition to fracture is the main disadvantage observed with these samples. This may result in difficulties in terms of their handling in FBC systems, due to intensified attrition and consequent elutriation from the reactor.     

蒸汽活化钙基吸收剂联合脱碳脱硫特性

利用管式炉（TF）、蒸汽发生器和热重分析仪（TGA）研究了钙基吸收剂联合脱碳脱硫以及水合特性,并通过N2吸附实验对不同烧结程度以及水合前后样品的孔隙结构进行了测量。结果表明,无水合时,40次碳化循环后的样品碳化活性降至18%,但仍具有44%的硫化活性,比新鲜剂仅低4%,说明脱碳失效剂仍是良好的脱硫剂。碳循环失效剂经蒸汽活化后其碳化活性可提高至68%左右,且具有与新鲜剂类似的活性下降规律。每两次碳化循环后进行一次蒸汽活化,可使样品保持65%的平均转化率。蒸汽活化后吸收剂硫化率可提高至80%,远高于新鲜剂,由电镜扫描实验发现这是由于水合时颗粒产生了大的裂缝和破碎,提供了大量产物可自由生长的外表面积。不考虑颗粒磨损,利用钙基吸收剂先循环脱碳再蒸汽活化最后脱硫是一项联合脱除烟气中CO₂和SO₂的新方法。     

Reinvestigation of hydration/reactivation characteristics of two long-term sulphated limestones which previously showed uniformly sulphating behaviour

Two Canadian limestones, Calpo and Luscar, were fully sulphated, and the residues were hydrated with liquid water and steam and then re-sulphated with synthetic flue gas in a thermogravimetric analyzer (TGA). Both sulphated limestones were previously classified as showing uniform sulphation patterns, and it was expected that they would not demonstrate significant reactivation by hydration. However, the current work demonstrates that both spent sorbents can be reactivated by steam hydration, while one of them, Luscar limestone, can also be reactivated by hydration with liquid water, whereas water hydration is less effective with Calpo limestone. Long-term sulphation was employed on the fresh limestones to ensure that the sorbents were fully sulphated to levels typical of full-scale units during the first sulphation. No evidence was found that the SO₂ concentration for first sulphation influenced the degree of reactivation, indicating that these sulphation times are sufficient. Total calcium utilization after re-sulphation was markedly improved – up to 90% by hydration. Possible explanations for the failure to reactivate these limestones by previous workers may well be that they chose unsuitable hydration conditions and/or that there are wide variations in limestone properties between different batches, even from the same supplier. It is also evident that it may be premature to categorize the sulphation patterns of a given limestone on the basis of limited tests.     

Reactivation and remaking of calcium aluminate pellets for CO2 capture

CaO-based pellets supported with aluminate cements show superior performance in carbonation/calcination cycles for high-temperature CO₂ capture. However, like other CaO-based sorbents, their CO₂ carrying activity is reduced after increasing numbers of cycles under high-temperature, high-CO₂ concentration conditions. In this work the feasibility of their reactivation by steam or water and remaking (reshaping) was investigated. The pellets, prepared from three limestones, Cadomin and Havelock (Canada) and Katowice (Poland, Upper Silesia), were tested in a thermogravimetric analyzer (TGA). The cycles were performed under realistic CO₂ capture conditions, which included calcination in 100% CO₂ at temperatures up to 950 °C. Typically, after 30 cycles, samples were hydrated for 5 min with saturated steam at 100 °C in a laboratory steam reactor (SR). Moreover, larger amounts of pellets were cycled in a tube furnace (TF), hydrated with water and reshaped, and tested to determine their CO₂ capture activity in the TGA. It was found that, after the hydration stage, pellets recovered their activity, and more interestingly, pellets that had experienced a longer series of cycles responded more favorably to reactivation. Moreover, it was found that conversion of pellets increased after about 70 cycles (23%), reaching 33% by about cycle 210, with no reactivation step. Scanning electron microscope (SEM) analyses showed that the morphology of the low-porosity shell formed at the pellet surface during cycles, which limits conversion, was eliminated after a short period (5 min) of steam hydration. The nitrogen physisorption analyses (BET, BJH) of reshaped spent pellets from cycles in the TF confirmed that sorbent surface area and pore size distribution were similar to those of the original pellets. The main alumina compound in remade pellets as determined by XRD was mayenite (Ca₁₂Al₁₄O₃₃). These results showed that, with periodic hydration/remaking steps, pellets can be used for extended times in CO₂ looping cycles, regardless of capture/regeneration conditions.     

Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO2 capture

The lime enhanced gasification (LEGS) process uses CaO as a CO₂ carrier and consists of two coupled reactors: a gasifier in which CO₂ absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO₂ gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity (a_ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO₄ if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.     

烟气中水蒸气对钙基吸收剂碳酸化的影响特性

在循环煅烧/碳酸化反应系统上考察煅烧气氛和碳酸化气氛中水蒸气含量以及CO₂分压对钙基吸收剂成型颗粒碳酸化的影响,通过对钙基吸收剂微观结构分析(扫描电镜、氮吸附分析)以理解水蒸气影响碳酸化特性的机理。结果表明,煅烧气氛和碳酸化气氛中的水蒸气均可提高钙基吸收剂的碳酸化转化率,水蒸气含量分别为10%和5%时,吸收剂的碳酸化性能较好;水蒸气在碳酸化气氛中对高铝水泥改性吸收剂的改善作用较石灰石显著。煅烧气氛中的CO₂分压越高,烧结现象越严重,降低钙基吸收剂的捕集效率;碳酸化气氛CO₂分压提高,有利于提高钙基吸收剂的碳酸化转化率。烟气中水蒸气丰富了吸收剂的微观孔隙,使得吸收剂捕集CO₂性能得到改善。     

An investigation of the kinetics of CO2 uptake by a synthetic calcium based sorbent

This study examines the kinetics of carbonation by CO₂ at temperatures of ca. 750 °C of a synthetic sorbent composed of 15 wt% mayenite (Ca₁₂Al₁₄O₃₃) and CaO, designated HA-85-850, and draws comparisons with the carbonation of a calcined limestone. In-situ XRD has verified the inertness of mayenite, which neither interacts with the active CaO nor does it significantly alter the CaO carbonation–calcination equilibrium. An overlapping grain model was developed to predict the rate and extent of carbonation of HA-85-850 and limestone. In the model, the initial microstructure of the sorbent was defined by a discretised grain size distribution, assuming spherical grains. The initial input to the model – the size distribution of grains – was a fitted parameter, which was in good agreement with measurements made with mercury porosimetry and by the analysis of SEM images of sectioned particles. It was found that the randomly overlapping spherical grain assumption offered great simplicity to the model, despite its approximation to the actual porous structure within a particle. The model was able to predict the performance of the materials well and, particularly, was able to account for changes in rate and extent of reaction as the structure evolved after various numbers of cycles of calcination and carbonation.     

CO2 Capture Using CaO Modified with Ethanol/Water Solution during Cyclic Calcination/Carbonation

The calcium‐based sorbent cyclic calcination/carbonation reaction is an effective technique for capturing CO₂ from combustion processes. The CO₂ capture capacity for CaO modified with ethanol/water solution was investigated over long‐term calcination/carbonation cycles. In addition, the SEM micrographs and pore structure for the calcined sorbents were analyzed. The carbonation conversion for CaO modified with ethanol/water solution is greater than that for CaO hydrated with distilled water and is much higher than that for calcined limestone. Modified CaO achieves the highest conversion for carbonation at the range of 650–700 °C. Higher values of ethanol concentration in solution result in higher carbonation conversion for modified CaO, and lead to better anti‐sintering performance. After calcination, the specific surface area and pore volume for modified CaO are higher than those for hydrated CaO, and are much greater than those for calcined limestone. The ethanol molecule enhances H₂O molecule affinity and penetrability to CaO in the hydration reaction so that the pores in CaO modified are obviously expanded after calcination. CaO modified with ethanol/water solution can act as a new and promising type of calcium‐based regenerable CO₂ sorbent for industrial applications.     

Hydrogen production via sorption enhanced reforming of methane: Development of a novel hybrid material—reforming catalyst and CO2 sorbent

This study presents the development and the evaluation of a new hybrid material, NiO–CaO–Ca₁₂Al₁₄O₃₃, which functions simultaneously as reforming catalyst and CO₂ sorbent for application in sorption enhanced reforming. CaO–Ca₁₂Al₁₄O₃₃ acts as an effective CO₂ sorbent and also as a support for the active metallic Ni particles. This idea aims to overcome the complex problems related with handling of the two different solids (catalyst and CO₂ sorbent) required for sorption enhanced reforming and also to increase the industrial potential of the process by decreasing the cost of the total solid material used. The CO₂ fixation ability of the new material (56% carbonation conversion) remains unchanged for 45 cycles of sorption-desorption. It is assumed that the presence of NiO acts synergetically with Ca₁₂Al₁₄O₃₃ to the excellent stability of the novel material compared with other CaO-based sorbents. The effect of NiO loading on the catalytic functionality of the material, under methane sorption enhanced reforming conditions was studied and the results showed that conversion goes through a maximum (80%) in the presence of the hybrid material with 16 wt% Ni loading. The hybrid material with the optimum metallic loading was tested at 650 °C and methane to steam ratio equal to 3.4. The presence of free CaO resulted in the capture of CO₂ formed, producing a stream rich in H₂ (90%) and poor in CO₂ (2.8%) and CO (2%).     

Continuous experiment regarding hydrogen production by Coal/CaO reaction with steam (II) solid formation

Integration of coal gasification and CO₂ separation reactions in one reactor may produce a high concentration of hydrogen. To design a reactor for this new reaction system, it is necessary to know all reaction behaviors in the integrated reaction system. In our previous study, we performed a continuous reaction experiment of coal/CaO under high steam pressure and confirmed that H₂ concentration higher than 80 vol% with little CH₄ was produced; and that almost all CO₂ was fixed by adding CaO. In this study, the behaviors of solid products during the continuous experiment were investigated. It was found that, CaO first reacted with high-pressure steam to form Ca(OH)₂ (hydration), then the Ca(OH)₂ absorbed the CO₂ generated by coal gasification to form CaCO₃. The hydration of CaO restored sorbent reactivity. Eutectic melting of Ca(OH)₂/CaCO₃ was found to occur in the experiment at 973 K, and this eutectic melting led to the growth of large particles of solid materials. However, at the relatively low temperature of 923 K, eutectic melting could be avoided. Carbon conversion of the coal in the continuous reaction of the coal/CaO mixture with steam was high as 60–80%, even at the lower temperature of 923 K.     

Hydrogen production by sorption-enhanced steam methane reforming process using CaO-Zr/Ni bifunctional sorbent–catalyst

A bifunctional CaO-Zr/Ni (13, 18, and 20.5 wt% NiO) sorbent–catalyst was developed using the wet-mixing/sonication technique and applied for hydrogen production by sorption-enhanced steam methane reforming (SESMR), an intensified process that integrates hydrogen production with CO₂ capture. The material was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and N₂ physisorption (BET). CO₂ sorption efficiency of the developed materials was evaluated during 25 CO₂ sorption/regeneration cycles. The prepared sorbent–catalysts were then applied in the SESMR during 10 reaction cycles. The results showed that the bifunctional sorbent–catalyst with 20.5 wt% NiO loading presented the most suitable activity. The H₂ yield of ∼91% at the end of the 10th SESMR cycle is considerably higher than equilibrium H₂ yield that could be obtained by traditional steam methane reforming.     

Cyclic calcination/carbonation looping of dolomite modified with acetic acid for CO2 capture

The dolomite modified with acetic acid solution was proposed as a CO₂ sorbent for calcination/carbonation cycles. The carbonation conversions for modified and original dolomites in a twin fixed-bed reactor system with increasing the numbers of cycles were investigated. The carbonation temperature in the range of 630 °C–700 °C is beneficial to the carbonation reaction of modified dolomite. The carbonation conversion for modified dolomite is significantly higher than that for original sorbent at the same reaction conditions with increasing numbers of reaction cycles. The modified dolomite exhibits a carbonation conversion of 0.6 after 20 cycles, while the unmodified sorbent shows a conversion of 0.26 at the same reaction conditions, which is calcined at 920 °C and carbonated at 650 °C. At the high calcination temperature over 920 °C modified dolomite can maintain much higher conversion than unmodified sorbent. The mean grain size of CaO derived from modified dolomite is smaller than that from original sorbent with increasing numbers of reaction cycles. The calcined modified dolomite possesses greater surface area and pore volume than calcined original sorbent during the multiple cycles. The pore volume and pore area distributions for calcined modified dolomite are also superior to those for calcined unmodified sorbent during the looping cycle. The modified dolomite is proved as a new and promising type of regenerable CO₂ sorbent for industrial applications.     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

CO2 uptake of modified calcium-based sorbents in a pressurized carbonation-calcination looping

This study focuses on enhancing CO₂ uptake by modifying limestone with acetate solutions under pressurized carbonation condition. The multicycle tests were carried out in an atmospheric calcination/pressurized carbonation reactor system at different temperatures and pressures. The pore structure characteristics (BET and BJH) were measured as a supplement to the reaction studies. Compared with the raw limestone, the modified sorbent showed a great improvement in CO₂ uptake at the same reaction condition. The highest CO₂ uptake was obtained at 700 °C and 0.5 MPa, by 88.5% increase over the limestone at 0.1 MPa after 10 cycles. The structure characteristics of the sorbents on N₂ absorption and SEM confirm that compared with the modified sorbent, the effective pores of limestone are greatly driven off by sintering, which hinders the easy access of CO₂ molecules to the unreacted-active sites of CaO. The morphological and structural properties of the modified sorbent did not reveal significant differences after multiple cycles. This would explain its superior performance of CO₂ uptake under pressurized carbonation. Even after 10 cycles, the modified sorbent still achieved a CO₂ uptake of 0.88.     

Regenerable MgO-based sorbents for high-temperature CO2 removal from syngas: 1. Sorbent development, evaluation, and reaction modeling

Highly reactive and mechanically strong low-cost regenerable MgO-based sorbents were prepared by modification of dolomite which involved partial calcinations followed by impregnation with a potassium-based salt. The sorbents are capable of removing CO₂ from gasification-based processes such as Integrated Gasification Combined Cycle (IGCC). The sorbents have high reactivity and good capacity toward CO₂ absorption in the temperature range of 300-450 °C at 20 atm. and can be easily regenerated at 500 °C. The reaction appears to be first order with respect to CO₂ concentration with an activation energy of 44 kJ/mol. The reactivity and the absorption capacity of the sorbents increase with increasing temperature, as long as the partial pressure of CO₂ is above the equilibrium value for sorbent carbonation. The reactivity of the sorbents appears to improve in the presence of steam, which is likely due to the increase in the BET surface area and the porosity of the sorbent. A two-zone expanding grain model, consisting of a high-reactivity outer shell and a low-reactivity inner core is shown to provide an excellent fit to the TGA experimental data on sorbent carbonation at various operating conditions.     

Effect of the porous structure in carbon materials for CO2 capture at atmospheric and high-pressure

Activated carbons prepared from petroleum pitch and using KOH as activating agent exhibit an excellent behavior in CO₂ capture both at atmospheric (∼168 mg CO₂/g at 298 K) and high pressure (∼1500 mg CO₂/g at 298 K and 4.5 MPa). However, an exhaustive evaluation of the adsorption process shows that the optimum carbon structure, in terms of adsorption capacity, depends on the final application. Whereas narrow micropores (pores below 0.6 nm) govern the sorption behavior at 0.1 MPa, large micropores/small mesopores (pores below 2.0–3.0 nm) govern the sorption behavior at high pressure (4.5 MPa). Consequently, an optimum sorbent exhibiting a high working capacity for high pressure applications, e.g., pressure-swing adsorption units, will require a poorly-developed narrow microporous structure together with a highly-developed wide microporous and small mesoporous network. The appropriate design of the preparation conditions gives rise to carbon materials with an extremely high delivery capacity ∼1388 mg CO₂/g between 4.5 MPa and 0.1 MPa. Consequently, this study provides guidelines for the design of carbon materials with an improved ability to remove carbon dioxide from the environment at atmospheric and high pressure.     

High selectivity of TiC-CDC for CO2/N2 separation

A series of carbide-derived carbons (CDC) have been prepared starting from TiC and using different chlorine treatment temperatures (500–1200 °C). Contrary to N₂ adsorption measurements at −196 °C, CO₂ adsorption measurements at room temperature and high pressure (up to 1 MPa) together with immersion calorimetry measurements into dichloromethane suggest that the synthesized CDC exhibit a similar porous structure, in terms of narrow pore volume, independently of the temperature of the reactive extraction treatment used (samples synthesized below 1000 °C). Apparently, these carbide-derived carbons exhibit narrow constrictions were CO₂ adsorption under standard conditions (0 °C and atmospheric pressure) is kinetically restricted. The same accounts for a slightly larger molecule as N₂ at a lower adsorption temperature (−196 °C), i.e. textural parameters obtained from N₂ adsorption measurements on CDC must be underestimated. Furthermore, here we show experimentally that nitrogen exhibits an unusual behavior, poor affinity, on these carbide-derived carbons. CH₄ with a slightly larger diameter (0.39 nm) is able to partially access the inner porous structure whereas N₂, with a slightly smaller diameter (0.36 nm), does not. Consequently, these CDC can be envisaged as excellent sorbent for selective CO₂ capture in flue-gas streams.     

SO2 retention on CaO/activated carbon sorbents. Part III. Study of the retention and regeneration conditions

The retention of SO₂ on CaO/activated carbon sorbents is studied. The effect of several variables such as the reaction temperature, partial pressure of SO₂ for different calcium loads, and O₂ presence are analysed. Additionally, the regeneration and reutilization of spent sorbents is investigated. In all cases presence of well-dispersed CaO in the sorbents improves SO₂ retention in comparison with the activated carbon. In absence of O₂ in the gas mixture, the amount of SO₂ retained does not depend on the SO₂ partial pressure in the range of partial pressures studied and, as expected, SO₂ physisorption on the activated carbon support occurs at room temperature. SO₂ retention occurs in surface CaO between 100 °C and 250 °C, and in bulk CaO above 300 °C. The total calcium conversion is reached at 500 °C. Above 550 °C calcium-catalysed carbon gasification by SO₂ occurs. In presence of O₂ in the gas mixture, the studied sorbents are very effective for SO₂ removal. However, the SO₂ retention process in presence of oxygen must be carried out at temperatures lower than 300 °C to avoid carbon gasification by O₂. The thermal regeneration of the spent sorbents can be done under inert atmosphere (880 °C) with only 20% activity loss after the first regeneration cycle due to sintering and formation of CaS. No additional activity loss is detected in the subsequent cycles.     

Experimental studies and thermodynamic modeling of the carbonation of Portland cement,metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91 days and exposure to 1% (v/v) CO₂ at 20 °C/57% RH for 280 days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO₂ binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO₂ binding capacity. The limestone blend has higher CO₂ binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO₂ binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.