Enhancement in stability of air bubbles in mortar at fresh state with different SCMs

doi:10.21203/rs.3.rs-4846466/v1

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Enhancement in stability of air bubbles in mortar at fresh state with different SCMs

https://doi.org/10.21203/rs.3.rs-4846466/v1

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The stability of air bubbles is a crucial factor in determining the workability, strength, durability and surface quality of concrete. There is a growing interest in the regulation of air bubble stability in concrete industry. This study examines the influence of various supplementary cementing materials (SCMs, 20% by weight in replacement of cement) on the foam/air bubble stability. The time-dependent evolutions of foaming height and air bubble size distribution were tested, which reflected the trend of the influence of different SCMs on the foam/air bubble stability in solutions or mortars. The air bubble size distribution in mortar was tested using AVA (air void analyzer) from 5 minutes to 60 minutes and X-CT from 60 minutes to 180 minutes after the mixture were prepared. The results demonstrated that over time, the number of small bubbles decreased, while the number of large bubbles increased. The primary change occurred within the initial 60 minutes. The results of the wettability test were combined with those of the X-ray diffraction (XRD) analysis to identify a correlation between the stability of air bubbles and the wetting angle of the supplementary cementitious material (SCM). The wetting angle of the SCM was found to be within 90° in cases where the air bubbles were more stable. Furthermore, the XRD patterns revealed significant differences in the mineral compositions between the air bubble shells and the screened pastes from fresh mortar. The presence of more SCMs and hydration products on the air bubble shells than in the paste was identified as a potential reason for the observed differences in air bubble stability. The utilization of specific SCMs has the potential to enhance the air bubble stability in the context of concrete construction engineering, in addition to chemical admixtures.

air bubble

stability

fresh mortar

SCMs

air bubble shell

mineral compositions

Concrete is composed of solid, liquid and vapor phases at multiple scales. The two primary representation forms of vapor phase are air bubbles in fresh concrete and air voids in hardened concrete. Air bubbles in fresh concrete are generated by high-speed shearing during forced mixing, which is facilitated by the presence of surface active agents such as superplasticizer and air-entraining agent (AEA) [1]. The characteristics of air content and air bubble/void size distribution can significantly affect concrete performance [2]. Researchers have devoted to further investigation into the relationship between air bubble/void features and fresh/hardened concrete properties. It was discovered that the air content of fresh concrete has a significant impact on its rheological behaviour [3–5]. Zhang [4] noted that small bubbles ranging from 10–600 µm, greatly contributed to the flowability of fresh concrete. However, higher air content might pose a risk of weak mechanical strength, as there was a strong linear correlation between air content and 28d compressive strength, as reported in literature [6]. In civil construction engineering, unconnected micro air voids entrained by AEA could reduce freeze-thaw damage of concrete at low temperatures, which has been proven effective widely [7].

Air voids in hardened concrete originate from air bubbles in fresh concrete, which are thermodynamically unstable systems with a tendency toward increase in size due to coalescence and Oswald ripening [8]. From a macroscopic perspective, the production and evolution of air bubbles can be affected by multiple factors such as raw materials, chemical admixtures, mix proportions, mixing processes, environmental temperature and atmospheric pressure [9–13]. The issue of air bubble stability is so complicated that different research results may vary and even be controversial among research groups [2, 14, 15]. Therefore, more detailed microstructure information of air bubbles formation and evolution in fresh concrete are worthy of collecting and analyzing to accurately describe the dynamic processes.

In recent years, researchers have discovered that the air bubble is not simply composed of air wrapped by a pure liquid membrane. Instead, it is a liquid membrane with solid particles adhered to it, forming a shell [16, 17]. It suggested that the opaque shell was formed after a selective adhesion of certain solid particles, which could resist coalescence and stabilize air bubbles. Some AEA can combine with calcium ions to form solid precipitation, which adheres to the air bubble surface and improves its stabilization [18, 19]. Some researchers have adopted nano-sized particles to enhance mechanical strength of bubble shell to inhibit coalescence degree of fine bubbles, especially when the wetting angle of nano-sized particles are around 90 degrees, which shows a promising potential for future application [20, 21]. As it can be seen, the solid phases on the bubble surface are critical to air bubble stabilization, yet most of the existing studies focused on qualitative observation rather than quantitative evaluation.

Supplementary cementing materials (SCMs) are widely used by partially replacing cement in concrete preparation nowadays. They can improve concrete performance by optimize the rheological properties of fresh mixture, reducing hydration heat in the early age, and increasing long-term mechanical strength and durability [22]. Besides, researchers have found different binder compositions of concrete could obviously affect the air bubble stability in fresh state. Puthipad et al [23, 24] revealed that the use of fly ash in the self-compacting concrete (SCC) could result in a higher degree of coalescence of the fine air bubbles. The phenomenon of unstable air content of concrete containing fly ash was also discovered from other researchers [25, 26]. However, rice husk ash seemed not to affect the air-void stability in fresh concretes [27]. Anyway, there are still lack of deep studies of air bubble stability evaluation of concrete containing other many different types of SCMs. Furthermore, the interaction between different SCMs particles and air bubble shell is worthy of investigation.

This paper focuses on provide insight into the air bubble stability and the relationship between it and air bubble shell compositions. In this study, experiments were conducted with fresh mortar prepared with eight types of SCMs. The SCMs were carefully selected for their different wetting angles and particles diameters. Mortar instead of concrete was chosen since the influence of coarse aggregates should be avoided to minimize the testing error. Quantitative evaluation of time-dependent air bubble size distribution evolution of fresh mortars was conducted first. Then a new experimental method is proposed to collect the solid phases for composition analyzation on the surface of air bubble in fresh mortar. Authors also attempt to investigate the relationship between the mineral compositions of air bubble shell of each mortar sample and SCM particle features.

2.1 Materials

P·II 52.5 Portland cement (C), Class F fly ash (FA), S95 grade ground granulated blast furnace slag (GGBS), 400-mesh quartz powder (Qtz400), 800-mesh quartz powder (Qtz800), 400-mesh limestone powder (LS400), 800-mesh limestone powder (LS800), 400-mesh talc powder (Talc400) and 800-mesh talc powder (Talc800) were chosen as binder materials in this paper. The physical property parameters of each binder material including specific surface area (SSA), apparent density (ρ) and average particle size (D_p) were presented in Table 1. XRD patterns of binder materials were shown in Fig. 1. ISO standard sand, deioned water and a polycarboxylate superplasticizer (PCE) with solid content of 15% were used to prepare mortar mixtures.

Table 1

Physical property parameters of binder materials
Binder	C	FA	GGBS	LS400	LS800	Talc400	Talc800	Qtz400	Qtz800
SSA/(m²/g)	1.05	1.31	0.49	1.95	2.08	3.18	3.11	1.18	1.81
ρ(g/cm³)	3.09	2.15	2.80	2.75	2.70	2.80	2.70	2.65	2.65
D_p(µm)	34.24	34.61	25.35	41.72	18.18	32.21	16.71	37.41	12.72

2.2 Mix proportions and sample preparation

Cement mortar samples were prepared with water to binder ratio of 0.35, SCM replacement of 20% and sand to binder ratio of 1.5 as shown in Table 2. Before preparing cement mortars mixtures, the cement and SCM were dry mixed uniformly and then were poured into the planetary mixer which was met the requirement of Chinese standard JC/T 681–2022. Then, an automatic stirring program according to Chinese standard GB/T 17671 − 2021 was adopted for cement mortars preparation. After the mixing of cement mortars, fluidity and air content were measured immediately.

Table 2

Mortar proportions and fresh properties
Sample	C/g	SCM/g	Sand/g	Water/g	PCE/g	Air content/%	Fluidity/mm
C	1000	0	1500	350	5	9.7	250
FA	800	200	1500	350	5	8.6	265
GGBS	800	200	1500	350	5	10.1	245
LS400	800	200	1500	350	5	9.3	255
LS800	800	200	1500	350	5	9.8	250
Talc400	800	200	1500	350	5	10.2	245
Talc800	800	200	1500	350	5	10.6	250
Qtz400	800	200	1500	350	5	8.3	260
Qtz800	800	200	1500	350	5	8.6	260

2.3 Test methods

2.3.1 Contact angle of binder materials

Washburn capillary rise experiment (WCR) [28] was introduced to quantify the wettability of binder powders through a Kruss K100 tensionmeter. When performing the measurement, a glass tube with diameter of 10 mm was used. The same amount around 1.5 g of each binder material (vacuum heated at 40 degrees centigrade for 24h) was divided into three equal parts and filled into the glass tube in sequence. After filling each part of SCM into the glass tube, tapping 10 times at a constant tapping height of 20 mm was conducted. Then the glass tube containing the binder material was slowly moved to the wetting liquid.

It is based on the assumption that powder materials can be described as bundles of capillary tubes of constant radius, which was derived from Poiseuille's Law principle and it is defined assuming a linear relationship between the squared mass (m²) of wetting liquid versus measurement time (t) (Eq. 1). The contact angle of each binder material could be calculated according to reference [29].

$$\:{m}^{2}=\frac{C{\rho\:}^{2}\sigma\:\text{c}\text{o}\text{s}\left(\theta\:\right)}{2\eta\:}\times\:t$$

where C is the material constant, θ is the contact angle and ρ, σ and η are the density, surface tension and viscosity of the wetting liquid, respectively.

$$\:C=r{A}^{2}{\epsilon\:}^{2}$$

C is a geometric factor that is related to the geometry of the pores, where r is the effective pores radius, A is the cross-section area of the tube and ε is the porosity of the powder material. C is determined by using a reference liquid with low σ (hexane was used in this study) for which is assumed that completely wets the powder material (θ = 0°). Once C is defined, θ can be calculated between the binder materials powders and deioned water.

2.3.2 Foaming height and half-life time

To investigate the impact of SCM type and dosage on the quantity and stability of foam produced by the superplasticizer solution, the authors adapted the foam index and stability index test methods as reference [30–32]. The binder material and superplasticizer solution were placed into a 100-mL measure cylinder jar. The binder material was vacuum heated at 40 degrees Celsius for 24h. The superplasticizer solution had a solid content of 0.25%, which is a typical concentration in the liquid phase of fresh concrete regardless of adsorption. Table 3 showed the proportions of the suspensions investigated for foam index tests with each binder material. The jar was capped and thoroughly shaken for 10 seconds to fullly wet the powder. Subsequently, it was shaken for additional 60 seconds to create foam. The process was recorded using a high-definition camera to capture the changes in foam height.

Table 3

The proportions of suspensions with each binder material for foam index tests
Binder material/g	PCE solution (wt, 0.25%)/mL	Solid phase ratio (g/100mL)
2.4	60	4
4.8	60	8
7.2	60	12
9.6	60	16

Foam height (p₀) and half-life time of foam (t_1/2) were adopted as shown in Fig. 2. p₀ is the difference between the suspension level and the top surface of the foam in the measuring cylinder, as soon as it became distinguishable after 60 seconds of vigorous shaking. The half-life time (t_1/2) is the duration of time it took for the foam height to decrease to half of its initial value (½p₀). The foam index tests were replicated twice, and the reported values (p₀ and t_1/2) are the averages of the replicates.

2.3.3 Air bubble size distributions of fresh mortars

The entrained air bubble size distributions in fresh mortars were measured using a Germann air-void analyzer (AVA 3000) at the initial state and 60 minutes after mixing. The AVA apparatus has been demonstrated to be effective and widely used in quantifying air bubble distribution in fresh concrete or mortar [4, 20, 33, 34]. Once the mortar mixture was uniformly mixed, a syringe was used to extract approximately 20 mL of fresh mortar sample, taking care to avoid trapping any excess air. The syringe was carefully installed at the bottom of the riser column, and the mortar sample was then added into the column to initiate the AVA testing program. Air bubbles in fresh mortar might be released and rise to the water surface where there was a buoyancy pan for recording the impact force of air bubbles. Then the AVA analyzer automatically ran a program to measure and calculate the air volume of bubbles with varying size (chord length).

In this study, it was found that air bubbles in fresh mortar were difficult to separate after 60 minutes of mixing due to the loss of fluidity in the mortar mixture. The step is essential for the AVA air-void analyzer. Therefore, a ZEISS Xradia Context micro-CT [3] was used to non-destructive examine the the characteristics of air bubbles in fresh mortar from 60 to 180 minutes after mixing. The fresh mortar mixture was carefully placed into a 1.5 cm diameter and 8.5 cm height plastic test tube to prevent macro defects. The test tube was then sealed for examination at 60, 120 and 180 minutes after mixing. The X-ray tube voltage and current were set to 80 kV and 0.088 mA, respectively. A total of 966 2D CT images were obtained in the X-Y direction, with an image size of 13695×14043 pixels and a resolution of approximately 9.17 µm. The image exposure time of each image was 340 ms. The air bubble microstructure was reconstructed and the air bubble size characteristics were analyzed from the X-CT images using VG Studio software.

2.3.4 Air bubble shell separation and mineral composition characterization

It was quite difficult to separate the air bubbles with the shell around them for further investigations. The researches on the air bubble shell in literatures [16, 17, 35, 36] adopted either indirect or equivalent testing methods. In this study, a novel experimental method for air bubble shell separation from fresh mortar mixtures was proposed. Inspired by air-void analyzer (AVA 3000), a quite similar apparatus was designed and assembled as shown in Fig. 3, only with larger size. The operation steps were almost the same as air-void analyzer as described in section 2.3.3. The fresh mortar sample volume for each separation was around 100 mL instead of 20 mL for air-void analyzer. The majority of the bubbles adhered to the top of the column and then the solid phases from the bubbles were collected. The same procedure was repeated for several times to collect enough solid phases.

Afterwards, the fresh mortar was also collected and screened through a 75µm mesh sieve at 5min and 60min after mixing. The screened bulk pastes and the solid phases collected from air bubbles were separately soaked into isopropanol for 24h to terminate the hydration. Subsequently, the samples were filtrated with filter papers and placed in a vacuum desiccator at 40 degrees centigrade for another 24h.

2.3.5 Mineral composition characterization

A D8 Discover X-Ray diffractometer from Bruker Inc. was applied to probe into the mineral compositions of binder materials, air bubble shell and screened bulk paste. All samples were analyzed from 5° to 70° 2θ, with a step size of 0.02° and scan step of 4°/min. A traditional copper (Cu) x-ray tube with working voltage 40 kV and working current 30 mA was used. The diffractometer was configured with a 0.3° divergent slit, a 2.5° soller slit on the incident-beam side. Each pattern was analyzed using the whole pattern fitting method developed by [37] using the TOPAS Academic software package.

2.3.6 Micro morphology

The microstructure images of each binder materials were examined with a QUANTA 250 SEM from FEI Inc. The operation voltage was 15 kV and working distance was 10mm. Before imaging, the samples were sputter-coated with a thin layer of gold to enhance conductive continuity.

3.1 Micro morphology

SEM images of the binder materials selected for this study, as shown in Fig. 4, revealed that the particles were mostly irregular in shape and size, with the exception of fly ash, which was mostly spherical. The particle size of each binder material generally corresponded to the average particle diameter test results in Table 1. It is challenging to differentiate between the binder material types based on the SEM images, with the exception of fly ash.

3.2 Wettability of binder materials

Figure 5 presents the profiles of deionised water penetration into tubes packed with each binder material. According to the literature [29], the curve of the penetration rate can be categorized into three main stages. The first stage is governed by inertia forces which are related to the penetration of the wetting liquid into the glass tube through its base. In this study, the first stage was hardly to tell in Fig. 5. Next, the second stage is controlled by capillary forces which are associated with the wetting liquid penetrating into the binder materials and it corresponds to the. Eq. (1) and Eq. (2) were applied to this stage for contact angle characterization of each binder material. The third stage is determined by the equilibrium of these forces, corresponding to the oblique part of the curve.

Table 3 shows the measured capillary constants and contact angles of cement and SCMs. It is evident that the wetting angle of each binder material varied significantly. The binder material exhibits the lowest wetting angle with 800-mesh quartz powder and the highest wetting angle with 800-mesh talc powder. However, it is noteworthy that all the contact angles of the commonly used powder materials in concrete are below 90°, indicating their hydrophilic nature. The differences in wettability of the binder materials can be attributed to the physical-chemical nature of the mineral compositions [38]. Additionally, the use of grinding aids in mineral powder manufacturing may also affect the wettability of these binder materials [39].

Table 4

Capillary constant and contact angle test results
Binder material	Capillary constant	Contact angle/°
C	3.365E-6	40.0
FA	1.353E-6	43.7
GGBS	2.573E-6	56.0
LS400	1.436E-6	51.6
LS800	1.305E-6	56.3
Talc400	2.306E-6	83.2
Talc800	2.439E-6	86.6
Qtz400	3.435E-6	46.9
Qtz 800	5.257E-6	30.3

3.3 Foaming height and half-life time

Figure 6 presented the foaming height and half-life time results of PCE solutions with cement and SCMs. Foaming height is a parameter that characterizes the foaming ability of a given solution. Figure 6a demonstrates that the foaming height generally decreased as more binder material was added to the PCE solutions. In the cases of cement and GGBS, the foaming height decreased immediately and then stabilized. The addition of quartz or fly ash to the PCE solutions resulted in an initial increase in foaming height, followed by a decrease. Talc and limestone had little effect on foaming height in this study. The foaming height of PCE solutions ranged from 23 mm to 35 mm when the binder material content was between 0 g/100mL and 16 g/100mL.

In contrast, the foam half-life time, which characterizes the foam stability of a given solution, was more influenced by the type and content of binder materials, as shown in Fig. 6b. Except for talc powder, the foam half-life time decreased slightly as other binder materials were added to the PCE solutions and became stable once the solid phase ratio reached 8 g/100mL. It is important to note that the addition of Talc powder, regardless of particle diameter, significantly improves the foam stability of PCE solutions. Talc increased the foam stability by three to four orders of magnitude. The 800-mesh powder showed the greatest improvement, with a half-life time of over 10,000 seconds at a solid phase ratio of 4 g/100mL. However, the half-life time of PCE solutions was so high that the actual foam height could not be accurately measured after three days of observation. The record of foam height results was terminated at that point. Based ons the half-life time results shown in Fig. 5b, the contribution to foam stability of each binder material could be ranked as talc800 > talc400 > LS800 > LS400 > FA > GGBS > Qtz800 ≈ Qtz400 > C.

3.4 Air bubble structure evolution

3.4.1 5min ~ 60min

Figure 7 presents the size distribution of air bubbles and the degree of coalescence in fresh mortar from 5 to 60 minutes after mixing. The initial air bubble structure in fresh mortar can be clearly distinguished in Fig. 7a. Over time, the number of small bubbles decreased while the number of large bubbles increased in all fresh mortar mixtures. The critical chord length of air bubbles was approximately 500 µm [33]. Air bubbles in fresh mortar are a thermodynamically unstable system and tend to reduce their surface free energy, leading to spontaneous coalescence, Ostwald ripening, and collapse. To evaluate the extent of air bubble size change quantitatively, we introduced the air bubble coalescence degree (Eq. 3), as referenced in [10].

$$\:C=\frac{{A}_{s1}-{A}_{s2}}{{A}_{s1}}$$

C is the degree of coalescence of fine air bubbles, A_s1 is the content of small bubbles (up to 500 µm) in the initial state (%) and As2 is the content of small bubbles (up to 500 µm) at 60 min (%).

Figure 7 shows that the air bubble coalescence degree of the reference sample reached 19.3%, the highest value among all samples. This suggests that the stability of air bubbles was improved to varying degrees by introducing different supplementary cementitious materials (SCMs) in the mortar mixtures. Talc powder demonstrated the most effective reinforcement in air bubble stability within 60 minutes, regardless of its fineness. The size of air bubbles hardly increased when talc powder was added to mortar mixtures. GGBS also improved air bubble stability, with a coalescence degree of 2.5% within 60 minutes. The coalescence degree of the Qtz800 sample was 17.8%, while that of Qtz400 was only 6.2% when it came to quartz powder. By combining the coalescence degree results in Fig. 7b with the foam half-life time results in Fig. 6b, it can be observed that the stability of the air bubble/foam is in line with the contact angle sequence of the binder materials. The greater the contact angle of the binder material, the more stable the air bubble/foam will be.

3.4.2 60min ~ 180min

This study observed a distinct loss of fluidity within 60 minutes after mixing, indicating that air bubbles in mortar mixtures could not be released by the air void analyzer. X-CT was used to characterise air bubble size and shape evolution from 60 to 180 minutes after mixing. The results of the analysis of air bubble size distribution are presented in Fig. 8. After 60 minutes, the evolution of air bubble size tended to slow down. The Ref sample and Talc800 sample were the most and least stable mortar mixtures in terms of air bubble stability, respectively, as shown in Fig. 7. Additionally, except for the LS800 sample, which showed a significant change in air bubble content (< 500 µm), the main changes in air bubble content occurred at sizes above 1000 µm in the other samples. Between 60 and 180 minutes after mixing, small bubbles remained stable while larger bubbles underwent size evolution. It is important to note that the air bubble size calculated from X-CT images was based on equivalent diameter, whereas AVA results used chord length. The measured chord length is equal to 2/3 of the true air void diameter [33].

The reconstruction of air bubble microstructure from X-CT scanning images provided both size distribution and micro morphology details. Figure 9 presented the distribution of sphericity of air bubbles in fresh mortar from 60 to120 minutes. Sphericity of a bubble refers to the ratio of the surface area of the equivalent-volume sphere to that of the bubble. It can reflect the degree of deformation and coalescence of bubbles within a specific size range. The greater the sphericity, the more irregular the air bubbles. Figure 8 showed that all air bubbles within the scanning scope had a sphericity between 0 and 0.6, which gradually increased over time. In addition to the typically unimodal curve of bubble sphericity in the 800-mesh Talc sample, bimodal distribution curves of bubble sphericity were found in all other samples. The change in bubble sphericity over time reflected the evolution of air bubble stability.

The micro morphology of bubbles in the reference sample underwent a significant shift from 60 to 120 minutes. Additionally, the bubble sphericity curves moved towards the right from 60 to 90 minutes in both the FA and GGBS samples, but remained relatively unchanged from 90 to 120 minutes. In the 400-mesh LS sample, the air bubble content with sphericity ranging from 0.2 to 0.3 decreased, while the content with sphericity ranging from 0.35 to 0.45 increased correspondingly. Conversely, the opposite phenomenon was observed in the 800-mesh sample. The sphericity curves shifted mainly from 60 to 90 minutes in the Qtz samples and the 400-mesh Talc sample. In the case of the 800-mesh Talc, the curve barely shifted from 60 to 120 minutes.

3.5 Mineral composition of air bubble shell

The XRD patterns of the separated air bubble shell and screened paste from each fresh mortar sample were presented in Fig. 10. In general, the clinker content in the air bubble shell was quite lower than it in the paste. Because of the presence of fine aggregates in mortar samples, quartz can be recognized in air bubble shell although careful screening was conducted before XRD measurement. Furthermore, much more quartz was found in air bubble shell than in the paste, which reflected that the fine particles in fine aggregate were more likely to adhere to air bubble than cement particles. More calcite was observed in air bubble shell than in paste for all samples, which might be ascribe to the inevitable carbonation during the air bubble shell collection process. It also can be inferred that the calcium hydroxide produced from the cement hydration tended to grow on the surface of air bubbles. Additionally, Fig. 10 demonstrated mullite and quartz in FA sample, calcite in LS samples, talc and dolomite in Talc samples, quartz in Qtz samples, tended to more adhere to the air bubble shell as well. It is believed that all the SCMs selected in this study would much more easily adhere to air bubble than cement clinkers.

Comparing the mineral compositions of air bubble shell and paste with SCMs with different fineness, authors revealed some interesting results. The least content of cement clinker was found in the samples with limestone powders. As 20% limestone was introduced into the mortar, the peak intensity at 29° (mainly C₃S and calcite) and 31° (dolomite) of pastes was much higher than them of other samples. However, the peak intensity at the same positions of bubble shell with 400-mesh limestone was apparently lower than them of bubble shells in other samples, yet the peak intensity at 29° and 31° of bubble shell with 800-mesh limestone was higher than them of bubble shells in other samples. As for Talc samples, stronger peak intensity for talc in bubble shell was recognized in 800-mesh talc sample than in 400-mesh talc sample. It can be inferred that the 800-mesh limestone/talc was more inclined to adhere to the surface of air bubbles in fresh mortar than the 400-mesh ones. Also, the fineness of quartz seemed to hardly affect the XRD patterns of bubble shells.

In addition, the XRD patterns for all samples varied little within 60 minutes. It was because the cement particle barely hydrates in such short period. Correspondingly, changes of bubble shell composition can be distinguished in Fig. 9b. The authors speculated that hydration products were more likely to precipitate on air bubbles although the whole hydration degree was pretty low and the bubble shell compositions might change as air bubble coalescence happened.

The air bubble stability in fresh concrete is one of the key properties to determine the air void structure characteristics after hardening, which is critical to the strength, freeze-thaw durability and even surface aesthetics of hardened concrete. This study focuses on the time-dependent air bubble structure evolutions and the interactions between SCMs and air bubble film.

Through the foam stability and air bubble structure evolution test results, it can be seen that the SCMs had a great influence on the foam/air bubble size distribution and its time-dependent changes. Besides, the trend of the influence of different SCMs on the foam/air bubble stability in solutions or mortars is almost the same. XRD patterns demonstrated that the binder materials particles had tendency to adhere to the air bubble film and all the SCMs used in this study were more likely to adhere to the air bubble film than cement. The adhesion of the solid particles was able to strengthen the air bubble stability in concrete [16]. Combining the wettability test results, the larger the wetting angle of the SCM (within 90°), the more stable the air bubbles. It was observed the talc powder with wetting angle around 90° had the most air bubble stability improvement among all the SCMs [40]. 800-mesh talc and limestone powders were easier to adhere to the air bubble film than 400-mesh powders. However, no obvious difference could be distinguished when it came to the mortars containing quartz, maybe due to the smaller wetting angle of the 800-mesh quartz powder.

It can be inferred that, careful binder material compositions design with certain types of SCMs was able to strengthen the air bubble stability in concrete other than air entraining agents or other chemical admixtures [41]. This work is a preliminary experimental study on the effects of SCMs on the air bubble stability in fresh mortar. More mineral admixtures and the combining use of mineral and chemical admixtures will be investigated in the future to reveal the deep interaction mechanism between admixtures and air bubble shell compositions.

In this study, the influences of various SCMs on the air bubble stability and the interactions between them and air bubble shell were investigated. From the test results, the following main conclusions were obtained:

1 The incorporation of 20% cement replacement SCMs into PCE solutions or fresh cement mortars did not influence the stability of air bubbles. However, the higher the wetting angle of the SCM, the more stable the air bubbles. In this study, talc powder exhibited the highest enhancement, while quartz powder exhibited the least.

2 In all fresh mortars, the air bubble undergoes a process of growth over time. Specifically, within 60 minutes of mixing, the content of air bubbles with a diameter below 500µm decreased, while the content of air bubbles with a diameter above 500µm increased. Between 60 minutes and 120 minutes after mixing, the content of air bubbles with a diameter below 500µm remained relatively stable, while the content of air bubbles with a diameter above 1000µm continued to change.

3 Significant differences of mineral compositions between the air bubble shells and screened pastes from fresh mortar. More SCMs and hydration products were found on the air bubble shells than in the paste, which revealed that the SCMs tend to adhere to the air bubble shell and the cement hydration were more likely to occur on the air bubble shell than in the paste.

The use of foaming height in solutions and air bubble structure in fresh cement mortars allows for the intuitive evaluation of the influence of supplementary cementing materials (SCMs) on the stability of foam/air bubbles. The air bubble stability difference among the different binder compositions was sequenced. This demonstrated that not only can chemical admixtures (i.e. air entraining agents) be used to enhance the air bubble stability, but also that some mineral powders can be employed in this regard. The use of these materials has the effect of enriching the technologies employed in the regulation of concrete durability and surface quality.

Competing interests:

The authors have no competing interests to declare that are relevant to the content of this article.

Acknowledgements:

This research was supported by National Natural Science Foundation of China (Grant Numbers: 52308253) and Jiangsu Association for Science and Technology Youth Talent Support Project Funded Program (JSTJ-2023-007).

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Enhancement in stability of air bubbles in mortar at fresh state with different SCMs

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Abstract

Figures

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Mix proportions and sample preparation

2.3 Test methods

2.3.1 Contact angle of binder materials

2.3.2 Foaming height and half-life time

2.3.3 Air bubble size distributions of fresh mortars

2.3.4 Air bubble shell separation and mineral composition characterization

2.3.5 Mineral composition characterization

2.3.6 Micro morphology

3 Results and discussion

3.1 Micro morphology

3.2 Wettability of binder materials

3.3 Foaming height and half-life time

3.4 Air bubble structure evolution

3.4.1 5min ~ 60min

3.4.2 60min ~ 180min

3.5 Mineral composition of air bubble shell

4 Discussions

5 Conclusions

Declarations

Competing interests:

Acknowledgements:

References

Status:

Version 1