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Cement powder shapes have a pivotal role in particle packing and microstructural development, while its effect on capillary pore structure formation in three-dimension has not been fully understood. In this study, the modified CEMHYD3D model building on previous work of irregular-shaped cement particles is firstly used to simulate the evolution of capillary pore structures in cement pastes at various water-to-cement ratios. Pore networks at different curing time and degree of hydration are extracted and visualized. Subsequently, some home-made programs for determining three-dimensional pore structure characteristics including porosity, pore size distribution, pore connectivity and pore tortuosity are carried out on these simulated pore network extractions in hardened cement pastes. The results indicate that shape-induced larger surface area in more non-equiaxed irregular-shaped cement particles can improve pore structure parameters in hardened cement pastes, but this effect will be slight in the later curing period and at a low water-to-cement ratio. In addition, the less considered geometric difference plays a role in pore structure evolution especially for extremely non-equiaxed cement particle. However, the geometric attribute has a weak effect on pore structure parameters overall. It can also be concluded that the pore-to-solid ratio is still the most pronounced influence factor for pore structure parameters in hardened cement pastes. [ABSTRACT FROM AUTHOR]

The effects of three commonly used superplasticisers (SPs) - sulfonated naphthalene formaldehyde (SNF), sulfonated acetone formaldehyde (SAF) and polycarboxylates (PCB) - on the hydration process, products and microstructure of tricalcium aluminate (C3A) pastes in the presence of gypsum were systematically investigated using isothermal calorimetry, x-ray diffraction, environmental scanning electron microscopy, infrared spectroscopy and magic angle spinning-nuclear magnetic resonance spectroscopy. The results show that the hydration reaction rate of C3A paste at the initial stage (0-2 h) was promoted by the incorporation of SPs, especially in the C3A-SNF paste. Hydration was rapidly reduced at the early stage (2-12 h), but an acceleration in C3A-SNF/PCB pastes occurred again at the late stage. The morphology of the C3A-SNF/SAF paste changed from thin needle-shaped crystals at the initial stages to thick and coarse rod-like crystals at late stages. In the C3A-PCB paste, very few crystals occurred at an early age, while a large amount of prismatic crystals were observed at the later age. Chemical environment studies revealed that the SPs did not visibly change the position or shape of the characteristic bands of aluminates, but only influenced their relative intensities. A predominant resonance, assigned to six-coordinated aluminium as [AlO6] type, was observed for all the pastes with and without SP. The interaction mechanism between SP and C3A was also explored. [ABSTRACT FROM AUTHOR]

In this study, a novel type of lime-based low carbon cementitious material (LCM) was designed and prepared using low contents of hydrated lime and Portland cement and a high content of mineral admixtures. The mechanical properties, workability, heat of hydration, hydration degree, porosity, hydration products, and microstructure of LCM were systematically investigated. The test results indicated that LCM showed good mechanical properties and workability compared with conventional blended cement. The compressive strength of LCM can achieve 50–60 Mpa at curing 28 d and more than 70 Mpa at curing 180 d. The hydration products of LCM were mainly composed of ettringite, portlandite, and C–(A)–S–H gel according to X-ray powder diffraction results. Analysis suggests that a small amount of Portland cement in LCM accelerated the pozzolanic reaction of mineral admixtures due to the alkalinity, and the Ca 2+ ion concentration in the pore solution increased. Meanwhile, the hydrated lime reacted with mineral admixtures in long-term hydration and increased the mechanical property of LCM. In addition, LCM exerted a notable reduction in carbon emission, energy consumption and environmental burden compared with Portland cement. The quantitative calculation suggested that the energy intensity and carbon emission of the LCM are only approximately 24–45% and 13–38% that of Portland cement, respectively. [ABSTRACT FROM AUTHOR]

Abstract To reduce the carbon footprint and energy consumption from the cement manufacturing industry, lime-based low carbon cementitious materials (LCM) has caught strong attention. LCM as a novel low-carbon cement shows impressive performance and promising prospects; however, its mechanical properties are inferior to those of ordinary Portland cement (OPC). In this study, different dosages of sulfate sodium was incorporated into LCM to investigate the effects of sodium sulfate on LCM performance. The properties and hydration of LCM with and without sulfate sodium were systemically investigated and analyzed. The results revealed that LCM blending with 2–3 wt% sodium sulfate showed the best mechanical performance. The compressive strength of LCM containing 3 wt% sodium sulfate was increased by 57.0% and 20.8% relative to the plain LCM at 3 d and 90 d, respectively. Microstructural characterization showed that a great amount of ettringite had formed at 3 d, which effectively improved the mechanical performance of LCM at early stage. Moreover, the addition of sodium sulfate effectively accelerated the hydration of the solid waste in LCM, and more hydrated lime was consumed in the hydration process. The ettringite became embedded in C(A)SH gel with increasing curing age, which resulted in a dense microstructure of hydrated paste with fewer coarse pores and an enhancement in the mechanical performance of LCM. Thus, the sodium sulfate effectively increased the strength of LCM at both early and later stage. [ABSTRACT FROM AUTHOR]

Highlights • The sodium hydroxide enhanced the mechanical properties of LCM at the early age. • The increase of strength of LCM with sodium hydroxide was little at the later stage. • The C-(N)-A-S-H gel formed in the paste due to the Na uptake in the C-A-S-H gel. Abstract To increase the recycling of industrial solid waste in cement-based materials, lime-based low carbon cementitious materials (LCM) as green binder materials have shown bright prospects. To further improve the properties of LCM, different dosages of sodium hydroxide as alkali activators were mixed with LCM in the test, and this study focused on the influence of sodium hydroxide on the performance and hydration of LCM. The strength, workability, hydration and microstructure of LCM in the presence of sodium hydroxide were investigated in detail and compared to those of the original LCM. The test results indicated that the minor amounts of sodium hydroxide remarkably improved the mechanical properties of LCM at early ages (3 and 7 days). Additionally, the further hydration of mineral admixtures was affected by the presence of sodium hydroxide, which resulted in a slight improvement of the mechanical properties of LCM at the later stage. The formation of ettringite in the hydrates of LCM was suppressed by the addition of sodium hydroxide. Moreover, the Ca/Si ratio of the C-(A)-S-H gel formed in the sodium hydroxide activated LCM was lower than that in the plain LCM, and a minor amount of C-(N)-A-S-H gel formed in the hydrated paste due to the Na uptake in the C-A-S-H gel. Therefore, sodium hydroxide modified the composition of hydrates in the LCM to a certain extent during the hydration process. [ABSTRACT FROM AUTHOR]

A novel green cement which was prepared with superfine Portland cement and high volume solid wastes (fly ash and blast furnace slag) was investigated in this study. The mechanical performance, heat of hydration, particle packing density, evolution of hydration products and microstructure of this novel blended cement were studied and compared to the conventional blended cement. Moreover, the energy consumption and carbon emission of novel green cement were calculated. The effects of superfine cement in blended cement were systematically studied and compared to Portland cement (PC). Test results showed that a notable improvement in the mechanical properties of novel green cement was achieved compared to conventional blended cement. Even the mechanical properties and workability of novel green cement were very close to PC. The calculation based on mixtures indicated that the energy consumption and carbon emissions of novel green cement containing 70 wt% mineral admixtures only reached 47.3% and 40.9%, respectively, of those of PC. The analysis suggested that the superfine cement increased the packing density of blended cement and effectively accelerated the hydration of mineral admixtures to form a refined and dense microstructure, which is key for preparing high-performance green cement. This research provided guidance for developing low carbon and environmentally friendly cement. [ABSTRACT FROM AUTHOR]