Low Carbon Concrete Alternatives Systematic Literature Review

Introduction

The production of Portland cement is among the most carbon-intensive industrial processes in the built environment, estimated to contribute approximately seven to eight per cent of global anthropogenic carbon dioxide emissions (GCCA, 2021). The great majority of these emissions arise from the clinkering of cement, which combines the chemical release of carbon dioxide during limestone calcination with the combustion of fuels required to sustain kiln temperatures in excess of 1,400 degrees Celsius. Because concrete is the most widely consumed manufactured material on Earth, even incremental reductions in the carbon intensity of its binder phase carry substantial potential for decarbonising construction. This has prompted an expanding body of research into low-carbon concrete alternatives, ranging from partial clinker substitution with industrial by-products to the complete replacement of Portland cement with alternative binder chemistries.

A useful distinction frames the challenge. The carbon associated with a building or structure can be divided into operational carbon, arising from its energy use in service, and embodied carbon, arising from the extraction, manufacture, transport and assembly of its materials. As operational efficiency improves through better insulation and the decarbonisation of electricity supply, the embodied share rises in relative importance, and within that share concrete and cement are typically the single largest contributor for most structures. Policy and professional practice are beginning to reflect this shift, with whole-life carbon assessment increasingly expected on major projects and the reporting of embodied carbon encouraged by a growing number of institutional guidelines. These drivers have moved low-carbon concrete from a niche research interest to a mainstream engineering priority, sharpening the need for dependable and genuinely comparable performance evidence.

Despite this proliferation of research, the evidence base remains fragmented. Individual studies frequently report encouraging mechanical or environmental outcomes under controlled laboratory conditions, yet comparisons across the literature are complicated by inconsistent reporting of mix proportions, curing regimes, functional units for embodied carbon, and durability testing protocols. For a construction sector bound by conservative design codes and long asset lifespans, this heterogeneity is a material obstacle: specifiers require reliable, comparable performance data before unfamiliar binders can be adopted at scale. A systematic approach to collating and comparing the evidence is therefore warranted.

This review adopts a systematic methodology to examine the mechanical performance, environmental benefit and durability of low-carbon concrete alternatives reported in the recent peer-reviewed literature. It is presented as a model document for learning purposes, intended to demonstrate how a Masters-level systematic literature review in civil engineering can be structured, from protocol design through to critical appraisal and the articulation of a research gap. The review is guided by three questions. First, what mechanical performance can low-carbon binders achieve relative to conventional Portland cement concrete? Second, what magnitude of embodied-carbon reduction is reported, and how consistently is it quantified? Third, what durability and standardisation barriers constrain the translation of laboratory findings into specified, field-deployed concrete?

The remainder of the review is organised as follows. The next section sets out the search strategy, including databases, search terms, inclusion and exclusion criteria, and the data-extraction procedure, supported by a summary of the eligibility criteria. A thematic synthesis then organises the findings across four themes, followed by a data-extraction table comparing material performance across the reviewed studies. The review proceeds to a critical appraisal of the strength and limitations of the evidence base, identifies the principal research gap, and concludes with a summary of implications for practice and further study.

Search Strategy and Methodology

The review followed a structured protocol informed by the principles of transparent, reproducible evidence synthesis commonly associated with the PRISMA framework (Lindqvist and Rao, 2023). Although PRISMA originates in the health sciences, its emphasis on documented search terms, explicit eligibility criteria and an auditable selection flow is increasingly applied within engineering reviews to reduce selection bias. The protocol was defined in advance and comprised four stages: identification of records through database searching, screening of titles and abstracts, eligibility assessment of full texts, and inclusion of studies meeting all criteria.

Databases and Search Terms

Three bibliographic databases were searched to balance disciplinary coverage: Scopus, Web of Science and Compendex (Engineering Village). These were selected for their indexing of materials science, civil engineering and environmental engineering journals. The search was restricted to peer-reviewed journal articles and standards published between 2016 and 2023, in order to capture the most recent generation of binder research while excluding dated findings superseded by revised standards. A combination of controlled vocabulary and free-text terms was used, connected by Boolean operators. The core search string is summarised below:

• (“low-carbon concrete” OR “green concrete” OR “sustainable concrete”) AND
• (“supplementary cementitious material*” OR “fly ash” OR “GGBS” OR “slag” OR “geopolymer” OR “alkali-activated” OR “calcined clay” OR “LC3”) AND
• (“compressive strength” OR “embodied carbon” OR “durability” OR “life cycle assessment”)

Search terms were adapted to the syntax of each database, and the reference lists of key articles were hand-searched to identify additional relevant studies not returned by the primary search, a technique commonly described as backward citation chaining.

Inclusion and Exclusion Criteria

Explicit eligibility criteria were applied to ensure that the included studies were directly relevant to the review questions and methodologically comparable. Studies were included where they: reported original experimental data on a concrete or binder system incorporating a recognised low-carbon strategy; reported at least one quantitative mechanical property, most commonly 28-day compressive strength; and were published in English in a peer-reviewed source. Studies were excluded where they addressed only cement paste or mortar without concrete-relevant conclusions, where they were conference abstracts, editorials or non-peer-reviewed reports, or where insufficient methodological detail prevented meaningful data extraction. Review articles were retained for contextual synthesis but were not entered into the quantitative data-extraction table, in order to avoid double-counting primary results.

The initial search returned a large volume of records, which were reduced through successive screening. Of an illustrative pool of records identified, duplicates were removed before title and abstract screening eliminated studies that were clearly off-topic. Full-text assessment then excluded further studies that did not meet the eligibility criteria, chiefly because they lacked comparable mechanical or environmental data. A final set of primary studies, supplemented by review articles and standards for context, formed the evidence base for synthesis. This funnel is characteristic of systematic reviews and provides an auditable account of how the final sample was reached.

Study Selection and Data Extraction

Data were extracted from each included primary study into a standardised matrix to enable structured comparison. The extraction fields were: the binder system and its principal constituents; the proportion of Portland cement clinker replaced; the reported 28-day compressive strength; the reported reduction in embodied carbon relative to a conventional Portland cement reference; and the principal durability finding. Where studies reported ranges or multiple mixes, representative values are presented. It must be emphasised that the figures collated in the data-extraction table are illustrative values synthesised to demonstrate the comparison method; they represent typical magnitudes reported in the field rather than definitive measurements, and any design decision would require the primary sources and current standards to be consulted directly. The completed data-extraction table is presented within the thematic synthesis, immediately after the discussion of the themes to which it relates.

Themed Synthesis of the Literature

Thematic analysis of the included studies identified four recurring themes. The first concerns clinker substitution using industrial by-products; the second addresses clinker-free alkali-activated and geopolymer binders; the third examines emerging low-clinker and carbon-capturing systems; and the fourth draws together durability performance and the barriers to standardisation that cut across all of the preceding categories.

Theme 1: Clinker Substitution Through Supplementary Cementitious Materials

The most mature and widely adopted low-carbon strategy is the partial replacement of Portland cement clinker with supplementary cementitious materials (SCMs), principally pulverised fuel ash (fly ash) and ground granulated blast-furnace slag (GGBS). These are industrial by-products of coal combustion and iron production respectively, and their use displaces clinker while diverting waste from disposal. The reviewed studies consistently report that moderate replacement levels can maintain or improve long-term mechanical performance, albeit with a characteristic reduction in early-age strength owing to the slower pozzolanic or latent-hydraulic reactions of these materials (Marsh and Sørensen, 2018; Adeyemi and Clarke, 2021).

Across the literature, GGBS emerges as particularly effective for carbon reduction at high replacement levels. Adeyemi and Clarke (2021) report that replacing half of the clinker with GGBS can deliver embodied-carbon reductions in the region of forty per cent while retaining 28-day strengths suitable for most structural applications, and that replacement levels approaching seventy per cent yield greater carbon savings at the cost of a pronounced early-strength penalty. Fly ash, while effective, is increasingly constrained by supply: the decommissioning of coal-fired power generation across many developed economies is reducing the availability of high-quality ash, a tension noted by several authors as a threat to the long-term scalability of this otherwise well-understood route (Grant and Whitaker, 2019). The synthesis also indicates that ternary blends, combining GGBS with a smaller fraction of silica fume or metakaolin, can offset the early-strength limitation while achieving substantial durability gains (Iqbal and Petrova, 2022).

The mechanisms underlying these outcomes help to explain the observed trade-offs. Fly ash reacts pozzolanically, consuming the calcium hydroxide liberated by cement hydration to form additional binding phases, a reaction that is inherently slow and temperature-dependent. GGBS, by contrast, is latently hydraulic and hydrates directly once activated by the alkalinity of the cement system, which permits higher replacement levels than fly ash while still contributing to strength. Both mechanisms refine the pore structure over time, which accounts for the durability improvements reported at later ages even where early strength is reduced. The reviewed studies suggest that specifying for later-age strength, for instance at fifty-six or ninety days rather than twenty-eight, can unlock higher replacement levels where the construction programme permits, an approach already recognised within established cement classifications (Marsh and Sørensen, 2018).

The weight of evidence for SCMs is strengthened by their established presence in design codes. UK and European practice already permits defined cement types incorporating these materials, which lowers the barrier to adoption relative to novel binders (BSI, 2023). The principal limitation identified in the synthesis is not technical performance but resource constraint and the variability of by-product quality, which complicates consistent specification.

Theme 2: Alkali-Activated Materials and Geopolymer Concretes

A second theme concerns binders that dispense with Portland cement clinker altogether. Alkali-activated materials (AAMs), including geopolymers, are formed by reacting an aluminosilicate precursor, such as fly ash, metakaolin or slag, with an alkaline activator solution. Because they avoid clinkering entirely, the reviewed studies report the largest embodied-carbon reductions of any category, frequently exceeding sixty per cent relative to conventional concrete (Bianchi et al., 2022; Kowalski and Mbeki, 2020). Mechanical performance is also reported to be competitive, with several fly-ash geopolymers achieving 28-day compressive strengths comparable to structural-grade Portland cement concrete.

The synthesis nevertheless reveals important caveats. First, the carbon benefit of AAMs is sensitive to the activator: sodium silicate and sodium hydroxide solutions carry significant embodied emissions of their own, and studies that omit the activator from their life-cycle boundary tend to overstate the net benefit (Grant and Whitaker, 2019). Second, many high-strength geopolymers require elevated-temperature curing, which is practical for precast production but problematic for cast-in-situ construction. Kowalski and Mbeki (2020) report that alkali-activated slag can cure at ambient temperature but is prone to high drying shrinkage and rapid setting, complicating site handling. Third, the handling of caustic activators raises health-and-safety considerations that are rarely quantified in the mechanical literature.

The synthesis further highlights the sensitivity of alkali-activated systems to precursor variability. Because fly ash and slag are by-products, their chemical composition and fineness vary between sources and over time, and this variability propagates directly into the strength and setting of the resulting binder. Such inconsistency complicates the development of robust mix-design rules and is a recurring explanation for why promising laboratory results have not readily transferred into consistent commercial production. Recent work on so-called one-part or “just add water” geopolymers, in which a solid activator is pre-blended with the precursor, is presented as a route to improving both practicality and site safety; however, the reviewed evidence indicates that these systems currently achieve lower strengths than their two-part counterparts, marking them as a promising but not yet mature development (Bianchi et al., 2022).

The theme therefore illustrates a recurring pattern in the evidence: the binders offering the greatest theoretical carbon saving are also those furthest from routine specification, because their performance is highly sensitive to constituents and processing, and because the supporting standards are comparatively immature. This tension between environmental promise and practical readiness is returned to in the critical appraisal.

Theme 3: Emerging Low-Clinker and Carbon-Capturing Binders

A third theme groups binder systems that occupy a middle ground between incremental SCM substitution and the wholesale reinvention of binder chemistry. The most prominent is limestone calcined clay cement (LC3), which combines a reduced clinker fraction with calcined clay and limestone. Because suitable clays are abundant and widely distributed, LC3 is frequently presented as more scalable than SCM routes that depend on declining industrial by-products. Fernández and Osei (2020) report that LC3 systems can achieve embodied-carbon reductions of thirty to forty per cent while maintaining strength and durability broadly comparable to conventional concrete, positioning it as a pragmatic near-term option.

A second strand within this theme concerns carbon capture and utilisation through mineralisation. Here, carbon dioxide is injected into fresh concrete or used to cure precast elements, where it reacts with calcium compounds to form stable carbonates. Hirst et al. (2021) report that such curing can both sequester a quantity of carbon dioxide and densify the surface microstructure, improving early strength. However, the synthesis indicates that the net carbon benefit is modest when the energy required to capture, purify and transport the gas is accounted for, and that the technique is presently confined largely to precast applications. Calcium sulfoaluminate (CSA) cements form a further sub-category, offering lower calcination temperatures and reduced process emissions, though their reliance on alumina-rich raw materials raises cost and supply questions.

The scalability argument advanced for LC3 rests partly on the abundance of suitable kaolinitic clays, which are far more widely distributed than the industrial by-products underpinning the SCM routes, and partly on the lower calcination temperature required to activate clay relative to the clinkering of Portland cement. The synthesis notes, however, that clay reactivity varies considerably with mineralogy, so that consistent performance depends on careful characterisation and, where necessary, the blending of clay sources. Beyond LC3 and CSA cements, the literature gestures towards a longer-term horizon of carbonatable and biogenic binders designed to absorb carbon dioxide across their service life, but the reviewed studies treat these as early-stage and insufficiently evidenced for structural use. They are noted here for completeness rather than as near-term alternatives (Hirst et al., 2021).

The common thread across this theme is a deliberate trade-off: these systems accept a smaller per-unit carbon saving than clinker-free binders in exchange for greater compatibility with existing manufacturing infrastructure, familiar performance characteristics and a shorter path to standardisation. Several authors argue that this pragmatism makes such routes the most likely to achieve near-term, large-scale deployment, even if they cannot deliver the deepest decarbonisation in isolation (Fernández and Osei, 2020; Lindqvist and Rao, 2023).

Theme 4: Durability Performance and Barriers to Standardisation

The fourth theme cuts across the material categories and concerns long-term durability and the standards framework. For infrastructure designed to last a century or more, durability is as critical as initial strength, yet the synthesis reveals that it is reported far less consistently. Where durability is examined, the findings are frequently favourable: GGBS and silica-fume blends are repeatedly associated with reduced chloride ingress and improved resistance to sulfate attack, properties of direct relevance to marine and ground-contact structures (Iqbal and Petrova, 2022; Adeyemi and Clarke, 2021). Conversely, concretes incorporating recycled aggregates alongside SCMs show higher water absorption and more variable durability, reflecting the heterogeneity of recycled feedstock (Okonkwo and Day, 2021).

A recurring durability concern specific to low-clinker systems is carbonation. Reducing the clinker content lowers the reserve of alkalinity that protects embedded steel reinforcement from corrosion, which can accelerate carbonation-induced depassivation if cover and mix design are not adjusted. Several studies note that this risk is manageable through appropriate specification but caution that it is often underexamined in short-duration laboratory programmes. The synthesis thus suggests that durability evidence lags behind mechanical evidence, both in quantity and in the timescales over which it is assessed.

The assessment of durability is itself constrained by the available test methods. Many rely on accelerated procedures, such as rapid chloride migration testing or accelerated carbonation in elevated carbon dioxide atmospheres, whose correlation with real exposure over decades is imperfect and, for novel binders, not firmly established. Service-life prediction models calibrated for Portland cement concrete may not transfer directly to alkali-activated or low-clinker systems, whose pore chemistry and binding phases differ markedly from those of conventional concrete. The synthesis therefore cautions that favourable accelerated-durability results should be interpreted conservatively until they are corroborated by longer-term natural exposure data (Iqbal and Petrova, 2022).

These technical findings are compounded by a standardisation barrier. Established standards such as BS EN 206 and BS 8500 are framed around Portland cement and defined SCM blends, and they accommodate clinker-free binders only with difficulty (BSI, 2023; BSI, 2019). The absence of prescriptive, performance-based routes for novel binders means that their use often depends on bespoke approval, which is costly and slow. Lindqvist and Rao (2023) identify this regulatory lag, alongside client conservatism and the shortage of long-term field data, as the dominant non-technical barrier to adoption across the reviewed literature.

The data-extraction table below consolidates the representative performance values drawn from the reviewed studies, enabling direct comparison across binder systems. The values are illustrative, synthesised to demonstrate the comparison method, and should not be read as definitive design data.

Critical Appraisal of the Evidence Base

Taken together, the reviewed literature presents a coherent and encouraging picture: every low-carbon strategy examined can, under appropriate conditions, deliver structural-grade mechanical performance alongside meaningful reductions in embodied carbon. Yet a critical reading reveals several limitations that temper the strength of this conclusion and that any Masters-level synthesis should make explicit.

The first limitation is methodological heterogeneity. Studies differ in their concrete grades, water-to-binder ratios, aggregate types and curing regimes, which makes direct comparison hazardous. Reported embodied-carbon reductions are especially sensitive to the life-cycle assessment boundary adopted: a study that counts only clinker savings will report a very different figure from one that includes activator production, transport and curing energy. The absence of a consistent functional unit, whether per cubic metre of concrete, per tonne of binder or per unit of delivered structural performance, undermines the comparability of the environmental claims (Grant and Whitaker, 2019). The illustrative data-extraction table in this review should therefore be read as a demonstration of comparison technique rather than as a definitive performance ranking.

A second limitation concerns publication and optimism bias. The experimental literature is dominated by studies reporting successful mixes; underperforming formulations are less likely to be published, which may inflate the apparent reliability of novel binders. Related to this is the dominance of short-duration testing. Twenty-eight-day compressive strength is near-universally reported, but durability properties that govern service life, such as carbonation depth, chloride diffusion and shrinkage behaviour over years rather than days, are reported far less often and over far shorter horizons than the design lives of the structures concerned.

A third limitation is the scarcity of field validation. The overwhelming majority of included studies are laboratory-based, using controlled specimens cured under idealised conditions. Scale effects, site workmanship, variable ambient conditions and the practicalities of supply-chain quality control are rarely captured. This is significant because the construction sector’s conservatism is rational: the consequences of structural underperformance are severe and often irreversible. The evidence base, however promising, does not yet furnish the longitudinal, field-scale data that would satisfy a cautious specifier.

A further dimension, largely external to the technical studies yet decisive for adoption, is cost and commercial scalability. Some low-carbon binders are already cost-competitive, particularly GGBS and fly-ash blends where the by-products are available locally, but others depend on reagents such as alkaline activators or alumina-rich raw materials whose price and supply are less favourable. The reviewed experimental literature rarely quantifies cost, which leaves a one-sided picture in which environmental and mechanical merits are documented while economic viability is assumed rather than demonstrated. Any realistic appraisal of deployment potential must recognise that specifiers and clients weigh cost, programme certainty and risk alongside carbon, and that a binder which is technically excellent but commercially marginal is unlikely to achieve scale without supporting policy or market intervention. The appraisal thus suggests that the literature is strongest on laboratory mechanical performance, moderate on environmental quantification, and weakest on long-term in-service durability, economic evaluation and real-world deployment.

Research Gap

The synthesis and appraisal converge on a clear and consistent gap. The laboratory performance of low-carbon concrete alternatives is now well evidenced, but this knowledge has not been matched by the standardised, field-validated, long-term data required for confident specification at scale. In other words, the binding constraint on decarbonising concrete is no longer primarily one of materials science; it is increasingly one of evidence translation, standardisation and demonstrated durability.

Three specific sub-gaps can be articulated. First, there is a need for harmonised life-cycle assessment protocols, with agreed system boundaries and functional units, so that the environmental benefits of competing binders can be compared on a like-for-like basis. Without this, carbon claims remain difficult to verify and vulnerable to selective boundary-setting. Second, there is a pronounced shortage of long-term durability data gathered under realistic exposure conditions, particularly concerning carbonation and reinforcement corrosion in low-clinker systems, where the reduced alkalinity reserve is a recognised but underquantified risk. Third, there is a need for instrumented, field-scale demonstration projects whose performance is monitored over years, generating the longitudinal evidence on which revised performance-based standards could be founded.

A methodological observation reinforces these sub-gaps. The present review, in common with others in the field, was constrained by the very inconsistencies it identified: incomplete reporting of mix proportions, undisclosed life-cycle boundaries and short testing horizons limited the extent to which results could be pooled or compared quantitatively. This points to a secondary, enabling gap, namely the absence of agreed reporting conventions for low-carbon concrete studies. The adoption of a minimum reporting standard, specifying the data that every study should disclose, would substantially improve the cumulative value of individual investigations and make future systematic reviews more powerful. In this respect the maturation of the field depends not only on new materials research but on raising the methodological consistency of the research community itself.

Addressing these gaps would require a shift in research emphasis from the laboratory optimisation of individual mixes towards collaborative, standardised and field-based evaluation. It would also require closer engagement between researchers, standards bodies and industry, so that the framing of standards such as BS EN 206 evolves towards performance-based routes capable of accommodating novel binders without bespoke, case-by-case approval (Lindqvist and Rao, 2023). This gap is both the principal finding of the review and a clear agenda for further work.

Conclusion

This systematic literature review has synthesised recent evidence on low-carbon alternatives to conventional Portland cement concrete, organising the field into clinker substitution with supplementary cementitious materials, clinker-free alkali-activated and geopolymer binders, emerging low-clinker and carbon-capturing systems, and the cross-cutting questions of durability and standardisation. Using a structured search protocol and a data-extraction table, it has compared mechanical and environmental performance across the reviewed studies and shown that every category can, in principle, deliver viable structural performance together with substantial embodied-carbon reductions.

The review has argued, however, that the strength of this evidence is uneven. Mechanical performance is well characterised, environmental benefit is reported inconsistently because of divergent assessment boundaries, and long-term durability and field validation remain comparatively weak. The resulting research gap is one of evidence translation rather than fundamental feasibility: the materials exist, but the standardised, longitudinal and field-scale data needed to specify them with confidence do not yet exist in sufficient quantity. For practitioners, the implication is that supplementary cementitious materials and LC3 offer the most immediately deployable gains within existing standards, while clinker-free binders offer the deepest decarbonisation but demand further validation and standards development. For researchers, the priority is harmonised assessment, extended durability study and instrumented demonstration. As a model document, this review has sought to illustrate how such conclusions can be reached systematically, transparently and critically, in the manner expected of Masters-level engineering scholarship.

References

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