Rheological properties are of significant importance in successful placement and performance of self-compacting concrete (SCC). In this work, the rheological properties of SCC, combined with a powder-viscosity modifying admixture, were investigated based on a series of experimental studies. The modified Bingham model was applied to determine the rheological parameters and shear thickening behavior of SCC. The effects of mixing procedures, including charging sequence, mixing time and mixing speed, were analyzed. The results show that the shear thickening of SCC is reduced by first mixing the aggregate and water and then adding other raw materials. The direct contact between aggregate and water leads to a large amount of free water to be adsorbed by the aggregate system, resulting in an increase of the yield stress. In order to ensure an excellent fluidity and a low shear thickening behavior of SCC, the mixing time should be controlled at 4-5 min and the mixing speed at 30-45 r/min.

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Lagged settlement is a typical accident induced by shield construction in sandy cobble strata. This paper analyzed the process and mechanism of lagged settlement, results show that all phases are in accord with the characteristics of ellipsoid theory of particle flows. Based on this theory, a method for calculating coefficient of lateral earth pressure and loosened earth pressure is proposed in this research. For the coefficient of lateral earth pressure, the boundary of loosened ellipsoid is divided into two parts, the arch zone and the excavation zone, and the lateral pressure coefficients are derived respectively according to the stress state. For loosened earth pressure on tunnel, the Terzaghi earth pressure theory and Protodyakonov earth pressure theory are adapted in different conditions according to the state of loosened cobble soil. Theories developed in this study can be applied on determination of shield excavation parameters, as well as calculation of loosened earth pressure and control of tunnel support.

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The internal relative humidity (RH) plays a crucial role in most of the concrete properties. Self-desiccation caused by continuous cement hydration is a major factor affecting the RH of concrete. This paper investigates the relationship between RH and microstructure for cementitious systems in the case of self-desiccation. A series of paste specimens prepared with different binder and water-binder-ratio (w/b) were cured under sealed conditions from 1 day to 1.5 years. The RH and microstructure of the paste specimens were measured. The microstructure characteristics under study include porosity, pore size, evaporable and non-evaporable water content. The results reveal that the RH of cementitious system drops to a great extent in the first 105 days' hydration and decreases slowly afterwards. The blended materials such as fly ash, slag or limestone powder have different influences on the RH. A mathematical model between RH and the average pore diameter is proposed for cementitious systems under self-desiccation, regardless of age, w/b or cement type.@en

The production of cement leads to a large amount of CO₂ emission. Using industrial waste slag, such as ceramic polishing powder (PP), to replace part of Portland cement can reduce the pollution caused by the cement industry and solid waste disposal. In order to use PP as a replacement for cement, its effects on the properties of cement paste need to be clarified. In this study, the effect of PP on the nucleation and growth of hydrates in cement paste at very early ages was investigated. Quartz was used as a reference. The interactions of their surface with various ions in cement paste solution, which has an important effect on the nucleation and growth of hydrates, were studied by using the zeta potential test. The morphology of the nucleus and crystal of hydrates was investigated by using SEM. The zeta potential measurements showed that the affinity of the surface of PP and quartz to ions in the pore solution of cement paste is similar. The image of SEM indicated that there is also not much difference in the crystallization of hydrates on the surfaces of PP and quartz. These evidences suggested that PP has similar surface charge properties as quartz, and its effects on the properties of cement paste are the same as that of quartz. From the viewpoint of the effect on very early hydration, PP can be used in cement paste, similar to quartz power.

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Capturing the long-term performance of concrete must be underpinned by a detailed understanding of the pore structure. Mercury intrusion porosimetry (MIP) is a widely used technique for pore structure characterization. However, it has been proven inappropriate to measure the pore size distribution of cementitious materials due to the ink-bottle effect. MIP with cyclic pressurization-depressurization can overcome the ink-bottle effect and enables a distinction between large (ink-bottle) pores and small (throat) pores. In this paper, pressurization-depressurization cycling mercury intrusion porosimetry (PDC-MIP) is adopted to characterize the pore structure in a range of cementitious pastes cured from 28 to 370 days. The results indicate that PDC-MIP provides a more accurate estimation of the pore size distribution in cementitious pastes than the standard MIP. Bimodal pore size distributions can be obtained by performing PDC-MIP measurements on cementitious pastes, regardless of the age. Water-binder ratio, fly ash and limestone powder have considerable influences on the formation of capillary pores ranging from 0.01 to 0.5 μm.@en

An analytical model for predicting the relative chloride diffusion coefficient in cementitious materials at different degrees of water saturation is presented in this paper. The model is developed based on the Nernst-Einstein equation and conductivity of cementitious electrolyte, as well as on moisture distribution in the pore structure. Good agreement is found between the model and the experimental data. With the help of the model, the chloride diffusion coefficient of cementitious material at unsaturated state can be determined according to the chloride diffusion coefficient at saturated state, the degree of water saturation and the average pore diameter of the material. A detailed discussion about the inputs and outputs of the model is given in order to facilitate its application for engineering practice.

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The combination of photocatalyst with cementitious materials for air pollution abatement and self-cleaning has gained considerable attention. However, the most popularly used photocatalytic cementitious composites based on TiO₂ achieve the purification function under ultraviolet sunlight, significantly impeding a broader application of photocatalytic cementitious composites. Here, the photocatalytic cementitious composites were prepared by incorporating g-C₃N₄ nanosheets (CNNs) in Portland cement. With the increase of CNNs content, the photocatalytic NO_x abatement and self-cleaning performance was notably improved, while the micro-hardness of cementitious composites increased first and then decreased. Among the three mixing dosages (0.5%, 1% and 2% by weight of cement), the cementitious composite admixed with 2% CNNs showed the highest photocatalytic NO_x abatement efficiency of 227.3 μmol m⁻² h⁻¹, and could degrade the rhodamine B within 40 min under visible light. Additionally, its microscopic hardness was 5.4% higher than the one without CNNs, suggesting that a moderate amount of CNNs (below 2%) in cementitious composites was effective in achieving an improved photocatalytic depollution performance.

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Reliable durability design of marine concrete structures requires a critical understanding of the chloride diffusion in unsaturated concrete. A mathematical model for predicting the relative chloride diffusion coefficient has been established in a previous work [1]. In the present paper, that model will be extended to analyze the time-dependent chloride diffusion coefficient in unsaturated cementitious materials. The role of the degree of water saturation in service life prediction is discussed. It is emphasized that the long-term sharp decrease of the chloride diffusion coefficient in marine concrete is not primarily caused by densification of the microstructure, but by the decreasing degree of water saturation. @en

Experiments have been carried out to study the influence of moisture condition, including moisture content and its distribution, on the chloride diffusion in partially saturated ordinary Portland cement mortar. The mortar samples with water-to-cement (w/c) ratios of 0.4, 0.5 and 0.6, cured for 1 year, were preconditioned to uniform water saturations ranging from 18 to 100%. The interior relative humidities of these partially saturated cement mortars, i.e. water vapour desorption isotherm (WVDI), were measured. The WVDI results in relation to the pore structures obtained from the mercury intrusion porosimetry tests of paste samples with the same w/c ratios were analyzed, which provided a basic insight into the moisture distribution in the non-saturated cement mortars. The relative chloride diffusion coefficients of cement mortars at various water saturations were determined based on the Nernst-Einstein equation and conductivity technique. It is found that the relative chloride diffusion coefficient D_rc depends on the degree of water saturation S_w and WVDI. At a given S_w level, the D_rc is larger for a higher w/c ratio. The role of the w/c ratio in the D_rc–S_w relation, however, becomes less pronounced with increasing w/c ratio. There exists a critical saturation, below which the water-filled capillary pores are discontinuous and the D_rc-value tends towards infinitely small. An increase of the w/c ratio results in a decrease of the critical saturation level.

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Chloride-induced reinforcement corrosion, caused by chloride diffusion in the unsaturated concrete cover, is a major durability problem of concrete structures. Current concepts for concrete mixture design and for service life prediction are generally based on the understanding of the chloride diffusion coefficient of saturated concrete. This will introduce uncertainties and give rise to misjudgement of the actual serviceability of concrete structures, especially when supplementary cementitious materials (SCMs) are added in the concrete mixture. This thesis developed a numerical tool for predicting the chloride diffusion coefficient in cementitious materials at different degrees of water saturation. The tool accounts for the microstructure and moisture distribution in cementitious materials. The tool provided a basis for service life design based on unsaturated chloride diffusion. The results of the thesis emphasize the importance of looking at the chloride diffusion coefficient at unsaturated state, rather than at saturated state, in order to more effectively utilize the SCMs in concrete mixture design.@en

In this paper, a new analytic model for predicting chloride diffusivity in unsaturated cementitious materials is developed based on conductivity theory and Nernst-Einstein equation. The model specifies that chloride diffusivity in unsaturated cementitious materials can be mathematically described as a function of chloride diffusivity in saturated state, water saturation and average pore diameter of the material. A series of experiments were conducted in order to validate the model. Mortar samples with varying cementitious mixtures were cast and cured for one year, followed by oven drying at 50 °C until desired water saturation levels (18 to 100 %) and homogeneous moisture distribution were obtained. The electrical conductivities of mortar specimens at various water saturations were measured and then converted into chloride diffusivities by using Nernst-Einstein equation. It is found that the experimental results can be well described by the analytic model proposed in this work. @en

Experiments are carried out to investigate the chloride diffusivity in partially saturated ordinary Portland cement mortars with water-to-cement (w/c) ratios of 0.4, 0.5 and 0.6. Based on resistivity measurement and Nernst-Einstein equation, the chloride diffusivities of cement mortars at various water saturations are estimated. The pore structures of cement pastes obtained from mercury intrusion porosimetry tests are analysed, which provide basic insight into the moisture distribution of non-saturated cement mortars. It is found that the chloride diffusivity is highly dependent on water saturation. The dependency is closely related to the pore size distribution and pore connectivity of the material. There exists a critical saturation level at which the pore solution becomes discontinuous and ionic diffusion is impossible. An increase of the w/c ratio results in a decrease of the critical saturation level.@en

The purpose of this paper is to investigate the pore connectivity and its influence on chloride transport property in cementitious materials under saturated condition. Mercury intrusion porosimetry experiment was carried out on a variety of hydrated cement pastes curing up to one year. By repeating pressurization-depressurization cycles, pore entrapment (namely ink-bottled effect) was expressed as a function of the equivalent diameter of penetrated throat neck pores. Accordingly, the small capillary pores that are present within patches of hydration products and the large capillary pores that are left out of hydration products were distinguished and their connections were ascertained, which provide evident basis for the study of transport property estimated by means of rapid chloride migration test on mortar samples. The results suggested that in hydrated cementitious system the capillary pore connections influencing mass transport should be mainly attributed to the connectivity of small capillary pores, whose relevance to chloride migration coefficient has also been testified by the well-fitted experimental plots in this study.@en

Due to environmental (low CO2 emission) and economic benefit, supplementary cementitious materials (SCMs) have been widely used in reinforced concrete structures. However, the question from engineering practice is to which extend these blended mixtures do meet the future durability criteria. Chloride ingression is nowadays considered as the main concern for reinforcement corrosion. Due to self-desiccation or drying process, cement-based materials are no longer saturated, which would greatly influence the chloride transport properties. This paper explores the effect of water saturation level on chloride diffusivity in cement-based materials by resistivity measurements. Experiments were performed on mortars made of different blended materials, i.e. fly ash, blast furnace slag and limestone powder. Mortar materials have been curing for 200 days conditioning in humid room at 20±1°C, followed by oven drying at 50°C until the samples reach targeted water saturation levels. The results showed that chloride diffusivity is highly dependent on the water saturation level and water-vapour desorption isotherm of the mortar material. The dependency varies with cement type, and is significantly related to pore size distribution characteristics. In addition, the effect of water content on chloride diffusivity is more evident in blended cement mortars than reference ordinary Portland cement mortar. For the materials studied in this research, the capillary pores with diameter range of 7.1-73 nm plays dominated role in chloride diffusion under non-saturated state.@en