Rheological Properties and Structural Build-Up of Cement Based Materials with Addition of Nanoparticles: A Review

: Nanoparticles improve the mechanical properties and durability of cement-based materials. However, owing to the high surface energy and speciﬁc surface areas of nanoparticles, the packing characteristic of cementitious particles will be affected. With the action of the electrostatic attraction and Van der Waals force, the cementitious particles are agglomerated into ﬂocculation structures, and the free water is entrapped. Furthermore, as the water consumption of cement-based materials increases, the ﬂowability gets worse, which is thought to be one of the reasons limiting its application in practical engineering. In addition, nanoparticles increase the viscosity and thixotropy of fresh cement-based materials and provide more nucleation sites in cement paste, accelerating the cement hydration process in early hydration. In this paper, the research progress on the rheological properties and structural build-up of cement-based materials with the addition of nanoparticles was reviewed. The applicability of rheological test methods and rheological models was summarized. The variation of rheological parameters of fresh cement-based materials affected by nanoparticles species, contents, dispersion method, superplasticizer, etc., were discussed. Based on the packing density, water ﬁlm thickness, and ﬂocculation structures, the action mechanism of nanoparticles on the rheological properties of cement-based materials was analyzed. Further research topics on the rheology and structural build-up of nano-modiﬁed cement-based materials are suggested as well.


Introduction
Nanomaterials, which refer to a three-dimensional structure with at least one dimension, are at the nanoscale (1-100 nm) or by them as the basic constituent unit [1].Nanomaterials are regarded as the most perspective materials in the 21st century due to their excellent properties such as interface effects, small size effects, surface effects, and quantum size effects.In recent years, nanomaterials have assumed an extremely important position in many fields due to the demand for the production of enhanced materials [2,3].Nanoparticles (nano SiO 2 , nano clay, nano ZnO, nano TiO 2 , nanofiber, etc.) have been introduced into cement-based materials to improve the mechanical properties, durability, and self-cleaning function, etc., of cement-based materials and obtained good results [3][4][5][6][7][8].Nanoparticles are fabricated through chemical and physical methods; the chemical methods involve vapor deposition, precipitation method, microemulsion method, solution-gel method, etc.; the physical methods involve ball milling method, etc. [9,10].Nanoparticles with small size, high specific surface area, and surface energy affect the packing density of cementitious particles and free water and flocculation structures in fresh cement paste.Meanwhile, nanoparticles provide more nucleation sites in fresh cement paste, influencing the process of cement hydration [2,11].The small particle size of nano-SiO 2 , nano-TiO 2 , and carbon nanotubes, etc., perform filling roles in cement-based materials to improve the compactness of internal structure, but with the increase of nanoparticle content, the water absorption of particles increases; furthermore, the water consumption of fresh cement paste increases [8,11,12].Moreover, the rheology of cement-based materials is significantly affected, and the workability worsens.In addition, during the preparation of fresh cement paste, the trapped air in paste incorporating nanoparticles is much higher than that of ordinary cement paste, which weakens the development of the strength of cement-based materials [13].For the moment, the adverse effects of nanoparticles on the flowability of cement-based materials are becoming increasingly evident, which is thought to be one of the reasons limiting its application in practical engineering.
However, nanoparticles can significantly increase the viscosity of fresh cement paste, which is used as viscosity modifiers in concrete, and nano clay can improve the thixotropy of fresh cement-based materials and reduce the formwork pressure of self-compacting concrete [14][15][16][17].In addition, nanoparticles provide more nucleation sites and accelerate the structural build-up of cement paste in early hydration.Experimental studies and theoretical analyses were carried out successively to reveal the effects of nanoparticles on the rheological properties and the structural build-up of fresh cement-based materials [18][19][20].In the experimental studies, the conventional fluidity method and the rheological method were used to describe the rheological properties of fresh cement-based materials and fluidity, yield stress, and plastic viscosity obtained from the tests, respectively.Rheological devices automatically acquit data during the rheological test and obtain more rheological parameters to quantitatively evaluate the rheological properties of fresh cement-based materials, which have obvious advantages over the traditional fluidity test method [18][19][20][21].In terms of theoretical research, the study of rheological models for fresh cement-based materials is relatively mature, and the existing rheological models are equally applicable in describing the rheological characteristics of nano-modified cement-based materials [5][6][7][8]22].The rheological mechanism of fresh cement-based materials was revealed by particle packing density, water film thickness, flocculation structures, etc. [23,24], but the rheological mechanism of nano-modified cement-based materials is rarely studied.The species of nanoparticles, admixture, and dispersion method have significant effects on the rheology of fresh cement-based materials [11,15,[25][26][27][28].However, this is still being debated over the effects of nanoparticle species and admixture on the rheological properties of cementbased materials [5,8,11,15,25,26,28]. Jiang et al. [8] concluded that the plastic viscosity of fresh cement paste increases first and then decreases with nano-SiO 2 content.Guneyisi [5] believed that the plastic viscosity of cement paste increases with the increase of nano-SiO 2 content.Furthermore, the rheology of nano-modified cement-based materials is also related to superplasticizers, dispersion methods, and other factors [8,21,29].The evolution of the internal structural build-up of fresh cement paste with hydration time is usually characterized by the variety of storage modulus, loss modulus, phase angle, and static yield stress with hydration time using the small amplitude oscillation shear test method or the static yield stress test method [30,31].Since concrete can be considered as a dispersion of coarse aggregates in a fine-grained matrix, its rheological properties are mainly determined by fine components; therefore, numerous studies are mostly based on the cement paste to investigate the rheology and structural build-up [8,30,31].Furthermore, whether rheology or the structural build-up investigation of fresh cement-based materials was mainly studied by the experimental method, and some computer numerical simulation has been used in civil engineering [32,33], and it is necessary to use these methods to simulate the rheology and the structural build-up of cement-based materials in detail.
In this paper, the research results on the rheology of cement-based materials with nanoparticles are reviewed in terms of rheological methods, rheological models, rheological mechanisms (packing density of cementitious particles, water film thickness, flocculation structures), and various factors (nanoparticles species, superplasticizer, dispersion method, etc.).Moreover, the effect of nanoparticles on the internal structural build-up of cement paste is summarized.The problems in the current research are pointed out, and the next step of the research direction is put forward.

Fluidity Measurement
The rheological properties of cement-based materials are generally obtained through fluidity test methods and rheological test methods.Among them, the fluidity test methods include the slump test, flowability test, secondary pour test, Thaulow test, etc. [3,34].Moreover, measurement devices such as micro-slump cone, Marsh cone, V-funnel, microcone, etc., were used in these tests [8,15,22,[34][35][36].The fluidity test for mortar works similarly to the fluidity test for the cement paste with different dimensions of the cone, which aims to measure the flowability of the composite materials by gravity [37].These conventional fluidity methods are easily, quickly, and widely applicable; however, the experiment process is greatly affected by manual operation, and it is rough, the repeatability of experimental data is poor, and it only qualitatively evaluates the flowability of cementbased materials.

Rheological Measurements
Rheological experimental methods are proposed according to the internal friction, and rheological instruments include a capillary rheometer, falling-ball viscometer, rotational viscometer, rotational rheometer, etc.Among them, rotational viscometers and rheometers are generally used to obtain the rheological parameters of cement-based materials.The rotational viscometer is affordable, has ease of operation, and quickly obtains the apparent viscosity of the non-Newtonian fluid.Rheometers include cone-plate, flat plate, and coaxial cylinder rheometers according to the geometric configuration of the rotor.The shear stress, shear rate or rotational speed, and torque were measured by rheometer, and then the rheological parameters: yield stress, plastic viscosity, etc., were calculated by rheological models.An oscillatory rheometer was used to measure the dynamic viscoelastic properties of materials, and the experiments of small amplitude oscillation were used for evaluating the structural build-up in fresh cement paste [26,27].Rheological testing methods allow for a quantitative and more comprehensive evaluation of the rheological properties of fresh cement-based materials; the current rheological method is applied equally to cement-based materials with nanoparticle addition [2][3][4]6,19,21].

Static Rheological Measurement
The static yield stress is the required stress for fluid to begin to flow, and the static yield stress is related to the connection between flocculation structures in cement paste [38].The shear rate should be small enough to minimize the break to the microstructures of fresh cement paste, and the shear rate is generally in the range of 0.001-0.05s −1 , and the peak stress is considered as the static yield stress [30,39,40].The lower the shear stress, the higher the yield stress, and the longer time it takes to reach the peak or plateau stress during static yield stress testing, a typical development curve of shear stress, as shown in Figure 1 [30].When the shear rate is not selected appropriately, the shear stress does not reach a peak, and the shear stress gradually increases with time; the static yield stress cannot be obtained by the peak shear stress.If the shear stress does not reach a peak value throughout the static shear test, the static yield stress is determined by the stress value at the end of the linear region of the shear stress curve [39].The evolution of static yield stress with time can be used to characterize the evolution of structural build-up of cement-based materials [30,38].Kim and Qian et al. [15,16] found that nanoclay increases the static yield stress and contributes to the reduction of formwork lateral pressure of self-compacting concrete.Yuan et al. [30,39] demonstrated that the static yield stress increase with the increase of nano clay, nano-CaCO 3, or nano-SiO 2 content, wherein the static yield stress of cement paste is significantly increased by nano-SiO 2 .Moreover, the evolution of the structural build-up of cement paste is evaluated by the change of static yield stress with the increase of hydration time, and it is concluded that nanoparticles can promote structural build-up [40].
cement paste is significantly increased by nano-SiO2.Moreover, the evolution of the structural build-up of cement paste is evaluated by the change of static yield stress with the increase of hydration time, and it is concluded that nanoparticles can promote structural build-up [40].

Dynamic Rheological Measurement
In the dynamic rheological test, the shear stress/torque at various shear rates/rotation speeds are obtained; furthermore, rheological parameters such as dynamic yield stress, plastic viscosity, and hysteresis loops area are obtained based on the shear stress-shear rate or rotation speed-torque curves [3].The dynamic yield stress obtained by a dynamic shear test is the minimum stress required to maintain the flow state of the fluidity.Step shearing and linearity shearing were used in the dynamic rheological test.Jiao et al. [40] conducted a dynamic rheological test using a plate rheometer; to eliminate the effect of the shear history of paste, the pre-shear period lasted for 30 s with a shear strain rate of 240 s −1 ; for the data-logging period, reduce the shearing rate in steps (20 s/step).Only the last 10 s were used to record data (as shown in Figure 2a).Jiang et al. [8] used a vane rheometer with a pre-shearing stage, and the rotational speed changed linearly over time, setting 30 s and then entered the data-logging cycle, where the rotational speed was increased linearly within 75 s, and then decreased to 0 in the next 75 s (as shown in Figure 2b).Yuan et al. [30] used a rotor rheometer with a dynamic shear and tested the dynamic shear stress, viscosity, and hysteresis loops area of fresh cement paste with the addition of nano clay, nano-CaCO3, and nano-SiO2, a linearly shear rate was used for the data-logging period.Ghafari et al. [7] chose a coaxial rheometer to investigate the effect of Zinc oxide and Al-Zinc oxide nanoparticles on the yield stress and viscosity of cement paste.Dejaeghere et al. [27] studied the effect of nano-attapulgite clay on the rheological properties of fresh cement mortar using a vane viscometer.Nano clay increases the yield stress and plastic viscosity of cement mortar.

Dynamic Rheological Measurement
In the dynamic rheological test, the shear stress/torque at various shear rates/rotation speeds are obtained; furthermore, rheological parameters such as dynamic yield stress, plastic viscosity, and hysteresis loops area are obtained based on the shear stress-shear rate or rotation speed-torque curves [3].The dynamic yield stress obtained by a dynamic shear test is the minimum stress required to maintain the flow state of the fluidity.Step shearing and linearity shearing were used in the dynamic rheological test.Jiao et al. [40] conducted a dynamic rheological test using a plate rheometer; to eliminate the effect of the shear history of paste, the pre-shear period lasted for 30 s with a shear strain rate of 240 s −1 ; for the data-logging period, reduce the shearing rate in steps (20 s/step).Only the last 10 s were used to record data (as shown in Figure 2a).Jiang et al. [8] used a vane rheometer with a pre-shearing stage, and the rotational speed changed linearly over time, setting 30 s and then entered the data-logging cycle, where the rotational speed was increased linearly within 75 s, and then decreased to 0 in the next 75 s (as shown in Figure 2b).Yuan et al. [30] used a rotor rheometer with a dynamic shear and tested the dynamic shear stress, viscosity, and hysteresis loops area of fresh cement paste with the addition of nano clay, nano-CaCO 3, and nano-SiO 2 , a linearly shear rate was used for the data-logging period.Ghafari et al. [7] chose a coaxial rheometer to investigate the effect of Zinc oxide and Al-Zinc oxide nanoparticles on the yield stress and viscosity of cement paste.Dejaeghere et al. [27] studied the effect of nano-attapulgite clay on the rheological properties of fresh cement mortar using a vane viscometer.Nano clay increases the yield stress and plastic viscosity of cement mortar.2.2.3.Small Amplitude Oscillatory Shear (SAOS) Test Small amplitude oscillatory shear test uses an amplitude less than the ultimate shear strain (stress) value and aims to investigate the evolution of the viscoelasticity of the fresh cement pastes over hydration by continuously testing, and the parameters: storage mod-

Small Amplitude Oscillatory Shear (SAOS) Test
Small amplitude oscillatory shear test uses an amplitude less than the ultimate shear strain (stress) value and aims to investigate the evolution of the viscoelasticity of the fresh cement pastes over hydration by continuously testing, and the parameters: storage modulus, loss modulus, and phase angle were obtained [29,30,[41][42][43].Due to the use, the amplitude is less than the ultimate shear strain (stress) value, and the SAOS test can be considered a non-destructive rheological method [29].To date, the variation of storage modulus with hydration was used to describe the structural build-up of cement paste at rest [41,42].Jiao et al. [41] demonstrated that nano Fe 3 O 4 promoted the structural build-up of cement paste, and the storage modulus and loss modulus of cement paste increased.Nano CaCO 3 and nano SiO 2 increased the storage modulus of cement paste and accelerated the structural build-up [29,30].SAOS method was used to monitor the evolution of the storage modulus of cement paste with nanoclay, and the nanoclay species make for significant differences [37]; some clay decreased the storage modulus [39].However, some scholars pointed out that the evolution of structural build-up of cement pastes evaluated by static rheological measurement and the SAOS method is not consistent.

Rheological Models Determined by Dynamic Rheological Test
The rheological model reflects the rheological characteristic of fresh cement-based materials.However, the rheological properties of fresh cement-based materials are related to the volume fraction of solid particles, packing density, particle size, particle shape, etc. Herein, some factors are difficult to quantify (shape of the particle, etc.), then there are few theoretical rheological models in line with reality, most of which are empirical models [19,[44][45][46][47].The empirical models are established through the relationship of shear rate-shear stress obtained by dynamic rheological test.To date, the rheological model used in cement-based materials mainly includes the Bingham model, modified Bingham model, Herschel-Bulkley model, and Power-law model.[19][20][21].Fresh cement-based material is a kind of material between viscous, elastic, and plastic; the rheological characteristic of most cement-based materials conforms to the Bingham model (the paste belongs to Bingham fluid, Figure 3), whereas the Bingham model does not take into account the effects of particle migration, thixotropy, and cement hydration, and it is not appropriate for assessing the nonlinear relationship of shear rate-shear stress [48,49].Afterward, the modified Bingham model and the Herschel-Bulkley model were proposed [50,51].Moreover, the modified Bingham model considers the nonlinear relationship between shear stress and the shear rate of fresh cement-based materials under the influence of a high shear rate or low water-to-binder ratio [52].The Herschel-Bulkley model quantifies the nonlinear behavior (shear thickening or shear thinning, as shown in Figure 3) of the shear stress-shear rate curve [52][53][54][55], and the larger the flow index deviates, the more significant of shear thickening or shear thinning behavior [52,54].If the rheological curve crosses the origin and there is nonlinear behavior, the power-law model can be chosen to describe the rheological characteristics of cement-based materials [21].The Bingham model, modified Bingham model, Herschel-Bulkley model, and Powerlaw model were the most common models for fresh cement-based materials.In addition, pseudoplastic fluid or dilatant fluid, according to the parameters of the rheological equation, is determined [55][56][57][58].The power-law model can reflect the rheological characteristics of non-Newtonian fluids with rheological curves going through the origin.The Power-law model can be expressed as: where K is the consistency coefficient, Pa•s n .n is the power index; if n = 1, the paste is Newton fluid; if n > 1, the growth rate of apparent viscosity increase with the increase of shear rate, and the paste belong to dilatant fluid; if n < 1, the growth rate of apparent viscosity decrease with the increase of shear rate, the paste is pseudoplastic fluid.
The Bingham model assumes a uniform dispersion of solid particles of the fluid and a linear relationship between the shear stress and the shear rate of the fluid; when the shear stress on the fluid is lower than the yield stress, the fluid undergoes only elastic deformation; when the shear stress on the fluid is higher than the yield stress, the paste follows Newton's fluid and undergoes viscous flow [59].The Bingham model can be formulated as [46]: where τ is shear stress, Pa; τ 0 is yield stress, Pa; µ is plastic viscosity, Pa•s; . γ is shear rate, s −1 .The modified Bingham model modified for the low shear rate region of Bingham model, the modified Bingham model has significant advantages for the shear thickening behavior of non-Newtonian fluids.The modified Bingham model can be represented as [50,51]: where c is second-order, Pa•s 2 .
Herschel-Bulkley model is proposed based on the nonlinear relationship between shear stress and shear rate, which can characterize the shear thickening and shear thinning behavior of fresh cement-based materials and quantitatively evaluate the degree of deviation from Newtonian fluid [54].The Herschel-Bulkley model can be presented as [51,60,61]: where n is the power index; if n < 1, the fluid exhibited shear thinning; if n > 1, the fluid exhibited shear thickening.
When the Bingham model, modified Bingham model, and Herschel-Bulkley model are used to analyze the same experimental data, the Bingham model obtained the lowest yield stress, and the Herschel-Bulkley model obtained the highest yield stress value [48].These rheological models are suitable for describing the rheological characteristics of fresh cementbased materials with nanoparticles addition [7,8,22,48].The rheological characteristics of cement paste with nano-SiO 2 , nano-TiO 2, and carbon nanotubes conform to the modified Bingham model [2,8].The rheological characteristic of self-compacting concrete with nano-SiO 2 conforms to Herschel-Bulkley model, in which the power index exceeds 1, and the power index increases with the increase of nano-SiO 2 addition [5].The rheological characteristics of nano-CaCO 3 cement paste conform to the Herschel-Bulkley model, the power index n is less than 0, and n decreases with the increase of nano-CaCO 3 content, and the shear thinning behavior becomes more significant with the increase of nano-CaCO 3 content [22].The rheological characteristic of cement paste with the addition of nano-ZnO conforms to the Bingham model [7].The rheological characteristic of fresh cement paste with nano-metakaolin conforms to the modified Bingham model, and the paste follows shear thinning as the superplasticizer is added, which conforms to the Herschel-Bulkley model, and the power index decreases with the increase of nano-metakaolin contents [11].It can be seen from the above that nanoparticles affect the rheological characteristics of cement-based materials, especially for the shear thickening or shear thinning behavior.

Mechanism Analysis of Rheology
Researchers investigate the packing density and water film thickness of cementitious particles and the flocculation structures to reveal the rheological mechanism of fresh cementbased materials.Overall, studies on the rheological mechanism of fresh cement-based materials with nanoparticles are not systematic enough.

Packing Density
In the fresh cement paste, water fills the space between the cementitious particles.The higher the packing density of cementitious particles, the less the void between particles; then, the less water used to fill the void between particles, the more free water in the paste and the better the flowability of the fresh cement-based materials [23,24,[62][63][64][65].The experimental methods of packing density mainly include the dry packing density method and the wet packing density method.The dry packing density test method is largely related to the state of particles compaction; when particle size is less than 100 µm, under the effects of electrostatic attraction and Van der Waals force, the agglomeration of cementitious particles, the dry packing density method is no longer applicable.Cementitious particles contain small size (<100 µm) particles.The wet packing density methods, such as the standard consistency water consumption method [64] and the maximum particle concentration method [65], were generally used.The packing density of the maximum particle concentration method can be expressed as [11,65]: where φ is packing density; vs. is the volume of mixed particles of nanoparticles and cement particles; ρ w is the density of water; M is the mass of paste in the container; V is the volume of the container; ρ C and ρ N are the density of cement and nanoparticles, respectively; R C and R N are the ratio of the volume of cement and nano particles to the volume of mixed particles, R C + R N = 1.Small-size particles have a significant effect on the packing density of cementitious particles, and when nano-metakaolin particles are added to cement particles, the packing density of cementitious particles decreases [11].Indeed, the reduction of the packing density of the cementitious particles caused by small particles is mainly related to factors such as the wall effect and the loosening effect of cementitious particles [23,24].The wall effect means the existence of smaller voids between large particles, which other size particles cannot fill, thus leaving voids around large particles, and the loosening effect refers to the existence of a large number of small particles between large particles, which cause the separation of large particles from each other [23,24].The effect of the wall effect and loosening effect on the packing density of the solid particle is shown in Figure 4.The appearance of voids around the particles causes a decrease in the packing density of the cementitious particles.Nanoparticles are prone to aggregate after being added to water or cement paste; furthermore, voids are present between nanoparticles.However, the effect of nanoparticles on wall effect and loosening on cementitious particles is poorly investigated.Small-size particles have a significant effect on the packing density of cementitious particles, and when nano-metakaolin particles are added to cement particles, the packing density of cementitious particles decreases [11].Indeed, the reduction of the packing density of the cementitious particles caused by small particles is mainly related to factors such as the wall effect and the loosening effect of cementitious particles [23,24].The wall effect means the existence of smaller voids between large particles, which other size particles cannot fill, thus leaving voids around large particles, and the loosening effect refers to the existence of a large number of small particles between large particles, which cause the separation of large particles from each other [23,24].The effect of the wall effect and loosening effect on the packing density of the solid particle is shown in Figure 4.The appearance of voids around the particles causes a decrease in the packing density of the cementitious particles.Nanoparticles are prone to aggregate after being added to water or cement paste; furthermore, voids are present between nanoparticles.However, the effect of nanoparticles on wall effect and loosening on cementitious particles is poorly investigated.

Water film thickness
In fresh cement paste, the excess water, after filling voids between particles, wraps the surface of cementitious particles, then the water film thickness forms.Water film has

Water Film Thickness
In fresh cement paste, the excess water, after filling voids between particles, wraps the surface of cementitious particles, then the water film thickness forms.Water film has a lubricating effect on particles when fluid is flowing, and the larger the thickness of the water film, the better the flowability of the cement-based material.The formation and variation of the water film thickness of cementitious particles are illustrated in Figure 5 [11].As Figure 5a shows, the solid particles are closely packed with each other before water is added; when water is added to the cementitious particles, the water gradually fills in the voids between particles, and at this point, the mixture can not flow.When the water has completely filled the voids between solid particles (in the critical state), the water film thickness is exactly zero (Figure 5b).If the water is higher than the required for the voids between particles, the free water wraps the surface of the cementitious particles, at which time the paste begins to flow (Figure 5c), and when the water continues to increase, the water film thickness increases and the flowability of the paste improved.Significantly, the water film thickness is obtained from the average specific surface area of the cementitious particles [66].Water film thickness is the ratio of excess water over the specific surface area of the particles in cement paste, and it can be expressed as [11,36,67,68]: where WFT is the water film thickness of particles; u is the volume ratio of the excess water and solid particles; A is the total surface area of cementitious particles; A N and A c are the specific surface areas of nanoparticles and cement particles, respectively.The water film thickness of cement paste decreases with the increase of nano-metakaolin addition, and superplasticizer increases the water film thickness of ordinary/nano-metakaolin cement paste (as illustrated in Figure 6) [11].The water film thickness of cementitious particles increases with the increase of fly ash and ultrafine cement (micron-scale) in cement paste [31,52].For a water-solid ratio of 0.40, the water film thickness of cement paste with 40% fly ash is about 3.4 times that of ordinary cement paste [31].When the water-to-cement ratio is 0.20, the water film thickness of cement paste with 20% ultrafine cement particles is about two times that of the particles in ordinary cement paste [52].For the cement paste with silica fume (specific surface area of 1.82 m 2 /g) and limestone powder (specific surface area of 1.007 m 2 /g), the water film thickness of particles was slightly reduced compared with that of ordinary cement paste [36,68].The water film thickness of particles in cement paste with 50% limestone addition was about 90% of that The water film thickness of cement paste decreases with the increase of nano-metakaolin addition, and superplasticizer increases the water film thickness of ordinary/nano-metakaolin cement paste (as illustrated in Figure 6) [11].The water film thickness of cementitious particles increases with the increase of fly ash and ultrafine cement (micron-scale) in cement paste [31,52].For a water-solid ratio of 0.40, the water film thickness of cement paste with 40% fly ash is about 3.4 times that of ordinary cement paste [31].When the water-to-cement ratio is 0.20, the water film thickness of cement paste with 20% ultrafine cement particles is about two times that of the particles in ordinary cement paste [52].For the cement paste with silica fume (specific surface area of 1.82 m 2 /g) and limestone powder (specific surface area of 1.007 m 2 /g), the water film thickness of particles was slightly reduced compared with that of ordinary cement paste [36,68].The water film thickness of particles in cement paste with 50% limestone addition was about 90% of that of ordinary cement paste [68].The reason for this is that small particles filled the voids between large particles yielding an increase in particle packing density, and the excess water increased in the paste; meanwhile, small particles have a higher specific surface area and surface energy, which are prone to agglomeration and an increase in interparticle voids under the effect of wall effect and loosening effect, and a decrease in particle packing density [23,24,31,52].Therefore, more water was needed to fill the interparticle voids, and the excess water in the paste decreased.Additionally, the agglomerate also wraps a certain amount of free water, yielding a further reduction of free water in the paste.To date, there are few studies on the water film thickness of particles in fresh cement paste with nanoparticle addition.There are a couple of remaining issues for the calculation methods of water film thickness: (1) It is too idealistic for water film thickness to be treated as the average particle size.Different sizes of particles should have different water film thicknesses.(2) The effect of the external specific surface area of solid particles is only considered, while for some porous nanoparticles, there are more internal pores and a higher internal specific surface area.

Flocculation Structures
The dissolution of mineral particles and the generation of hydration products yield different charges on the surface of cementitious particles and flocculation structures formed under electrostatic attraction and Van der Waals forces.Some water was wrapped [69][70][71][72][73][74].Furthermore, the free water in cement paste decreases and is inferior in fluidity.Optical microscope and scanning electron microscope were used to investigate the flocculation structures; two aggregation states of cement particles exist in aqueous solutions, small particles adhered to the surface of larger particles and small particles bonded to each other to form aggregates; the smaller the particle size of cement particles, the larger the surface energy [69,75].They observed by laser confocal microscopy that small-volume flocs adsorbed on large-volume flocs, and large and small flocs overlapped each other to form a network-like flocculation structure [69,70].In addition to the optical microscope and scanning electron microscope, the agglomerated structure can also be observed by an EDS map [71].Nanoparticles have a high specific surface area and surface energy, and the larger the addition of the nanoparticles, the more significant the agglomeration of the flocculation structure [11].As shown in Figure 7, the size and quantity of flocculation structures increased with nano-metakaolin addition.Superplasticizer adsorbs directionally on the surface of cementitious particles in fresh cement paste, yielding a like charge (negative charge) on the surface of the particles [72].Superplasticizer plays the role of electrostatic repulsion and steric hindrance in cement paste, which disperses the flocculation structures, the wrapped water in the flocculation structures is released, and the frictional re-

Flocculation Structures
The dissolution of mineral particles and the generation of hydration products yield different charges on the surface of cementitious particles and flocculation structures formed under electrostatic attraction and Van der Waals forces.Some water was wrapped [69][70][71][72][73][74].Furthermore, the free water in cement paste decreases and is inferior in fluidity.Optical microscope and scanning electron microscope were used to investigate the flocculation structures; two aggregation states of cement particles exist in aqueous solutions, small particles adhered to the surface of larger particles and small particles bonded to each other to form aggregates; the smaller the particle size of cement particles, the larger the surface energy [69,75].They observed by laser confocal microscopy that small-volume flocs adsorbed on large-volume flocs, and large and small flocs overlapped each other to form a network-like flocculation structure [69,70].In addition to the optical microscope and scanning electron microscope, the agglomerated structure can also be observed by an EDS map [71].Nanoparticles have a high specific surface area and surface energy, and the larger the addition of the nanoparticles, the more significant the agglomeration of the flocculation structure [11].As shown in Figure 7, the size and quantity of flocculation structures increased with nano-metakaolin addition.Superplasticizer adsorbs directionally on the surface of cementitious particles in fresh cement paste, yielding a like charge (negative charge) on the surface of the particles [72].Superplasticizer plays the role of electrostatic repulsion and steric hindrance in cement paste, which disperses the flocculation structures, the wrapped water in the flocculation structures is released, and the frictional resistance between solid particles or flocculation structures decreases, and the flowability is promoted [73].However, the formation of flocculation structures in cement paste is not only affected by electrostatic attractions or Van der Waals forces but related to the bridged by the hydration product of C-S-H gel for cementitious particles.Roussel [74] and Qian [75] et al. idealized the C-S-H gel as a uniform connection of nanoscale cylinders and bridging between cementitious particles.The more flocculation structures in the fresh cement paste, the higher the thixotropy, and the reversible behavior of destruction-recovery of flocculation structures is mainly related to the colloidal bonding and the bridging action of C-S-H gel [74][75][76][77].Up to now, the effects of nanoparticles on the agglomeration of flocculation structures, the amount of flocculation, and the flocculated water have not been quantitatively evaluated [11], and it should be investigated in the next research work.

Analysis of Influencing Factors
The characteristics (dispersion, species, superplasticizer) of nanoparticles and matrix factors are the key factors affecting the rheology of cement-based materials, and the specific influence and mechanisms are described as follows.

Effect of Dispersion Method
Nanoparticles have high surface activity and a large number of unsaturated bonds.Whether they are directly mixed with cement or dispersed in water before mixing with cement, they agglomerate inevitably under the action of electrostatic attractions and Van der Waals forces, reducing the properties improvement of cement-based materials [78].At this point, the dispersion methods of nanoparticles mainly include: mechanical agitation, ultrasonic dispersion, adding surfactants, and changing solvents [25].The characterization method for nanoparticle dispersion such as ultraviolet-visible spectrophotometry method, dynamic light scattering method, the zeta potential method, and nanoparticle sedimentation method [78][79][80][81][82].Among these dispersion methods, ultrasonic dispersion method and surfactant are the most effective; ultrasonic dispersion methods can effectively weaken the interaction energy between nanoparticles and open the agglomerated nanoparticles; surfactant adsorbs on the surface of nanoparticles, the surface electrification of nanoparticles changes, and the repulsive forces generated between particles make nanoparticles dispersion effectively [25,81,82].The dispersion method of nanoparticles affects the rheology of cement-based materials, and the viscosity of nano-modified cement paste decreases with surfactants added [26].For the mechanical agitation method, the However, the formation of flocculation structures in cement paste is not only affected by electrostatic attractions or Van der Waals forces but related to the bridged by the hydration product of C-S-H gel for cementitious particles.Roussel [74] and Qian [75] et al. idealized the C-S-H gel as a uniform connection of nanoscale cylinders and bridging between cementitious particles.The more flocculation structures in the fresh cement paste, the higher the thixotropy, and the reversible behavior of destruction-recovery of flocculation structures is mainly related to the colloidal bonding and the bridging action of C-S-H gel [74][75][76][77].Up to now, the effects of nanoparticles on the agglomeration of flocculation structures, the amount of flocculation, and the flocculated water have not been quantitatively evaluated [11], and it should be investigated in the next research work.

Analysis of Influencing Factors
The characteristics (dispersion, species, superplasticizer) of nanoparticles and matrix factors are the key factors affecting the rheology of cement-based materials, and the specific influence and mechanisms are described as follows.

Effect of Dispersion Method
Nanoparticles have high surface activity and a large number of unsaturated bonds.Whether they are directly mixed with cement or dispersed in water before mixing with cement, they agglomerate inevitably under the action of electrostatic attractions and Van der Waals forces, reducing the properties improvement of cement-based materials [78].At this point, the dispersion methods of nanoparticles mainly include: mechanical agitation, ultrasonic dispersion, adding surfactants, and changing solvents [25].The characterization method for nanoparticle dispersion such as ultraviolet-visible spectrophotometry method, dynamic light scattering method, the zeta potential method, and nanoparticle sedimentation method [78][79][80][81][82].Among these dispersion methods, ultrasonic dispersion method and surfactant are the most effective; ultrasonic dispersion methods can effectively weaken the interaction energy between nanoparticles and open the agglomerated nanoparticles; surfactant adsorbs on the surface of nanoparticles, the surface electrification of nanopar-ticles changes, and the repulsive forces generated between particles make nanoparticles dispersion effectively [25,81,82].The dispersion method of nanoparticles affects the rheology of cement-based materials, and the viscosity of nano-modified cement paste decreases with surfactants added [26].For the mechanical agitation method, the higher the stirring rate, the lower the yield stress (unless the static yield stress is indicated, the others are dynamic yield stress).The yield stress of cement paste with nano-SiO 2 at a stirring rate of 1000 r/min is about 60% of that at 500 r/min, but if the stirring rate exceeds 1000 r/min, the yield stress will not decrease more [8].Jiang et al. [8] believed that when the ultrasonic time for nanoparticle dispersion was less than 5 min, the flowability of the paste increased with the increase of ultrasonic time; when the ultrasonic time was over 10 min, the yield stress and plastic viscosity decreased.The action mechanism of the effect of dispersion on the rheology of fresh cement-based materials is summarized as follows: (1) Surfactant causes electrostatic repulsion between the cementitious particles, yields an increase in the distance between particles, the number of flocculation structures decreases; furthermore, the resistance generated by particles and flocculation structures reduced, and the viscosity decreased.( 2) Mechanical agitation and ultrasonic dispersion methods break the nanoparticle agglomerates and weaken the adsorption and friction of particles.The higher the mechanical stirring rate, the better the dispersion, and the smaller the yield stress (Jiang et al. [8] proposed that too high a stirring rate and long ultrasonic time will increase the temperature of paste, and yield water evaporation, increase the viscosity of cement paste).Up to now, the study of nanoparticle dispersion methods and dispersion effects has been relatively mature, but the effects of different dispersion methods and dispersion effects on the rheological properties of cement-based materials with the addition of nanoparticles have not been quantitatively evaluated.

Effect of Species and Contents of Nanoparticles
The effects of different nanoparticles on the rheological properties of fresh cementbased materials have been widely investigated, and the variation of the growth rate of the yield stress of cement paste with nanoparticles compared with ordinary cement-based materials is shown in Figure 8.As Figure 8 shows, nano-CaCO 3 , nano-SiO 2 , nano-ZnO, nano-TiO 2, and nano clay increased the yield stress of cement paste [7,8,11,19,22].However, the same nanoparticle has a different effect on yield stress on fresh cement paste [3,8,12].Jiang [8] and Guneyisi [5] investigated that the yield stress of cement paste increased with nano-SiO 2 addition.Tobon [12] believed that nano-SiO 2 decreases the yield stress of cement paste when the nanoparticle is over 5% addition, which attributes to the "ball-bearing" effect of nano-SiO 2 particles.Nanoparticles also affected the plastic viscosity of the fresh cement paste.Nano-SiO 2 can improve the cohesiveness of fresh concrete.The plastic viscosity of concrete mixed with 1% nano-SiO 2 increased by 64.74% compared to ordinary concrete [4].The plastic viscosity of cement paste increased with the increase of nanometakaolin addition (1-10%); the cohesiveness and plastic viscosity of cement paste with nano-attapulgite clay increased [15,27].However, due to the lubricating and rolling effect of the spherical shape of nanoparticles, nano-SiO 2 reduces the plastic viscosity of cement paste [12].Owing to the interparticle collision and friction, nano-Fe 3 O 4 increased the yield stress and plastic viscosity of cement paste [44].Makwana et al. [2] revealed that the plastic viscosity of cement paste is little affected by nano-SiO 2 , and the yield stress is affected significantly.Moreover, nano-SiO 2 was more plentiful and effective at reducing fluid loss as compared to nano-Al 2 O 3 and nano-TiO 2 [8].Growth rate of yield stress/% Nanoparticles contents/% Nano SiO 2 [8] Nano TiO 2 [8] Nano SiO 2 [5] Nano CaCO 3 [22] Nano ZnO [7] Nano Metakaolin [11] Nano clay [15] Nano Fe 4 O 3 [44] Nano SiO 2 [12] Figure 8.The relationship between the growth rate of yield stress and the content of nanoparticles [5,7,8,11,12,15,22,44].
The high specific surface area of nanoparticles makes more water be wrapped and moisten the particle surface, causing a decrease of free water in cement paste and an increase in the interparticle frictional resistance, then the yield stress increased (spherical nanoparticles may reduce yield stress) [8,12].The plastic viscosity characterizes the internal structures of the fresh cement-based material impeding its flow, and there are differences in the influence of nanoparticles species on plastic viscosity.On the one hand, nanoparticles are prone to agglomeration, promote the formation of flocculation structures, and yield an increase in frictional resistance between particles or flocculation structures.On the other hand, due to the complex morphology of nanoparticles, some spherical particles act as "lubrication" and "ball" during paste flow, which increases the flowability.Plastic viscosity changed under the combined effect of the above two factors.Due to the high specific surface area and surface activity of nanoparticles, they prefer to adhere to the surface of large particles or bond to each other to form agglomerates, causing changes in flocculation structures; furthermore, the thixotropy of cement-based materials was affected [4,11,22,[83][84][85][86]. The hysteresis loop area of concrete with 1% nano-SiO2 increased by 235.96% compared to ordinary concrete [4].The thixotropy of fresh cement paste increased with the increase of nano-metakaolin addition (1-15%) [11].The addition of 0.25-0.5% nano clay facilitated the formation of flocculation structures, yielding an increase in the thixotropy of the cement paste [85].Morsy [4] believed that the hydroxyl groups on the surface of nano-SiO2 or the hydrogen bonding between adjacent hydroxyl groups attributed to the increase in thixotropy.However, Xiao [22] believed that the nanoparticles provided more connection points for the net structure of the paste, making the flocculation structure difficult to be broken, then the thixotropy of fresh cement paste decreased.Differences remained in particle size and morphology of different nanoparticles.The variation of the rheological properties of nano-modified cement-based materials is disputed and needs to be further verified.

Effect of Superplasticizer
The particle characteristics of nanomaterials (high surface activity, high specific surface area) adversely affect the flowability of fresh cement-based materials, and increasing the water-to-binder ratio and incorporating superplasticizer is effective in improving the flowability [11,15,87].Superplasticizer acts as adsorption-dispersion and lubrication-wetting effects in fresh cement paste [34].When superplasticizer is added, superplasticizer molecules with anions adsorbed on the surface of cement and hydration products, electrostatic repulsion is generated between particles, which breaks and inhibits the formation The relationship between the growth rate of yield stress and the content of nanoparticles [5,7,8,11,12,15,22,44].
The high specific surface area of nanoparticles makes more water be wrapped and moisten the particle surface, causing a decrease of free water in cement paste and an increase in the interparticle frictional resistance, then the yield stress increased (spherical nanoparticles may reduce yield stress) [8,12].The plastic viscosity characterizes the internal structures of the fresh cement-based material impeding its flow, and there are differences in the influence of nanoparticles species on plastic viscosity.On the one hand, nanoparticles are prone to agglomeration, promote the formation of flocculation structures, and yield an increase in frictional resistance between particles or flocculation structures.On the other hand, due to the complex morphology of nanoparticles, some spherical particles act as "lubrication" and "ball" during paste flow, which increases the flowability.Plastic viscosity changed under the combined effect of the above two factors.Due to the high specific surface area and surface activity of nanoparticles, they prefer to adhere to the surface of large particles or bond to each other to form agglomerates, causing changes in flocculation structures; furthermore, the thixotropy of cement-based materials was affected [4,11,22,[83][84][85][86]. The hysteresis loop area of concrete with 1% nano-SiO 2 increased by 235.96% compared to ordinary concrete [4].The thixotropy of fresh cement paste increased with the increase of nano-metakaolin addition (1-15%) [11].The addition of 0.25-0.5% nano clay facilitated the formation of flocculation structures, yielding an increase in the thixotropy of the cement paste [85].Morsy [4] believed that the hydroxyl groups on the surface of nano-SiO 2 or the hydrogen bonding between adjacent hydroxyl groups attributed to the increase in thixotropy.However, Xiao [22] believed that the nanoparticles provided more connection points for the net structure of the paste, making the flocculation structure difficult to be broken, then the thixotropy of fresh cement paste decreased.Differences remained in particle size and morphology of different nanoparticles.The variation of the rheological properties of nano-modified cement-based materials is disputed and needs to be further verified.

Effect of Superplasticizer
The particle characteristics of nanomaterials (high surface activity, high specific surface area) adversely affect the flowability of fresh cement-based materials, and increasing the water-to-binder ratio and incorporating superplasticizer is effective in improving the flowability [11,15,87].Superplasticizer acts as adsorption-dispersion and lubricationwetting effects in fresh cement paste [34].When superplasticizer is added, superplasticizer molecules with anions adsorbed on the surface of cement and hydration products, electrostatic repulsion is generated between particles, which breaks and inhibits the formation of flocculation structures in cement paste, and the free water increases significantly compared to the paste without superplasticizer [88].Lavergne [13] believed that the required amount of superplasticizer to disperse the nanoparticles is related to the specific surface area of particles.Polycarboxylic superplasticizer is composed of polycarboxylic esther groups, the anionic main chain is rapidly adsorbed on the surface of cement particles, and electrostatic repulsion occurs.The side chains reach into the solution of cement paste to form a thicker layer of polymer molecules adsorbed on the surface of cement particles, in which the dispersion action is mainly controlled by steric repulsion forces [3,[88][89][90].Moreover, superplasticizer interferes with the hydration kinetics of cement paste [3,13].Superplasticizer adsorbs on the surface of cement particles and retard cement hydration, resulting in less frictional resistance generated by the internal structures of the cement paste [88,91,92].The adjustment of the superplasticizer to the fluidity of the cement paste exists a saturation point over the content, and the flowability of the cement paste increases slowly [8].The relationship between superplasticizer and rheological parameters (yield stress, plastic viscosity) of cement-based materials with nanoparticles is shown in Figure 9 [2,19,80].It can be observed that the yield stress and plastic viscosity both decrease with increasing superplasticizer content.On the one hand, superplasticizer acts as the effect of electrostatic repulsion in cement paste.The number of flocculation structures decreases, the number of free water increases, and then the viscosity and yield stress decreases.On the other hand, the steric hindrance of the superplasticizer decreases the interparticle frictional resistance, and the yield stress decreases [85,93].Nanoparticles adsorb more strongly on the superplasticizer than cement particles.Nanoparticles reduce the dispersion effect of superplasticizers [94].At present, the research on the effect of superplasticizers on the rheological properties of nano-modified cement-based materials is more mature, and the adsorption and compatibility of nanoparticles and superplasticizers are needed in future studies.
of flocculation structures in cement paste, and the free water increases significantly compared to the paste without superplasticizer [88].Lavergne [13] believed that the required amount of superplasticizer to disperse the nanoparticles is related to the specific surface area of particles.Polycarboxylic superplasticizer is composed of polycarboxylic esther groups, the anionic main chain is rapidly adsorbed on the surface of cement particles, and electrostatic repulsion occurs.The side chains reach into the solution of cement paste to form a thicker layer of polymer molecules adsorbed on the surface of cement particles, in which the dispersion action is mainly controlled by steric repulsion forces [3,[88][89][90].Moreover, superplasticizer interferes with the hydration kinetics of cement paste [3,13].Superplasticizer adsorbs on the surface of cement particles and retard cement hydration, resulting in less frictional resistance generated by the internal structures of the cement paste [88,91,92].The adjustment of the superplasticizer to the fluidity of the cement paste exists a saturation point over the content, and the flowability of the cement paste increases slowly [8].The relationship between superplasticizer and rheological parameters (yield stress, plastic viscosity) of cement-based materials with nanoparticles is shown in Figure 9 [2,19,80].It can be observed that the yield stress and plastic viscosity both decrease with increasing superplasticizer content.On the one hand, superplasticizer acts as the effect of electrostatic repulsion in cement paste.The number of flocculation structures decreases, the number of free water increases, and then the viscosity and yield stress decreases.On the other hand, the steric hindrance of the superplasticizer decreases the interparticle frictional resistance, and the yield stress decreases [85,93].Nanoparticles adsorb more strongly on the superplasticizer than cement particles.Nanoparticles reduce the dispersion effect of superplasticizers [94].At present, the research on the effect of superplasticizers on the rheological properties of nano-modified cement-based materials is more mature, and the adsorption and compatibility of nanoparticles and superplasticizers are needed in future studies.

Structural Build-Up of Cement Paste in Early Hydration
Due to the small particle size and high surface activity of nanoparticles, they provide more nucleation sites during cement hydration, which affects the structural build-up of cement paste [11,85].Static yield stress method and small amplitude oscillatory shear method are the main methods to evaluate the structural build-up of fresh cement paste, and extensive studies have been conducted.Jiao et al. [95,96] believed that nano Fe3O4 increased the storage modulus of cement paste and promoted the structural build-up of cement paste.Yuan et al. [29,30] monitored the storage modulus and static yield stress of

Structural Build-Up of Cement Paste in Early Hydration
Due to the small particle size and high surface activity of nanoparticles, they provide more nucleation sites during cement hydration, which affects the structural build-up of cement paste [11,85].Static yield stress method and small amplitude oscillatory shear method are the main methods to evaluate the structural build-up of fresh cement paste, and extensive studies have been conducted.Jiao et al. [95,96] believed that nano Fe 3 O 4 increased the storage modulus of cement paste and promoted the structural build-up of cement paste.Yuan et al. [29,30] monitored the storage modulus and static yield stress of fresh cement paste in early hydration by small amplitude oscillatory shear method and static yield stress test method and investigated the effects of nano-CaCO 3 and nano-SiO 2 on the structural build-up of cement paste.Moeini et al. [37] concluded that the species of nanoclay were important factors affecting the storage modulus of nano-clay cement paste.Ma et al. [39] found that nano-clay yields an increase in static yield stress and a decrease in the storage modulus of fresh cement paste.The growth rate of the static yield stress of cement paste with the addition of nanoparticles compared with ordinary cement paste is shown in Figure 10.However, the static yield stress method obtained the variation of the structural build-up of cement paste with time, and the small amplitude oscillatory shear method was not consistent [37,39].Nano-SiO 2 accelerates cement hydration at an early age, and the cement paste with nano-SiO 2 releases more heat before 11 h with contact between cement and water [13].Nano-metakaolin promoted an increase in the impedance modulus and pore solution resistance of cement paste at hydration for 15 min-12 h and accelerated the cement hydration [97].Nano-SiO 2 and nano-TiO 2 accelerate the cement hydration, and the peak of exothermic heat increases significantly within 4-7 h after cement contacts water, and the intensity of the main exothermic peak increases [3].The reason for this was that nanoparticles provide more nucleation sites and a seeding surface for hydrate deposition, accelerating the formation of hydration products at an early age in cement paste [48,49,98,99].fresh cement paste in early hydration by small amplitude oscillatory shear method and static yield stress test method and investigated the effects of nano-CaCO3 and nano-SiO2 on the structural build-up of cement paste.Moeini et al. [37] concluded that the species of nanoclay were important factors affecting the storage modulus of nano-clay cement paste.Ma et al. [39] found that nano-clay yields an increase in static yield stress and a decrease in the storage modulus of fresh cement paste.The growth rate of the static yield stress of cement paste with the addition of nanoparticles compared with ordinary cement paste is shown in Figure 10.However, the static yield stress method obtained the variation of the structural build-up of cement paste with time, and the small amplitude oscillatory shear method was not consistent [37,39].Nano-SiO2 accelerates cement hydration at an early age, and the cement paste with nano-SiO2 releases more heat before 11 h with contact between cement and water [13].Nano-metakaolin promoted an increase in the impedance modulus and pore solution resistance of cement paste at hydration for 15 min-12 h and accelerated the cement hydration [97].Nano-SiO2 and nano-TiO2 accelerate the cement hydration, and the peak of exothermic heat increases significantly within 4-7 h after cement contacts water, and the intensity of the main exothermic peak increases [3].The reason for this was that nanoparticles provide more nucleation sites and a seeding surface for hydrate deposition, accelerating the formation of hydration products at an early age in cement paste [48,49,98,99].[30] 2% nano CaCO 3 [30] 0.3% nano clay [39] Figure 10.The growth rate of static yield stress of cement pastes with addition of nanoparticles compared with ordinary cement paste [30,39].

Further Development
According to this review, a great deal of work is in the field of rheology and structural build-up of cement-based materials with nanoparticle addition, and the current studies mainly focus on experimental studies.The effects of nanoparticle species, contents, superplasticizers, etc., on the rheological properties of fresh cement-based materials, were investigated.However, the action mechanism on rheological performance affected by nanoparticles and the structural build-up of fresh cement-based materials in early hydration has not been fully investigated, the nanoparticle's characteristics receive more attention, and the particles dissolution and water film thickness of particles should also be further investigated.In addition, it is necessary to quantitatively evaluate the effect of nanoparticles on the flocculation structures of fresh cement paste and the effect of the dispersion mode of nanoparticles on rheological performance.Further research should obtain better flowability of fresh nano-modified cement-based materials.Last but not least, the correlation between rheological properties and hardening properties should be established, and .The growth rate of static yield stress of cement pastes with addition of nanoparticles compared with ordinary cement paste [30,39].

Further Development
According to this review, a great deal of work is in the field of rheology and structural build-up of cement-based materials with nanoparticle addition, and the current studies mainly focus on experimental studies.The effects of nanoparticle species, contents, superplasticizers, etc., on the rheological properties of fresh cement-based materials, were investigated.However, the action mechanism on rheological performance affected by nanoparticles and the structural build-up of fresh cement-based materials in early hydration has not been fully investigated, the nanoparticle's characteristics receive more attention, and the particles dissolution and water film thickness of particles should also be further investigated.In addition, it is necessary to quantitatively evaluate the effect of nanoparticles on the flocculation structures of fresh cement paste and the effect of the dispersion mode of nanoparticles on rheological performance.Further research should obtain better flowability of fresh nano-modified cement-based materials.Last but not least, the correlation between rheological properties and hardening properties should be established, and the final nano-modified cement-based materials obtained with excellent rheology, mechanical properties, and durability and applied to practical engineering.

Conclusions
The rheological properties and the structural build-up in cement paste at an early age have been widely investigated.Based on the discussion and summary, some main conclusions were obtained: (1) The Bingham model, modified Bingham model, and Herschel-Bulkley model are the commonly used models to describe the rheological characteristics of cement-based materials with nanoparticles.Nanoparticles affect rheological characteristics.The cement pastes with nano-SiO 2 , carbon nanotubes, nano-CaCO 3 , and nano-metakaolin show shear-thinning behavior, and nano-TiO 2 yields shear-thickening for cement paste.(2) Nanoparticles affect the rheology of cement-based materials by changing the particle packing density, water film thickness, and flocculation structures of cement paste.Small particles fill the voids between cementitious particles and increase the packing density.The smaller size particles tend to trigger agglomeration, resulting in the increase of interparticle spaces and the decrease of the particle packing density of the cementitious system.(3) Nanoparticles have a high specific surface area, surface energy, and high surface activity, which cause the increase of flocculation structures in cement paste.Flocculation structures wrap free water, which causes the free water to reduce, and the paste's flowability worsens.Nanoparticles increase the flocculation structures and the thixotropy of cement paste.(4) The nanoparticles' dispersion mode affects cement-based materials' rheology.Mechanical agitation, ultrasonic dispersion, and incorporating surfactant are effective methods to improve the dispersibility of nanoparticles in cement-based materials.
When the surfactant is used to disperse nanoparticles, the viscosity decreases.For the mechanical agitation method, the higher the stirring rate, the lower the yield stress of cement paste; when ultrasonic dispersion of nanoparticles is used, the yield stress of nano-modified cement paste decreases.(5) The species and contents of nanoparticles and the amount of superplasticizer significantly affect the rheological properties of cement-based materials with nanoparticles.The yield stress of cement-based materials increases with the increase of nanoparticle content.(6) Nanoparticles promote structural build-up and accelerate cement hydration at an early age.Nanoparticles provide more nucleation sites and provide a seeding surface for hydrate deposition.

Figure 4 .
Figure 4.The wall effect and loosening effect on the packing density of grains [23,24].

Figure 4 .
Figure 4.The wall effect and loosening effect on the packing density of grains [23,24].

Figure 5 .
Figure 5. Formation and changes of WFT [11,65,66].(a) The close packing of cementitious particles; (b) The mixed water fills the voids between particles; (c) the excess water forms a water film on the particles' surface.

Figure 5 .
Figure 5. Formation and changes of WFT [11,65,66].(a) The close packing of cementitious particles; (b) The mixed water fills the voids between particles; (c) the excess water forms a water film on the particles' surface.

Figure 10
Figure10.The growth rate of static yield stress of cement pastes with addition of nanoparticles compared with ordinary cement paste[30,39].