The study, led by Maureen Denu of the Karlsruhe Institute of Technology with colleagues from KIT, TU Munich, and Heidelberg Materials, sifted through 200 published studies and distilled 57 that met strict criteria into a single quantitative dataset. The team focused on laboratory-prepared recycled cement paste, the reactive heart of construction and demolition waste, and compared the two most prominent activation routes: heating the material to drive off water and rebuild reactive minerals, or treating it with carbon dioxide to lock the gas away and create pozzolanic gel. To make the comparison fair, the authors introduced two indices. The Activity Index measures a property such as strength or heat release against ordinary Portland cement, while the Impact Index measures the same property against unactivated recycled paste, isolating what the activation itself actually contributes.

Thermal activation is the older and better-mapped of the two techniques, with roots stretching back to work on hardened cement paste in the early 1980s. The chemistry unfolds in stages. Around 110 degrees Celsius, free and chemically bound water begin to escape from calcium silicate hydrate, ettringite, and related phases. Between 400 and 500 degrees, portlandite, or calcium hydroxide, dehydrates into reactive calcium oxide. From roughly 600 degrees onward, calcium carbonate decomposes and releases carbon dioxide. Crucially, between about 390 and 490 degrees the dehydrated silicate gel reorganizes into belite-like phases, and depending on the thermal regime these can form reactive polymorphs similar to alpha-prime C2S. Above roughly 800 degrees, however, those reactive forms collapse into the stable but sluggish beta-C2S, and rapid cooling can trigger the same loss of reactivity. The sweet spot, the analysis confirms, sits near 600 degrees.

Carbonation activation works in the opposite chemical direction. In wet carbonation, the most efficient variant, carbon dioxide is bubbled as fine bubbles through an aqueous suspension of the paste, dissolving and reacting with portlandite, calcium silicate hydrate, and aluminate phases within hours, even at ambient temperature and pressure. The products are calcium carbonate in its various forms and an amorphous alumina-silica gel. That gel is the star of the show: it has a high specific surface area and reacts with portlandite in cement to form additional binding hydrates, behaving like a fast-acting pozzolan. The calcium carbonate, while not reactive on its own, acts as a filler and nucleation surface, and can react with aluminate phases to stabilize ettringite, nudging early strength upward in a manner familiar from limestone-blended cements.

The spectroscopic fingerprints of the two routes could hardly be more different. Silicon-29 nuclear magnetic resonance spectroscopy classifies silicate units by their connectivity, from isolated Q0 species in unreacted clinker through chain-ending Q1 and middle-chain Q2 units in C-S-H gel to the branching Q3 and fully polymerized Q4 networks. In thermally activated paste, the Q1 and Q2 signals collapse toward zero as the C-S-H chains break apart, while Q0 rises dramatically, reaching an impact index of about 10 at 750 degrees, reflecting the birth of modified belite. In carbonated paste, the opposite happens: Q3 and Q4 populations soar, with a combined impact index of 16.1, marking the growth of a highly polymerized silica network as C-S-H decalcifies into gel. Thermal activation depolymerizes; carbonation polymerizes.

Microstructure tells a similar story of divergence. Heating narrows the pore distribution of the paste, concentrating pores between 50 and 300 nanometers, and raises the BET surface area, apparently because the modified C2S phase carries an intrinsic microporosity. Carbonation, by contrast, floods the material with fine calcium carbonate particles between 0.1 and 2 micrometers and generates extensive gel pores from the silica gel, pushing the BET surface area impact index to roughly 5, five times that of untreated paste, while leaving overall density essentially unchanged. Particle size responds differently too: heating at 550 to 650 degrees typically coarsens the median particle size by about half again, whereas carbonation can either refine the powder dramatically or agglomerate it, a scatter the authors flag as a genuine research gap.

When it comes to performance, both routes beat doing nothing, but neither beats Portland cement outright. Thermally activated paste releases more heat in the first hours of hydration than untreated material, with cumulative heat impact indices reaching 1.2 and up to 3.0 at favorable water-to-binder ratios, though still below pure cement. In mortar, the strength activity index peaks near 600 degrees: reported values of 0.88 and 0.86 at around 30 percent replacement approach parity with cement, and one study reports a k-value of 1.0, effectively full credit as a cement constituent. Carbonated paste delivers rapid early strength thanks to its pozzolanic gel, but the gel is consumed within about a week, after which strength gains plateau, and the strength activity index generally stays between 0.6 and 1.0. The relative impact against untreated paste is around 1.1 for carbonation versus 1.3 for thermal treatment.

The analysis is candid about the elephant in the laboratory: real demolition waste is not pure cement paste. Recycled concrete waste carries quartz, calcite, dolomite, and other largely inert aggregate phases that dilute reactivity, and its silica content can run far higher than laboratory pastes. Pilot-scale carbonation facilities already exist, including an industrial-scale enforced carbonation plant, but thermal activation has so far been validated mainly at laboratory scale, even though the underlying calcination technology is proven for calcined clays. Early life-cycle modeling offers an encouraging signal: at 10 percent substitution, thermally activated waste fines could cut carbon dioxide emissions by nearly 10 percent compared with ordinary Portland cement, outperforming limestone powder or fly ash largely because the raw material is available locally. Still, the authors stress this is a first indication, not a transferable result, and no full life-cycle assessment of carbonation activation yet exists.

The paper’s most valuable contribution may be its roadmap. The authors call for side-by-side experiments on identical starting materials, studies combining activated recycled paste with other supplementary cementitious materials such as calcined clay, systematic testing on real demolition fines rather than laboratory pastes, a larger database to capture the scatter caused by feedstock variability, comprehensive durability investigations, and full techno-economic and life-cycle assessments of both activation plants. Their bottom line is measured: thermal activation currently delivers the stronger mechanical gains, carbonation offers the unique bonus of sequestering carbon dioxide, and only standardized, comparative research will reveal which route, or which hybrid of the two, can genuinely turn the rubble of demolished cities into the low-carbon concrete of the next one.

Subject of Research: Comparative analysis of thermal and carbonation activation of recycled cement paste as a supplementary cementitious material

Article Title: Thermal and carbonation activation of recycled cement paste: A comparative data analysis and future research directions

Article References: Denu, M., Höffgen, J. P., Niewöhner, F., Zajac, M., Patel, R. A., Machner, A., & Dehn, F. (2026). Thermal and carbonation activation of recycled cement paste: A comparative data analysis and future research directions. Case Studies in Construction Materials, 25, Article e06611. https://doi.org/10.1016/j.cscm.2026.e06611

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06611

Keywords: recycled cement paste, thermal activation, carbonation activation, supplementary cementitious materials, concrete recycling, CO2 sequestration, construction and demolition waste, cement chemistry, C-S-H, belite, pozzolanic reactivity, life-cycle assessment

The scale of the problem is difficult to overstate. The construction industry contributes an estimated 33 percent of global carbon emissions, driven by energy-intensive cement production, the depletion of non-renewable aggregates, and the sheer volume of debris generated by rapid urbanization. In regions experiencing construction booms, and in areas devastated by earthquakes or severe weather, mountains of broken concrete accumulate faster than they can be absorbed. Traditional disposal through landfilling is becoming untenable, as land grows scarce and disposal costs climb. Recycling, the authors argue, is the most viable circular-economy strategy available once a concrete structure reaches the end of its service life and direct reuse or refurbishment is no longer feasible.

The technical logic of concrete waste recycling is straightforward in principle. Demolished concrete is collected, sorted and processed through crushing, screening and contaminant removal, with steel reinforcement, impurities and off-specification particle sizes separated out. The output is a family of secondary materials: recycled coarse aggregates, recycled fine aggregates and recycled concrete powder. These can partially replace natural aggregates in new concrete mixes, and the fine powder can substitute for a portion of cement in suitable proportions. The environmental payoff is twofold: waste is diverted from landfill, and the demand for virgin aggregates and cement, both of which carry heavy carbon footprints, is reduced. Empirical studies cited in the review also point to economic benefits, including revenue generation, job creation and technological development.

Yet the reality on the ground is far less circular than the theory suggests. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses, or PRISMA, methodology, the team searched the Scopus and Web of Science databases for English-language peer-reviewed publications from 2000 onwards. An initial haul of 1,642 records was progressively filtered down through language and year screening, duplicate removal and full-text assessment, ultimately yielding 65 scholarly publications for detailed analysis. These studies, published between 2004 and 2025, spanned 25 countries, with China contributing the most research (14 studies), followed by Australia (8), Malaysia (5), and the United Kingdom and Hong Kong (4 each). The geographic distribution reveals a striking imbalance: research on concrete recycling is heavily concentrated in parts of Asia, Europe and Oceania, while African and many developing-country contexts remain largely underrepresented in the literature.

From these 65 studies, the researchers identified 30 distinct barriers to concrete waste recycling, grouped into six categories: regulatory and policy, financial and economic, technical and technological, social and behavioural, market, and organizational and operational. Five barriers dominated the literature. High initial investment costs for recycling equipment and infrastructure were the most frequently reported, appearing in 14 studies. Close behind were poor quality and limited applications of recycled concrete products (12 studies), lack of comprehensive governmental regulations and guidelines (10), limited availability of advanced recycling technologies (10) and insufficient recycling facilities (10).

The quality problem deserves particular attention because it is fundamentally rooted in materials science. Recycled aggregates typically carry a layer of adhered cement mortar on their surfaces, which increases water absorption and porosity. This porous structure degrades the mechanical strength and durability of new concrete made with these aggregates, which is why recycled products are frequently restricted to lower-grade applications such as road base, pavements and drainage systems rather than structural concrete. Various treatment techniques have been developed to strip away the adhered mortar, including mechanical, thermal, chemical, microbial and supplementary cementitious material-based approaches. Among these, carbonation-based conditioning, in which recycled aggregates are exposed to carbon dioxide, has attracted growing interest because it can simultaneously improve aggregate performance and lock away carbon, contributing to emissions reduction.

Social and behavioural factors compound the technical ones. The review found that stakeholder resistance, ingrained preference for traditional practices, skepticism about recycled material quality and limited awareness of recycling benefits all slow adoption. Theoretical framing in the study draws on Diffusion of Innovation theory, which explains how new practices spread through social systems at different rates depending on perceived benefits, compatibility and complexity, and on Attitude theory, which holds that individual and collective attitudes shape behaviour. If industry leaders champion concrete recycling, the authors note, others are more likely to follow; conversely, misconceptions about recycled product quality can harden into industry-wide reluctance.

The study’s answer to these barriers is a set of 19 critical success factors organized into four domains. Legal and regulatory drivers include clear technical standards, quality certification systems, recycling targets, landfill restrictions, minimum recycled-content requirements, public procurement criteria and landfill taxes, all backed by enforcement, audits and accountability mechanisms. Economic and market drivers encompass grants, subsidies, tax reductions, competitive pricing of recycled aggregates and the strategic location of recycling centers near demolition sites to optimize transport logistics. Infrastructure and technology drivers involve government-supported land allocation for regional recycling hubs, investment in advanced processing technologies, and digital tools such as Building Information Modelling, RFID-enabled material tracking, material passports and AI-supported logistics platforms. Knowledge and education drivers include targeted professional training, public awareness campaigns, collaborative knowledge-sharing platforms, and pilot and demonstration projects that validate recycled concrete performance.

The most novel contribution of the review is its integrated conceptual framework, which links these success factors to the specific barriers they mitigate and assigns roles to the stakeholders across the concrete recycling value chain. Governments and regulatory authorities set policy direction and enforce standards. Construction and demolition enterprises practice selective demolition and waste segregation. Clients and project owners stimulate demand by writing recycled-content requirements into project briefs and tenders. Consultants and built-environment professionals influence material selection through design specifications. Recycling enterprises process the waste, concrete producers incorporate the resulting aggregates into new mixes, and educational institutions generate the evidence base and train the workforce. The public and non-governmental organizations round out the picture through awareness-building and implementation monitoring.

The authors are candid about the limitations of their work. The review drew only on Scopus and Web of Science, restricted to English-language publications from 2000 onwards, which may have narrowed the temporal, geographic and linguistic diversity of the evidence. The frequency analysis indicates how often barriers and success factors appear in the literature, not their relative severity or causal influence. Future research, they suggest, should empirically validate the framework across different regulatory and economic contexts, pursue longitudinal studies of policy instruments and market-development strategies, and pay far more attention to developing countries, where weak infrastructure and enforcement may shape recycling outcomes very differently.

Still, the message of the review is unambiguous. Concrete waste recycling is not merely a technical challenge to be solved with better crushers. It is a systemic problem requiring coordinated interventions across policy, finance, technology, markets and human behaviour. With the right mix of regulation, incentives, advanced processing and stakeholder collaboration, the rubble of yesterday’s cities could become the raw material of tomorrow’s, closing one of the largest and most stubborn material loops in the global economy.