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.

Construction and demolition waste (CDW) puts significant pressure on landfills and natural resources worldwide. Although partial substitutions of Portland cement (PC) with various recycled materials have been reported, a complete (100 %) replacement using recycled concrete remains problematic. In the present study, a novel binder was produced entirely from recycled concrete fines (RCF), while recycled aggregates were incorporated to replace virgin aggregates. High-energy grinding in a planetary ball mill was applied to the RCF fraction below 150 µm. Particle size reduction occurred after grinding, resulting in a decrease in d50 from 139 μm to 3.18 μm. The intensity of the X-ray diffraction (XRD) peaks corresponding to muscovite, microcline, and portlandite decreased significantly. On the other hand, quartz was only slightly affected. Activated RCF was used as a 100 % PC replacement in paste and concrete mixes. Compressive strengths of approximately 20 MPa were obtained after 28 days in laboratory tests for concrete with w/c equal to 0.6. A preliminary life cycle assessment (LCA) was performed to evaluate the environmental impacts, focusing on carbon emissions and resource depletion. The LCA results suggested over 90 % reduction in CO2 emissions compared to traditional cement-based mixes. Even at the laboratory-scale milling with high energy consumption per 1 kg of RCF, the impacts correspond to 50 % of traditional concrete (TC). Benefits remain significant over short transporting distances of the processed recycled ingredients; however, resource depletion exceeds TC after about 200 kilometers, and carbon emissions go beyond TC after approximately 800 kilometers.

Concrete is the most used building material. With the increasing growth of industries and urbanization globally; the demand for concrete is increasing significantly. Ordinary Portland Cement (PC) is the binder used to produce typical concrete. Unfortunately, for every ton of manufactured cement about 0.61-ton CO2 is emitted into the earth’s atmosphere. As a result, several solutions have been implemented to reduce the usage of this material in the production of concrete. This includes its partial or full replacement with supplementary cementitious materials (SCMs) or alternative binders. Some of these combinations could be problematic to be used in cold climates due to a lower developed hydration heat, slower setting, or worse frost durability.

In winter the immediate exposure of fresh concrete to freezing temperatures results in pore ice formation and could delay or completely stop the hydration process. This is commonly prevented by using an additional heating system installed in concrete or the formwork. Unfortunately, usually, it adds complexity, increases the price, and depending on the used power source, could increase the CO2 footprint. Another potentially simpler and more sustainable solution is to modify the concrete itself by adjusting the mix design, by using certain chemical admixtures and special cementitious binders.

This research aimed to better understand how partial replacement of Portland cement with GGBFS and/or CSA cement affects the properties of concretes exposed to freezing and subfreezing temperatures in a fresh state and at a young age. The secondary aim was to evaluate a possible application of UHPC to protect new and existing concrete structures from frost damage.

The research included a literature review of methods used to produce concrete structures at zero and subzero temperatures. A special emphasis was on the application of chemical and mineral admixtures that could eliminate the need to use heat treatments. The output of this analysis enabled to narrow the scope of the research.

The experimental program focused on the optimization, testing, and analysis of mixes containing various combinations of chemical admixtures, CSA cement, and Portland cement. Tests included exposure to freezing and subfreezing temperatures. The aim was to lower the freezing point of water and promote faster hydration and strength gain. Fresh and hardened properties were determined for all produced concretes. The phase transition of pore water into ice, the ice-forming temperature, and their effects on the binder matrix were studied using differential scanning calorimetry (DSC). Other tests included ultrasonic pulse velocity measurements (UPV), bond test (pull-off), scanning electron microscope (SEM) for analysis of the microstructure and phase composition, frost durability evaluation with Båras test, semi-adiabatic calorimetry to study hydration processes, compressive strength measurements.

Mortar beams incorporating carbon nanofibers (CNFs), which were synthesized in situ on Portland cement particles, were used to produce nanomodified Portland cement sensors (SmartCem sensors). SmartCem sensors exhibited an electrical response comparable to a thermistor with a temperature coefficient of resistivity of − 0.0152/ °C. The highest temperature sensing was obtained for the SmartCem sensor, which contained ~ 0.271 wt.% of CNFs. The calculated temperature sensitivity was approximately 11.76% higher in comparison with the mortar beam containing only unmodified Portland cement. SmartCem sensors were used to monitor the cement hydration in large-scale self-compacting concrete beams. The measurements were conducted after casting for 7 days. Additionally, commercially available thermocouple and humidity sensors were used as references. The results showed that changes in electrical resistivity measured by the SmartCem sensor were well aligned with the ongoing hydration processes.

This study determined how replacing sodium nitrate-based antifreeze admixture (AF) with belite-calcium sulfoaluminate (belite-CSA) cement affects the early age properties of ecological concretes based on ordinary Portland cement (OPC) and ground granulated blast-furnace slag (GGBFS). Concrete specimens were cured at −15 °C and treated in various ways before testing, i.e., no treatment, stored at 20 °C for 12 and 24 h. Generally, the addition of belite-CSA cement shortened the setting time due to the rapid formation of ettringite. The incorporation of 25 wt% of antifreeze admixture (AF) to the OPC-GGBFS concrete cured at −15 °C partially inhibited ice formation and enabled the continuation of hydration processes. This trend was observed for all samples, independent of the applied AF after freezing curing. On the contrary, the addition of 20 wt% of CSA failed to inhibit the ice formation and increased the risk of frost damage for concretes despite the treatment after freezing. These concrete specimens had lower hydration, lower strength, and a more porous binder matrix. The microstructure of the binder matrix was significantly affected by the amount of CSA and extreme negative curing, followed by no notable recovery post-curing at room temperature. Therefore, pre-curing at room temperature for at least 6 h has the potential to avoid frost damage. Concrete containing 25 wt% AF combined with 12 h and 24 h of curing at 20 °C after removal from freezing and prior to testing could enhance the compressive strengths of all concretes. The renewed hydration was indicated as the main influencing factor.

The effects of a partial replacement of Ordinary Portland cement (OPC) with three types of calcium sulfoaluminate (CSA) cements (40 wt% and 20 wt%) were investigated. The obtained results were generally in agreement with previously published data but with few interesting exceptions. Setting times were shortened due to the formation of ettringite. The maximum hydration temperature increased for concretes containing 40 wt% of CSA but decreased when 20 wt% replacement was used. The decrease was related to the deficiency of the available sulfates, which limited the formation of ettringite. The presence of extra anhydrite and calcium oxide was associated to the delayed establishment of the second temperature peak in contrast to OPC-based concretes. Their surplus delayed calcium aluminate and belite reactions, and triggered renewed formation of ettringite, C-S-H and portlandite. Effects of aluminum hydroxide were also indicated as possibly important, although not proved experimentally in this research. The slightly lower compressive strength measured for mixes containing 40 wt% of CSA were linked with more formed ettringite. The same factor was indicated as the key to the reduction of the total shrinkage in mixes containing 40 wt% of CSA and increased for the lower CSA replacement level. In that case, the insufficient amount of formed ettringite caused too small expansion, which could not efficiently mitigate or compensate the developed shrinkage.

Lowering the freezing temperature of the mixing water is crucial for concrete works at subzero temperatures. In this study, formation of ice was examined for various pastes and concretes of OPC-GGBS based, while exposed to a constant temperature of −15 °C. Sodium nitrate antifreeze admixture was added as 0, 6, 10, 15, 20, 25, 30 wt% by the total binder amount. The ice formation and its effects on the binder matrix microstructure was studied using differential scanning calorimetry (DSC), ultrasonic pulse velocity (UPV) and Scanning Electron Microscopy – Energy Dispersive Spectrometry (SEM-EDS). Several curing procedures were applied to samples before commencing tests. Results showed that, addition of 25 wt% of the sodium nitrate caused the most extensive delay of the ice growth. Mixes containing less admixture showed an increasing amount of the forming ice which in some cases lead to the development of the false strength. The hydration rate has been the highest for the mix with 25 wt% of the sodium nitrate and tended to be limited at lower additions. The porosity of the hydrated binder matrix tended to be lower for mixes characterized by a lower amount of the forming ice. In general, application of above freezing temperature resulted in resuming of the hydration process that led to densification of the microstructure and strength increase. This trend was more pronounced for mixes having lower amounts of the formed ice.

Concrete structures exposed to harsh environments, especially including bridges, harbor structures are often suffered from durability problems. Typical external signs include surface deterioration, cracking caused by for example sulphate attack, frost action or reinforcement corrosion. All are strongly linked to the porous microstructure of the binder matrix and chemical decomposition of certain phases. Full replacement of deteriorated concrete structures is costly and can be troublesome for their users. The increasing demand to reduce the carbon footprint and to prolong the service life of concrete structures adds yet another argument to restore the existing structures. One alternative is to use very dense Ultra-High-Performance concrete (UHPC) as an external protective coating. The goal of this study was to determine the interfacial bonding characteristics between a damaged normal concrete (NC) and the applied thin layer of the UHPC. To curb the CO2 emission, UHPC is produced by substituting 50 wt% of Portland cement with a fine limestone powder. Fresh and hardened properties, shrinkage and frost durability have been evaluated. Mechanical properties were determined on a full-scale hybrid element using ultrasonic pulse velocity (UPV) and bond test (pull-off test). The results showed a significant increase of mechanical properties. Despite the applied thin layer of UHPC and volumetric restrain from the substrate normal concrete (NC) only limited surface shrinkage cracks were observed. The bond test and UPV showed good excellent values.

Most of the currently used concretes are based on ordinary Portland cement (OPC) which results in a high carbon dioxide footprint and thus has a negative environmental impact. Replacing OPCs, partially or fully by ecological binders, i.e., supplementary cementitious materials (SCMs) or alternative binders, aims to decrease the carbon dioxide footprint. Both solutions introduced a number of technological problems, including their performance, when exposed to low, subfreezing temperatures during casting operations and the hardening stage. This review indicates that the present knowledge enables the production of OPC-based concretes at temperatures as low as −10 °C, without the need of any additional measures such as, e.g., heating. Conversely, composite cements containing SCMs or alkali-activated binders (AACs) showed mixed performances, ranging from inferior to superior in comparison with OPC. Most concretes based on composite cements require pre/post heat curing or only a short exposure to sub-zero temperatures. At the same time, certain alkali-activated systems performed very well even at −20 °C without the need for additional curing. Chemical admixtures developed for OPC do not always perform well in other binder systems. This review showed that there is only a limited knowledge on how chemical admixtures work in ecological concretes at low temperatures and how to accelerate the hydration rate of composite cements containing high amounts of SCMs or AACs, when these are cured at subfreezing temperatures.

A reduced carbon footprint and longer service life of structures are major aspects of circular economy with respect to civil engineering. The aim of the research was to evaluate the interfacial bond properties between a deteriorated normal strength concrete structure and a thin overlay made of Eco-UHPC containing 50 wt% of limestone filler. Two types of formwork were used: untreated rough plywood and surface treated shuttering plywood. The normal strength concrete elements were surface scaled using water jets to obtain some degradation prior to casting of the UHPC overlay. Ultrasonic pulse velocity (UPV), bond test (pull-off test), and Scanning Electron Microscopy (SEM) combined with Energy Dispersive Spectrometry (EDS) were used for analysis. Elements repaired with the Eco-UHPC showed significantly improved mechanical properties compared to the non-deteriorated NSC sample which was used as a reference. The bond strength varied between 2 and 2.7 MPa regardless of the used formwork. The interfacial transition zone was very narrow with only slightly increased porosity. The untreated plywood, having a rough and water-absorbing surface, created a surface friction-based restraint which limited microcracking due to autogenous shrinkage. Shuttering plywood with a smooth surface enabled the development of higher tensile stress on the UHPC surface, which led to a more intensive autogenous shrinkage cracking. None of the formed microcracks penetrated through the entire thickness of the overlay and some were partly self-healed when a simple water treatment was applied. The project results showed that application of UHPC as repair material for concrete structures could elongate the lifespan and thus enhance the sustainability.