Many solutions are being explored to reduce the greenhouse effect, especially solutions regarding carbon capture and storage. This is because reducing greenhouse gas emissions alone is no longer sufficient to meet 1.5 or even 2 °C warming targets, and it will take a significant cumulative CO₂ uptake of 200–400 gigatonnes CO₂ over the course of the century to limit global temperature increases to 2 °C or less [1]. In view of this, there is a need to develop higher-density, highly permanent forms of carbon storage, to reduce the volume required on land or sea to store the captured CO₂. Carbon-neutral concrete is one such solution.

Two forms of high-density carbon storage within concrete are biochar and calcium carbonate. Biochar, which is pyrolyzed biomass, is typically 50–93% carbon by mass [2]. The International Biochar Initiative classifies biochar as class 1 if it has a C_(organic) mass fraction ≥ 60% [3]. The energy needed to make it varies by feedstock; for example, biochar from pine requires 1.6 +/− 0.3 MJ/kg [3]. Biochar is commonly used as a soil amendment, with agronomic biochar research alone being the subject of 15,000 publications thus far [4]. However, only so much biochar can go into agricultural soils; application rates do not seem to exceed 20 tonnes/ha [5]. Furthermore, biochar can remain in soil longer (>1000 years) if its oxygen-to-carbon (O/C) molar ratio is less than 0.2. If this ratio is greater than 0.6, its half-life is <100 years [3]. Due to the dependence of biochar’s durability on its chemistry when used in soil, the use of biochar in construction materials where it is more permanently bonded can provide a much-needed alternative to its use in soils.

Similarly, calcium carbonate, known as agricultural lime, is also used as a soil amendment. Although calcium carbonate offers a lower carbon percentage by weight than biochar, it is also known to be a highly permanent form of carbon storage, barring acid attack or exposure to extremely high calcination temperatures [6]. However, as with biochar, only so much calcium carbonate can stay absorbed in agricultural soils.

Ideally, both high-density forms of carbon storage might find use in the construction industry, such as in a mixture that partially replaced the cement, sand, and aggregates used to make concrete. A target volume of such a size is needed to ensure adequate storage for the gigatonnes of CO₂ that need to be taken up to meet climate targets [1]. Concrete is already the most abundant manufactured product by mass; it outweighs all living biomass on Earth, is mostly made of aggregates, and aggregate production alone was described as increasing from 24 gigatonnes/year in 2011 to 55 gigatonnes/year in 2060 [7]. If a significant percentage of concrete could be replaced by high-density carbon storage, this would enable concrete to be carbon-neutral, offering great environmental benefits. A carbon-negative concrete has even been demonstrated, but it reveals a definite tradeoff between carbon storage content and compressive strength [8].

The application of both biochar and calcium carbonate in cement has been explored. For the application of biochar in cement, two conclusions can be drawn from the many research findings available ^{[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]}[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. First, biochar can be effectively used as an additive and as a replacement for cement in cement mortars. There, it adds a small amount of carbon storage while raising the compressive strength. Second, biochar can also be used as a sand and/or aggregate replacement in concrete where it is possible to use it in larger percentages, thus resulting in significant carbon storage and an increase in compressive strength, up to a limit. Calcium carbonate is already a permitted additive in Portland cement at up to 5% weight [25].

The interaction between both biochar and calcium carbonate with cement has not been well explored. Thus, this papentryr will provide an overview of the literature on biochar and calcium carbonate regarding carbon storage and compressive strength, examine the potential benefits of combining both materials with cement in a carbon storage system, and help elucidate new areas of research needed to identify the optimal amounts of both biochar and calcium carbonate with cement. Even if this high-density carbon storage system achieves only lower compressive strength values, there are still many uses in other areas rather than structural building materials. However, as will be seene will see later in this entrypaper, it may be possible to have high compressive strength as well as high carbon storage using optimal amounts of both biochar and calcium carbonate with cement.

2. Research Directions

The study of biochar and calcium carbonate in carbon-neutral concrete and how it can scale to meet climate goals is still a very new field worthy of further research. Of all the carbon dioxide removal (CDR) areas discussed in the 2021 critical review by Terlouw et al. ^[26][80], there was only one mention, under “other promising CDR technologies”, about the use of infrastructure to store CO₂. Timber use and concrete carbonation were both cited as examples, but not the use of carbon-negative materials such as biochar within concrete infrastructure. This ignores the finding that if less than 1% of biochar by weight of concrete were included in concrete materials, 0.5 gigatonnes of CO₂ could be realistically sequestered every year by the modified concrete, an amount equivalent to ~20% of the annual total emissions of CO₂ produced by the cement-based industries ^[27][44]. In China alone, where most of the Earth’s concrete is manufactured, Yang et al. ^[28][81] found that over 920 kg CO₂-eq could be sequestered through the conversion of 1 tonne of crop residues into biochar. Based on crop residue availability for China in 2014, the estimated annual carbon sequestration potential in China could be as high as 0.50 gigatonnes CO₂-eq from this method alone. A difference with the use of biochar in concrete is that there is no crop yield for the trapped biochar to promote. Thus, the following figure of −870 kg CO₂-eq/tonne dry feedstock from Roberts et al. ^[29][45] derived from corn stover and yard waste biochar might be more accurate since they did not incorporate the promoting effect of this biochar on crop yield. The key areas of further research within this section are as follows: (1) a thorough exploration of the material properties of biochar–calcium carbonate–cement composites and their impact on compressive strength and carbon storage, (2) creating a comprehensive model featuring the properties of individual materials within carbon-neutral concrete and their interactions, (3) examining long-term durability of carbon-neutral concretes in a variety of harsh conditions, (4) drawing upon life-cycle assessments for use in concrete mix optimization, (5) exploring enhancements to biochar’s CO₂-adsorbent ability through functionalization, and (6) developing strategies to overcome economic and social obstacles to scaling the use of carbon-neutral concrete. Additionally, Akinyemi and Adesina ^[27][44] highlight the following research efforts which will be rather useful: (1) regulating pyrolysis conditions, (2) developing biochar aggregates to make a more lightweight concrete, (3) developing a nano-biochar to enhance cement composites, (4) understanding durability of biochar–concrete composites, and (5) using biochar for accelerated carbonation given its CO₂ adsorption capability.

There are many factors that can affect the properties of composite materials such as carbon-neutral concrete, and such factors are worthy of continued exploration. According to Restuccia et al. ^[30][82], there is not currently an ideal mix design for the use of biochar, since it comes from different raw materials and production plants with different characteristics. Furthermore, the curing conditions of the cement-based specimens influence the benefits of incorporating biochar. Lastly, any treatment of the biochar particles (sieving, grinding, pre-soaking) prior to their addition to the cement/concrete can also lead to different results. This was realized in their researchtudy—the percentages of biochar addition that led to the best results for flexural strength and fracture energy were not the percentages found in previous studies, perhaps because of the different production processes for different biochars as well as different biomass source. This variation in performance can be challenging when it comes to broad adoption within ready-mix concrete where the mix is periodically changed for specific concrete jobs. Consistency is extremely important for ready-mix plants and therefore the use of standardized biochar–calcium carbonate concrete mix formulations is critical.

If carbon-neutral concrete is to be fiber-reinforced, the following studies can provide insight on the best means of doing so, as well as provide suggestions for additional research directions. Gupta, Kua, and Cynthia ^[31][38] explored using biochar as a coating on polypropylene (PP) fibers to improve the mechanical properties and permeability of mortar. The biochar was from wood saw dust pyrolyzed at 300 °C, with or without being saturated with CO₂ prior to applying it as a coating. The fresh biochar coating offered the best performance in terms of a significant improvement in compressive and flexural strength of mortar. Coating fresh biochar on PP fibers makes the surface hydrophilic rather than hydrophobic, and thus the PP fibers were more stable in the wet mix. This reduced the agglomeration of fibers, which is a factor in reducing the strength of fiber-reinforced cementitious composites. The biochar coating also roughened the PP fiber surface, improving the anchoring of fibers in the mortar matrix. CO₂-saturated biochar, on the other hand, added as a coating lowered both the 7- and 28-day compressive strength. This was due to carbonation induced by the CO₂ molecules adsorbed by the pores of the biochar particles. The CO₂ reacted with the portlandite or C-S-H, compromising the bonding of fibers. Carbonation is typically very slow, but it was observed quickly since the CO₂ source was coming from inside the mortar (i.e., the saturated biochar) rather than from the atmosphere. Furthermore, Kua et al. ^[32][83] showed that when polypropylene (PP) fibers used in reinforcement were coated with CO₂-dosed biochar, there was a 13 and 16% reduction in compressive and flexural strength. Fibers not saturated with CO₂ had an improvement of 19% for both compressive and flexural strength when compared to control samples containing PP fibers not coated with biochar. This was suggested to be due to the filler effect, strengthening the bond between the PP fiber surface and the mortar matrix. This papentryr also highlighted the potential usefulness of biochar as a carbon sink as well as a strength enhancement for cement mortar.

Further research can also be conducted on the chemical modification of carbon-neutral concrete. For example, Haque, Khan, Ashraf, and Pendse [13] describe how chemo-mechanically modified biochar (using stearic acid) was used to create a super-hydrophobic carbonaceous powder (SHCP) that could partially replace (up to 15 wt%) OPC in paste and mortar samples. This caused up to a 70% reduction in the rate of water absorption while accelerating cement hydration due to the fine particle size distribution of the SHCP. However, most chemical modifications cannot be performed without affecting one or more material properties through a number of possible interactions. The next research direction below will cover this in greater detail.

This section describes 16 reported interactions between material properties, many of which contain multiple interactions between multiple properties. If each property is represented as a node, edges can be drawn between them, one for each interaction between respective properties. The result would be a graph network that can be weighted according to the number of sources found for each interaction/edge. Such a graph network could be used as the backbone of a comprehensive model for predicting the compressive strength and carbon storage of carbon-neutral concretes.

Other modeling methods exist and can be employed alongside the above. For example, Liu et al. ^[33][84] described a pore-connectivity-changing model to predict how carbon capture ability would improve in biochar-containing cementitious composites. This model works with water-to-cement ratios of 0.25–0.4 and demonstrates improvements of biochar on carbon capture ability from 10% to 148%. A layered structure compared to an even structure also leads to a 44% increase in compressive strength and a 28% increase in three-point flexural strength when a W/C ratio of 0.25 is used. Such a layered structure may be good for improving compressive strength, but it is likely unworkable for ready-mix applications, except for possibly the 3D printing of concrete. Pre-cast applications might be able to take advantage of this. Lastly, Boumaaza et al. ^[34][85] developed ANN (artificial neural network) and RSM (response surface method) models designed to provide much useful information with the least amount of test mixtures—these have been effective in predicting flexural strength, displacement, and flexural modulus. Between the two, the ANN model outperformed RSM by an R² of 0.9980 ^[34][85]. It would be ideal to apply this or other models to the data in Table 1 of ^[35][86] to better understand what improvement in CO₂ storage might be expected when including biochar-based CO₂ adsorbents in concrete.

The 16 reported interactions between material properties which can be investigated in future research work are as follows:

Concrete Shrinkage vs. Expansion—Effects on Carbon Storage and Strength: Ye et al. ^[36][87] describe a balance between shrinkage and expansion that affects both carbonation capacity as well as strength performance of the concrete. There is typically a shrinkage during carbonation in mortars made with plain OPC. However, there is an expansion in OPC with alkali enrichment. Thus, a volume change in OPC due to carbonation can occur that is a balance of shrinkage induced by dissolution and expansion induced by crystallization.

Pyrolysis Temperature, Volatile Matter, and Carbon Storage: Mensah et al. [16] demonstrate that higher pyrolysis temperatures were found to decrease the amount of volatile matter in the pores, leaving behind the carbon. Figure 1 of [16] shows that for wood, straw, green waste, and dry algae, volatile matter content, which is at 70–80 wt% at 350 °C, drops to 20–30 wt% at 450 °C and continues to drop, but not as quickly, when 600 °C and 800 °C are reached.

Pyrolysis Temperature, Porosity, Moisture Retention, and Concrete Shrinkage: Wang et al. [22] created biochar blocks produced at 500 °C and 700 °C, and the higher-temperature one had larger pores (due to removal of volatile matter) and higher specific surface area to facilitate increased bonding due to increased matrix infiltration. However, this also increased the moisture retention ability of the mortar, which can reduce the risk of concrete shrinkage.

Cement and Water Content, Aggregate Packing/Shape, Porosity, Additives, Admixtures, Carbon Storage: Tahanpour Javadabadi and Hajmohammadian Baghban ^[37][88] elaborate on the importance of optimizing concrete mix design for the development of sustainable concrete. Important components to such a design are the amount of cement, water-to-binder ratio, aggregate packing, additives, and admixtures. As the amount of water increases, particles move more easily but too much water will lead to density stratification, with heavier/larger particles falling to the bottom. Aggregate packing is also important—if there are too few fine aggregates and too many coarse aggregates, there is a larger void fraction in the concrete mix. Aggregate shape is also important—rounded grains will slide more easily onto another, but angular grains tend to stick to each other, reducing mass movement and workability. With good aggregate packing, the amount of cement can be reduced; its use is to bind the aggregates together and fill voids, so fewer voids = less cement = less CO₂ emissions.

Compressive Strength, Cumulative Heat of Hydration, Water Content: Gupta and Kashani ^[38][89] found strong positive correlations (R² = 0.96 and 0.94 at 3-day and 7-day ages, respectively) between compressive strength development and cumulative heat of hydration (J/g of binder). This finding suggests that a higher rate of hydration in biochar–cement pastes contributes substantially to compressive strength development; however, this may only be applicable to high water–binder ratio (W/B > 0.40) cementitious composites because there, the total heat evolution is fairly consistent as W/B increases. Lower W/B composites would have a lower heat release at later ages due to self-desiccation and less space for hydration products.

Particle Size, Porosity, Pore Structure Connectivity, and CO₂ Adsorption: Liu, Xiao, Guan, Zhang, and Yao ^[33][84] describe how biochar particles offer a filler effect while coarser biochar particles allow resulting building materials to have a higher porosity and pore structure connectivity. The larger size of biochar particles, coupled with the larger pores in the larger-sized biochar, helps explain why larger biochar sizes improve CO₂ capture.

Water and Pozzolan Content; Aggregate Size/Texture/Shape/Roughness/Porosity: Mrad and Chehab ^[39][65] describe how the additional water offered by biochar over the curing process promotes pozzolanic reactions where the aggregates and cement paste meet, thus strengthening the bond between the two interfaces. Important properties governing bond strength between these interfaces are the size, texture, shape, roughness, and porosity of the aggregates.

Particle Size, Water Permeability, Hydrophobicity, Durability, Compressive Strength, and Carbon Storage: Haque, Khan, Ashraf, and Pendse [13] found that a low permeability improves the resistance of the composites against chemical attack by detrimental ions, and thus improves the durability of the composite. This was likely due to the fine particle size and the super-hydrophobic nature of the particles (SHCP) they used in their researchtudy, which served to block pores in the mortar to reduce the moisture permeability. However, the lower permeability came at the cost of performance: incorporation of SHCP caused gradual decreases in both compressive and flexural strength upon increasing SHCP addition from 2.5 to 15%. The addition of 2.5% SHCP did not have any significant effect on the mortar compressive strength and was associated with an embodied CO₂ reduction in the binder mix (OPC + SHCP) by 10%.

Pyrolysis Temperature, Surface Area, Chemical Stability, Low Flammability, and Carbon Storage: Akinyemi and Adesina ^[27][44] examine how pyrolysis temperatures affects chemical stability. Chemical stability of biochars in cementitious materials is likely enhanced using fast pyrolysis at 800 °C due to higher carbon contents, aromaticity of the feedstock, and increased surface area needed for sorption while reducing reactive oxygen and hydrogen volumes. Reducing these reactive zones leads to better chemical stability and minimizes the occurrence of destructive chemical reactions when mixed with cementitious materials. This finding points out a tradeoff for the use of biochar-containing concrete in buildings. Low flammability is desirable ^[40][90] but so are higher carbon content and higher CO₂ sequestration. However, higher temperatures are needed to deliver the higher carbon content, which also uses more fuel, which reduces CO₂ sequestration.

CO₂ Adsorption, Surface Area, Pyrolysis Temperature/Rate, Pressure, Porosity, Quantity of Biochar Produced: Gupta and Kua ^[41][91] indicate that CO₂ adsorption capability is determined by the structure of biochar, particularly its total surface area. This is affected by pyrolysis temperature, pyrolysis rate, and pressure. Temperature is important since it affects volatile release, formation of the carbon skeleton, formation of pores, and widening of pores. Pyrolysis rate and pressure govern the mass transfer of volatiles at a particular temperature. However, at higher temperatures, biochar does undergo a secondary reaction increasing the yield of gas and liquid and decreasing the actual proportion of biochar. For example, in pine undergoing slow pyrolysis at 300 °C a 58% char yield is possible. At 450 °C under the same conditions, the char yield is 26%. Corn stover biochar made at 500 °C has a char yield of only 16.80.

Pyrolysis Temperature, Surface Area, Porosity: Gupta and Kua ^[41][91] found that too-high a pyrolysis temperature can cause a loss of structural complexity—this happens in a pronounced manner when the pyrolysis temperature is the same as the ash melting point of the feedstock. For pine, a decrease in biochar surface area was found at a pyrolysis temperature of 1000 °C. This is likely due to pore widening/coalescence with neighboring pores, and perhaps softening and melting.

Pyrolysis Temperature/Rate, Carbon Storage, Quantity of Biochar Produced: Gupta and Kua ^[41][91] also describe how the pyrolysis process not only affects the carbon content of different biochars but also the net amount of solid char produced instead of liquid or gaseous byproducts. In fast pyrolysis, the heating rate ranges from 100 to 1000 °C/s, causing thermal cracking, but produces about 15–25% biochar, with the rest being liquids and gases. In slow pyrolysis, there is a low heating rate (10 °C/min) between 300 and 700 °C. This produces more char, but the longer vapor residence time and lower heating rate provide an improved environment for secondary reactions. Fast pyrolysis biochars have lower carbon content and higher oxygen content compared to those produced by slow pyrolysis.

Low Flammability, Pyrolysis Temperature/Rate: Zhao, Enders, and Lehmann ^[40][90] describe how, although biochar does not qualify as flammable according to UN criteria, it can support a propagating combustion front of about 200 mm, which is important when it comes to the use of biochar-containing concrete in buildings, The presence of a propagating combustion front was much more likely for biochars made using fast pyrolysis (5 of 7 samples) than those made with slow pyrolysis (5 of 24 samples). More short-term flammability was also observed in biochar produced at higher pyrolysis temperatures compared with those produced at lower temperatures, but this short-term flammability reduced to negligible levels within hours, likely due to the removal of free radicals through reaction with air and the reordering of the carbon structure. Both of these reduce flammability.

Hydration, Porosity, Compressive and Flexural Strength: An advantage of biochar-containing concrete is the ability to use internal curing, as described by Mrad and Chehab ^[39][65]. Internal curing consists of supplying a well-dispersed, water-saturated material throughout the hydrating Portland cement paste to increase the degree of hydration over time. This is as opposed to conventional curing, where the surface of the concrete is wetted after it is placed, allowing water to penetrate only a few millimeters down. High-porosity aggregates are most favorable for internal curing, and internal curing improves compressive and flexural strength of mortar, particularly at later stages. This occurs because the additional water offsets the empty matrix pore spaces created during the shrinkage that takes place at the early stages of hydration.

Heat of Hydration, Workability, Setting Time, Strength, Shrinkage, Permeability, Chemical Resistance, Serviceability, Sustainability: Al-Mansour, Chow, Feo, Penna, and Lau ^[42][34] also discuss ternary vs. binary concrete systems. In a ternary system, the goal is to obtain the most benefits out of each material and overcome the shortfalls of each material. Properties of concern are properties while fresh (workability, setting time, heat of hydration), when hardened (strength, shrinkage, permeability, resistance to sulfate attack), serviceability, and sustainability. The papentryr described some ternary systems with OPC and observations of how properties varied in those systems.

Carbon Storage, Hydration, and Compressive Strength: Gupta, Kashani, Mahmood, and Han [12] concluded that adding biochar to cement mortar has a positive influence on rapid carbonation and subsequent carbon sequestration. There is an initial loss in compressive strength after 7 days due to higher initial moisture loss from carbonation, but after 28 days, increased carbonate mineralization and a reduced depth of carbonation lead to a 24% improvement in compressive strength.

There is already a relatively good understanding of the short-term (<180 days) mechanical performance of biochar-containing concretes, but there is very little literature on how these will perform when exposed to harsh environments with high salinity, high alkalinity, freeze–thaw conditions, high temperature, or high sulfates. Thus, Tan, Wang, Zhou, and Qin [21] have pointed out the need for additional studies examining the long-term durability of biochar-containing concretes. For example, any CO₂ captured in biochar could cause later concrete carbonation, which could cause corrosion problems for reinforced concrete. This and other interactions between biochar and other concrete admixtures also remain unknown.

In contrast to the potential detrimental effects of biochar on reinforced concrete in the long term, Gupta and Kua ^[43][92] found that sorptivity was also reduced by about 70% after 28 days, demonstrating how the addition of biochar can play a key role in promoting the durability and strength of concrete infrastructure. Furthermore, there are many undeniable short-term benefits: similar to many other papers, the authors report how up to a 4% cement replacement by biochar can yield a slight compressive strength improvement due to the fine particle size and microfiller effect of biochar. When both the short- and long-term benefits of biochar-containing concrete are examined, which include reduced vulnerability to damage and fewer repairs over its lifespan, it becomes clear how such concrete can promote the economic and environmental sustainability of buildings.

The incorporation of other materials besides biochar can also affect long-term durability. Rostami et al. ^[44][93] describe how shrinkage in fiber-reinforced mortars (and thus susceptibility to crack formation) can be counteracted by the use of superabsorbent polymers. A reduction in plastic shrinkage by 30–75% and a reduction in autogenous shrinkage by 30–124% occurred when superabsorbent polymers (SAP) were added. Furthermore, de Souza et al. ^[45][94] examined the effects on durability of incorporating graphene, a form of carbon-like biochar, into concrete. Graphene-based nanosheets (GNS) include graphene, graphene oxide, reduced graphene oxide, and graphene nanoplatelets. A very small addition of GNS (0.01–0.05% of the weight of the Portland cement) can impart >80% increases in compressive/tensile strength and >500% improvements in water-penetration resistance to the cementitious material. Though the total cost of graphene oxide-reinforced concrete might be ~2–7% higher than a reference concrete mix, the compressive strength/cost per m³ was increased by 25–40%. The enhanced durability must also be factored in since this will lengthen the service life of the concrete, ultimately lowering costs over that service life.

The wider use of LCA may very well drive improvements in the way embodied energy and carbon are defined, measured, and ultimately used when revising architectural codes and standards for carbon-neutral concrete. Santos et al. ^[46][95] describe how LCA is crucial for optimizing mortars, which have many applications in modern construction and that are used throughout the service life of buildings. Using improved mortars can significantly reduce the embodied energy and embodied carbon in buildings. LCA aids such optimization by allowing users to research and select the best materials for their mortar mix, whether based on embodied energy, embodied carbon, or other environmental data. For example, Table 1 of Adesina ^[47][74] provides the embodied carbon for concrete constituents and indicates that, by far, the constituent with the highest embodied carbon is Portland cement at 0.83 kg CO₂/kg. Through the use of LCA software with such data, a user can design a carbon-neutral mix.

There are many software programs to aid in the performance of LCA; these will not be reviewed here except to mention how extensive their capabilities and data extents can be. An example of what goes into developing such a program is offered by Kim et al. ^[48][96], which discusses the development of a software program to assess GHG emissions incurred over the life cycle of a concrete product. Data collection, maintenance, and the frequency of updates are key to ensuring accurate LCA results.

System scope is also critical for performing an accurate and meaningful LCA, as demonstrated by the following example. Feiz et al. ^[49][97] examined the CO₂ performance of different ways of producing cement, in cooperation with the cement-producing company CEMEX. They found that cement products containing a large proportion of byproducts (such as GGBS from the iron/steel industry) had the lowest unit emissions of CO₂-eq. However, the authors also mentioned that the LCA results did not include any allocation of the impact from the iron/steel industry via the GGBS to the cement products.

Biochar-based CO₂ adsorbents can improve carbon storage and minimize the amount of biochar needed to achieve carbon neutrality, which may be advantageous from a cost perspective depending on the balance of other ingredients within the concrete mix. There are four ways of activating biochar for CO₂ adsorption, as reported in ^[50][78]: (1) physical, (2) chemical, (3) surface functionalization, and (4) heteroatom doping and metal/metal oxide impregnation. These can either be used alone or in combination. Examples of achievable increases in CO₂ adsorption after activation range from 1.9 to 4.4 mmol/g at 25 °C and 1 bar, with the maximum being 6.78 mmol/g at 30 °C and 1 bar for biochar impregnated with copper oxide ^[50][78].

Given the wide variety of biochar feedstocks, pyrolysis temperatures, surface areas, and CO₂ adsorption capacities possible, it is desirable to improve the understanding of how biochar-based adsorbents could be used for CO₂ capture. This was addressed by ^[35][86], which also indicated a good performance measure for CO₂ adsorption: the oxygen/carbon ratio (O/C). Both low H/C and O/C ratios (≤0.2) suggest a high amount of aromaticity and fixed carbon, which are chemically stable. White oak biochar had a very low O/C of 0.051, which was associated with high hydrophobicity, low polarity, and enhanced CO₂ capturing capability of biochar. Hydrophobicity and non-polar characteristics were also suggested as facilitators of improved CO₂ adsorption capacity due to lowering the competition of H₂O molecules for the CO₂ adsorbing sites ^[35][86].

Another strong determinant of CO₂ adsorption capacity appears to be the closeness in biochar pore size to the kinetic diameter of CO₂ molecules. There are many routes to achieving this desired pore size, and a fruitful research direction would be to enumerate these and determine how these may interact. For example, Gupta and Kua ^[41][91] describe how biochar can be treated with potassium or sodium hydroxide to create a very high surface area useful for enhancing CO₂ adsorption. They also describe how treating biochar with low oxygen at 3–5% at a temperature range of 550–650 °C can produce micropores conducive for CO₂ capture under ambient conditions. The selectivity of those micropores is yet another factor for improving CO₂ adsorption. To make biochar select for CO₂ more than water vapor or nitrogen, the isosteric heat of adsorption for CO₂ must be much higher than that of N₂. A narrow pore distribution with diameters closer to the molecular diameter of CO₂ will do this. A doubling of the isosteric heat of adsorption for CO₂ over that of N₂ will make CO₂ adsorption dominant at room temperature and pressure. A CO₂ adsorption of 4.80 mmol/g at 25 °C was recorded for a KOH-to-biochar ratio of 2, yielding many narrow micropores (<1 nm diameter) at an activation temperature of 600 °C. During that 2-minute adsorption time, N₂ adsorption was only 0.89 mmol/g, much lower than the CO₂ adsorption, which implies high CO₂ selectivity.

CO₂ adsorption can decrease at higher activation temperatures, likely due to reduced pore filling, as found by Gupta and Kua ^[41][91]. Although higher temperatures yield higher surface areas, it may also produce pores that are larger than optimal. The kinetic diameter of CO₂ molecules is 0.33 nm and CO₂ adsorption by pore filling is reduced when pore diameters are much larger or smaller than this. With bamboo-derived biochar, a CO₂ adsorption of 7 mmol/g was measured. Additionally, four reuse cycles were performed at 25 °C with no regeneration loss. Further research might be performed on the effects of activation temperature and pore size on CO₂ adsorption.

Reaching consensus on guidelines for CO₂ storage within concrete is important to promote the spread of carbon-neutral concrete. Habert, Miller, John, Provis, Favier, Horvath, and Scrivener ^[51][77] provide examples of such benchmarks from Europe that can help scale-up the use of concrete CO₂ storage. These include the following. For cement producers: tonnes CO₂/tonnes clinker < 0.7. For concrete producers: <3.5 kg clinker/m³/MPa for a standard 30–50 MPa concrete mix. For engineering offices designing concrete structures: <250 kg CO₂/m² floor area for the concrete allocated to the structure. For construction companies: <500 kg CO₂/m² floor area for the whole building.

Other future directions have been described by Adesina ^[47][74]: (1) a high use of alkali-activated binders in developing countries due to abundant aluminosilicate precursors at those locations; (2) the use of more demolition and construction wastes in concrete to create a more circular economy; (3) standards development supporting the use of waste materials in concrete; (4) designing new concrete mixtures that use low cement; (5) significant use of alternative fuels such as biomass to offset the heavy use of coal and pet coke at present; (6) developing a carbon-neutral concrete construction process involving such techniques as CO₂ curing; (7) CO₂ management; (8) stricter policies regarding carbon footprints (environmental taxes, approval delays); and (9) incentives for green concrete use (grants, tax rebates, lower development fees).

Finally, since there is much more ocean on Earth than land, coastal and oceanic applications for carbon-neutral concrete are much more plentiful and thus worthy of further investigation. Pradhan, Poh, and Qian ^[52][40] suggest an important point if such concrete is to be used for seawater applications: the salinity of seawater is approximately 3.5%, translating into a 0.6 M NaCl solution. Thus, chloride tests should be conducted using similar concentrations of NaCl to determine the effects on long-term durability of carbon-neutral concrete used at sea.