Cement Suspensions Research Articles

Mechanisms of air bubble rise in cement suspensions studied by X-ray analysis

The de-airing of fresh concrete is a crucial step in ensuring sustainable and durable concrete structures. Currently, proper de-airing is ensured by adjusting the fresh concrete consistency i.e. its rheological properties, without any knowledge whether this will actually guarantee sufficient de-airing. The exact relationship between the rheological properties and air bubble dynamics in concrete is not yet understood. In this paper, a detailed study was carried out in which air bubbles of different size and volume were injected in fresh cement suspensions with varying solid volume fractions (i.e. varying rheological behaviour). The air bubbles were visualised by means of X-ray radiography and the rise behaviour was evaluated with image based algorithms. It was found that in more viscous suspensions (i.e. higher particle contents) the bubble speed increases with decreasing suspension viscosity. For suspensions with higher flowability (i.e. less particle content), the inertial forces dominate the viscous forces, with the viscosity playing a subordinate role. The shear rate induced by an ascending air bubble is sufficient to locally reduce the dynamic viscosity of the suspension. In addition, it leads to shear-induced particle migration, with less concentrated regions in the ascending channel. Consequently, subsequent bubbles are influenced by changes of viscosity in this channel, rising faster and having different bubble shapes.

Study on mechanical properties of large deformation segmental cement-based honeycomb structure

Honeycomb structures provide a new means of controlling and supporting the tunnel envelope. However, traditional honeycomb structures have low strength and poor stability, and are prone to stress concentration and instability, further limiting their application in deep tunnel support projects. In this paper, a new type of segmental cementitious honeycomb structure is investigated, its performance under different loading rates is tested, and its application in deep large deformation tunnel support is discussed. Firstly, the honeycomb model was drawn and the honeycomb skeleton was prepared. Then, the cement suspension technique was optimised. Secondly, the effects of different loading rates on the performance of segmented cement-bonded honeycomb structures were investigated by laboratory experiments. The results show that when the loading rate is 3 mm/min, the structure has the maximum load capacity and the best energy absorption performance. It is worth noting that too fast or too slow loading rate will affect the performance of the structure. Finally, the damage mechanism of the segmented honeycomb structure was further investigated by using an acoustic emission system, and the acoustic emission characteristics showed that the segmented cementitious honeycomb structure firstly went through a relatively stable stage of microcrack development under the action of the loads in all bands, and then a large area of damage was observed in the top layer of the honeycomb skeleton when the peak load was reached, resulting in the collapse of the whole layer of the honeycomb structure, which led to the collapse of the whole layer of the honeycomb skeleton. This led to the collapse of the whole layer of the honeycomb structure and a significant decrease in the bearing capacity, which confirmed the layer-by-layer damage characteristics of segmental cementitious honeycomb structures. In addition, the RA-AF values show that the loading rate has little effect on the crack type, which is almost unchanged with the increase of loading rate. These studies verify the feasibility of using honeycomb structure to support roadway with fast deformation speed and large deformation. It is of great significance to guide the application of honeycomb structure in deep roadway support engineering.

Stabilization and Solidification of Beryllium Waste: Influence of the Cement Composition on the Corrosion of Be Metal.

Beryllium metal is used as neutron moderator and reflector or multiplier in certain types of fission or fusion reactors. Dismantling of these reactors will produce radioactive beryllium waste, classified as low- or intermediate-level waste, that will need to be stabilised and solidified before being sent to disposal. The cementation process is under consideration because it may offer a good compromise between simplicity of implementation, cost, and quality of the final cemented wasteform. Nevertheless, knowledge of the corrosion behaviour of Be metal in a cement-based matrix is still limited, partly due to the high toxicity of Be that complicates testing. This study thus investigates Be corrosion in cement suspensions using potentiometry, voltammetry, and electrochemical impedance spectroscopy. Among the five different investigated systems (Portland cement blended without or with 40 wt.% silica fume, calcium sulfoaluminate clinker blended without or with 15% anhydrite, and calcium aluminate cement), Portland cement blended with 40% silica fume and calcium sulfoaluminate cement comprising 15% anhydrite are the most effective in mitigating beryllium corrosion. They allow reduction in the corrosion current by factors of 4 and 50, respectively, as compared to Portland cement.

Analysis of thixotropy of cement grout based on a virtual bond model

Thixotropy of cementitious materials is a crucial intrinsic property that determines the flowability and workability of cement-based grout. A novel virtual bond model of cement particles is developed in this paper to depict the thixotropy of cement grout. A particulate description of the reversible and erasable interparticle bonds is established based on experimental observations with a focus on the non-contact interactions mainly contributed in practice by calcium silicate hydrates (C–S–H). The structural breakdown of the cement network is realized through bonds breakage under applied motion, and the bonding network recovers with regeneration of interparticle connections that involve reversible hydrate reactions in the mixture. The balance between bond rupture and rebuilding can be tuned by assigning different strength limits for bond breakage. We have implemented this model in the open-source code Yade to carry out 3D discrete element method simulations of a rotational vane system filled with spherical particles, and the results show good agreement with experimental data. The modelling results reveal the transition from a solid-like structure to a fluid-like medium within cement suspensions caused by the evolution of broken interparticle bonds. The results also provide a distinct view of thixotropic variation upon disturbance. This model is extendable to other cohesive materials providing an explicit physical definition of the interparticle interactions. It also provides a theoretical explanation for the empirical estimations of thixotropy common in engineering industries and a potential means of measuring cementitious granular flow that may be useful in future studies.

Temperature induced fast-setting of cement based mineral-impregnated carbon-fiber reinforcements for durable and lightweight construction with textile-reinforced concrete

Developing durable and sustainable mineral-impregnated carbon-fiber (MCF) reinforcement system is today an effective measure to solve common service issues of conventional steel or FRP reinforcement in building sector. This study introduces a novel methodology for the design and realization of fast-setting cement based MCF reinforcements via targeted thermal activation. The process involves impregnating continuous yarns with a micro-sized particle cement suspension utilizing custom-built manufacturing equipment. Subsequently, the impregnated yarns undergo controlled heating at moderate temperatures to accelerate the cure process and strength development. Flexural and tensile performance of the MCFs exhibits progressive improvements with longer curing durations (from 2 to 20 h) and higher temperatures (from 40 °C to 60 °C). Enhanced mechanical properties are attributed to advanced hydration reactions and microstructural densification, as proven by thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), scanning electron microscopy, isothermal calorimetry and micro-computed tomography (μCT). When heating at 60 °C for 20 h, as-produced MCFs demonstrate optimal tensile strength of 2747 MPa and flexural strength of 482 MPa, with exceptional bond with concrete substrate, comparable to conventional FRPs. The proposed post-treatment shows promising potential for significantly enhancing the flexibility of mineral matrix composites, making them suitable for a wide range of industrial and field applications.

Nanocrystalline Cellulose to Reduce Superplasticizer Demand in 3D Printing of Cementitious Materials.

One challenge for 3D printing is that the mortar must flow easily through the printer nozzle, and after printing, it must develop compressive strength fast and high enough to support the layers on it. This requires an exact and difficult control of the superplasticizer (SP) dosing. Nanocrystalline cellulose (CNC) has gained significant interest as a rheological modifier of mortar by interacting with the various cement components. This research studied the potential of nanocrystalline cellulose (CNC) as a mortar aid for 3D printing and its interactions with SPs. Interactions of a CNC and SP with cement suspensions were investigated by means of monitoring the effect on cement dispersion (by monitoring the particle chord length distributions in real time) and their impact on mortar mechanical properties. Although cement dispersion was increased by both CNC and SP, only CNC prevented cement agglomeration when shearing was reduced. Furthermore, combining SP and CNC led to faster development of compressive strength and increased compressive strength up to 30% compared to mortar that had undergone a one-day curing process.

New insights into the interaction between sorbitol-based liquid-type temperature rise inhibitor and cement hydration: From experiments to molecular dynamic simulations

The use of temperature rise inhibitors (TRIs) holds significant promise in mitigating thermal cracking issues in modern concrete by controlling the precipitation of C-S-H gel, the main product of cement hydration. However, the complexity of their interaction with the non-classical nucleation process of C-S-H remains unclear. This study systematically investigated the influence of a sorbitol-based liquid-type temperature rise inhibitor (L-TRI) on the hydration kinetics of cement suspension using a combination of methods, including conductivity testing, pore solution analysis, and molecular dynamic simulations. It is revealed that the admixture-to-water ratio, rather than admixture-to-cement ratio, governs the effect of L-TRI on cement hydration. The disturbance of L-TRI molecules in the pore solution, mainly calcium complexation and water stabilization, plays a decisive role in inhibiting the secondary nucleation of C-S-H and decelerating the following growth. In contrast, L-TRI has a negligible influence on cement dissolution and the formation of C-S-H precursor.

Storing CO2 while strengthening concrete by carbonating its cement in suspension

Cement is a key constituent of concrete and offers a large sequestration potential of carbon dioxide (CO2). However, current concrete carbonation approaches are hindered by low CO2 capture efficiency and high energy consumption, often resulting in weakened concrete. Here, we conceptually develop and experimentally explore a carbonation approach that resorts to injecting CO2 into a cement suspension subsequently used to manufacture concrete, turning the carbonation reaction into an aqueous ionic reaction with a very fast kinetics compared to traditional diffusion-controlled approaches. This approach achieves a CO2 sequestration efficiency of up to 45% and maintains an uncompromised concrete strength. The study shows that the CO2 injection rate influences the polymorph selectivity of mineralized calcium carbonate (CaCO3) depending on the local environmental conditions and impacts the strength of concrete. The technological simplicity of the proposed approach enables a reduced carbon footprint and promising prospects for industrial implementation.

Load-bearing behavior of short-fiber-reinforced textile-reinforced concrete for a wrapping process

In the research project titled Wrapping process for highly fiber-reinforced concrete using the example of a pump sump (WiFaPu), a new production process for components made of short-fiber-reinforced textile-reinforced concrete was developed. The idea of wrapping concrete continuously around a formwork evokes high-level requirements on the workability of concrete while matching reinforcement properties. In this study, the load-bearing behavior of wrappable short-fiber-reinforced textile-reinforced concrete with an uncoated textile reinforcement made of AR-glass is investigated via tensile and bending tests. The concrete inserted into the wrapped layers by casting, laminating or spraying and the short fiber content and warp/weft direction of the uncoated fabric have a major influence on the load-bearing behavior of the tested specimens with regard to the utilized tensile strength and crack spacing. Members produced by inserting the concrete in a spraying or laminating process showed low tensile strength and unsatisfactory cracking patterns in the warp direction of the fabric due to the lacing of rovings, which led to lower matrix penetration. For rovings in the weft direction without lacings, higher utilized tensile strengths and fine crack patterns are observed. The casting of specimens leads to significantly higher utilized textile tensile strengths if the reinforcement is slightly tightened in the formwork. Compacting fine-grained concrete leads to better roving penetration than does the spraying process. The addition of short fibers generally benefits the cracking pattern and partially compensates for the negative effects of the textile weave on the utilized tensile strength as well as on the warp direction of the crack pattern. The load increase in tensile and bending tests is proportional to the increase in short fiber content and fiber length. Finally, the influence of different coatings is examined for another uncoated textile reinforcement made of AR-glass with a more favorable textile weave. The utilized tensile strength of the reinforcement is almost doubled when impregnated with a fine cement suspension and directly installed in the formwork and cast. An impregnation process such as this can also be integrated into the wrapping process.

Study on Replacement of Fine Aggregate with Light Weighted Super Absorbent Material in Internal Curing Concrete: A Review

The research on internal post-remediation approaches to high-performing concrete (HPC) for various materials is analyzed in this paper. To achieve the necessary internal hardness, lower the cement suspension's tendency to dry on its own, and drop the chance of cracks in the hardened concrete, common materials are required. Additionally, the behavior of HPC is the primary concern of this study, along with the microstructure of hydrated cement paste, density, strength (compression, tensile, and bending), and shrinkage (self-shrinking and drying). The findings indicated that internal hardness, instead of compressive strength, had a stronger effect on tensile and flexural strength in old age. The interfacial transition area gets denser and stronger as a result of internal solidification. Understanding the effect of waste on internal concrete (ICC) presents the primary challenge in analyzing prior research.

Dispersion behavior of silica fume in cementitious suspensions

The dispersion characteristics of silica fume are pivotal factors influencing the rheological properties, mechanical properties, and durability of cement-based materials. This study investigated the dispersion behavior of different types of silica fume including highly densified silica fume (HDSF), moderately densified silica fume (MDSF), and raw silica fume (RSF) in water and cement paste filtrate mediums. Optical microscopy (OM) was utilized to monitor the silica fume agglomerations in suspensions, and scanning electron microscopy (SEM) was employed to observe the agglomerates in hardened ultra-high performance concrete (UHPC) containing 25 wt% silica fume. The results revealed that RSF, MDSF, and HDSF have similar particle size distributions in water and cement paste filtrate, with D50 values of 10.9 μm, 15.0 μm, and 112.2 μm, respectively. The agglomeration degree of silica fume is virtually unaffected by superplasticizer. Additionally, the effects of dispersion methods (i.e., ultrasonic, aerodynamic, and aggregate premixing) on silica fume dispersions were also investigated. It was shown that the aerodynamic dispersion method significantly reduced silica fume agglomeration, and adequate ultrasonic treatment almost eliminated physical agglomeration. By contrast, premixing silica fume with aggregates and steel fibers partially mitigated agglomeration. This study provides a theoretical foundation and technical guidance for the effective utilization of different types of silica fume in concrete applications.

Fresh cement as a frictional non-Brownian suspension

Cement is an essential construction material due to its ability to flow before later setting, however the rheological properties must be tightly controlled. Despite this, much understanding remains empirical. Using a combination of continuous and oscillatory shear flow, we compare fresh Portland cement suspensions to previous measurements on model non-Brownian suspensions to gain a micro-physical understanding. Comparing steady and small-amplitude oscillatory shear, we reveal two distinct jamming concentrations, ϕμ and ϕrcp, where the respective yield stresses diverge. As in model suspensions, the steady-shear jamming point is notably below the oscillatory jamming point, ϕμ<ϕrcp, suggesting that it is tied to frictional particle contacts. These results indicate that recently established models for the rheology of frictional, adhesive non-Brownian suspensions can be applied to fresh cement pastes, offering a new framework to understand the role of additives and fillers. Such micro-physical understanding can guide formulation changes to improve performance and reduce environmental impact.

Investigation on the early proceeding of cement hydration containing dispersed nano Calcium Silicate Hydrated (CSH) seeds

Steam curing process is a traditional method for accelerating the progression of cement hydration to satisfy the rapid development of compressive strength in precast concrete industry. However, steam curing would release significant amount of CO2 into the atmosphere. In order to avoid the serious environmental pollution caused by the steam curing process, Calcium Silicate Hydrate (CSH) dispersed by comb-like polycarboxylate superplasticizer (PCE), shorted as dispersed nano CSH seed, was utilized to promote the early cement hydration at normal temperature. This investigation aims at disclosing the hydration process of cement and the microstructure evolution of cement paste in the presence of dispersed nano CSH seed during the early stage via setting time, ultrasonic pulse velocity (UPV), compressive strength, conductivity, pH, thermogravimetric (TG), 29Si NMR and nitrogen adsorption/desorption. The results demonstrated that with the dispersed nano CSH seed increasing, the setting process of cement paste was shortened dramatically, the compressive strength and UPV of cement mortar increased significantly especially at 12 h. The variation of conductivity and pH of cement suspension indicated that the ion dissolution process was promoted by dispersed nano CSH seed, which therefore conducive to the precipitation of cement hydration products. TG curves exhibited that the formation of cement hydration products, including CH (Calcium Hydrated), AFt and CSH gel were accelerated by dispersed nano CSH seed. The 29Si NMR (Nuclear Magnetic Resonance) and nitrogen adsorption/desorption results validated that the incorporating dispersed nano CSH seed promoted considerably the polymerization of CSH gel. The aforementioned results are correlated well with the fast formation of microstructure and in turn the rapid development of compressive strength at early age.

On the Flow of a Cement Suspension: The Effects of Nano-Silica and Fly Ash Particles.

Additives such as nano-silica and fly ash are widely used in cement and concrete materials to improve the rheology of fresh cement and concrete and the performance of hardened materials and increase the sustainability of the cement and concrete industry by reducing the usage of Portland cement. Therefore, it is important to study the effect of these additives on the rheological behavior of fresh cement. In this paper, we study the pulsating Poiseuille flow of fresh cement in a horizontal pipe by considering two different additives and when they are combined (nano-silica, fly ash, combined nano-silica, and fly ash). To model the fresh cement suspension, we used a modified form of the power-law model to demonstrate the dependency of the cement viscosity on the shear rate and volume fraction of cement and the additive particles. The convection-diffusion equation was used to solve for the volume fraction. After solving the equations in the dimensionless forms, we conducted a parametric study to analyze the effects of nano-silica, fly ash, and combined nano-silica and fly ash additives on the velocity and volume fraction profiles of the cement suspension. According to the parametric study presented here, larger nano-silica content results in lower centerline velocity of the cement suspension and larger non-uniformity of the volume fraction. Compared to nano-silica, fly ash exhibits an opposite effect on the velocity. Larger fly ash content results in higher centerline velocity, while the effect of the fly ash on the volume fraction is not obvious. For cement suspension containing combined nano-silica and fly ash additives, nano-silica plays a dominant role in the flow behavior of the suspension. The findings of the study can help the design and operation of the pulsating flow of fresh cement mortars and concrete in the 3D printing industry.

Volumetric surface area of natural coarse aggregates by interferometry-3D scanning (microscale) method

Surface (microscale) roughness of coarse aggregates influence the cement suspension’ viscosity and the interfacial bonding strength of concrete, but its characterization method is not consolidated and still a research challenge. 3D scanner determines accurately the shape, volume, but it is incapable of describing the roughness at microscale. Roughness at microscale (surface index – cm2/cm2) can be measured accurately using interferometry, but it cannot describe shape and volume of the particles. Once the volume of aggregate particles may vary 200% and it seems crucial to establish the (micro)roughness per volume (cm2/cm3) to better understand the effects in the properties of cementitious materials. This paper presents a method of obtaining volumetric surface area of coarse aggregates at micrometric scale by joining (cm2/cm3) both techniques. Three types of aggregates were assessed: gneiss, low-grade meta-carbonate (dolomite) and quartzite (gravel), with one hundred particles in the size range of 4.75 mm to 37.5 mm. For coarse aggregates, the larger the particles, the more of their surface area will be influenced by (mesoscale) shape and less by surface (microscale) roughness. However, the smaller the particles, SI index will differ considerably. The surface area determined by the method proposed in this paper provides a good correlation with the methylene blue method.

Shear thickening of cementitious suspensions: effect of high-volume fraction and hydration

The lubrication layer (LL) formed at the concrete–pipe interface plays a crucial role in facilitating concrete pumping. Shear thickening in this layer significantly affects concrete pumping. However, very few studies are available for understanding the onset and intensity of shear thickening of the LL. In this study, the effect of solid volume fraction (SVF), superplasticiser dosage, supplementary cementitious materials and hydration on the shear thickening (continuous and discontinuous) behaviour of cementitious suspensions were investigated. Results show that an increase in SVF reduces the shear thickening intensity for systems using ordinary Portland cement (OPC), whereas the intensity is amplified for systems with fly ash (FA) or ground granulated blast furnace slag (GGBS). An increase in superplasticiser dosage results in an early onset of shear thickening and increases the shear thickening intensity, regardless of the binder used. Systems including GGBS exhibit the highest shear thickening intensity, followed by OPC- and FA-based systems. Based on these results, it is evident that the superplasticiser dosage for OPC-based systems needs to be optimised based on the structural buildup, while, for FA- or GGBS-based systems, the superplasticiser dosage needs to be optimised based on shear thickening behaviour with respect to hydration.

Investigation of shear band strengthening by using different strengthening criteria

To prevent a critical failure mechanism, it is possible to simply strengthen the potential shear bands, which is a patented idea. The localization of potential shear bands that are most likely to form shear can be calculated. Before these shear bands can appear, they can be strengthened by injecting a cement suspension or using jet grouting at the locations where the most likely shear bands would occur. As a result, it is possible to increase the bearing capacity of the geotechnical system by focusing solely on the shear bands that are moste likely to form, minimizing the use of materials. Numerical modeling is employed to analyze the increase in bearing capacity. Four criteria for strengthening the shear bands are applied by incorporating them into a hypoplastic soil model, taking into consideration the material transition from soil to cement, as well as intermediate materials that can arise from jet grouting, which are mixtures of soil and cement. Two fundamental geotechnical systems, namely a shallow foundation and a retaining wall, are analyzed using ABAQUS/Standard for finite element analysis. The results demonstrate the potential of numerical modeling to identify the most critical localized areas in the soil that require strengthening, as well as the substantial increase in bearing capacity achieved after reinforcing the identified areas.

Mechanisms of thixotropy in cement suspensions considering influences from shear history and hydration

AbstractThe rheological properties of fresh concrete are a direct function of the interaction behaviour of the granular inventory of the concrete (i.e., gravel, sand and cement) and especially of the colloidal fractions of cement. Under low shear stresses, agglomeration of colloidal particles is observed, while at high shear stresses, dispersion of these agglomerates occurs. Besides the agglomeration state, the formation of shear banding, zones with concentrated shear flow, is another controlling mechanism of the flow behaviour of cement suspensions. Rheological creep tests in this study are focused on investigating the influence of shear history and hydration process on thixotropy of cement suspension. In this paper, the meaning of the word thixotropy is slightly extended to additionally encompass rheological aging and hydration effects. Selected samples were analyzed by coupling a rheometer to synchrotron X‐ray tomography to gain insight into the shear‐induced microstructural changes during shear start‐up tests. The observations show heterogeneities in the velocity profile in the shear gap and the development of shear banding.

Resting time effect on the rheological behaviour of glass beads and cement suspension: The role of PCE size and ionic strength

AbstractStudies have shown that in an aqueous suspension of inert glass beads, the yield stress is increased as a function of resting time. This study investigates this unexpected phenomenon, focusing on solid surface potentials, PCE size in ionic solutions, and their possible relations to rheological behaviour with resting time. Results indicated that PCE in artificial pore solution will result in a higher yield stress with time. PCE with lower anionic charges and molecular weight can significantly increase the yield stress after 2 h of resting. The absolute value of the zeta potential grows to a certain degree after resting phase. Dynamic light scattering (DLS) results showed that the size of PCE molecules in ionic solutions increases over time, suggesting an enhanced depletion force or bridging force among glass bead particles, which could be the reasons for the increase in yield stress over time. Furthermore, the possible impacts of the depletion force or bridging force resulting from the PCE size change on the rheological properties of cement suspension are discussed.

Influence of in situ ettringite formation on the rheological properties of quartz suspensions

Cement suspensions are subject to hydration processes causing dissolution of cement clinker, precipitation of hydration products and changes in the ion content of the carrier liquid. These mechanisms take place in parallel and control the workability of cement suspensions. The precipitation of nano-granular ettringite is currently believed to be one of the key influencing factors for rheological properties of fresh cement suspensions. In order to quantify the effect of ettringite precipitation from other precipitation or dissolution processes onto the rheological behaviour, a model suspension consisting of quartz powder and in situ formed ettringite is used, enabled by implementing a new upscalable synthesis route of ettringite. The quantity of formed ettringite in the model suspension is varied and derived qualitatively from X-ray diffraction as well as quantitatively from inductively coupled plasma optical emission spectrometry and thermogravimetric analysis. The influence of the ettringite content on the rheological properties is determined and linked to the specific surface of the formed ettringite particles. This allows a quantification of the controlling mechanism of ettringite and the dependency of these mechanisms on particle properties. The correlation of the results to a reference suspension without ettringite verifies the statement of influence.