Flowability Of Cement Paste Research Articles

The Relationship between Molecular Structure and Foaming of Poly(ethylene glycol)—Poly(propylene glycol) Triblock Surfactants in Cementitious Materials

This study investigates the relationship between the molecular structure and foaming of poly(ethylene glycol) and poly(propylene glycol) triblock copolymers in Portland cement pastes. Four copolymers with different molecular structures were studied at varying concentrations. All copolymers showed a reduction in surface tension of the cement pore solution; however, only some of them demonstrated foaming and air entraining in cement paste. The results indicated that the molecular structure parameter, hydrophilic-to-lipophilic balance (HLB), has a direct relationship with the foaming and air-entraining performance of the copolymers. The total organic carbon measurements showed very small adsorption of these non-ionic copolymers on hydrating cement particles due to the lack of surface charge needed to interact with the heterogeneously charged surface of hydrating cement. In addition, these copolymers did not seem to affect the flow of cement paste due to a lack of adsorption on cement particles. The cement paste modified with the copolymers showed increased water sorption compared to the control paste due to the increased capillary porosity and slight increase in pore surface hydrophilicity. However, the freeze-thaw resistance was shown to improve with an increase in the number of air voids in the modified cement pastes. The findings establish the relationship between molecular properties of copolymers and their air-entraining performance in cement paste to mitigate the damages caused by freeze-thaw action.

Evaluation of the acid degradation properties of colloidal nanosilica‐based cement pastes

AbstractThe present study examined the effects of colloidal nanosilica on the performance of cement paste samples exposed to nitric acid for a continuous period of 28 days. Compressive strength and weight losses in paste samples as a result of this attack were determined to be 66% and 27%. The inclusion of nanosilica up to 6% by weight reduced these sample losses by densifying the microstructure with its pore filling and pozzolanic effects. The addition of nanosilica decreased the flow of cement paste by 30%, but improved compressive and flexural strengths at the end of 28 days by up to 45% and 118%. The acid attack on the cement sample resulted in the formation of a degraded layer surrounding the unaltered core material. X‐ray tomography images showed this zone formation, and the scanning electron microscopy images revealed the degraded microstructure. The results of the x‐ray diffraction and nanoindentation tests confirmed the complete decalcification of the main cement hydrates, including calcium silicate hydrate and portlandite, as a result of acid attack. The results obtained suggest that the incorporation of nanosilica strengthens the microstructure of the cement matrix, thus improving the service life of structural concrete operating in chemical environments.

A new orientational molding method for ultra-high performance concrete with high content of steel fiber and investigation on its flexure and axial tensile properties

Although ultra-high performance concrete (UHPC) has ultra-high mechanical properties (compressive strength up to 150 MPa), the tensile strength to compressive strength ratio is even lower than that of ordinary concrete (less than0.1). Therefore, improving the flexural and tensile properties of UHPC is an important issue in civil engineering. In this paper, based on the vibration and cement paste flow control, a fiber orientational extrusion nozzle was designed. Then, a new orientational molding method for UHPC was developed. Four groups of UHPC with different steel fiber contents were designed (0 vol%, 2 vol%, 4 vol%, and 6 vol%) and fabricated by the new method. The flexural and axial tensile performance of UHPC were systematically studied. The results indicated that the new orientational molding method was suitable for manufacturing UHPC with high content of steel fibers, which reached even 6 vol%. With the orientational molding method, the flexural strength and axial tensile strength of UHPC with 4 vol% content of steel fiber reach 55.6 MPa and 14.1 MPa, respectively. In addition, the results show that the flexural and tensile strengths of the specimens formed using the oriented technique were maximum at 4 vol%. The flexural strength increases with increasing fiber content as the fiber content increases from 0 vol% to 4 vol%. However, the flexural strength decreases at 6 vol% fiber content. Besides, X-ray micro-computed tomography (CT) was used for the analysis of the orientation of steel fiber. Results indicate that most of the fibers showed parallel to the casting path orientation when using the orientational molding technique. The orientational rates were 72%, 81%, and 75% for fiber contents of 2 vol%, 4 vol%, and 6 vol%, respectively.

Effect of chain transfer agents in polycarboxylate superplasticizer on slump‐retention of concrete

AbstractTwo types of slump‐retention polycarboxylate superplasticizers (PC‐M and PC‐S) were synthesized by using 3‐mercaptopropionic acid or sodium hypophosphite as chain transfer agents, the effects of polycarboxylate superplasticizers on slump‐retention of cement and concrete were investigated, and the mechanism of action and the structure of polycarboxylate superplasticizers were investigated by using conductivity, complexing Ca2+ concentration, gel permeation chromatography (GPC), nuclear magnetic resonance hydrogen spectroscopy (1H‐NMR), and nuclear magnetic resonance phosphorus spectroscopy (31P‐NMR). The results indicated that the slump flow of cement paste mixed with PC‐M was increased quickly within 60 min and then lost gradually, while PC‐S increased slowly and evenly within 240 min. The slump flow of concrete mixed with PC‐S reached 405 mm after 150 min, but the concrete mixed with PC‐M was almost no fluidity after 150 min. The conductivity of the PC‐M aqueous solution decreased quickly within 45 min, then slowly decreased, while the conductivity of the PC‐S aqueous solution decreased slowly within 180 min. PC‐M had a much greater complexation ability of Ca2+ than PC‐S. The composition and structure of the two slump‐retention polycarboxylate superplasticizers were altered by different chain transfer agents, and the hydrolysis‐adsorption capacity of each component was different. The rate of hydrolyzation‐adsorption of each component was PC‐M3 ≈ PC‐S3>PC‐M1>PC‐M2 ≈ PC‐S1>PC‐S2. The faster the adsorption rate, the faster the slump flow loss of concrete.

Effect of the structure of macromonomer in polycarboxylate superplasticizer on slump-retention of concrete

Three kinds of slump-retention polycarboxylate superplasticizers(52HPC4, 52IPC4, and 52EPC4) were synthesized by using HPEG, IPEG, or EPEG as macromonomers. The effects of polycarboxylate superplasticizers on the slump-retention of cement and concrete were investigated. The mechanism of action was explained by the composition of PCE polymers and monomer sequences (EHE, HHE, or HHH), which were investigated by using conductivity, GPC, 1H-NMR, and 13C-NMR. The results indicated that the slump flow of cement paste mixed with 52EPC4 was increased quickly within 120 min, while 52IPC4 doesn’t change within 240 min. Concrete mixed with 52IPC4 had the best slump-retention performance reaching 440 mm after 150 min, but concrete mixed with 52HPC4 was almost no fluidity after 120 min which had the largest slump flow loss through time than that containing other PCEs, and the conductivity of 52IPC4 aqueous solution decreased more slowly than other PCEs. Because of the highest content of effective PCE polymers, and the largest steric hindrance to ester bond resulting in the concrete had the best slump-retention performance when mixed with 52IPC4, because of the smallest steric hindrance to ester bond resulting in the fastest increase spread flow of cement paste which mixed 52EPC4, because of the highest content of low molecular polymers and the lowest content of effective PCE polymers in 52HPC4 resulting in the smallest initial slump flow of concrete, and the largest slump flow loss through time of concrete, even though the steric hindrance effect was larger than 52EPC4.

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Circulating fluidized bed fly ash (CFA), by-product of coal gangue power generation industry based on circulating fluidized bed combustion technology, differs from ordinary fly ash (OFA) in its physical and chemical properties. This paper aims to investigate the influences of CFA with three particle sizes on the workability and rheological properties of cement paste. The results show that untreated raw CFA (RCFA) can significantly degrade many properties of fresh cement paste. Directly using CFA as a mineral admixture in cement-based materials is not suitable in terms of workability. However, CFA through grinding methods can significantly mitigate the detrimental effect of CFA on the flowability of cement paste. When 15% ultrafine CFA (UCFA) was added, the cement paste exhibited relatively favorable workability and rheological properties. The rheological behavior of cement paste containing CFA with three particle sizes follow the shear thinning, which is in line with the modified Bingham model. Furthermore, the reasons for the impact of CFA on the microscopic level of cement workability are analyzed and discussed. The addition of CFA reduces the amount of free solution in the cement paste, leading to the formation of a flocculated structure and an increase in the thixotropic loop area. Based on the analysis of TOC, BET and TEM, it was determined that numerous polycarboxylate superplasticizer (PCE) molecules are adsorbed onto the rough and porous surface structure of CFA particles, hence the destructive ability of PCE to the flocculation structure caused by cement hydration is weakened. Therefore, the mechanism that CFA affecting the working performance of cement paste can be determined as: the adsorption of free water and PCE molecules.

Effect of hydrophobic polymers (HASE) as additives on the cement paste flow behavior

In the present study, the superplasticizing effect of HASE (Hydrophobic Alkali Swellable Emulsions) associative polymers on a cementitious formulation has been studied through a Design of Experiment (DoE) statistical approach. The main advantage of a study based on a DoE approach is that it gives the possibility of carrying out tests in which there is a combination of the parameters chosen on two extremes. Thus, the tests are driven by a combination of these parameters according to the design.The DoE approach was used to optimize the combination of HASE polymer synthesis reaction variables. As a final response from the design, or rather as an objective of our synthesis, the superplasticizing effect that each polymer has on the final diameter variation of the cementitious formulation after the Flow Table Test was chosen. The variation of the final diameter was considered as the difference with respect to the same formulation without the addition of the superplasticizer.The variables were chosen in such a way as to have the greatest impact on the reaction product, in particular the quantity of acidic methacrylic monomer (MMA), quantity of surfactant (SDS), quantity of free radical initiator (ASP) and reaction time (t). These variables were assumed to be independent variables. The design also included a central level, that is an experiment in which all the factors are set at an intermediate value between the minimum and the maximum. In this case, only one central level was added for a total of 17 experiments. By applying the DoE design it was possible to identify that the variable that has the greatest influence on the final response, that is the diameter of the cementitious formulation after the Flow Table Test, is the variable linked to the concentration of the initiator (ASP). The other variables showed a lesser influence on the final response.

Automatic Measurement Method of Slump Flow of Cement Paste Based on Dynamic Image Processing Technology

The slump flow and flow time T200 are the main parameters to evaluate the rheological properties of cement paste. Generally, the manual measurement method is used and it is difficult to get T200 accurately. In this paper, an automatic measurement method of slump flow based on dynamic image processing technology is studied. Firstly, the cement paste flowability test video is divided into frames every 100 ms; secondly, based on the principle of perspective transformation, an automatic point selection method is proposed to correct the frame image to solve the problem of shooting angle; thirdly, the watershed algorithm based on distance transformation is used to segment the boundary of the cement paste area, and the slump flow is calculated according to the polar coordinates of the boundary; lastly, the relationship curve between slump flow and time is automatically output, and the slump flow at any time is obtained. The method is verified by nine groups of cement paste flowability tests. The results show that this method can fastly and accurately obtain the slump flow and flow time T200. Compared with the traditional manual measurement method, the measurement error of slump flow is less than 2%, and the measurement error of flow time T200 is less than 8%. The method enables real-time dynamic measurement of the slump flow.

Synthesis and characterization of a polycarboxylate superplasticizer modified by sodium hypophosphite

AbstractA sodium hypophosphite (SHP) modified polycarboxylatesuperplasticizer (P‐PCE) was synthesized successfully and the synthesis parameters were optimized through orthogonal experiment. Gel permeation chromatography (GPC), X‐ray photoelectron spectroscopy (XPS), 31P magic spinning nuclear magnetic resonance (31P MAS NMR) spectroscopy and elemental analysis measurements were used to characterize the molecular structure of P‐PCE. Effects of P‐PCE on the dispersing performance of cement paste were systematically investigated via mini slump test. The results showed that P‐PCE exhibited superior dispersing performance, more retarding hydration effect and better sulfate and clay tolerance than conventional PCE (C‐PCE). Results showed that the maximum flow of cement paste could reach up to 305 mm, while that of the C‐PCE was 285 mm.The maximum variation of adsorption of P‐PCE was 9% and 15% in the presence of sulfate and clay, whereas that of C‐PCE was 15% and 25%, respectively, leading to better sulfate and clay tolerance. These significant advantages could be ascribed to the high anion density and high calcium complexing capacity induced by introducingphosphinate group and phosphonate group into the molecular structure of P‐PCE. The as‐synthesized P‐PCE can be used as a kind of high performance superplasticizer in concrete engineering.

Effect of carbon nanotubes/clay hybrid composite on mechanical properties, hydration heat and thermal analysis of cement-based materials

Carbon nanotubes (CNTs) are known for their superior mechanical properties and have been found to increase the strength of cement-based materials. CNTs can also be synthesized on clay mineral by chemical vapour deposition (CVD). In this work, CNTs/Clay hybrid composite were added at 0.02 wt% up to 0.20 wt% in mortars and the effect on cement-based materials properties were investigated. Cumulative heat release and heat flow of cement paste were measured using isothermal calorimetry technique. Phase identification was carried out on samples at 28 days by X-ray diffraction and thermogravimetric analysis. Compressive strength, flexural strength and density of the mixes were determined at 28 days. Compressive strength was found to increase with increase CNTs/Clay hybrid composite and the optimal was found at 0.10 wt%. This is due to their ability to both reinforce and fill the pores in the cement matrix. Thermogravimertric results showed that at 0.10 wt% of CNTs/Clay hybrid composite and at ≈120-125 °C a slight increase in the mass loss can be observed. Moreover, isothermal calorimetry results showed that CNTs/Clay hybrid composite was found to accelerate early cement hydration.

Assessment and prediction of cement paste flow behavior; Marsh-funnel flow time and mini-slump values

In this study, the parameters affecting Marsh-funnel flow time and mini-slump of the paste mixtures were determined through experimental and modelling studies. Marsh-funnel flow times were modelled through artificial intelligence and regression methods. A novel model was used to train the coefficients of artificial neural networks (ANN) with the Teaching-Learning Based Artificial Bee Colony (TLABC) Algorithm. Accuracy of this method was investigated through ANN-Back Propagation, ANN-Teaching Learning Based Optimization Algorithm, ANN-Artificial Bee Colony, Multivariate Adaptive Regression Splines and Classical Regression Analysis methods. ANN-TLABC method showed the best results among the applied models. The admixture content, cement fineness, solid material content of admixture and C3A content of cement were found to be the most important parameters affecting the flowability of the paste. However, C2S, equivalent alkali, C4AF and C3S contents of the cement were observed to have no considerable effect on the Marsh-funnel flowability of paste.

Influence of dispersants and dispersion on properties of nanosilica modified cement-based materials

Properties of cement pastes containing nanosilica (NS) dispersed with seven different dispersants were evaluated. The dispersants used were anionic surfactants, sodium dodecyl sulfate (SDS) and polycarboxylate ether (PCE) based superplasticizer, and two groups of non-ionic surfactants, Tweens (T20, T40) and Tritons (TX114, TX100, TX405). The degree of NS dispersions in de-ionized water and cement pore solutions with ultrasonication and dispersants were quantified based on their average hydrodynamic particle size (Z-avg.) and zeta potential obtained from dynamic light scattering (DLS) measurements. The properties of cement-NS pastes assessed were flowability, heat of hydration, and compressive strength. The results showed that except for PCE, all surfactants reduced the flow of cement paste at a given dosage, and therefore, they were subsequently used along with PCE to achieve adequate flow. The combination of PCE and another surfactant accelerated cement-NS hydration. There were strong correlations between the Z-avg. value of cement-NS suspensions made with non-ionic surfactants and the strength and total heat of hydration of the corresponding cement-NS pastes. Surfactant TX405 provided the cement-NS paste significantly high increment in strength (33%, 41%, and 54% at 1, 3, and 28-day, respectively). SEM and TGA results revealed that enhanced nucleation of CSH fibrils and higher gel-space ratio in TX405-cement-NS paste might be responsible for the strength improvement.

Non-isothermal Bingham fluid flow between two horizontal parallel plates with Ion-slip and Hall currents

This study presents the numerical solution of velocity and temperature fields based on mass conservation, momentum and energy balances for the time-dependent Couette-Poiseuille flow of Bingham materials through channels. The channel flow of Bingham fluid concerns the flow of cement paste in the building industry and the mudflow in the drilling industry. The specific aim is to introduce the magnetohydrodynamic (MHD) phenomena specified by both Ion-slip and Hall currents into the non-isothermal channel flow in a theoretical approach. The Bingham constitutive equation is formulated by the generalized Newtonian fluid technique and solved by employing the explicit Finite Difference Method (FDM) using the MATLAB R2015a and Compaq Visual FORTRAN 6.6a both. For the exactness of numerical performance estimations, the criteria for stabilization and the convergence factor are analyzed. The velocity and temperature profiles are discussed individually at the moving and stationary walls of the channel. It is observed that magnetohydrodynamic phenomena accelerate the flow, and the temperature distributions reach the steady-state situation earlier than velocity distributions. Furthermore, the dominance of MHD parameters on the velocity distributions, shear stress, temperature distributions, and Nusselt number are discussed.

Artificial Neural Network and NLR techniques to predict the rheological properties and compression strength of cement past modified with nanoclay

One of the most important industries and building operations that cause carbon dioxide emissions is the cement and concrete-related industries that consume about 6 million Btus per metric ton and release about 1 metric ton CO2. Reducing cement consumption while using nanomaterials as cement replacement is favored for environmental protection reasons. In this study, the effect of nanoclay (NC) as an additive to the cement paste was evaluated and quantified. Scanning Electronic Microscope (SEM), X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), Fourier-Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy analysis were used to identify the cement and nanoclay. Experimental tests and modeling were conducted to predict the cement paste's flow properties like yield stress, shear strength (shear stress limit), viscosity, and stress at the failure stress of cement paste. The cement paste modified with nanoclay was tested at a water-to-cement ratio (w/c) of 0.35 and 0.45 and temperatures ranging from 25⁰C to 75⁰C. The addition of NC increased the ultimate shear strength (τmax) and the yield stress (τo) from 22.5% to 54.4% and from 26.3% to 203%, respectively based on the NC content, w/c, and temperature. TGA tests showed that the 1% nanoclay additive reduces the weight loss of the cement at 800⁰C by 74% due to the interaction with the nanoclay with the cement paste. The nonlinear regressions model (NLR), and Artificial Neural Network (ANN) technical approaches were used for the qualifications of the flow of slurry and stress at the failure of the cement paste modified with nanoclay. Based on the static analysis assessments, the rheological properties and compressive strength of cement paste modified with nanoclay can be well predicted in terms of w/c, nanoclay content, temperature, and curing time using two different simulation techniques. Among the used approaches and based on the experimental data set, the model made based on the NLR models is the most reliable model to predict rheological properties and compression strength of the cement and it is performing better than the ANN model. The coefficient of the correlation (R), mean absolute error (MAE), and root mean square error (RMSE) concluded that the nanoclay content is the most important parameter for rheological estimation and compression strength of cement paste.

Rheological properties for fresh cement paste from colloidal suspension to the three-element Kelvin–Voigt model

In the present work, rheological behaviors of fresh cement paste are studied based on multi-disciplinary approaches, i.e., colloidal suspension using attractive van der Waals force, rheology using the Bingham model and Bingham–Papanastasiou model, which are able to describe the behavior of cement pastes before and after yield stress, and finally the continuum mechanics based on Maxwell and Kelvin–Voigt models. To achieve this, the fresh cement paste with different water-to-cement ratios of 0.3 up to 0.6 is prepared. The attractive van der Waals forces are estimated based on the distances between solid cement particles, which vary at every single water-to-cement ratio. The rheology experiments of all water-to-cement ratios are performed using a rheometer. According to our experimental outcomes, the Bingham and Bingham–Papanastasiou models are applied in the modeling of the experimental curves and determination of yield stress and viscosity. Maxwell and the Kelvin–Voigt models are utilized in describing solid-like behavior before yield stress and fluid-like behavior beyond yield stress. It is observed that the increase of water generates a decrease in the viscosity, yield stress, and packing concentration of solids. It also increases the distances between two cement particles in the cement pastes. According to the modeling results, the Bingham–Papanastasiou model is well adapted for the cement paste flow due to its additional modeling parameter, which is known as m. The role of m is understood and described by linking the van der Waals interaction, rheology, and three-element Kelvin–Voigt model as a whole in function of water-to-cement ratio. m is understood as a key parameter in which the distance between particles affects the rheological behavior of fresh cement pastes. Lastly, the two-phase flow simulations have been successfully achieved and compared with the experiments. The conclusion and outlooks are summarized and discussed at the end of the paper.

Design of 3D printable concrete based on the relationship between flowability of cement paste and optimum aggregate content

This paper deals with the mixture design of 3D printable concrete (3DPC) based on the theoretical analysis of the yield stress of mortar. It has been found that there is a linear relationship between the flowability of paste and optimum aggregate content in 3DPC mixes which comply well with printability requirements. Additionally, the linear relationship obtained from the 3DPC with coarser natural sand can be also applied to the 3DPC with finer sand in accordance with the excess slurry theory. It is revealed that if the excess slurry thickness and the mix proportion of paste are the same, the mortar has similar rheological behavior. The verification experiment confirms that combined with the linear relationship between the flowability of cement paste and optimum aggregate content, the flowability of paste is a useful parameter for the mixture proportion design of 3DPC.

Comparative roles between aragonite and calcite calcium carbonate whiskers in the hydration and strength of cement paste

Aragonite CaCO3 whisker (CW) could improve the mechanical properties of cementitious composites. The microstructure of calcite is different from that of aragonite, but only a few studies reported different effects in cementitious composites between incorporating aragonite and calcite. In order to fully use CW as high-performance, low-cost microfiber and clearly understand different mechanisms between aragonite and calcite CW, in this study, the heat treatment method was used to convert aragonite CW to calcite CW, and the effect of calcite and aragonite on the hydration and strength of CW reinforced cement paste (CWRC) was studied to bridge the gaps in this area. The aragonite and calcite CWs exhibited the overall physical dilution effect in Portland cement, significantly decreasing the hydration heat of cement and the total amount of non-evaporable water and Ca(OH)2. Calcite CW presents a more significant nucleation effect than aragonite CW. Compared to aragonite CWRC, the hydration of silicon phase in calcite CWRC is faster and the hydration heat release is higher; however, there is no significant difference in the hydration of aluminum phase. The Ca(OH)2 content in calcite CWRC was more than that in aragonite CWRC. More hydration products formed on the surface of calcite CW than that on aragonite CW, improving the bonding strength between calcite CW and cement matrix. Hence, both the compressive strength and flexural strength of calcite CWRC are higher than those of aragonite CWRC. The rheological test of fresh aragonite and calcite CWRC proved that both aragonite and calcite CWs significantly decreased the flow of cement paste, presenting viscosity effect. Compared to aragonite CW, calcite CW improved the flowability of fresh cement paste, indicating the lubricant effect of calcite CW. Aragonite and calcite CW present chemical effect similar to limestone powders, i.e., aluminum phase in Portland cement interacted with CO32- dissolved from CW to form mono- and semi- carbonate. The Ca(OH)2 orientation indexes of cement decreased by the introduction of CWs. Aragonite and calcite CWs can also be used as microfiber and filler in cementitious composites. These effects improved the micro-structure, thus enhanced the flexural and compressive strength of cement paste.

Analysing limitations of the FlowCyl as a one-point viscometer test for cement paste

The FlowCyl is a simple flow viscometer – a modification of the Marsh Cone test apparatus – developed to quantify the flow behaviour of cement pastes. The FlowCyl gives a one-parameter characterisation of rheology called the flow resistance ratio or λQ, which is defined as the average ratio between the flow loss of a measured fluid and theoretical flow of an ideal fluid. This paper reports a study on the limitations of the FlowCyl and appurtenant flow resistance ratio. The investigation includes rheological measurements of cement pastes incorporating crushed aggregate fines with a diameter below 125 µm and development of a numerical model in order to analyse the flow condition inside the FlowCyl. The numerical simulations are carried out both with the Bingham- and Herschel-Bulkley material model of the rheometer data. A comparison with the experimental λQ results illustrates that only a minor error is introduced when describing the flow of cement paste in the FlowCyl with a two-parameter model (Bingham material model) as compared to a three-parameter model (Hershel-Bulkley model). The results also show that the one-parameter characterisation (i.e. λQ) mainly correlates to the plastic viscosity in the Bingham material model, while the yield stress only correlates if the dosage of superplasticizer per mass of cement is kept constant. The numerical simulations show that high shear rates at the outlet of the FlowCyl are responsible for the difference in the correlations.

A modified LBM for non-Newtonian effect of cement paste flow in 3D printing

Purpose The screw extruder is applied in cement-three-dimensional (3D) printing. The cement paste flow in 3D printing is the typical Herschel–Bulkley fluid. To understand the flow in the channel, the improved lattice Boltzmann method (LBM) is proposed. Design/methodology/approach For Herschel–Bulkley flow, an improved LBM is presented to avoid the poor stability and accuracy. The non-Newtonian effect is regard as a special forcing term. The Poiseuille flow is taken to discuss the detailed process of the method. With the method, the analytical solution and numerical solution are obtained and compared. Then, the effect of the initial yield stress on the numerical solution is both explored by the shear-thickening fluid and the shear-thinning fluid. Moreover, the variations of the relative errors under different lattice nodes and different power-law indexes are analyzed. Finally, the method is applied into the simulation of the flow in the extruder of cement-3D printing. Findings The results show that the improved method is effective for Herschel–Bulkley fluids, which can simulate the flow in the extruder stably and accurately. Practical implications The simulation can contribute to understand the cement paste flow in the screw extruder, which helps to optimize the structure of the extruder in the following periods. Originality/value The improve method provide a new way to analyze the flow in the extruder of cement-3D printing. Also, in the past research, LBM for Herschel–Bulkley fluid is ignored, whereas the study can provide the reference for the numerical simulation.

How does adsorption behavior of polycarboxylate superplasticizer effect rheology and flowability of cement paste with polypropylene fiber?

Nowadays, polycarboxylate (PCE) superplasticizer and polypropylene fiber (PPF) are widely used to improve the flowability and ductility of modern concrete. In this study, the rheology and flowability of cement paste with PPF have been investigated. With increasing length and dosage of PPF, the apparent viocosties, plastic viscosities and the yield stress of the cement paste increase gradually. The flowability of the cement paste decreases irrespective whether PCE has been added to the cement paste. However, with increasing length and dosage of PPF, the flowability ratio (i.e. the flowability of cement paste with PCE divided by the flowability of cement paste without PCE) becomes higher. That means the dispersibility of PCE increases with increasing length and dosage of PPF. In this study, the independent, total and synergistic adsorption amount of PCE has also been investigated. It shows that with increasing length and dosage of PPF, both the independent adsorption amount of PCE on the surface of PPF, and the total adsorption amount of PCE on the surfaces of PPF and cement particles increase. When PPF was added into cement paste, they would be intertwined by themselves, more PCE was adsorbed by the three-dimensional mesh of PPF. Therefore, due to the electrostatic repulsion and steric hindrance effects of PCE, the dispersion of cement paste with PPF is improved by adding PCE, and the dispersibility of PCE increases with increasing length and dosage of PPF.