Evolution Of Paste Research Articles

Distinctive rheological and temporal viscoelastic behaviour of alkali-activated fly ash/slag pastes: A comparative study with cement paste

This paper presents the results of the investigation on the fundamental differences in the viscoelasticity between alkali-activated fly ash/slag materials (AAMs) and cement pastes. The effects of the precursor and the activator on the rheological behaviour of AAM pastes were studied. Given the specific precursor, the activator viscosity significantly affected the AAM paste viscosity. The high viscous activator increased the plastic viscosity of AAM pastes by 4–8 times higher than that of cement paste and drastically decreased the yield stress due to viscous effects and weak colloidal interactions. Temporal changes in viscoelasticity showed that the negligible colloidal interactions between particles in AAM paste made the system non-percolated until the initial setting. This resulted in very different viscoelastic behaviour compared to the one from the well-percolated network in cement paste. Considering all results obtained in this study, the paper describes the short-term evolution of the AAM paste from the fresh condition to the initial setting.

The role of metakaolin in pore structure evolution of Portland cement pastes revealed by an impedance approach

Metakaolin has been shown to play a significant role in the pore structure evolution of cementitious materials due to its high pozzolanic reactivity and small fineness. In this work, the early-age evolution of pore structure of moderate heat Portland cement (MHPC) pastes incorporating metakaolin is studied using an innovative impedance approach. Based on the fractal modeling, the pore structure parameters of MHPC blended with metakaolin of various mass percentages up to 20%, including porosity, pore size distribution, pore tortuosity and its fractal dimension, and mean and maximal pore diameters, are determined. The results from the impedance measurements show that the porosity of metakaolin-blended MHPC at both initial hydration and pozzolanic reaction stages increases with the hydration time, which is, respectively, attributed to the dissolution of cement and metakaolin grains. The maximal porosities and the saturation states of MHPC-metakaolin blended pastes occur concurrently due to the dissolution behavior of cement grains. It is concluded that metakaolin incorporation at the 10% replacement ratio is optimal in terms of pore structure refinement of blended cement pastes.

Water evolution and hydration kinetics of cement paste under steam-curing condition based on low-field NMR method

The presented study focuses on the water evolution and hydration characteristics of Portland cement paste under steam-curing condition. The detailed information concerning water consumption and micro-pore network as well as the corresponding hydration kinetics of hydrating cement paste (HCP) during steam-curing process were discussed based on the low-field nuclear magnetic resonance measurement. The results indicate that, within steam-curing period, most of the free water in HCP are distributed in smaller pores less than 500 nm. The water evolution rate of paste system can be divided into three stages, i.e. acceleration stage within heating period and the first 2 h of constant-temperature period, rapid low-down stages in later constant-temperature period and diffusion stage in rest period. The curing temperature predominantly controls the water evolution rate. Finally, the hydration kinetics model of paste within steam-curing period is established and verified by experiment.

Effect of a liquid-type temperature rise inhibitor on cement hydration

A liquid-type temperature rise inhibitor (L-TRI) based on modified sorbitol was developed to solve the thermal cracking issue by modulating the heat release from cement hydration during the early age. Both the exothermic process and phase evolution of cement pastes blended with/without L-TRI were investigated by the combination of isothermal calorimetry, in-situ XRD, ICP-OES and SEM. Also, the adsorption of the L-TRI in the cement paste was identified by total organic carbon analysis. The testing results suggested that the L-TRI affected the heat flow curve in two ways. It prolonged the induction period as a retarding effect. It also reduced the heat flow during the acceleration period and decreased the maximum heat flow as a depressing effect. The retarding effect was mainly caused by the complexation of unabsorbed molecules in the pore solution, and the depressing effect was caused by inhibiting the growth of C-S-H.

Interfacial adhesion mechanism between organic polymer coating and hydrating cement paste

Surface coatings have proved to be one of the most effective methods to protect the concrete structures in aggressive environments. However, coating the structures with polymer after 28 days of curing/hydration to get the best interfacial performance has little scientific backing. The measured surface free energy of the hydrating cement paste showed that the surface energy is high for a 2-day hydrated paste compared to cement paste cured for 7, 28 days. This opens up the possibility for exploration into coating an acrylic based polymer on a concrete surface before it reaches 28 days of hydration, which has been investigated in this study. The practical work of adhesion between the hydrating cement paste and polymer as measured from macro mechanical tests showed to have a dependency on the surface evolution of the cement paste. However, it was also observed that it depends equally on the polymer functional groups. Poly (methyl methacrylate) (PMMA), when coated on a 2-day hydrated cement paste, showed higher interfacial strength than when coated on later days (7, 14, 28 days). In the case of epoxy, irrespective of when the polymer was coated, it had the same interfacial strength. The influence of the evolving cement paste and the polymer functional group (PMMA in particular) on the interfacial strength at the micro- and molecular-length scales are discussed. It is found that the free/adsorbed calcium form a metal-complex with the ester functional group of the polymer in addition to the physical interactions, resulting in higher interfacial strength. The observations shed light on the importance of interfacial science and act as a guideline for the design of effective polymer-cement coatings.

The Impact of the Amount of Water Used in Activation Solution and the Initial Temperature of Paste on the Rheological Behaviour and Structural Evolution of Metakaolin-Based Geopolymer Pastes

This study aimed to determine the impact of the initial temperature of the paste (from 5 °C to 35 °C) and the addition of water, which reflects a decrease in the molarity of activation solutions (AS) by diluting 10 M NaOH with distillate water, on the rheological properties of geopolymer pastes. Additionally, this resulted in changes to the physical–mechanical properties of geopolymers after curing. A higher amount of water in the AS composition and higher initial paste temperature led to an increase in the spread values up to 28% and decreases viscosity. A smaller amount of water in the AS composition and a higher initial paste temperature accelerated the speed of the geopolymer structure formation up to 1.5 times during the curing period, increased compressive strength and reduced apparent porosity and pore size. X-ray diffraction confirmed the compressive strength test results and revealed that the lower amount of water in the AS and the higher initial paste temperature for the geopolymer preparation significantly affected the mineral formation and physical and mechanical properties of the samples.

Hydration heat evolution of Portland cement paste during unsteady steam curing process: Modelling and optimization

The hydration process under unsteady steam curing condition is difficult to monitor in real time by experimental method. This study developed a numerical method to describe the hydration heat release characteristics of Portland cement under atmospheric steam curing condition. Instantaneous activation energy and an iterative algorithm were employed in this method to improve the accuracy. It was found that prolonging the precuring time to over 7 h or heating time to over 3 h would both decrease the peak value of hydration heat release rate, while a treatment time over 8 h would slightly affect the total heat release during steam curing. The validity of the method was confirmed by the experimental results. This proposed method can rapidly analyze the effect of steam curing regimes on hydration heat release behavior of Portland cement and thus optimize the hydration process of Portland cement under steam curing process in order to mitigate or avoid heat damage of steam curing concrete.

Sintered Nanocopper Paste for High-Performance 3D Heterogeneous Package Integration

Fifth-generation (5G) communications have been driving major package innovations to enable low-loss interconnects between the integrated circuits (IC) and other system components such as antenna arrays. Antenna-in-package with three-dimensional (3D) or double-side component is widely pursued as the front-up architecture to realize this vision. The interconnect height and losses are critical parameters in these 3D package structures. Copper sintering paste is emerging as an ideal candidate to replace solders for both off-chip and on-package interconnects because of the resulting higher electrical conductivity and relatively simple manufacturability with additive processes. This article focuses on computational modeling, optimization of sintering conditions, and electrical conductivity measurements of highly conductive copper paste to verify the proposed model. The model is based on two-particle sintering theory, which describes the neck growth evolution of copper paste at the initial stage of sintering. The neck-growth model shows good consistency with the measured neck size of copper, and hence can be used to provide guidelines for the sintering conditions of copper paste. The celectrical conductivity measurements suggest that the copper paste sintered at 260°C for 30 min shows a electrical conductivity of 1.4 × 107 S/m, which is 82% higher than that of solder. In addition, this article investigates the potential of copper paste interconnect technology in high-frequency applications such as in 5G millimeter-wave communications. The transmission lines patterned with the copper paste show good correlation with simulated results in the millimeter-wave frequency band. The high-frequency characterization also indicates that the copper paste enables simple circuit patterning, offering equivalent signal losses with plated copper. The detailed analyses discussed in this article suggest the eligibility for multi-applications of copper sintering paste in IC packaging.

Effect of carbonation on cement paste microstructure characterized by micro-computed tomography

The carbonation of cement paste changes its microstructural characteristics. The porosity of the cement paste microstructure is reduced when calcite is produced from the reaction between portlandite and carbon dioxide. In this study, such microstructural changes were investigated using image analyses of the 3D microstructures obtained with synchrotron micro-computed tomography (CT). When two sets of samples were subjected to different degrees of carbonation, the porosity and pore size reductions were identified from the micro-CT images. The formation of calcite caused by the reaction of portlandite was indirectly identified by the reduction of the mean grayscale value or linear attenuation coefficient (LAC) of the solid phase voxels. It was confirmed that micro-CT could also be used as a supplementary tool for the analysis of the microstructural evolution of cement paste under carbonation.

Study on the early cement hydration process in the presence of cationic asphalt emulsion

Cement-asphalt (CA) paste could be regarded as the Portland cement (PC) paste with enhanced flexibility due to the addition of asphalt emulsion (AE) and could be utilized as the semi-flexible pavement and repairing materials. In order to extensively explore the process of early cement hydration and the microstructure evolution of PC paste in the presence of cationic AE, various measurements including setting time, hydration heat, conductivity, compressive strength and ultrasonic pulse velocity, Fourier transform infrared spectroscopy (FTIR), chemically bound water, thermogravimetric (TG) and field emission scanning electron microscopy (FESEM) were adopted in the investigation. The experiment results showed that the setting process of PC paste was gradually prolonged with the increase of A/C, implying that the formation of microstructure of PC paste was inhibited by cationic AE, which is in agreement with the hydration heat and conductivity test. The compressive strength and pulse velocity of ultrasonic wave of cement mortar reduced with the incorporation of cationic AE, which thus implied the porosity increases with the addition of A/C, and in turn indicating the generation of microstructure of cement paste was suppressed gradually with the increase of cationic AE, this could also be confirmed by the chemically bound water, FTIR and FESEM results. Finally, the possible retardation mechanism of cement hydration, caused by the cationic AE, was proposed.

Recycling of raw water treatment sludge in cementitious composites: effects on heat evolution, compressive strength and microstructure

Water treatment sludge (WTS) is produced daily and represents a globally significant solid waste stream. The application of this sludge as construction materials has been studied although most studies have modified the sludge before its incorporation, hence involving significant energy consumption. This study aims to use raw sludge as a novel cementitious material, by determining the effects of sludge addition on the composition and performance of cementitious composites. Important aspects such as the physicochemical interaction of the raw sludge with the Portland cement, the heat evolution of the cement paste and the compressive strength of the composite cement were carefully studied. The results show that for 1-2% of WTS addition, the compressive strength and heat evolution of the cement paste was well maintained being close to the reference specimen after 28 days of curing. However, for sludge addition above 5%, a delay in the hydration reaction was observed, together with about 25% reduction in compressive strength at 28 days of curing. The mineralogical and thermal analysis showed decreasing portlandite content and increasing calcite in the WTS-amended composites. Scanning electron microscope analysis demonstrated that the addition of sludge induced more porous and weak surface structures compared to the reference specimen.

Properties evolution of high-early-strength cement paste and interfacial transition zone during steam curing process

At present, concrete prefabricated elements commonly used in railway engineering in China were produced by steam curing. Steam curing is a complex hygro-thermal coupling process that has a significant impact on the hydration process and microstructure formation of cement-based materials. This paper investigated the macroscopic mechanical properties, hydration process, pore structure, microstructure formation and micromechanical properties of cement-based materials during steam curing process. It is discovered that the evolution of cement-based materials under steam curing is a dynamic process that can be divided into three stages. The fastest evolution stage of the performance is the rising stage and the first 2 h of treatment stage. At this time, the microstructure of the pastes is rapidly formed, and the mechanical properties are significantly improved. Porosity and interfacial transition zone widths also rapidly decrease. The reaction activity of cementitious material is one of the critical factors affecting the properties evolution of steam-cured paste. In addition, an evolution model of the pore structure was established based on the change in the hydration degree during steam curing, which is proved to be well consistent with the measured results.

In situ monitoring of hydration of magnesium oxysulfate cement paste: Effect of MgO/MgSO4 ratio

This work systematically investigated the early hydration process of magnesium oxysulfate (MOS) cement pastes by non-contact impedance measurement (NCIM) and other microstructural tests. According to the evolution of impedance response, the hydration process of MOS cement pastes can be divided into three stages (dissolution, acceleration and hardened stages). The identification of hydration is strongly related to the transformation of solid phases and alkalinity of pastes. The presence and crystalline degrees of 517 phase and 318 phase are evaluated by DTG and XRD tests, respectively. It seems that MgO/MgSO4 ratio does not affect crystalline degree of hydrated products much. MH crystals and cracks are considered to result in the low mechanical strength of MOS cement paste. It can be inferred from experimental results that NCIM is a good method to analyze the hydration evolution of MOS cement pastes in-situ.

Numerical Validation of a Population Balance Model Describing Cement Paste Rheology.

Rheology control is essential during the period in which cement and concrete pastes are encountered in the fresh state, due to the fact that it directly affects workability, initial placement and the structural performance of the hardened material. Optimizations of clinker formulations and reductions in cement-to-water ratios induced by economic and environmental considerations have a significant effect in rheology, which invokes the need for mechanistic models capable of describing the effect of multiple relevant phenomena on the observed paste flow. In this work, the population balance framework was implemented to develop a model able to relate the transient microstructural evolution of cement pastes under typical experimental conditions with its macroscopic rheological responses. Numerical details and performance are assessed and discussed. It was found that the model is capable of reproducing experimentally observed flow curves by using measured cluster size distribution information. It is also able to predict the complex rheological characteristics typically found in cement pastes. Furthermore, a spatially resolved scheme was proposed to investigate the nature of flow inside a parallel-plates rheometer geometry with the objective of assessing the ability of the model of qualitatively predicting experimentally observed behavior and to gain insight into the effect of possible secondary flows.

Optical method to characterize and assess setting evolution of cement pastes: publisher's note.

This publisher's note amends the author affiliations in Appl. Opt.59, D1 (2020)APOPAI0003-693510.1364/AO.59.0000D1.

Role of shear stress at rest on the viscoelastic response of fresh cement pastes

Fresh concrete is composed of aggregates and cement paste, which itself is a dense suspension that exhibits a yield stress and serves as a binder that solidifies through cement hydration. Although there is relatively little reaction during the fresh state (i.e., before setting), there is still the formation of early hydration products that will influence the paste rheology. Nonzero stress at rest widely exists in concrete practice, e.g., interruption during pumping, static formwork casting, and 3D concrete printing, but its impact on eventual viscoelastic properties is not well understood. In this study, we investigated the influence of different levels of applied shear stress at rest on fresh cement pastes and show that it dramatically alters the eventual solid performance (i.e., static yield stress and stiffness). With increasing applied shear stress at rest, the cement paste exhibited increasing static yield stress and increasingly elastic behavior, where the origin was tied to the progression of hydration. Cement pastes were compared against magnesium oxide suspensions, which served as an inert model system, and an accelerated rate of static yield stress increase over time was observed in the former but not in the latter. This highlights the role of early cement hydration on the solidification behavior of fresh cement pastes in the presence of nonzero shear stress at rest. We further support this by showing that the structural evolution of cement paste under applied stress can primarily be attributed to an enhanced elastic limit, which is associated with early hydration. The influence of two common viscosity modifying admixtures, nanoclay and diutan gum, on these properties was also examined. This work demonstrates the importance of taking the shear stress at rest into consideration for laboratory rheological experiments and industrial concrete applications, as it can substantially impact the eventual solidification behavior.

In situ examination of engineered local additives in cement paste via neutron based scattering techniques

Hydration behavior of cements with additives is complicated due to the multifarious nature of hydration products formed at various stages of hydration. Neutron-based scattering techniques are useful in detecting both the microstructural evolution and water dynamics of hydrating cement pastes. This work investigates the effect of early-age hydration on microstructural evolution of cement paste with local volcanic ash using neutron-based beamline techniques. Early-age hydration dynamics of Portland cement paste with volcanic ash was examined via inelastic neutron scattering (INS), while the evolution of microstructure was observed with small angle neutron scattering (SANS). The data obtained from the ratio of volume fractal to surface fractal clearly showed that greater than 30% substitution of volcanic ash leads to unreacted volcanic ash and coarser morphology, which could influence the chemo-mechanical properties of the resulting hydration products. INS results showed that the effect of finer-particle-sized volcanic ash contributes to initiation of gelation, which accommodates higher conversion of free water to bound water. The multi-scale analysis that combines a time-resolved study of water dynamics along with microstructure is found to provide a basis for effectively utilizing engineered additives for the local cement industry.

Optical method to characterize and assess setting evolution of cement pastes.

In this work, a nondestructive and noninvasive technique, based on laser technology and the use of the Kubelka-Munk model to calculate the dynamic behavior of the cement paste from the diffuse reflection properties of both cement components and hydration products, is proposed. Also, the Powers-Brunauer model is used to explain this behavior during the first 9 h of the hydration process. This method allows us to obtain the initial and final cement setting times from the diffuse reflection measurements.

Effect of the Sodium Silicate Modulus and Slag Content on Fresh and Hardened Properties of Alkali-Activated Fly Ash/Slag

This paper presents the results of an experimental study performed to investigate the effect of activator modulus (SiO2/Na2O) and slag addition on the fresh and hardened properties of alkali-activated fly ash/slag (AAFS) pastes. Four activator moduli (SiO2/Na2O), i.e., 0.0, 1.0, 1.5, and 2.0, and five slag-to-binder ratios, i.e., 0, 0.3, 0.5, 0.7, 1.0, were used to prepare AAFS mixtures. The setting time, flowability, heat evolution, compressive strength, microstructure, and reaction products of AAFS pastes were studied. The results showed that the activator modulus and slag content had a combined effect on the setting behavior and workability of AAFS mixtures. Both the activator modulus and slag content affected the types of reaction products formed in AAFS. The coexistence of N–A–S–H gel and C–A–S–H gel was identified in AAFS activated with high pH but low SiO2 content (low modulus). C–A–S–H gel had a higher space-filling ability than N–A–S–H gel. Thus, AAFS with higher slag content had a finer pore structure and higher heat release (degree of reaction), corresponding to a higher compressive strength. The dissolution of slag was more pronounced when NaOH (modulus of 0.0) was applied as the activator. The use of Na2SiO3 as activator significantly refined the pores in AAFS by incorporating soluble Si in the activator, while further increasing the modulus from 1.5 to 2.0 prohibited the reaction process of AAFS, resulting in a lower heat release, coarser pore structure, and reduced compressive strength. Therefore, in view of the strength and microstructure, the optimum modulus is 1.5.

In situ monitoring of pore structure of magnesium oxysulfate cement paste: Effect of MgSO4/H2O ratio

This work systematically presented the pore structure evolution of different magnesium oxysulfate (MOS) cement pastes by non-contact impedance measurement (NCIM), fractal theory and other microstructural experiments. According to the evolution of electrical impedance response, three hydration stages (dissolution, acceleration and hardened stages) can be identified in the pastes. Furthermore, based on the results obtained from NCIM and fractal theory, the porosity, incremental pore volume, pore tortuosity, average pore diameter and maximal pore diameter of MOS cement pastes are investigated. The influence of MgSO4/H2O ratio on pore structure of MOS cement paste is primarily explained. It can be inferred from predicted and experimental results that NCIM is a good method to analyze the microstructure evolution of MOS cement pastes in-situ.