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Population and development megatrends will drive growth in cement production, which is already one of the most challenging-to-mitigate sources of CO 2 emissions. However, availabilities of conventional secondary cementitious materials (CMs) like fly ash are declining. Here, we present detailed generation rates of secondary CMs worldwide between 2002 and 2018, showing the potential for 3.5 Gt to be generated in 2018. Maximal substitution of Portland cement clinker with these materials could have avoided up to 1.3 Gt CO 2 -eq. emissions (~44% of cement production and ~2.8% of anthropogenic CO 2 -eq. emissions) in 2018. We also show that nearly all of the highest cement producing nations can locally generate and use secondary CMs to substitute up to 50% domestic Portland cement clinker, with many countries able to potentially substitute 100% Portland cement clinker. Our results highlight the importance of pursuing regionally optimized CM mix designs and systemic approaches to decarbonizing the global CMs cycle. In this paper we report the maximum potential for cement substitution with secondary materials to reduce CO2 emissions globally (1.3 Gt CO2-eq. in 2018) and on a country-by-country basis.

The demand for production of cements is ever increasing to meet the infrastructure development globally. The energy and emission factors available for cements in most of the life cycle assessment (LCA) databases may not exactly suit for all the geographical locations. The main challenge in Indian scenario is the absence of database for LCA study. This study attempts to develop the energy and emission factors for the manufacturing of cements in Indian context. In the present study, five different cement manufacturing plants located in north, south, east, west and central zones of India are considered to assess the energy dissipation and carbon dioxide emission involved during the production of ordinary Portland cement (OPC). Most of the data is collected from the field, so that the energy and emission factors determined will be suitable for the zonal study. The study is then extended to assess the energy consumption and carbon dioxide emission for three blended cements, viz. Portland Pozzolan cement (PPC), Portland slag cement (PSC) and composite cement (CC) with permissible known replacement levels of fly ash, granulated blast furnace slag and both fly ash and slag, respectively. The average energy use and carbon emission is found to be on higher side in India by 15.14% and 12.64%, respectively, compared to other countries in manufacturing of cements. An average energy consumption in manufacturing of PPC, PSC and CC is found to be respectively 24.5%, 35.3% and 43.13% less compared to that of OPC. The CO 2 emission intensity for OPC is found to vary between 893 and 940 kg/tonne of cement from five different zones, and an average of respectively 24.8%, 40.97% and 47.18% lower CO 2 emission was observed from PPC, PSC and CC compared to OPC. From the inventory results, CC has proven to be a more sustainable cement with low energy consumption and lower CO 2 emission compared to other cements.

Cement production causes 7.5% of global anthropogenic CO 2 emissions, arising from limestone decarbonation and fossil-fuel combustion 1 – 3 . Current decarbonation strategies include substituting Portland clinker with supplementary materials, but these mainly arise in emitting processes, developing alternative binders but none yet promises scale, or adopting carbon capture and storage that still releases some emissions 4 – 8 . However, used cement is potentially an abundant, decarbonated feedstock. Here we show that recovered cement paste can be reclinkered if used as a partial substitute for the lime–dolomite flux used in steel recycling nowadays. The resulting slag can meet existing specifications for Portland clinker and can be blended effectively with calcined clay and limestone. The process is sensitive to the silica content of the recovered cement paste, and silica and alumina that may come from the scrap, but this can be adjusted easily. We show that the proposed process may be economically competitive, and if powered by emissions-free electricity, can lead to zero emissions cement while also reducing the emissions of steel recycling by reducing lime flux requirements. The global supply of scrap steel for recycling may treble by 2050, and it is likely that more slag can be made per unit of steel recycled. With material efficiency in construction 9 , 10 , future global cement requirements could be met by this route. Recovered cement paste can be reclinkered if used as a partial substitute for the lime–dolomite flux used in steel recycling, which can reduce waste and carbon emissions.

Investigating the effects of lithium slag (LS) as an admixtures on the properties of different cement pastes can expand its potential applications in construction materials. This study examines the influence of LS content (0%, 10%, 20%, and 30%) on the performance of ordinary Portland cement (OPC), ultra-fine ordinary Portland cement (UPC), and calcium sulfoaluminate cement (CSC) pastes. The underlying mechanisms are explored using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). The results indicate that incorporating LS reduces the fluidity, setting time, and bleeding rate of cement pastes but enhances their compressive strength at 1, 7, and 28 days. Specifically, LS improves the long-term strength of OPC pastes, whereas it primarily enhances the early-age strength of UPC pastes. For CSC pastes, LS significantly promotes both early and later-age strength. SEM, XRD, and FTIR analyses reveal that the effects of LS stem from a filling effect and the activation of hydration reactions, where reactive components and gypsum in LS promote the formation of hydration products such as calcium silicate hydrate (C-S-H) and ettringite. These findings provide a theoretical basis for the application of LS in diverse cementitious systems.

The inherent quasi-brittleness of cement-based materials, due to the disorder of their hydration products and pore structures, present significant challenges for directional matrix toughening. In this work, a rigid layered skeleton of cement slurry was prepared using a simplified ice-template method, and subsequently flexible polyvinyl alcohol hydrogel was introduced into the unidirectional pores between neighboring cement platelets, resulting in the formation of a multi-layered cement-based composite. A toughness improvement of over 175 times is achieved by the implantation of such hard-soft alternatively layered microstructure. The toughening mechanism is the stretching of hydrogels at the nano-scale and deflections of micro-cracks at the interfaces, which avoid stress concentration and dissipate huge energy. Furthermore, this cement-hydrogel composite also exhibits a low thermal conductivity (around 1/10 of normal cement) and density, high specific strength and self-healing properties, which can be used in thermal insulation, seismic high-rise buildings and long-span bridges. Despite widely used in the construction sector, Portland cement’s high brittleness and low toughness still pose challenges in some applications. Here, authors apply an ice-templating method to fabricate a cement-hydrogel composite with alternating layered microstructure resulting in significantly increased toughness.

Low-heat Portland (LHP) cement is a kind of high-belite cement, which has the characteristic of low hydration heat. Currently, it is extensively used in the temperature control of mass concrete. Based on the thermodynamic database of OPC-based materials, the thermodynamic software GEM-Selektor (noted as GEMS) is used for simulating the hydration products of the LHP cement paste. Then, according to the GEMS thermodynamic simulation results, MATLAB is used to visualize the initial and ultimate stages of LHP cement pastes; the effects of curing temperature and water to cement (w/c) ratio on hydration products are addressed; and the porosity, fractal dimension, and tortuosity of different pastes are calculated. It is found that an appropriately high curing temperature is important for reducing porosity, especially in the early hydration stage. Hydration time also has a significant impact on the hydration of LHP cement paste; long hydration time may reduce the impact of temperature on hydration products. The w/c ratio is another important consideration regarding the hydration degree and porosity of LHP paste, and under different curing temperatures, hydration times, and w/c ratios, the porosity varies from 5.91–32.91%. The fractal dimension of this work agrees with the previous findings. From tortuosity analysis, it can be concluded that the high curing temperature may cause significant tortuosity, further affecting the effective diffusivity of LHP cement paste. For cement pastes with low w/c ratio, this high curing temperature effect is mainly reflected in the early hydration stage, for ones with high w/c ratio, it is in turn evident under long-term curing.

This study investigates the early hydration characteristics and kinetics of ordinary Portland cement (OPC) and calcium sulfoaluminate cement (CSA) composite pastes. The hydration mechanisms of OPC-CSA systems with different proportions are analyzed through zonal analysis and the Krstulović–Dabić method. The experimental results show that in OPC-dominated systems, an appropriate amount of CSA promotes the rapid hydration of ye’elimite and optimizes the cumulative hydration heat and pore structure. However, excessive CSA inhibits hydration due to alkalinity imbalance. In CSA-dominated systems, 10% OPC increases the alkalinity, promoting ye’elimite to hydrate into ettringite. Higher OPC content hinders the hydration process due to ion concentration imbalance. The kinetics model indicates that CSA accelerates the interfacial reaction and diffusion in the OPC system, while OPC reduces the overall hydration rate of the CSA system. Microscopic analysis confirms that the composite system improves the pore structure through mineral interaction. In the OPC-dominated area, the pore structure is mainly composed of small and dense pores. In the CSA-dominated area, the characteristics of large pores are affected by the expansion properties of CSA and hydration heat. This study constructs a coupling mechanism of alkalinity regulation and crystal nucleus generation, providing a theoretical basis for the design of high-performance composite cement materials.

The present research focuses on an experimental investigation of the impact of Dimensional Lime Stone Slurry Waste (DLSW) on Self-Compacting Concrete (SSC) with Ordinary Portland Cement (OPC) and Pozzolanic Portland Cement (PPC). One of the primary environmental problems is the disposal or recycling of stone waste materials. Every year, stone crushers and limestone mines generate millions of tonnes of waste in the form of slurry and dust powder. These waste materials can be sustainably used to make SCC as fine aggregates. The DLSW was replaced by 15%, 30%, 45%, 60%, 80% and 100% with fine aggregate. The workability and mechanical performance of the SCC series were evaluated with both cements. The effect of DLSW on fresh properties was examined by slump flow tests, T 500 , V-funnel, and L-box tests, while the compressive strength and flexural strength tests were performed at 28 days in mechanical properties. The Ultrasonic Pulse Velocity test (UPV) and microstructure tests were also done. Compared to PPC-based SCC with the same mix design parameters, the OPC-based SCC highlighted greater strength. Workability parameters were improved by using DLSW; additionally, PPC-based SSC showed superior workability compared to OPC-based SSC. A constant water and cement content were kept for both SCC series. At the same time (superplasticizer), SP doses were varied to maintain the workability parameters, which were examined and ranged as per EFNARC guidelines 2005. The workability and mechanical parameters were increased in both cement SSC series up to a 30% DLSW replacement level.

Concrete sulfate attack is of great interest as it represents one of the main reasons of concrete deterioration and poor durability for concrete structures. In this research, the effect of different cement types on concrete sulfate resistance was investigated. This included three concrete classes, namely, low strength concrete, medium strength concrete, and high strength concrete. Blast furnace cement (BFC), sulfate resisting Portland cement (CEM I-SR5), and ordinary Portland cement (OPC) were used in a total of eighteen concrete mixes. Three binder contents of 250 kg/m3, 350 kg/m3, and 450 kg/m3 and a constant silica fume (SF) content were applied in this experimental study. The water/binder (w/b) ratio was varied between 0.4 and 0.8. Concrete specimens were immersed in highly severe effective sodium sulfate solutions (10,000 ppm) for 180 days after standard curing for 28 days. The fresh concrete performance was evaluated through a slump test to attain proper workability. Concrete compressive strength and mass change at 28 days and 180 days were measured before and after immersion in the solution to evaluate the long-term effect of sulfate attack on the proposed concrete durability. Scanning electron microscopy (SEM) analysis was conducted to study the concrete microstructure and its deterioration stages. The obtained results revealed that BFC cement has the best resistance to aggressive sulfate attacks. The strength deterioration of BFC cement was 3.5% with w/b of 0.4 and it increased to about 7.8% when increasing the w/b ratio to 0.6, which are comparable to other types of cement used. The findings of this research confirmed that the quality of concrete, specifically its composition of low permeability, is the best and recommended protection against sulfate attack.

Cement production is currently the largest single industrial emitter of CO₂, accounting for ∼8% (2.8 Gtons/y) of global CO₂ emissions. Deep decarbonization of cement manufacturing will require remediation of both the CO₂ emissions due to the decomposition of CaCO₃ to CaO and that due to combustion of fossil fuels (primarily coal) in calcining (∼900 °C) and sintering (∼1,450 °C). Here, we demonstrate an electrochemical process that uses neutral water electrolysis to produce a pH gradient in which CaCO₃ is decarbonated at low pH and Ca(OH)₂ is precipitated at high pH, concurrently producing a high-purity O₂/CO₂ gas mixture (1:2 molar ratio at stoichiometric operation) at the anode and H₂ at the cathode. We show that the solid Ca(OH)₂ product readily decomposes and reacts with SiO₂ to form alite, the majority cementitious phase in Portland cement. Electrochemical calcination produces concentrated gas streams from which CO₂ may be readily separated and sequestered, H₂ and/or O₂ may be used to generate electric power via fuel cells or combustors, O₂ may be used as a component of oxyfuel in the cement kiln to improve efficiency and lower CO₂ emissions, or the output gases may be used for other value-added processes such as liquid fuel production. Analysis shows that if the hydrogen produced by the reactor were combusted to heat the high-temperature kiln, the electrochemical cement process could be powered solely by renewable electricity.