Flowability and microstructure of fresh ­cement pastes in the presence of superplasticizer, latex and asphalt emulsion

1 Introduction

High performance concrete (HPC) is widely used in the construction industry at present due to its excellent properties [1]. The improved properties of HPC are closely related to the various additives that are incorporated in the concretes and mortars to achieve the desired properties. Superplasticizers are usually used to reduce the water/cement ratio (W/C) with the aim of enhancing the strength and durability of the concrete in order to satisfy the construction requirements [2–6]. Polymer latexes are often employed as cement mortar and concrete modifiers to improve the mechanical properties and durability of the mortars and concretes [7–11]. Asphalt emulsions are added to cement mortar to produce an inorganic-organic composite with high elasticity and toughness that serves as a vibration-absorbing layer in the slab track system of railroad structures [12–16]. The three types of additives usually possess different molecular/particle sizes in aqueous solution ranging from nanometres to microns, i.e. superplasticizers with a nanometre hydraulic radius (10–100 nm), polymer latex with submicron particle sizes (100–1000 nm) and asphalt particles of micron size (1–10 μm). Their effects on the pore structure and impermeability of HCPs have been reported in our previous study [16].

Practical experience has shown that in addition to improving the desired properties of hardened concrete or mortars, the inclusion of these additives also changes the flowability of fresh concrete or mortars. It is well known that adequate flowability of a construction material plays an essential role in ensuring successful construction and also has a considerable effect on the strength and durability of the hardened material [17, 18]. Studies on the flowability of fresh cement pastes containing superplasticizers and the working mechanism of superplasticizers have been well documented [19–25]. Superplasticizers could increase the flowability by adsorption on the cement grains and breaking down the flocculated structures. Much of the research relating to polymer latexes has focused on the properties of hardened polymer-modified pastes whereas little attention has been paid to the fresh pastes [26–31]. Some researchers have thought that the presence of latex in the paste could effectively enhance the flowability due to the plasticizing effect of the surfactant in the latex, the lubricating effect of spherical polymer particles and the air bubbles caused by the surfactant and stabilizer in the latex. However, Allan [29] found that the presence of polymers led to an apparent decrease in flowability accompanied by increasing yield stress and apparent viscosity. Li et al [31] reported that the addition of polymer latexes caused an initial increase followed by a decrease in the flowability of FCPs. The mechanism of polymer latexes in changing the flowability has not yet been clarified. In practice, cement asphalt mortar (CAM) is mixed on site and poured into relatively narrow spaces, so the flowability of fresh CAM or CA paste has attracted much attention and researchers have supposed that the adsorption and lubricating effect of spherical asphalt particles may influence the flowability of CAM [12–15].

A great deal of research has been carried out into cement pastes containing superplasticizers, polymer latexes and asphalt emulsions but few studies have dealt with their impact on the flowability of fresh cement pastes from the aspect of the microstructure, especially the varying impact originating from the particle size. The different particle sizes of the three types of additives may play an important role in changing the flowability.

This study has carried out a comparative analysis of the flowability of fresh cement pastes containing the three types of additives. For the first time the microstructure of FCPs in the presence of additives has been characterized by using a combination of environmental scanning electron microscope (ESEM), Morphologi G3 and 3-D laser scanning microscope. The similarities and differences in the working mechanisms of these additives were revealed by the changes in the microstructure and the flowability of the FCPs.

2 Experimental

2.1 Materials

Reference cement PI 42.5 with a fineness of 0.5 % and density of 3.10 g/cm³, consisting of 46.00 % C₃S, 27.14 % C₂S, 8.45 % C₄AF and 7.05 % C₃A, was employed in this study. The main chemical components were 62.83 % CaO, 21.56 % SiO₂, 4.44 % Al₂O₃, 2.78 % Fe₂O₃ and 3.14 % SO₃, etc. The particle size distribution was measured by laser particle size analyzer and is shown in Figure 1.

A self-synthesized polycarboxylate (PCE) superplasticizer with a hydrodynamic radius of about 10 nm measured by the method of dynamic light scattering was used. The superplasticizer is an aqueous solution of a copolymer of acrylic acid and methyl polyethylene glycol methacrylate. ­Styrene-acrylate copolymer latex with a particle size of 300 nm provided by BASF was employed. Also used was an anionic asphalt emulsion provided by the China Petrochemical Corporation with a particle size of around 3 μm.

An antifoaming agent Rhodoline DF 642 was supplied by Rhodia (China) Co., Ltd. Deionized water was used in all the experiments, including preparation of the fresh cement pastes.

2.2 Preparation and measurement of the test pieces

2.2.1 Fluidity measurement

The mini-cone test is usually carried out on cementitious materials to obtain an approximate idea of their rheological properties, which are represented by the spread diameter. The samples were first prepared in accordance with the formulations in ­Table 1, where P/C, L/C and A/C denote the solid/solid mass ratios of superplasticizer, latex and asphalt to cement respectively. In order to ensure the operability and the accuracy of the fluidity measurement, the mass ratios of water to cement were set to 0.29, 0.5 and 0.4 respectively when the superplasticizer, latex and asphalt emulsion were added to the pastes. The mass ratio of antifoaming agent to additive was 0.0005 in all the test pieces. In accordance with the mixing procedure stipulated in the Chinese standard GB/T8077, all the samples were prepared in a mixer at a slow mixing speed of 62 rpm for 2 min and a rapid mixing speed of 125 rpm for 2 min. After thorough mixing, the fresh paste was immediately poured into a cone with a top diameter of 36 mm, bottom diameter of 60 mm and height of 60 mm. The cone was then quickly inverted. When the paste stopped flowing the average value of four spread diameters crossing at right angles was recorded.

2.2.2 Measurement of the microstructure

Optical microscope

A high sensitivity and high resolution analytical tool, the Morphologi G3 (Malvern Instruments Limited, Malvern/UK), was employed for qualitative and quantitative characterization of the microstructure of the cement suspensions. The cement suspension samples with different additives were prepared as follows in accordance with the formulations in Table 2.

Each cement powder was introduced to the dispersion medium shown in Table 2 and mixed well. 2 ml of the suspension was then taken and promptly injected into a wet cell. Observation of the sample was initiated immediately. During the scanning of the particles a high quality image of each particle was captured and a 2-dimensional projection was carried out using geometrical calculations on the collected images. A global picture containing all the scanned particles as well as some magnified images of local particles was eventually obtained. Some structural parameters, including the particle size and the fractal dimensions of the particle spatial distribution (Dpd), were obtained from the image analysis software for quantitative characterization of the microstructure of the cement suspensions. It should be noted that the particle size is defined by the diameter of an equivalent circle (CE diameter) with an area equal to that of the particle image. This is different from the volume diameter measured by laser granulometry. Dpd provides information about the distribution of particles in a 2-dimension image, and a higher Dpd signifies a greater degree of dispersion.

ESEM

An environmental scanning electron microscope (ESEM) (FEI, Quanta 200FEG, USA) was used to observe the microscopic morphology of FCPs with different additives. After thorough mixing, the blank FCPs and the FCPs with 0.1 % super­plasticizer were observed in a low vacuum mode (300–500 Pa). The microstructures of FCPs containing 3 % and 9 % latex and 10 %, 35 % and 60 % asphalt were also measured in the same way.

3-D laser scanning microscope

A 3-D laser scanning microscope was employed to capture the global 2-D morphology images, 3-D morphology images and 3-D topographic maps at mass ratios of asphalt to cement of 0, 0.35, 0.6 and 1.0 so that the microstructure of the FCPs containing asphalt emulsion could be observed clearly. All the observations were initiated immediately after the pastes were prepared.

3 Results and discussion

3.1 Flowability

The effects of superplasticizer, latex and asphalt emulsion on the flowability of FCPs are shown in Figure 2. It is clear that when PCE superplasticizer is added the flowability of FCPs gradually grows to a maximum value at a P/C of 0.8 % and then remains stable. In the presence of polymer latex with its submicron particle size the flowability decreases at first and then increases to a maximum at an L/C of 12 %. The flowability starts to drop again on further addition of polymer latex. In contrast, the flowability of FCPs containing asphalt particles in the micron range increases with the increasing addition of asphalt and reaches a maximum at an A/C of 20 %. When the addition level exceeds 20 % the flowability of the FCPs decreases with increasing A/C.

Basically speaking, it is the changes in the microstructure of FCPs caused by the incorporation of additives that are responsible for the variations in flowability. It is well understood that the three additives with negative charges could be effectively adsorbed on the positively charged surface of the cement due to electrostatic attractive forces. The adsorption will induce electrostatic and/or steric effects, and even stereo effects, between the flocculated cement grains [32], thereby changing the dispersive state of the cement grains in the paste. It is deduced that when the coverage of additives on the cement surface reaches 100 %, i.e. saturated adsorption, the degree of dispersion of the cement grains reaches a maximum and the flowability of FCPs therefore achieves a maximum value. When the total surface area of cement grains and early hydrates (at 5 min) [33] that could be fully covered by additives is 590 m²/kg, the molecular radius of the superplasticizer is 10 nm and its density is 1 g/cm^³ it can be calculated that the addition level of superplasticizer at full coverage is 0.8 %. This is defined as the saturated adsorption addition level. Similarly, it was found that the saturated adsorption addition levels of polymer latex with a particle size of 300 nm and of asphalt particles of 3 μm are 12 % and 120 % respectively. Full coverage of the three additives on the cement surface is illustrated in Figure 3c, g and k.

When the adsorption of superplasticizer on the cement surface is not saturated (P/C < 0.8%), a higher addition level brings about an improved state of dispersion of the cement grains due to the electrostatic and/or steric effects. This is the so-called dispersive effect of the superplasticizers, as illustrated in Figure 3b. The degree of dispersion of the cement grains and the flowability of the FCPs gradually increase with increasing P/C and finally reach a maximum at saturated adsorption at a P/C of 0.8 %, i.e. 100 % coverage of the cement surface. After the adsorption reaches saturation most of any further superplasticizer added remains in the aqueous phase, which contributes little to the dispersion of the cement grains. The flowability of FCPs therefore remains stable at P/C > 0.8 %, regardless of any further addition of superplasticizer, as illustrated in Figure 3d.

It is supposed that the anionic latex particles could also be adsorbed in the same way on the cement surface and bring about a superior dispersive state of the cement grains due to its dispersive effect by inducing electrostatic repulsion between the cement grains. However, unlike superplasticizers, the polymer particles in latex exist in a condensed state with a size of 300 nm. At low addition levels (L/C ≤ 3%) the adsorption of polymer particles on the cement surface may lead to transformation of the zeta potential of the FCPs from a positive value to a negative one. It has been widely accepted that a suspension dispersion system is most unstable if the | ζ | of the system is in the vicinity of 0 mV, at which the particles tend to form the largest flocculated structures [34, 35]. In this case a low addition level of latex, i.e. low coverage of the cement surface, facilitates the formation of flocculated structures. This is defined as the neutralizing effect of latex, as illustrated in Figure 3e. As a result the flowability of the FCP decreases with increasing addition of latex if the L/C is lower than 3 %. Further addition of latex, i.e. more coverage of latex particles on the cement surface, results in a decrease in zeta potential to a more negative value. This enhances the electrostatic repulsion between cement grains and thus the degree of dispersion of the cement grains (Figure 3f). The flowability of FCPs therefore starts to increase with increasing L/C. The flowability of FCPs reaches a maximum value when full coverage of the latex particles on the cement surface has been achieved at an L/C of 12 %, i.e. saturated adsorption.

Above the saturated adsorption addition level of 12 %, those latex particles of submicron size remaining in the aqueous phase increase the content of solid phases in the paste, as shown in Figure 3h. This is defined as the filling effect of latex and is detrimental to increased flowability [16]. The addition of latex therefore reduces the flowability of FCPs when the L/C is greater than 12 %.

For FCPs containing asphalt emulsion with particles of micron size the flowability gradually grows with the increasing A/C. This could be explained by the dispersive effect of asphalt emulsion caused by the adsorption of asphalt particles on the cement surface, as shown in Figure 3i. The inclusion of asphalt emulsion apparently increases the content of solid phase in the pastes due to the existence of micron-sized asphalt particles. This is the filling effect of asphalt emulsion and is more significant than that of submicron latex particles. The flowability of FCPs therefore reaches a maximum value at an A/C of 20 %, which is far below the saturated adsorption addition level of 120 %. When the A/C exceeds 20 % the filling effect overrides the dispersive effect of the asphalt emulsion and the flowability begins to decrease with increasing A/C.

It should also be noted that, in contrast to the submicron latex particles, the asphalt particles do not reduce the flowability at low addition levels (< 3%). This phenomenon may be related to the stereo effect of asphalt particles, which was considered to be the third mechanism of the additives [32]. More efforts are being made to explain this mechanism.

To obtain a deeper understanding of the working mechanism of these additives the microstructure of FCPs containing superplasticizers, latex and asphalt emulsion was investigated in depth by a combination of different types of microscopy, as described in the next section.

3.2 Microstructure

3.2.1 FCPs containing superplasticizer

The microstructure of FCPs with and without superplasticizer recorded by ESEM has been reported in our previous research where the cement grains were well dispersed in the presence of superplasticizers (Figure 4) [16]. The microstructure of cement suspensions in the absence and presence of the superplasticizer is shown in Figure 5. A large number of flocculated structures of various sizes were found in the C-W suspension system (Figure 5a–b). These consisted of cement grains of different sizes and entrapped water, with irregularly shaped vesicles and blurred and spinous edges. In the presence of the superplasticizer, however, the separated cement grains with clear edges and hard corners were well dispersed in the water (Figure 5c–f). The degree of dispersion of the cement grains in the superplasticizer solution became greater with increasing concentration of superplasticizer.

The particle size distribution curves of the cement suspensions are shown in Figure 6 and the structural parameters of the microstructure, including the mean particle size and the fractal dimension of particle spatial distribution (Dpd), are listed in Table 3. As can be seen from Figure 6, the particle size distribution of the C-W-P system differs clearly from that of the C-W system, in which there is a sharp rise in the quantity of the particles smaller than 5 µm and there are fewer particles larger than 20 µm. The mean particle sizes of the systems were gradually reduced to 4.53 μm and 2.99 μm from 8.09 μm of the C-W system by the addition of superplasticizer. The dispersion indices Dpd were also increased to 1.80 and 1.94 from 1.68, which signifies the enhanced degree of dispersion of the cement grains in the presence of superplasticizer.

From both the qualitative and quantitative viewpoints the variations in the microstructure of the FCPs were in good agreement with the flowability of FCPs in the presence of superplasticizer, i.e. the superior state of dispersion of the cement grains corresponded to the greater flowability of the FCPs.

3.2.2 FCPs containing poly(styrene-acrylate) latexes

When latex was incorporated in the FCPs the modes of interaction of the latex particles with cement grains were recorded by ESEM, as shown in Figure 7. The adsorption of particles on the cement surface is clearly shown in Figure 7a. With increasing L/C the latex particles fill in the voids between the cement grains as well as being adsorbed on the cement surface (Figure 7b).

The microstructure of cement suspensions containing latex is shown in Figure 8. It is clear that the addition of 3 % latex particles leads to the formation of a large quantity of flocculated structures in the suspension (Figure 8a and 8b). The particle size distribution curves of the C-W-L systems (Figure 6) clearly show an increase in large particles in the 50–100 μm size range. This phenomenon is consistent with the flowability curves of the FCPs at low addition levels (≤3%); the incorporation of latex reduces the flowability of the FCPs by causing flocculation of the cement grains. Figure 8c and 8d show separated cement grains with clear edges and hard corners and a small quantity of flocculated structures at an L/C of 9 %. There is also a sharp drop in the content of particles with size of 3–50 μm (Figure 6). These phenomena indicate that at high addition levels the addition of latex helps to increase the degree of dispersion of the cement grains, thereby increasing the flowability of the FCPs.

A new peak at about 500 nm could also be seen in the particle size distribution curves of the C-W‑L systems (Figure 6), which is due to the presence of submicron latex particles in the suspensions. This means that although there are a large number of flocculated structures in the C-W-L-0.3 % system its mean particle size of 7.25 μm is lower than the 8.09 μm value of the C-W system and the dispersion index of 1.70 is slightly larger than the 1.68 value for the C-W system.

3.2.3 FCPs containing asphalt emulsion

Figure 9 shows the ESEM images of fresh cement asphalt pastes. At an A/C of 0.1 the cement paste functions as a continuous phase and the asphalt particles are well dispersed in the paste with respect to adsorption on the cement surface and filling in the voids between the cement grains. When the A/C increases from 0.35 to 0.6 the asphalt emulsion gradually becomes the continuous phase and encloses the disperse phase of cement grains.

Observation with the Morphologi G3 shows that some separate cement grains have adsorbed asphalt particles and that a few small flocculated structures and many spherical micron-sized asphalt particles are well dispersed in the system, as shown in Figure 10. Like the C-W-L system, the average particle size of the C-W-A system is 3.44 μm and the Dpd is 1.99 due to the existence of many asphalt particles in the paste. These approximate respectively to the size of asphalt particles of 3 μm and the Dpd of asphalt emulsion of 2.0, i.e. the two structural parameters of the C-W-A system tend to reflect the particles size of the asphalt particles and their state of dispersion in the suspension rather than those of the cement grains. It is therefore hard to establish a direct relationship between the flowability of the FCPs and the microstructure on the basis of the two structural parameters.

In view of the large addition level of asphalt in the cement paste (the A/C ranges from 0.2 to 1.0 in a typical cement asphalt paste) a 3-D laser scanning microscope was used to observe the distribution of the asphalt emulsion in the FCPs at different A/Cs. As is shown in Figure 11 it was possible to make a clear distinction between the grey cement paste and the brown asphalt emulsion when the two phases were mixed together. The 2-D morphology images, 3-D morphology images and 3-D topographic maps of FCPs at A/Cs of 0, 0.35, 0.6 and 1.0 are shown in Figure 12 and Figure 13. In the 3-D topographic maps the red, yellow, green, blue and purple colours are used to mark different heights of the solid phases in the FCPs in the sequence from high to low.

In the blank FCP the large flocculated structures are clearly visible and are marked in Figure 12a–c. At an A/C of 0.35 the large flocculated structures in the paste are broken down into small pieces as shown in Figure 12d–f, which means that the dispersive state of the cement grains in the paste has been effectively improved by the introduction of the asphalt emulsion. This phenomenon corresponds well to the changes of flowability of the FCPs containing asphalt emulsion in Figure 2.

An interpenetrating network is formed with further addition of the asphalt emulsion. The brown area is the asphalt and the grey area corresponds to the cement grains, as shown in Figure 13a and Figure 13b. There are no obvious flocculated structures in Figure 13c and the green areas are well distributed in the image, which signifies that the cement grains are well dispersed in the asphalt emulsion. The A/C is 1.0 so the asphalt emulsion behaves as the continuous phase and encloses all the cement grains, giving the dark global image shown in Figure 13d and Figure 13e. The larger areas of blue and purple in Figure 13f indicate that the cement grains are well dispersed while the continuous green area may be related to the agglomerated asphalt particles.

The microstructure of the FCPs in the presence of asphalt emulsion and its variations with A/C were characterized for the first time using a combination of methods. This directly validates the hypothesis for the working mechanism of asphalt emulsion on the changing flowability of the FCPs in section 3.1, namely the dispersive effect at low addition levels and the filling effect at high addition levels. Specifically, the quantity of flocculated structures gradually falls at low A/Cs (≤ 20%) due to the adsorption of asphalt particles onto the cement surface and there is a corresponding increase in the flowability of the FCPs. The high content of asphalt particles in the paste at high A/Cs definitely leads to a significant growth of the solid phase content as well as of the frictional forces between the dispersed phase, including the cement grains with adsorbed asphalt particles and the separate asphalt particles, thereby reducing the flowability of the FCPs.

4 Conclusion

Comparative studies were carried out to discover the effects of three types of additives, namely PCE superplasticizer, poly(styrene-acrylate) latex and asphalt emulsion, on the microstructure and flowability of fresh cement pastes. Based on the results, the following conclusions can be drawn:

(1) Microscope observation reveals that the three types of additives could be adsorbed on the surface of the cement grains and change the dispersive state of cement grains. Their different molecular/particle sizes, namely 10 nm, 300 nm and 3 μm for superplasticizer, latex and asphalt emulsion respectively, led to different surface coverages at a constant addition level and to different saturated addition levels of 0.8 %, 12 % and 120 % at full coverage of the cement surface. The degree of dispersion of the cement grains reaches its maximum when saturated adsorption is achieved, i.e. at full coverage of the cement surface,.

(2) When the adsorption of superplasticizer is not saturated (P/C < 0.8 %) the flowability of the FCPs rises gradually with the increasing addition level of superplasticizer. This is due to the dispersive effect of the superplasticizer. In contrast to the blank cement paste, in which the cement grains form flocculated structures due to the heterogeneous charge distribution, the cement grains in the FCPs containing superplasticizer are well dispersed. The degree of dispersion of the cement grains and the flowability of the FCPs reach maximum values at a P/C of 0.8 %. After the adsorption becomes saturated any further superplasticizer that is added tends to remain in the aqueous phase, which makes little contribution to the dis­persion of the cement grains. This means that at P/Cs > 0.8 % the flowability stays unchanged regardless of further addition of superplasticizer.

(3) Incorporation of polymer latex with a particle size of 300 nm first causes a drop in the flowability of FCPs and then increases it to a maximum when the adsorption of latex particles on the cement surface becomes saturated. Microstructural results demonstrate that at low L/Cs (≤ 3%) a low coverage on the cement surface facilitates the formation of a large quantity of flocculated structures. With further addition of latex the greater coverage of latex particles on the cement surface causes better dispersion of the cement grains and greater flowability of the FCPs. The largest flowability is achieved at an L/C of 12 %, i.e., 100 % coverage of the cement surface by polymer particles. Above the saturated adsorption addition level any further latex particles that are added cause a drop in flowability. This is because of the increased content of solid phases due to the presence of submicron particles in the aqueous phase, known as the filling effect.

(4) In contrast, in the case of FCPs containing asphalt emulsion with micron-sized particles the flowability rises with increasing asphalt addition level at low A/Cs (≤ 20 %) due to the dispersive effect of the asphalt particles. Addition of asphalt emulsion causes the flocculated structures of cement grains to break down into small pieces. In the meantime, the inclusion of asphalt emulsion causes a substantial increase in the solid phase content of the pastes due to the filling effect of the micron-sized asphalt particles. The flowability of the FCPs therefore reaches its maximum at an additional level of 20 %, which is far below the saturated adsorption addition level of 120 %. At A/Cs greater than 20 % the filling effect overrides the dispersive effect, with the result that the flowability of the FCPs starts to decrease.

Acknowledgements

The support of the National Natural Science Foundation of China (Grant No. 51608032) and the Fundamental Research Funds for the Central Universities (No. 2016JBM036) are appreciated.