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Effects of Anionic Asphalt Emulsion on Early-Age Cement Hydration

  • Jinxiang Hong
  • Wei Li
  • Kejin Wang
Conference paper

Abstract

Cement-asphalt mortar (CAM) is a cement-based, asphalt-modified, inorganic-organic composite. In this study, the effects of anionic asphalt emulsion on early-age cement hydration of CAM pastes are investigated. The setting behavior, heat of hydration, and electrical resistivity of CAM pastes with different asphalt-to-cement (A/C) ratios are evaluated. The results indicated that the set time of cement paste was prolonged and the heat released from cement hydration was delayed with increased A/C. During the early state of cement hydration, the resistivity of the CAM paste studied was higher than that of the corresponding Portland cement paste, indicating that the asphalt emulsion hindered the ion dissolution of the cement in the CAM paste. However, during the acceleration period of cement hydration, the resistivity of the CAM paste studied was lower than that of the corresponding Portland cement paste, implying that the formation of microstructure of the CAM paste was delayed. This retardation phenomenon could be attributed to two mechanisms: (i) the active sites of cement particles that could have been occupied by asphalt emulsion through anionic emulsifier via electrostatic interaction and (ii) the most surfaces of the cement particles that might have been covered by asphalt membrane due to the demulsification of asphalt emulsion.

Cement-asphalt mortar (CAM) is a cement-based, asphalt-modified, inorganic-organic composite. In this study, the effects of anionic asphalt emulsion on early-age cement hydration of CAM pastes are investigated. The setting behavior, heat of hydration, and electrical resistivity of CAM pastes with different asphalt-to-cement (A/C) ratios are evaluated. The results indicated that the set time of cement paste was prolonged and the heat released from cement hydration was delayed with increased A/C. During the early state of cement hydration, the resistivity of the CAM paste studied was higher than that of the corresponding Portland cement paste, indicating that the asphalt emulsion hindered the ion dissolution of the cement in the CAM paste. However, during the acceleration period of cement hydration, the resistivity of the CAM paste studied was lower than that of the corresponding Portland cement paste, implying that the formation of microstructure of the CAM paste was delayed. This retardation phenomenon could be attributed to two mechanisms: (i) the active sites of cement particles that could have been occupied by asphalt emulsion through anionic emulsifier via electrostatic interaction and (ii) the most surfaces of the cement particles that might have been covered by asphalt membrane due to the demulsification of asphalt emulsion.

1 Introduction

Cement-asphalt mortar (CAM) is a cement-based, asphalt-modified, inorganic-organic composite, which consists of Portland cement (hereafter PC), asphalt emulsion, water, and other related admixtures. This hybrid material combines the high compressive strength of cement paste with the high flexibility of asphalt, and the resulting composite possesses not only desirable strength, stiffness, temperature resistance but also much improved flexibility and ductility [1]. CAM has been extensively used as a grouting material in the cushion layer of the slab track system of high-speed railways (HSR) in China, primarily due to its excellent damping property.

In a CAM mixture, asphalt is often introduced as an emulsion form. During mixing, the droplets of asphalt often adsorb on the surfaces of cement particles. As a layer of asphalt droplet coats the cement particles, it influences the CAM mixture rheological property, cement hydration, and set behavior. As cement hydrates, water in the mixture reduces due to cement hydration and evaporation. This result in a thin membrane of asphalt that lines capillary pores and cracks interweaves with cement hydration products and wraps unhydrated cement and aggregate particles. Consequently, the hardened CAM has excellent mechanical and durability properties.

The properties of CAM can be strongly affected by the type of asphalt emulsion used. The anionic asphalt emulsion is primarily adsorbed onto the positively charged surface of aluminate phases, such as C3A and some small parts of silicate phases [2]. Therefore, the anionic asphalt emulsion could only exert some slight hindrances to the hydration of C3S and C2S, which are the main contributors to the strength development of PC paste. Therefore, one could deduce that anionic asphalt emulsion is more suitable for formulating CAM in the applications where high strength is required. However, little study has been reported to confirm this concept.

In the present study, an anionic asphalt emulsion was used to formulate a type II CAM, where a relatively high strength was required under the current Chinese standard for high-speed railway. The effects of this anionic asphalt emulsion on the early-age cement hydration process were characterized by set time, isothermal calorimetry, and electrical resistivity of the CAM. Based on these experimental test results, three possible mechanisms by which the anionic asphalt emulsion influences cement hydration in CAM were then discussed.

2 Raw Materials, Sample Preparation, and Test Methods

2.1 Raw Materials

Portland cement (PC), type P.I 42.5, complying with the Chinese standard GB8076-2008, was used in this study. An anionic asphalt emulsion (with a solid content of 60%) was supplied by Jiangsu Bote New Materials Co., Ltd.

2.2 Sample Preparation

Paste samples were used for all tests conducted in the present study. The CAM pastes had a fixed water-to-cement (W/C) ratio of 0.41 and different asphalt-to-cement (A/C) ratios, namely, 0, 0.16, 0.24, and 0.32. Based on their A/C, the CAM mixes studied were named as PC (i.e., CA-0, CA-0.16, CA-0.24, and CA-0.32). All these pastes were tested for both set time and isothermal calorimetry. However, the electrical resistivity tests were carried out only for two CAM pastes, with A/C of 0 and 0.24.

To prepare samples, water and asphalt emulsion was mixed first. (The amount of water in the anionic asphalt emulsion was considered in the calculation of W/C.) Cement was then added into the solution. The mixture was then mixed according to the standard practice for mechanical mixing of hydraulic cement paste.

2.3 Test Methods

2.3.1 Set Time Test

Set time was measured according to the standard of ISO 9597:2008 Cement-Test Method, determination of set time and soundness, NEQ. Both the initial and final set times of CAM pastes with different anionic asphalt emulsion were measured, and the test temperature was kept at 20 °C.

2.3.2 Calorimetry Test

The fresh CAM pastes were put into a plastic bottle within 10 min after mixing, and the heat released from the paste was measured by a self-regulated isothermal conduction calorimeter. The testing temperature was equilibrated at 20 °C between the specimen and the instrument before conducting the measurement, and the test lasted about 72 h.

2.3.3 Electrical Resistivity Measurement

This measurement was performed using a noncontact impedance measurement (NCIM CCR-II). In the test, a fresh CAM paste was cast in a ring-shaped plastic mold. AC currents with different frequencies were applied to the sample via a transformer core. The current going through the tested sample was recorded, and the resistivity of the sample was then computed.

3 Results and Discussion

3.1 Influence of Anionic Asphalt Emulsion on Cement Set Time

Figure 16.1 shows that both the initial and the final set times were prolonged with the increase of the A/C from 0 to 0.32, which implies that the PC hydration was retarded by the anionic asphalt emulsion. As mentioned previously, during CAM mixing, a layer of asphalt droplet often coats cement particles, which could prevent the cement particles to contact with water, thus retarding cement hydration and delaying the paste set times.
Fig. 16.1

Influence of anionic asphalt emulsion on set time [3]

3.2 Influence of Anionic Asphalt Emulsion on the Hydration Heat

Figure 16.2a presents the heat flow during the first 70 h of PC hydration at different A/C, and Fig. 16.2b shows the details of heat evolution during the first 10 h.
Fig. 16.2

Influence of asphalt emulsion on heat of cement hydration [3] (a) heat flow curve, (b, c) the initial part of (a) (time ≤ 10 h)

From Fig. 16.2b, it can be seen that the induction period of heat flow curve was prolonged with the increase of A/C. It is generally agreed that the surface of aluminate phase and certain part of silicate phases show positive charge after mixing with water, which is mainly due to the discrepancy of the migration rate of different kinds of ions from the cement particles into solution [4]. Hence, the anionic asphalt emulsion can be adsorbed onto the positively charged sites of cement particles through electrostatic interaction. Consequently, the surface of cement particles could be covered by an asphalt membrane, which would hinder the further dissolution of cement particles and resulted in the slow increase of the cumulative concentration of Ca2+ with the increase of A/C during the induction period. Therefore, the duration of the induction period was prolonged due to the lower concentration of Ca2+ in the presence of anionic asphalt emulsion, i.e., the PC hydration process was retarded by the anionic asphalt emulsion.

3.3 Influence of Anionic Asphalt Emulsion on the Electrical Resistivity

In this study, the electrical resistivity was employed to investigate the evolution of the early-stage hydration process of CAM paste. Figure 16.3 shows the evolution of electrical resistivity of the CAM paste at an A/C of 0.24 in comparison with the pure PC paste within the first 24 h.
Fig. 16.3

Influence of anionic asphalt emulsion on electrical resistivity [3]

It is evidenced that during the dissolution period, the electrical resistivity of both the CAM paste and the pure PC paste reduced quickly to a minimum value, which lasted for about 100 min and 50 min, respectively. Additionally, it can be seen that the electrical resistivity of the CAM paste was higher than that of the pure PC paste during the dissolution and induction periods, while the opposite can be observed during the acceleration period.

It is well known that, during the early dissolution stage, the electrical resistivity of fresh PC paste is mainly determined by the ion concentration in the fresh paste which, in turn, is controlled by the dissolution process. That is, the electrical resistivity of the fresh PC paste decreases with the increase of the ion concentration in the solution.

On the contrary, during the acceleration hydration period, the electrical resistivity of the CAM paste was lower than that of the PC paste, which would indicate that the microstructure formed in the CAM paste was more porous than that of the PC paste. This, again, could be attributed to the barrier formed by the asphalt membrane on the surface of cement grains which could have adversely affected the dissolution process of cement and, hence, retarded the hydration process and the formation of the microstructure of the CAM paste.

3.4 The Retardation Mechanism of Asphalt Emulsion on PC Hydration

Mechanism I: Selective adsorption of anionic emulsifier via electrostatic attraction

Anionic asphalt emulsion can be selectively adsorbed onto those positively charged sites on the surface of cement grains, such as those sites occupied by aluminate phase or certain part of silicate phases, through anionic emulsifier via electrostatic adsorption. This layer of adsorbed anionic asphalt emulsion would then hinder further dissolution of chemical ions from cement particles and subsequent deposition of hydration products on the surface of cement particles, thus retarding cement hydration.

Mechanism II: Formation of coating from demulsification of anionic asphalt emulsion

Once the counterions, such as Ca2+, are dissolved from cement particles into the double electrode layers of asphalt emulsion, along with the consumption of water during PC hydration process, the asphalt emulsion would demulsify. This would result in the formation of aggregation of asphalt particles or the formation of asphalt membrane. Due to the surfaces of cement particles covered by asphalt as a membrane, the ions releasing from the cement particles and the deposition reactions of the cement particles were hindered.

4 Conclusions

  1. 1.

    The set time of PC was prolonged with increased A/C.

     
  2. 2.

    The heat of the cement hydration was delayed with increased A/C, and so did set time of the CAM paste.

     
  3. 3.

    During the early hydration period, the electrical resistivity of fresh CAM paste was higher than that of the pure PC paste; but later, at the accelerated hydration period, it was lower.

     
  4. 4.

    The possible retardation mechanisms by which the anionic asphalt emulsion influences the PC hydration are (i) the active sites of aluminate phase and some silicate phases could have been occupied by asphalt emulsion particles through anionic emulsifier via electrostatic interaction and (ii) the surface of cement particles might have been covered by a layer of asphalt membrane due to the demulsification of asphalt emulsion.

     

Notes

Acknowledgments

The present study is a part of the research project supported by the National Natural Science Foundation of China (No.51708483). This paper is modified from a part of publication of Ref. [3].

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Jiangsu Research Institute of Building Science Co., LtdNanjingChina
  2. 2.Department of Civil, Construction, and Environmental EngineeringIowa State UniversityAmesUSA
  3. 3.College of Civil Science and EngineeringYangzhou UniversityYangzhouChina

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