This paper studies chemical composition of partially and fully hydrated slag grains in a (nearly) 40-year-old field concrete from the Netherlands. The concrete samples were assumed to be sufficiently ...aged to contain fully hydrated slag grains as well as partially hydrated large slag particles with thick rims. Our analysis showed that three different elemental zoning could be identified depending on the original slag grain size. Upon full hydration of a small slag grain (i.e., <8 μm), two distinct regions were identified corresponding to a hydrotalcite-like phase in the outer rim and a C–S–H gel phase in the core, respectively. As for medium (8–17 μm) and large (>15 μm) slag grains, three distinct regions were clearly visible. Hydrotalcite-like phase was mainly observed in the outer rim and the core. C–S–H gel phase was found to be precipitated in the region between the outer rim and the core.
Sample preparation is of utmost importance for any microscopy and microstructural analysis. Correct preparation will allow accurate interpretation of microstructural features. A well‐polished section ...is essential when scanning electron microscopy (SEM) is used in backscattering electron (BSE) mode and characteristic X‐rays are to be quantified using an energy‐dispersive spectroscopy (EDS) detector. However, obtaining a well‐polished section, especially for cementitious materials containing aggregates, is considered to be challenging and requires experience. A sample preparation procedure consists of cutting, grinding and polishing. Undercutting of soft and brittle paste between harder aggregates can be overcome by vacuum epoxy impregnation offering mechanical support in the matrix. Furthermore, most of the attention during the sample preparation is given to the polishing of the sample. There is a wide range of suggestions on polishing steps, ranging from grain sizes, time and applied force; however, the final assessment of a polish surface is often subjective and qualitative. Therefore, a quantitative, reproducible guidance on the grinding steps, effect of experimental parameters and the influence of different grinding steps on the surface quality are required. In this paper, the influence of grinding was quantitatively evaluated by a digital microscope equipped with optical profilometry tools, through a step‐wise procedure, including sample orientation, grinding time and the difference between cement paste and concrete. Throughout the grinding procedure, the surface profiles were determined after each grinding step. This showed the step‐wise change in surface roughness and quality during the grinding procedure. Finally, the surface qualities were evaluated using optical and electron microscopy, which show the importance of the grinding/prepolishing steps during sample preparation.
In 3D concrete printing (3DCP), it is necessary to meet contradicting rheological requirements: high fluidity during pumping and extrusion, and high stability and viscosity at rest to build the ...layered structure. In this paper, the impact of the hydroxypropyl methylcellulose (HPMC)-based viscosity-modifying admixture (VMA) on the 3D printability and mechanical performance of a limestone and calcined clay based cementitious material is investigated. A combination of VMA and superplasticizer was used for that purpose. In this case, controlling the competitive effects between VMA and superplasticizer becomes critical. The main strategy for 3D printing in this study was to add an optimal dosage of VMA in the solid suspension that was already mixed with water and superplasticizer. A lab-scale 3DCP setup was developed and demonstrated as well. A series of tests was performed to characterize the effects of VMA on flowability, extrudability, open time, buildability, green strength, hydration, compressive strength, and air void content and distribution. Experiments performed in this study showed that the mixture containing 0.24% (of the binder mass) of VMA exhibited satisfactory 3D printability and optimal mechanical performance. Finally, the results, limitations, and perspectives of the current research were discussed.
In this paper, the authors investigated the correlation between slag chemistry and CO2 binding capacity of the blended system. To simplify the composition of mixture, model paste containing C3S, slag ...covering the common composition range and gypsum was employed.
After accelerated carbonation test, three CO2-binding phases were identified in the system as: carbonated Ca-Al AFm phases, carbonated hydrotalcite-like phase, and calcium carbonate, irrespective of slag chemistry and the addition of gypsum. On the other hand, carbonated Ca-Al AFm phases played a minor role in absorbing CO2, sharing less than 5% of CO2 among all carbonate phases. Hydrotalcite-like phase was able to bind up to ∼10% CO2, depending on the Mg/Al atomic ratio of raw slag. CaCO3, originated from the carbonation of portlandite and C−S−H gel phase, took up more than 85% CO2 after carbonation. Moreover, the carbonation degree of C−S−H gel phase was found to be negatively related with the Al2O3 content and Ca/Si ratio of raw slag.
New additive manufacturing methods for cementitious materials hold a high potential to increase automation in the construction industry. However, these methods require new materials to be developed ...that meet performance requirements related to specific characteristics of the manufacturing process. The appropriate characterization methods of these materials are still a matter of debate. This study proposes a rheology investigation to systematically develop a printable strain hardening cementitious composite mix design. Two known mixtures were employed and the influence of several parameters, such as the water-to-solid ratio, fibre volume percentage and employment of chemical admixtures, were investigated using a ram extruder and Benbow-Bridgwater equation. Through printing trials, rheology parameters as the initial bulk and shear yield stress were correlated with variables commonly employed to assess printing quality of cementitious materials. The rheology properties measured were used to predict the number of layers a developed mixture could support. Selected mixtures had their mechanical performance assessed through four-point bending, uni-axial tensile and compressive strength tests, to confirm that strain hardening behaviour was obtained. It was concluded that the presented experimental and theoretical framework are promising tools, as the bulk yield stress seems to predict buildability, while shear yield stress may indicate a threshold for pumpability.
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•A quantitative methodology based on rheological parameters to develop printable cementitious composites is presented•The methodology was successfully applied to the development of printable strain hardening cementitious composites•A correlation between shape stability and buildability with the initial bulk yield stress was found•The use of VMA and the liquid/solid ratio are key factors to control mixture stability and fibre dispersion•Rheological properties measured with the ram extruder and Benbow-Bridgwater model can suggest the build height of an object.
Ultra-high performance concrete (UHPC) is an innovative cement-based composite with high mechanical performance under tensile and compressive loads, extremely low permeability, and excellent ...durability. Because of these features, UHPC has the potential to contribute to the development of new architectural perspectives and structural systems with prolonged service life; therefore, it is anticipated that the use of UHPC in cast-in-situ applications will increase in the near future. As a result of its high Portland cement dosage, the hydration heat of UHPC can be relatively high compared to that of conventional concrete. Thus, ground granulated blast furnace slag (GGBS) can be used in UHPC formulation for reducing Portland cement dosage thereby limiting hydration heat while also addressing ecological and engineering concerns. In the scope of this study, the effects of GGBS replacement (0%, 30%, and 60%) on the hydration heat, strength, and microstructural characteristics of UHPC were studied. Results showed that GGBS-bearing UHPCs are more sensitive to ambient temperature in respect to cumulative heat. 60% GGBS replacement reduced cumulative heat release by 36% and 28% at 20 °C and 35 °C respectively. So, the benefit of GGBS on reducing hydration heat is less pronounced in hot weather. Performance differences in strength depending on the replacement ratio were only noticeable on the first day of curing. Prolonged curing time and fiber inclusion eliminated strength differences. Microstructural investigations indicated that Ca(OH)2 can be lowered up to 0.5%, and the Ca/Si ratio of the C–S–H phase was reduced below the value of 1.0 after 90 days of curing as a result of GGBS replacement.
•GGBS replacement reduces both the total heat released and the main peak of the heat flow curves at 20 °C and 35 °C.•The benefits of GGBS on reducing hydration heat are less pronounced in hot weather.•The amount of calcium hydroxide can be lowered up to 0.5% by partially replacing cement with GGBS.•Ca/Si ratio of the C–S–H phase can be reduced below 1.0 under moisture-curing by partially replacing cement with GGBS.
This paper aims to investigate the influences of high Portland cement substitutions (>60 wt%) by low-grade calcined clay (CC) and limestone (LF) on 3D concrete printability, stiffness evolution and ...early-age hydration. Results show that, with the same dosage of admixtures (superplasticizer and viscosity modifier), increasing LF and CC content reduced the slump, flowability and initial material flow rate, and significantly improved the buildability of fresh mixtures, which can be attributed to the reduced water film thickness (WFT). Furthermore, the stiffness evolution and SSAtotal development up to the first 3 h were accelerated by increasing CC content, which can also be linked to the change of WFT, and consumption of superplasticizer for the dispersion induced by hydration products. Additionally, the dilution effect on compressive strength and hydration caused by the high cement replacement was observed.
This paper reports the carbonation characteristics of a cement-slag system exposed to accelerated carbonation testing, and its improved carbonation resistance with the increasing MgO content in blast ...furnace slag, in which hydrotalcite-like phase plays a key role.
Our research showed that the hydrotalcite-like phase started to carbonate upon contacting with the carbonate ions and bound more than 15 wt% CO2−3 in the mildly carbonated and transition areas. This value was positively associated with the magnesia content of slag. Additionally, the proportion shared by hydrotalcite-like phase decreased in the fully carbonated area, and more CO2 was fixed in the form of calcium carbonate. Consistent with the thermodynamic modelling, the ratio of CO2 bound in carbonated hydrotalcite-like phase to the total CaCO3 continued to decrease as the CO2 ingress progressed. On the other hand, the reaction between hydrotalcite-like phase and CO2 was found to be volumetrically stable due to binding CO2 in the interlayer space, and Mg was still distributed within the original slag grain region. Mg/Al atomic ratio of hydrotalcite-like phase remained nearly the same before and after carbonation. Results of this study quantitatively emphasized the favorable effect of hydrotalcite-like phase to improve the carbonation resistance of slag-rich cementitious systems.
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