Investigation on hardening mechanism and cement

Asphalt Pavements – Kim (Ed) © 2014 Taylor & Francis Group, London, ISBN 978-1-138-02693-3

Investigation on hardening mechanism and cement hydration of Cement Asphalt Emulsion Composites Xing Fang Empa, Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland

Alvaro Garcia Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham, UK Empa, Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland

Manfred N. Partl Empa, Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland Highway and Railway Engineering, School of Architecture and the Build Environment, KTH Stockholm, Stockholm, Sweden

Pietro Lura Empa, Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland Institute for Building Materials, Swiss Federal Institute of Technology Zurich (ETHZ), Zurich, Switzerland

ABSTRACT: Cement Asphalt Emulsion Composites (CAEC) are mixtures of bitumen emulsion, cement, water and aggregates that harden at ambient temperature. They are mixed at ambient temperature and harden due to breaking of the emulsion, water evaporation and cement hydration. Potential advantages of CAEC are lower temperature susceptibility than asphalt concrete and higher flexibility than cement concrete. To quantify the effects of cement hydration on the mechanical properties of CAEC, two different emulsions (cationic and anionic) mixed with either 0%, 3% and 6% Ordinary Portland Cement (OPC) by mass of dry aggregates were studied by isothermal calorimetry and Marshall tests. By monitoring the mass of the specimens and estimating the amount of water bound by the cement, the water content was calculated. This study shows that bitumen emulsion has no significant effect on the degree of cement hydration. Cement hydration, however, significantly contributes to the hardening of CAEC. Moreover, a higher amount of cement added to the mixture results in a higher amount of bound, adsorbed and capillary water in the CAEC. Keywords:

1

CAEC, bitumen emulsion, Portland cement, isothermal calorimetry

INTRODUCTION

Cold mix asphalt consists of bitumen emulsion, water, unheated aggregates and filler. This material has low environmental impact and is cost-effective [1–3]. However, it has rarely been used as structural layer for heavy-duty pavements [2,3], mainly because of the long time (several weeks) required to reach its full strength [4], resulting in inadequate performance (inferior early strength and high porosity) compared with conventional Hot Mix Asphalt (HMA) [3,5,6]. In order to improve the early performance of cold mix asphalt, cement can be added to the mixture. The addition of 1% to 2% wt. of Ordinary Portland Cement (OPC) to cold mix asphalt may improve its early-life mechanical properties [7] and the fully cured material acquires comparable mechanical properties to an equivalent HMA [3,8,9]. Research on these 1441

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composite materials, often called CAEC (Cement-Asphalt Emulsion Composite) [10,11], started in the 1970s. CAEC shares characteristics of both cement and asphalt concrete [12]. In particular, compared with HMA, CAEC has higher deformation resistance, lower temperature susceptibility [10], and better resistance to water damage [11]. Additionally, the introduction of cement in cold mix asphalt accelerates emulsion breaking [5], because cement hydration consumes water in the emulsion and meanwhile it increases its alkalinity. This has a special impact when a cationic bitumen emulsion is used [13]. However, because of the long time required to reach its full strength and an inadequate understanding of this material, CAEC has rarely been used in pavements. This research aims at obtaining a general perspective of CAEC. It focuses on OPC hydration and on water evaporation and their contribution to the mechanical properties of CAEC. With this purpose, two emulsions, one cationic and one anionic, have been mixed with Ordinary Portland Cement (OPC) in various amounts (0%, 3% and 6% by total mass of aggregates) and with aggregates to prepare CAEC. The evolution of the mechanical properties of the mixture with time was characterized with Marshall Stability tests. The effect of emulsion on the cement hydration has been investigated by means of Isothermal Calorimetry tests. Finally, the evolution of water content in CAEC has been quantified to characterize the hardening of the mixture.

2 2.1

MATERIALS AND METHODS Materials

A dense CAEC mixture (0/8) was used in this research [14]. The aggregates used to make CAEC were quarry material (size between 2 and 8 mm and density 2770 kg/m3), crushed sand (size between 0.063 and 2 mm and density 2688 kg/m3), and filler (size < 0.063 mm and density 2638 kg/m3). The total amount of filler in the mixture was 6% by mass of dry materials (aggregates + filler). Two types of commercially-available unmodified bitumen emulsion, with 60% of residual bitumen content, were used in this paper. The first one was a rapid-setting cationic emulsion, while the second one was a solvent-free, slow-setting anionic emulsion (Table 1). Additionally, to facilitate the mixture preparation, 2.52% wt. (by dry aggregates) tap water was added to the mixture. Ordinary Portland Cement (OPC) CEM I 42.5 N (chemical analysis and phase composition shown in Table 2) was added to the mixture, by replacing 0%, 3% and 6% (all filler replaced) of filler. Table 3 shows the mix compositions. In this paper, OPC designates ordinary Portland cement; 3 and 6 represent 3% and 6% cement by mass of dry aggregates, and A and C denote anionic and cationic bitumen emulsion, respectively. An example is OPC3C-7d, a cationic bitumen emulsion with 3% OPC by mass of dry aggregates 7 days after compaction. 2.2

Test specimens preparation

The raw materials were added to the bowl in this order: first the coarse aggregates, then water and emulsion, then the sand and finally the filler and cement. The materials were mixed during 1 minute. The temperature during the mixing process was 20 ± 1°C. One mix was used to make 9 cylindrical Marshall specimens with 101.6 mm diameter, approximately 7 cm height and 1190 g of mass. Immediately after placing the specimens in the mould, they were compacted with 100 blows of the Marshall hammer, 50 for each side of the specimens. Table 1. Properties of bitumen emulsion and residual bitumen binder. Properties

Anionic emulsion

Cationic emulsion

Softening point °C Penetration, @ 25°C, 0.1 mm

58.5 41

63.6 24

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Table 2.

Chemical analysis and phase composition of OPC.

Chemical analysis

CaO

SiO2

Al2O3

Fe2O3

MgO

K2O

Na2O

SO3

TiO2

P2O5

SrO

L.O.I.

(wt. %)

62

20

5.1

2.9

2.3

1.01

0.26

3

0.28

0.2

0.15

2.68

Phase composition

C3S

C2S

C3A

C4AF

MgO

K2SO4

Na2SO4

K2O

Na2O

CaO free

CaCO3

CSH2

(wt. %)

56

16

4.8

11.5

1

1.6

0.26

0.1

0.15

0.27

4.8

4

Total 99.4

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Table 3.

The amount of materials by 100 g of mixture.

Aggregate [g]

Water [g]

Emulsion [g]

Filler [g]

Cement [g]

Cement [%]a

78.99

2.52

13.45

5.04 2.52 0

0 2.52 5.04

0 3 6

a

Percentage of cement by mass of dry aggregate.


The specimens, still in the moulds, were left in a humidity-controlled room (relative humidity 90 ± 3% and temperature 20 ± 1°C) for one day. After this, the test samples were demoulded and the lateral and bottom surfaces were sealed with aluminum foil. Then, the specimens were left in the humidity-controlled room during the required curing time: 1, 3, 7, 14, 21 and 28 days. During the curing period, water in the specimens could evaporate from the specimens’ upper side only. Additionally, 3 hot mix asphalt specimens (made by residual bitumen from both anionic and cationic emulsion) were prepared for comparison purposes. 2.3

Isothermal calorimetry

Isothermal calorimetry tests were conducted at 20°C with a Thermometric TAM Air instrument calibrated at 600 mW. The rate of heat release was measured on mixtures containing the three types of cement and the two types of bitumen emulsion, with duplicate specimens for each mixture. The water-to-cement ratio (w/c) of the CAEC mixtures was 3.14 for mixtures with 3% cement and 1.57, for mixtures with 6% cement, including both the water in the emulsions and the extra water added for workability. In addition to the CAEC mixtures, cement paste with w/c 1.0 and cold mix asphalt mixtures without cement were measured for comparison purposes. Cement paste with w/c 1.0 was used in order to avoid settlement of cement paste in the case of high w/c. Furthermore, it has been confirmed in reference [15] that the increase of w/c has no significant influence on cumulative heat released in the case when w/c is beyond 0.42. 20 g of freshly-mixed CAEC or 6 g of cement paste were inserted into glass vials of internal diameter 22.5 mm, sealed with a tight lid and placed in the measuring cell. The rate of heat release was then measured during 72 hours and integrated to obtain the cumulative heat release. 2.4

Moisture losses and amount of trapped water

The moisture loss of the Marshall specimens during curing was monitored by weighing them regularly during 28 days. Simultaneously, the Marshall specimens without cement were cured in a humidity-controlled room (relative humidity 90 ± 3% and temperature 20 ± 1°C) for 4 months until mass loss due to evaporation stopped. Then they were crushed and oven-dried at 105°C to obtain the trapped water content. The trapped water was defined as the water adsorbed on the surfaces of aggregate and filler as well as the water trapped in the closed pores within the bitumen and that cannot evaporate. In this study, the amount of trapped water was determined from the average of 20 duplicate samples. Because the cationic emulsion broke immediately when fine aggregates were added to the mixture, a considerable amount of water was lost during the compaction process. For this reason, it was possible to quantify the water-content evolution only for the mixtures with anionic emulsion. 2.5

Marshall tests

The Marshall stability of cold mix asphalt samples was measured at 1, 3, 7, 14, 21 and 28 days. The tests were carried out immediately after the specimens had left the humiditycontrolled room and were finished in approximately 10 min. The testing room temperature 1444


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was strictly controlled at 20 ± 1°C. Marshall tests were conducted in room temperature other than 60°C treatment in water bath. The reason is that the water bath will increase the cement hydration. Furthermore, the water bath will raise the water content in the samples which and thus decrease the strength of samples [9]. Every Marshall stability value was obtained from the average results of 3 specimens. 3


3.1

RESULTS AND DISCUSSION Isothermal calorimetry

Mixtures without cement did not show any heat liberation, confirming that all the heat liberated from the CAEC mixture came from cement hydration. In addition to the CAEC, also the rate of heat liberation and the cumulative heat for the cement pastes are shown in Figure 1. Compared with the cement paste, the main hydration peak of mixtures with anionic emulsion was wider and occurred significantly later, which reveals a retardation of the OPC hydration. However, the main hydration peak of OPC in the presence of cationic emulsion occurred slightly earlier than for the OPC paste. The cumulative heat release of OPC in the presence of anionic emulsion was initially lower than for cement paste, but increased steadily and eventually surpassed the heat released by the cationic mixture and by the cement paste. From Figure 1 it can be observed that cement with anionic emulsion samples showed a longer dormant period than cement with cationic emulsion samples, while there is no significant difference between cationic emulsion samples and the cement pastes. Similar conclusion has been made in reference [13]. In that paper, the author indicated that the addition of small percentage of cationic to cement paste didn’t influence the rate of heat liberation or the cumulative heat released. However, the addition of anionic bitumen emulsion slightly increased the dormant period of cement hydration [13]. The initial retardation or acceleration observed in Figure 1 may be due to the pH of the bitumen emulsions [13], while no significant effect of the emulsion or of the bitumen could be observed on the later development of hydration. Similar conclusion has been made in reference [16] that emulsifier has no significant influence on cumulative heat released of cement hydration. The degree of hydration reached by the cement was obtained as described in [17]:

α ( ti ) =

Hti Hcem

(1)

where α(ti) is the degree of hydration at time ti Hti is the cumulative heat measured at time ti (J/g of cement) and Hcem is the potential heat of OPC hydration which is 433.6 J/g of cement [18].

Figure 1. Isothermal calorimetry of OPC: 3% cement (left) and 6% cement (right) with cationic emulsion (blue) and anionic emulsion (red), cement paste (black—w/c = 1.0). Rate of heat liberation as continuous lines; cumulative heat as dashed lines.

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Table 4.

Degrees of OPC hydration at 24 h, 72 h and 28 d.

Mixtures

OPC3C

OPC6C

OPC3A

OPC6A

Cement paste

W/C 24 h [%] 72 h [%] 28 days [%]

3.14 35.1 60.7 60.7

1.57 32.2 60.3 60.3

3.14 20.3 71.8 71.8

1.57 15.4 70.3 70.3

1.0 32.0 58.6 58.6

The degrees of hydration of OPC at 24 h and at 72 h, obtained from Eq. 1 are shown in Table 4. Additionally, the influence of different types of bitumen emulsion on the hydration degree of mixtures with 3% and 6% OPC hydration are shown in Figure 1. As the initial w/c in the CAEC is rather high (3.14 for 3% cement and 1.57 for 6% cement), it is assumed in this study that the loss of water due to evaporation in the Marshall specimens has little impact on cement hydration in the first few days. Thus, the degree of hydration reached in the first three days is assumed to be equal to that of the mixtures hydrating in sealed condition in the isothermal calorimeter (considering the moisture loss by evaporation for the OPC mixtures after 3 days the w/c of the Marshall specimens is approximately between 0.9 and 1.7). After 3 days, the degree of hydration was considered to remain constant up to 28 days. Of course this approach will lead to an underestimation of the amount of bound water, which in reality will continuously grow, albeit at a low rate. However, since the degree of hydration at 3 days is already rather high, and considering the unknown effect of evaporation on the rate of hydration in the CAEC, this approach is considered to yield less uncertainties than a possible estimation of the degree of hydration at 28 days based on the rate of hydration in the first three days. 3.2

Quantification of water content evolution

The water in the mixture comes mainly from the bitumen emulsion and partly from the extra water used to wet the coarse aggregates. In reference [9] it has been shown that the water content has a significant effect on the mechanical properties of CAEC. Thus in this section, the amount of water evaporated, the amount of water trapped and the amount of water bound by the cement is quantified. The water in the specimens can be classified in three categories: i) residual water consisting of residual evaporable water at time ti and of trapped water, ii) physically bound water on the surface of the cement hydration products and iii) chemically bound water within the hydration products [19]. Evaporable water refers to the capillary water within the capillary pores that are present in the hydration products of the cement and in the pores between the aggregates and the filler. The evaporated water is the water evaporated through the upper face of the specimens and is monitored by regular weighing. The driving force for water evaporation is the difference in the water potential between the interior of the CAEC specimens and the air in the climate chamber, which progressively decreases (due to evaporation and binding by cement hydration products) until an equilibrium with the ambient relative humidity is eventually reached. In this paper, the bound water (both chemically bound and physically bound, see categories ii) and iii) above) was determined according to Powers model [19]. In particular, 1 g of OPC that has fully reacted binds a total amount of 0.42 g of water [19]. The mass loss from all the mixtures with anionic emulsion as a function of time, corresponding to the amount of evaporated water, was measured by regular weighting. The results revealed that the evaporated water increased rapidly during the first week after compacting and very small increase was observed afterward until 28 days. It is very clear that samples showed higher evaporated water content in the absence of cement. This is result from the consequences that cement hydration bound water and reduced the amount of evaporable water. The amount of trapped water was 1.21%; in this paper, it is assumed that the amount of trapped water is the same for all mixtures, independent of the amount of cement. 1446


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The residual evaporable water in specimens used for the Marshall tests was calculated by subtracting the water that had already evaporated at time ti, the water bound by the cement and the trapped water from the initial water present in the samples. Thus the residual evaporable water content was quantified as: pev ,res ( ti ) = ptot

pbou ( ti )

ptrap a − pev ( ti )

(2)


where ptot is the total initial water content in the mixtures. It was the same for every type of material with anionic emulsion and was obtained from the water in the emulsion plus the extra water used to wet the aggregates. Its value was 7.90%: 2.52% (tap water) + 13.45% (amount of emulsion) × 40% (water from the emulsion), see Table 3. pev(ti) is the percentage of water by initial mass of mixture that has evaporated at time ti, pbou(ti) is the percentage of water bound by the cement and ptrap is the water trapped in the mixture (1.21%). The residual evaporable water was quantified by Eq. 2 other than by oven drying is due to the reasons that the heating will accelerate cement hydration and the loss of volatile components of bitumen. Furthermore, pbou(ti), the water bound by cement hydration, was quantified as: pbou

λ C ⋅ α ( ti )

(3)

where λ is the amount of water bound by 1 g of cement, which is 0.42 g/g; C is the percentage of cement in the mixture, 2.52% and 5.04% for mixtures with 3% and 6% of cement, respectively (see Table 3) and α(ti) is the degree of hydration at curing time, ti (Table 4). According to this equation, the residual evaporable water left in the specimens, the water bound by cement and the water evaporated at after 1 day, 7 days and 28 days were quantified and are shown in Figure 2. In this Figure, it can be observed that OPC6A bound more water than OPC3A. But at the same time, OPC6C contained more residual evaporable water. This result partly explained why the modulus of CAEC steadily increases even after several months [4,12]. It should be noted that on one hand, a higher amount of cement in the CAEC mixture will definitely bind more water. On the other hand, the amount of water held by capillary forces may be higher in the mixtures with more cement. This will result in the decrease of the evaporation rate and also in a higher content of residual evaporable water at equilibrium. Although this is just a hypothesis and it is pending of further research. 3.3

Marshall stability results

In the case of the reference HMA, the Marshall stabilities of samples, made with the residual bitumen obtained from the anionic and cationic emulsions, were 25.9 kN and 36.5 kN, respectively. The evolution of the Marshall stability with 0%, 3% and 6% of OPC is shown in Figure 4. A linear growth of the stability can be observed in the case of test samples made with cationic

Figure 2.

The water content evolution of Marshall specimens with anionic emulsion.

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bitumen emulsion, without cement (OPC0C). However, in the case of anionic emulsion without cement (C0A), the bitumen emulsion broke too slowly to provide enough strength. The dimension changed during curing because of gravity. For this reason, the measured stability was presented here as a reference. The Marshall stability of mixtures with both cationic and anionic emulsions increased significantly with the addition of cement. The stability increased steadily during the 28 days curing period except for OPC3C, which remained almost constant after 7 days curing. The stability of OPC6C at 28 days was lower than that of the corresponding reference HMA, while the stability of test samples made with anionic emulsion (OPC3A and OPC6A) was similar to that of the reference HMA.


3.4

The contribution of cement to CAEC

In the case of mixtures without cement, only bitumen acts as a binder. The Marshall stabilities of mixtures with cationic emulsion are lower than the stabilities of those containing cement and, in the case of slow setting anionic emulsion, the test samples collapsed after demoulding. When a small amount of cement was added to CAEC, it had an immediate effect on the mechanical properties of the cold mixed asphalt: the stability of mixtures containing 3% OPC and cationic emulsion was higher than for mixtures without cement 1 day after mixing (see Fig. 3a). However, in the case of mixtures with OPC and anionic emulsion, there was no significant difference in the Marshall stability after 1 day curing when compared to the test samples without OPC (see Fig. 3b). In Figure 1, it can be observed that in the presence of cationic emulsion, the main hydration peak of OPC occurred before 24 hours curing, while in the case of mixtures with anionic emulsion the main hydration peak took place after 24 hours curing. Besides, as the contribution of the emulsion to the Marshall stability

Figure 3.

Evolution of the Marshall stability with curing time (a. OPC-C, b. OPC-A).

Figure 4.

Marshall force-displacement curve of OPC6C (left) as a function of curing time.

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Table 5.

Degree of hydration VS Marshall stability of mixtures at 24 h.

Mixtures

OPC3C

OPC6C

OPC3A

OPC6A

Degree of hydration [%] Marshall stability [kN]

35.1 4.1

32.2 4.6

20.3 1.8

15.4 1.4

is very limited (see OPC6C in Fig. 4), the increase of strength could be mainly attributed to cement hydration. Table 5 shows the degree of cement hydration versus the Marshall stability of anionic emulsion mixtures and cationic emulsion mixtures after 24 h curing. It indicated clearly that mixtures with higher degree of hydration clearly show higher Marshall stability. This can be confirmed by reference [6] and [20], in which rapid hardening cement was used to accelerate the hardening of CAEC.


3.5

The contribution of bitumen emulsion to CAEC

The most important contribution of bitumen emulsion to the mixture is to supply the bituminous binder for increasing the flexibility of CAEC and to provide water for the cement hydration. In this article it has been concluded that bitumen emulsion has no significant effect on the ultimate degree of cement hydration. On one hand, bitumen emulsion provides water for the cement hydration and on the other hand, cement particles consume the water and trigger the flocculation of bitumen droplets. By quantifying the moisture losses in the mixtures, it was found that the residual evaporable water content in the mixtures is relatively high after 28 days curing (from 2.5% to 3.5% by initial mass of mixtures). Thus, the increase of strength in these mixtures may be caused by the hardening of bitumen. The contribution of bitumen emulsion to the asphalt concrete mixture has however not been fully understood and needs to be characterized in detail in the future. 4

CONCLUSIONS

In this article, the cement hydration in the CAEC mixtures was characterized by isothermal calorimetry and the results indicated that bitumen emulsion may slightly retard or accelerate cement hydration, but has no significant effect on the ultimate degree of hydration of cement. Cement hydration products act as a binder by forming bridges between aggregates and thus increasing the Marshall stability of cold mixed asphalt. Meanwhile, cement hydration products act as a stiffener of the binder substrate and consequently increase the stiffness of cold mixed asphalt. Bitumen emulsion provides water for cement hydration and some flexibility for the mixture. The quantification of water content indicates that although cement hydration consumed a part of water, a relatively high amount of water still existed in CAEC mixtures after 28 days. This is the main reason for later development of strength and stiffness. It can be concluded that CAEC has comparable mechanical properties to hot mix asphalt when cement is well distributed in the mixtures. The early strength of cold mixed asphalt can be substantially improved by adding a certain amount of cement. Although bitumen emulsion has no significant effect on the cement hydration process, the individual contributions of cement and bitumen emulsions to the mechanical properties of cold mix asphalt are still unclear. ACKNOWLEDGEMENTS The authors thank Hans Kienast, Walter Trindler, Axel Schöler and Dr. Mateusz Wyrzykowski for help with the experiments and CTW Strassenbaustoffe AG for providing bitumen emulsion. The first author was financed by a scholarship from the China Scholarship Council. 1449


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