The construction industry worldwide is heavily dependent on concrete, which is the second most used material in the world after water. Unfortunately, concrete is characterized by a very high CO2 footprint, mostly related to the usage of Portland cement. There are several approaches to improve that situation. Typical include reducing the amount of Portland cement by substituting it partly with secondary cementitious binders (SCMs) and alternative binders. The concrete mix designs and production technologies have been optimized to reduce emissions. Unfortunately, as of today, none of these approaches enabled the production of truly zero-emission neutral sustainable concrete. Test results showed that producing new high-quality structural concrete is possible using only materials recovered from old concrete, including coarse and fine aggregates, fillers, and binders. The recovery method is based on a combination of crushers and ball mills that enable the mechanical activation of fines. The method used old concrete waste from a precast concrete factory yard. The material was crushed and sieved into several fractions, i.e., particle sizes 4–8 mm, 2–4 mm, and a fine powder < 2 mm. The fine powder was MCA-treated (Mechanical activation using intensive ball milling). All these materials were mixed, and the produced concrete reached the 28-day compressive strength of 30 MPa. The only new ingredient for that concrete was water with a little superplasticizer to improve the workability. The application of this technology in Europe could result in the annual production of 34 million tons of ZERO-emission cementitious binder. 

The aim of the study was to investigate the effect of mesoporous silica MCM-41 (at 0–2% by cement mass) on the thermal degradation of cement pastes subjected to exposure up to 700 °C. The experimental programme included, among others, thermogravimetry (TG/DTG), isothermal calorimetry, SEM-EDS, ultrasonic pulse velocity (UPV) measurement, and compressive strength testing. The results indicated that MCM-41 accelerates and intensifies the cement hydration process while simultaneously reducing portlandite content after thermal exposure (500 °C: 17.44% → 13.07%). At 300 °C, the content of bound water increased compared to plain cement paste (from 3.77% to 5.03–5.20%), with comparable mass loss at 100 °C (17.33–17.60%). A higher susceptibility to secondary carbonation was also observed in pastes containing MCM-41. Despite the increase in microporosity, matrix continuity was better preserved (UPV: +16.9% at 500 °C and + 25.3% at 700 °C for 0.5% MCM-41). Moderate dosages enhanced the initial compressive strength by 26.9% for paste with 0.75% MCM-41 and mitigated strength losses after heating (700 °C: +7.1% for 1.0% MCM-41 and + 4.9% for 1.5% MCM-41). Excessive dosage (> 2.0%) diminished the benefits. It was demonstrated that MCM-41, when added in moderate amounts (up to 1% by cement mass), improves the thermal resistance of cement pastes by reducing portlandite, promoting secondary carbonation, and maintaining favourable mechanical performance after heating. Accordingly, the carefully selected dosage of MCM-41 constitutes one of the effective strategies for enhancing the thermal resistance of cement pastes.

Leveraging “end of life concrete” as a resource, not waste, is a critical milestone in achieving circularity and meeting the sustainability targets outlined in the Sustainable Development Goals. However, current industrial practices remain limited, with most concrete waste being crushed and discarded, including the fine fraction known as recycled concrete fines (RCFs). This study investigates the reactivity of mechanochemically activated (MCA) RCFs as a potential supplementary cementitious material. Reactivity was assessed using Frattini, R3 calorimetry test, and Strength Activity Index (SAI). All methods confirmed that mechanochemical activation effectively enhanced the reactivity of RCFs, with an optimal grinding duration of 20 min. Mortar results showed a compressive strength improvement of 35% (at 20% cement replacement) relative to the untreated RCF blend. The results further indicated that particle size distribution affects the measured reactivity, emphasizing the need to distinguish the filler effect from intrinsic chemical reactivity. Overall, the findings highlight the potential of MCA as an effective and sustainable strategy for valorizing fine concrete waste in circular cementitious systems.

Calcination is a common activation method for clays used as supplementary cementitious materials (SCMs). This study explores an ecofriendly and sustainable clay activation approach, namely mechanochemical calcination (MCC), which combines calcination with a high shear grinding process. Two natural mixed layer clays, dominated by kaolinite and illite, along with the respective virgin pure clays (kaolinite and illite), were treated via calcination, mechanochemical activation, and MCC. The study evaluated their mineralogical transformation, microstructure, and compressive strengths. The findings revealed that mechanochemical activation reduced particle size by up to 73% and increased surface area by 72%, particularly in illite-rich clays. MCC further reduced particle size, particularly in clays rich in illite and quartz. MCC enhanced 7 day compressive strength in illite-dominant natural clays and pure illite clays by 6% and 98%, compared to calcination. In contrast, for pure kaolinite, MCC doubled the particle size (from 5 μm in calcined to 10 μm in MCC), reduced reactivity (12% lower bound water), and tripled porosity, resulting in lower compressive strength. Al²⁷ MAS-NMR revealed that MCC increased Al(V) sites in illite from 0% in raw clays to 40% in MCC treated clays. This increase contributed to enhanced strength. In kaolinite, however, calcination increased Al(V) sites, increasing pozzolanic reactivity. Overall, MCC proves to be an efficient activation method for illite-rich natural clays, yielding a 14% increase in compressive strength at 28 days compared to calcination. Future research should consider combinations of calcination and mechanochemical activation parameters to identify the most energy efficient conditions. Emphasis should be placed on energy demand to optimise and transfer laboratory scale conditions (mechanical activation) to industrial applications.

Controlling early age thermal cracking in hardening Low-Carbon Concrete (LCC) casted in extreme weatherrequires precise modelling of its maturity, strength and heat development. The existing engineering-orientedmodels are useful for Portland Cement Concretes, but their accuracy for LCC is still unclear. Moreover, study-ing the dependence of LCC maturity parameters on whether mechanical or heat tests are implemented is also ofinterest. In response, this investigation proposed enhanced modelling techniques targeting a proper strength,heat, maturity, and cross-over effect assessment of LCC, including slag, limestone, superplasticizer, and air-entrainment. The models were calibrated using compressive, tensile, semi-adiabatic and isothermal calorim-etry tests. Moreover, a novel Activation Energy (Ea) function was proposed, accounting for its hydrationdependence, here referred to as aging. The proposed models represent improvements for the evaluation of LCC assuggested by comparison against classical approaches. For instance, they better capture the extended dormantperiod of LCC. Furthermore, the aging of the Ea was found imperative to accurately predict the heat developmentof LCC independently of the type of heat test selected for calibration. The models also predicted more severecross-over effect on the tensile than the compressive strength for low water-to-binder ratio LCC, which is relevantto decide on cracking risk mitigation measures. In general, the modelling techniques developed here evidencedgood performance for LCC and are worth of further exploration by researchers and practitioners. Their appli-cation will contribute to incentive wider use of LCC in infrastructure projects by providing more accurate pre-dictions of their cracking risk.

To lower the carbon footprint associated with the cement industry, a detailed understanding of the available conventional raw materials and identification of new and emerging non-conventional raw materials is crucial. This review paper critically examines recent scientific literature on the origin, processing, properties, and utilization of conventional and potential alternative non-conventional raw materials available in most Nordic countries (Finland, Sweden, Norway, and Iceland) in the development of low-carbon concrete. The primary raw materials that have generated significant interest are clay minerals, limestone, and volcanic pozzolans. In addition, apart from blast furnace slag and fly ash that has been widely studied and almost fully utilized as secondary raw materials in low-carbon concrete, ongoing research is considering a much wider array of by-products and waste materials as potential alternative non-conventional raw materials. These materials are often available in high volumes and originate from different industries in the Nordics such as metallurgical (steel slags, fayalitic slags, foundry sand, bauxite, goethite, and jarosite residues etc.), mining (waste rock and mining residues), construction and demolition (mineral wools and crushed concrete), energy (fly ashes, peat ashes, biomass ashes, waste incineration ashes and slags), forest (wood ashes, green liquor dregs and other forest residues), and chemical (phosphogypsum). Depending on the by-product, they may require different pretreatment and beneficiation processes to enhance their suitability and performance as raw materials for low-carbon concrete. While most of these materials have been investigated as potential construction materials with promising results, future research and development efforts are necessary to better understand their impact on cement and concrete properties. Overall, this review provides valuable insights, highlights the latest developments, and provides recommendations for the scientific and industrial community on the uptake of a wider range of alternative raw materials in cement and concrete.

Carpet waste degrades slowly, posing an environmental burden. Incorporating this waste as fibre reinforcement in concrete helps to reduce accumulation and improve the properties of concrete. However, varying fibre characteristics and their interaction in blends are not well established. Hence, understanding hybrid fibre effects is crucial for construction adoption. This study evaluates the mechanical performance, shrinkage, microstructure, pore structure, and interfacial characteristics of hybrid carpet fibre-reinforced concrete. Four fibres, nylon, polypropylene (PP), polytrimethylene terephthalate (PTT), and polyester were analysed using grey relation analysis in six hybrid combinations with three mix ratios at a 0.3% volume fraction. At 28 days, flexural and splitting tensile strengths increased by 4–11% and 8–26%, respectively. All mixtures exhibited reduced shrinkage compared to the control. The nylon/PP 2:1 mix showed the best overall performance. Microstructural analysis revealed pore refinement, reduced porosity, and improved fibre–matrix bonding influenced by fibre hydrophilicity or hydrophobicity. Nanoindentation indicated hydrophilic fibres minimized the interfacial transition zone thickness. The findings indicate fibre combinations exhibit additive effects. This suggests performance can be predicted for blended fibre mixes, potentially allowing the design of mixes using fibre combinations to compensate for poorer performing fibres, enabling their reuse and avoiding landfill disposal.

This study investigates the use of wood ash (WA3 and WA4), both unground and ground for 10 and 20 min, as a partial replacement for Portland cement in concrete. Fly ash (FA) is used as a reference. The effects of incorporating ground and unground wood ash on fresh and hardened properties, hydration behavior, microstructure, durability, and environmental performance were evaluated. Grinding wood ash significantly improved the compressive strength of concrete and increased the cumulative heat release during hydration. Air-entraining admixtures significantly improved the freeze-thaw resistance of concrete with ground wood ash and reduced surface scaling compared to unground ash. Furthermore, grinding reduced the mobility of key heavy metals, enhancing environmental compatibility. Overall, 10-minute grinding provided the optimal balance between performance and environmental safety. These results support the controlled use of ground wood ash as a sustainable supplementary cementitious material in concrete, contributing to both concrete performance and environmental benefits.

This study investigates the physical transformations and assesses the performance of blended-cement concretes exposed to temperatures ranging from ambient to 800°C. Three different grades of cement: 32.5, 42.5, and 52.5, with various contents of ground granulated blast furnace slag (GGBFS), were employed in this research. The primary focus is on understanding how variations in cement-slag ratios impact the structural characteristics of concretes exposed to elevated temperatures. Through a series of mechanical tests and matrix analysis, we examined the response of concretes incorporating supplementary cementitious materials to high temperatures. The study demonstrated that slag-cement blends exhibit superior mechanical performance compared to the conventional concrete reference sample. Notably, after exposure to 400°C, the compressive strength of the blends showed significant improvement. The results contribute to enhancing the understanding of the thermal behavior and overall performance of environmentally conscious concrete mixes in challenging conditions.