Early Carbonation Curing Research Articles

Study on the properties and eco-sustainability of early-age carbonation cured cement paste with recycled concrete slurry waste substitution

The reduced early-age properties of cementitious materials substituted recycled concrete slurry waste (RCSW) hinder its utilisation, which can be further improved via early-age carbonation curing with benefits from sequestering the CO2. This study aims to compressively investigate the effectiveness and environmental impact of early carbonation curing on three replacement ratios and three types of treatments of RCSW including sieving, shearing and ball milling with standard curing. In addition to mechanical properties, hydration properties, microstructure characteristics, and carbon sequestration of the treated-RCSW replaced cement paste after different curing methods were investigated and compared. The results showed that, compared to substitution ratios of 30 % and 45 %, cement paste with a 15 % RCSW substitution ratio exhibited higher strength. Furthermore, compared to standard curing, the carbonation curing method effectively improved the early strength and overall performance of the cement paste, with the early strength under carbonation curing at a 15 % RCSW substitution rate being about 89 % higher than that of standard curing. According to economic and environmental calculations, RCSW substitution could effectively reduce carbon dioxide emissions, and the use of carbonised curing methods enabled the cement paste to effectively absorb carbon dioxide.

Electrical resistivity of cement-based materials through ion conduction mechanisms for enhancing resilient infrastructures

Cement-based materials (CBM) in humid environments, influenced by their inherent defects and the presence of pore solution, exhibit poor electrical insulation performance. Low electrical resistivity of cement-based materials poses a threat to the safety of resilient infrastructures, shortens the lifespan of materials, and increases the costs of maintenance and repair. In this work, we first elucidate two primary mechanisms for enhancing electrical resistivity: (1) inhibition of ion electromigration and (2) disruption of conduction paths. We then systematically summarize and discuss 16 potential methods for improving their electrical resistivity based on these mechanisms. It is indicated that among these 16 methods, early carbonation curing, the addition of high-activity mineral admixtures, and surface hydrophobic modification are particularly effective approaches. The combination of two or more methods can simultaneously exert their functions, thus maximizing the overall effectiveness. Future work is outlined with the aim of meeting the growing demand for cement-based materials with high electrical resistivity in the construction of resilient infrastructures.

Dimensional stability of belite-rich cement subject to early carbonation curing

This study investigated the dimensional stability and physicochemical changes of belite-rich cement (BRC) subjected to carbonation curing with four distinct water-to-cement ratios. Shrinkage of carbonation and water-cured BRC was measured to assess the dimensional stability. The carbonation-cured BRC exhibited more shrinkage than the normal-cured specimen. The mass loss of the BRC paste subjected to normal curing was greater than that of the carbonation-cured BRC paste. The mechanism involved in the dimensional stability and mass loss of carbonation- and normal-cured specimens was investigated by physicochemical analysis. The large amounts of calcite were observed in carbonation curing, but negligible in normal cured specimen. Furthermore, carbonation of Ca(OH)2 and decalcification of C–S–H released water from gel in pores during carbonation curing. These reactions were exothermic in nature and led to the evaporation of some free water. This mechanism accelerated the consumption of water and increased the shrinkage of the carbonation-cured cement paste.

Mechanisms of carbonation hydration hardening in Portland cements

Carbonation hardening of Portland cement concrete can contribute to lower its CO2 footprint and to improve its strength. The reaction mechanisms involved in carbonation curing were investigated on fresh cement pastes. The quantitative results gained allowed to link the curing conditions to the reactions kinetics, phase assemblage and microstructure evolutions.During the early carbonation curing, alite and belite react with dissolved CO2. The main carbonation products are calcium carbonate, an alumina-silica gel and a C-S-H phase with a low Ca/Si ratio. Hydration curing following the carbonation transforms the silica-rich phase into a C-S-H phase typical of hydrated Portland cements. Additionally, anhydrous calcium aluminates from clinker react to form ettringite and monocarbonate phases. The structure and composition of these phases is governed by the local equilibria in the dense microstructure. The carbonation and hydration reactions decrease the pore volume and refine the porosity resembling the microstructure development in the composite cements.

Experimental investigation on the mechanical and chemical properties of lightweight aggregate concrete with CO2 curing

In the cement and concrete industry, enormous amounts of Carbon Dioxide (CO2) are emitted during their production processes. Carbon dioxide emission significantly contributes to the global climate change, which has been one of the biggest challenges of our times. Some novel solutions have been proposed for CO2 capture and storage, as well as reducing CO2 emission in concrete production. Carbonation curing is an effective alternative for conventional water curing for concrete. It can store CO2 in the hardened concrete and meanwhile improve early mechanical properties of concrete. Partial replacement of cement with fly ash shows environmental benefits, such as reducing greenhouse gas emissions and industrial waste destined for landfills. There has been some previous research studying on the effect of carbonation curing on normal Portland concrete in the past decade. Nevertheless, few studies have focused on the CO2 curing for lightweight aggregate concrete (LWAC). In this paper, the influence of early carbonation curing on LWAC is studied. LWAC specimens with two different water-to-cement ratios are cast and cured for a series of experimental investigations. The mechanical and chemical properties including the 1-day compressive strength, 28-day compressive strength, flexural strength, CO2 uptake, heat development, and pH level are investigated. Specimens with ordinary Portland cement are also tested as references in terms of compressive strength and CO2 uptake.

Microstructure of Portland cement paste subjected to different CO2 concentrations and further water curing

Understanding the effect of formed calcium carbonate and highly polymerization of silicates on the microstructure of cement paste exposed to early CO2 curing and further hydration process is crucial to comprehend the mechanisms of CO2 curing. In this paper, the cement pastes were further cured in water after CO2 curing under two CO2 concentrations (3% and 20%) and the carbonation depth, compressive strength, composition and morphology of the formed products were characterized. The results showed that calcite was the main product generated in CO2-cured pastes and its content was increased with the increasing CO2 concentration. The formed calcium carbonate provided additional nucleation sites and accelerated the hydration of C3S in further water curing. Furthermore, calcium carbonate was consumed by C3A to form calcium aluminate monocarbonate, which delayed the transformation of ettringite to monosulfate during further water curing. The decarbonation temperature of the formed calcium carbonate was increased with the increase of the CO2 concentration, but it was decreased in further water curing due to participating the further hydration process. The early carbonation curing improved the polymerization of silica gel, which was decreased in the subsequent hydration due to the formation of C-S-H. The CO2-cured sample showed higher early compressive strength and comparable long-term compressive strength compared to the conventional samples due to the lower porosity.

Microstructure of cement paste subject to ambient pressure carbonation curing

This study investigated the microstructure of ordinary Portland cement paste subject to early age ambient pressure carbonation curing in a flexible enclosure. The high-pressure carbonation at 5 bar and the normal hydration were used as references. Both ambient-pressure and high-pressure carbonation was carried out with pure gas (99.9% CO2) for 12 h. It was found that ambient pressure carbonation could achieve comparable carbon uptake and strength gain as high pressure at both early and late age. Nevertheless, ambient carbonation was more economic and practical for precast products of different sizes and shapes. In order to examine this mechanism, X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), 29Si nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption (NAD) were adopted to characterize the microstructural development after early carbonation curing under both ambient pressure and high pressure. Ambient pressure carbonation reaction took place more on the surface than in the core due to the limited CO2 diffusion so that the surface layer was more densified. The NAD results showed that ambient pressure performed better than high pressure in terms of reducing the cumulative pore volume and refining the capillary pore size. Ambient pressure could produce well crystalline carbonates as high pressure as deduced by XRD, TGA, FTIR and SEM images. The generation of calcium carbonate and its intermingling effect with hydration products were the main reasons for the strength gain and microstructural development of paste after early carbonation. Besides, SEM images showed that ambient pressure tended to produce calcium carbonates along the surface of C-S-H filling pore structure. As subsequent hydration proceeded, the increase of well crystalline carbonates was seen more in ambient pressure carbonation than high pressure based on TGA. NMR revealed that ambient pressure could enhance the polymerization of silicon in C-S-H promoting the early strength gain and maintain high Ca/Si ratio.

Early carbonation curing effects on the microstructure of high initial strength Portland cement pastes

The microstructure of high early strength and sulfate-resistant Portland cement carbonated pastes at two different durations of carbonation (1 h and 24 h) was analysed under the best conditions for carbon dioxide capture as indicated in previous studies by the authors. The aim was to determine, by means of X-ray diffraction analysis, nuclear magnetic resonance spectroscopy and scanning electron microscopy, how different carbonation times affect the microstructure of the pastes and to explain their respective related mechanical and porosity properties. The microstructure of the reference paste was homogeneous. The 1 h carbonated paste showed a more porous region near its external surface, surrounding a much higher inner non-porous homogeneous phase, while the 24 h carbonated specimen showed a single and more porous phase. The longer the carbonation time, the higher was the microstructural porosity of the carbonated regions and the higher the condensed silicate structure of the matrix, as characterised by morphology modifications. The resultant mechanical and physical properties of the pastes were affected by these chemical changes.

Stable conversion of metastable hydrates in calcium aluminate cement by early carbonation curing

The hydration of monocalcium aluminate in calcium aluminate cement forms metastable products such as CAH10 and C2AH8, which undergo phase transformations into stable products, inducing significant volumetric instability. The present study adopted carbonation curing, in which a calcium aluminate cement paste is exposed to a CO2 concentration of 10%. X-ray diffractometry, thermogravimetry and mercury intrusion porosimetry were conducted on the carbonation-cured and on reference samples at 28days. The results showed that the metastable phases in the carbonation-cured sample were converted into stable phases and calcium carbonate polymorphs without inducing any significant change in the pore structure. This study may provide new insight into innovative means of avoiding the unstable conversion of hydrates in calcium aluminate cement by utilizing CO2 collected from industrial sources.

Effect of early carbonation curing on chloride penetration and weathering carbonation in concrete

Early carbonation curing of concretes was developed for accelerating the strength gain and promoting carbon dioxide utilization in concrete. However, the effect of early carbonation curing on chloride penetration was not clear. It was reported that weathering carbonation of concrete could increase chloride penetration. The purpose of this paper is to examine if early carbonation curing would also accelerate the chloride penetration and the weathering carbonation depth in concrete during service. Both carbonation cured and hydration cured concretes with and without fly ash were exposed to two severe cyclic conditions: (1) chloride immersion/air drying cycles and (2) chloride immersion/accelerated weathering carbonation cycles. It was found through one-year cyclic tests that total chloride content was actually reduced by more than 50% in concrete subjected to carbonation curing comparing to hydration reference. Reduction in free chloride (water-soluble) in carbonation cured concrete reached more than 60%. It was also found that carbonation cured concrete was not more vulnerable to weathering carbonation. This was attributed to the carbonate-rich surface protective layer which was less permeable, less absorptive, and with comparable pH value, enabling the carbonation cured concretes higher resistance to chloride penetration.

A study of the fluidic movement of the hydrated products from the early carbonation curing in cementitious paste and the effects on the mechanical and porosity properties

A study of the fluidic movement of the hydrated products from the early carbonation curing in cementitious paste and the effects on the mechanical and porosity properties

Early carbonation curing of concrete masonry units with Portland limestone cement

The effect of carbonation curing on the mechanical properties and microstructure of concrete masonry units (CMU) with Portland limestone cement (PLC) as binder was examined. Slab samples, representing the web of a CMU, were initially cured at 25°C and 50% relative humidity for durations up to 18h. Carbonation was then carried out for 4h in a chamber at a pressure of 0.1MPa. Based on Portland limestone cement content, CO2 uptake of PLC concrete after 18h of initial curing reached 18%. Carbonated and hydrated concretes showed comparable compressive strength at both early and late ages due to the 18-h initial curing. Carbonation reaction converted early hydration products to a crystalline microstructure and subsequent hydration transformed amorphous carbonates into more crystalline calcite. Portland limestone cement could replace Ordinary Portland Cement (OPC) in making equivalent CMUs which have shown similar carbon sequestration potential.

Early carbonation behavior of high-volume dolomite powder-cement based materials

Combined with DTG analysis, X-Ray diffraction analysis (XRD) and field emission scanning electron microscopy analysis (FSEM) affiliated with energy dispersive spectrometer analysis (EDS), the early hydration and carbonation behavior of cement paste compacts incorporated with 30% of dolomite powder at low water to cement ratio (0.15) was investigated. The results showed that early carbonation curing was capable of developing rapid early strength. It is noted that the carbonation duration should be strictly controlled otherwise subsequent hydration might be hindered. Dolomite powder acted as nuclei of crystallization, resulting in acceleration of products formation and refinement of products crystal size. Therefore, as for cement-based material, it was found that early carbonation could reduce cement dosages to a large extent and promote rapid strength gain resulting from rapid formation of products, supplemental enhancement due to water release in the reaction of carbonation, and formation of nanometer CaCO3 skeleton network at early age.

The effects of the early carbonation curing on the mechanical and porosity properties of high initial strength Portland cement pastes

Mechanical and porosity related properties of high initial strength sulfate resistant Portland cement (HS SR PC) pastes subjected to early age carbonation curing were investigated. This work presents the characterization through mechanical properties, of carbonated HS SR PC pastes and of non-carbonated references. They were treated at two different times of carbonation (1h and 24h), at the best conditions of capturing, previously determined by the authors. The mechanical properties were characterized by compressive strength, tensile strength by diametrical compression, elastic modulus and Poisson’s coefficient. It was found that 1h of carbonation improved the mechanical properties, while the increase of carbonation time to 24h, decreased significantly the same properties in relation to the reference specimens. The porosity related properties were characterized by total absorption, absorption by capillarity, gas permeability, and mercury intrusion porosity. It was noticed that the increase of the carbonation time from 1h to 24h, increased substantially the absorption properties. Despite the mechanical properties of the 1h carbonated paste were better than those of the reference, the absorbed water content of the carbonated specimen was slightly higher than that of the reference.

Early carbonation for hollow-core concrete slab curing and carbon dioxide recycling

Early carbonation curing was developed for hollow-core concrete slab production to partially replace heat curing and recycle carbon dioxide recovered from the cement kiln. The cavities in hollow core slab were utilized as a CO2 gas chamber to facilitate a carbonation curing at a low gas pressure. Concrete performance was evaluated by carbon dioxide uptake, compressive strength, pH values and resistance to air permeation. The microstructure was examined by X-ray diffraction patterns, thermogravimetric analysis and scanning electron microscope. It was found that early carbonation curing could reduce the conventional heat curing duration and produce concrete with higher strength and lower permeability due to the precipitation of calcium carbonates on surface. The pH value of the carbonated concrete at the reinforcement could be kept above the corrosion threshold through a subsequent hydration and the delayed ettringite formation owing to heat curing was totally eliminated by early carbonation. The process offers economical, technical and environmental benefits.

Microstructure of cement paste subject to early carbonation curing

Microstructure of Ordinary Portland Cement paste subjected to early age carbonation curing was studied to examine the effect of early carbonation on performance of paste at different ages. The study was intended to understand the mechanism of concrete carbonation at early age through the microstructure development of its cement paste. Early carbonation was carried out after 18-hour initial controlled air curing. The microstructure characterized by XRD, TGA, 29Si NMR and SEM was correlated to strength gain, CO 2 uptake and pH change. It was found that early carbonation could accelerate early strength while allowing subsequent hydration. The short term carbonation created a microstructure with more strength-contributing solids than conventional hydration. Calcium hydroxide was converted to calcium carbonates, and calcium–silicate–hydrate became intermingled with carbonates, generating an amorphous calcium–silicate–hydrocarbonate binding phase. Carbonation modified C–S–H retained its original gel structure. The re-hydration procedure applied after carbonation was essential in increasing late strength and durability.