icshm extended - o. copuroglu (1).pdf

7/29/2019 ICSHM Extended - O. Copuroglu (1).pdf

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SELF HEALING OF CEMENT PASTE BY CALCIUM ALUMINATE

BASED AGENTS

O. Copuroglu*, K. Sisomphon, and S. Komatsu

Delft University of Technology, Faculty CiTG, Materials and Environme nt, Stevinweg 1, 2628 CN,

Delft, The Netherlands. *contact author, email: [email protected]

Introduction

Self-healing of concrete is a h ighly challenging subject mainly because ofits multi-component material nature, complex chemical reactions involved and

continuous interaction of the material with environment. It is therefore importantto consider several aspects when attempting to design concrete elements withself-healing ability. One disadvantage of concrete in comparison with relativelyuniform materials (e.g. polymers), is its heterogeneous structure. Concrete isbasically composed of cement paste, sand, gravel, and interfacial transitionzones between the solids. Pristine concrete microstructure has a pH ofapproximately 12,5 -13 depending on the cement type used. Furthermore thepaste contains significant amount of capillary porosity, which determinesessentially the transport proper ties of concrete. All these parameters amplify thecomplexity of designing concrete with self -healing ability.

Except utilization of biological agents [1], self-healing mechanisms in

cementitious materials are currently based on further hydration of unreac tedparticles. These particles can be ordinary cement minerals (alite/belite) as well asother types of cementitious particles, which had not been fully hydrated in theinitial hydration stage and remained partially hydrated in cement paste. Howeversuch a mechanism is not highly promising in healing typical cracks in concrete

because of the limited remaining potential and difficulties in optimizing theperformance. On the other hand non-native agents such as two componentpolymer-based agents encapsu lated by glycol dimethycrylate may not be suitablefor concrete because one should encapsulate the healing agent and mix the

catalyst in the cement paste. This may affect the hydration kinetics and strengthdevelopment of concrete. High er cost of these agents is another disadvantage.

Therefore the market needs low -cost cement paste compatible self -healingagents for sustainable material use.

Attempting to respond these requirem ents, a new project is being initiated

at Delft University of Technology. The aim is to use encapsulated inorganicternary blends for autogenous ettringite mineral production upon crack formationin concrete. The theory behind the healing agents to be used has been known fordecades in the cement and concrete science. However utilization of these agents

in the self -healing framework is a relatively new theme.


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This paper reviews existing literature on the self -healing of concrete by calcium

aluminate based agents and discusses the potential crack healing capacity ofettringite minerals.

Technical Background

Ettringite (6CaO.Al2O3.3SO4.32H2O) is a naturally occurring mineral aswell as one of the important main hydration products in ordinary portland cementpaste. Also called as primary ettringite, this mineral is not detrimental toconcrete and it is formed according to:

3CaO.Al2O3 + 3CaSO 4.2H2O + 26H2O 3CaO.Al2O3.3CaO.SO4.32H2O [eq. 1]

The ettringite crystals are pseudo hexagonal (trigonal) and can be

observed in the form of highly elongated needles and rods (Figure 1). Thecrystals can precipitate from a solution bearing Ca2+, Al(OH)-4, SO

2-4, and OH

-,

and the solubility product of ettringite is about K sp=2.80E-45 [2]. Ettringite isstable up to around 90 C but in cement paste this value is slightly lower.

Figure 1. (Left) Photomicrograph of a single imperfect ettringite crystal tip and(right) massive ettringite cluster produced by hydration of calcium -aluminate

based agents. (Images captured in ESEM/GS E mode).

In addition to its formation during cement hydration, ettringite can also beproduced by external sulfate attack which transforms monosulfoaluminate(Ca4Al2(OH)12.SO4.6H2O) that had been produced during the cement hydrationinto ettringite crystals. This transformation is an increase in solid volume, whichmay cause distress in concrete and eventually cause cracking. Another form of

ettringite damage is known as delayed et tringite formation (DEF), which isobserved in steam-cured concrete. This detrimental formation occurs by therecrystallization of decomposed primary ettringite crystals.

Although ettringite is associated with detrimental processes in concrete, itis also being used for favorable purposes. Many special concrete applicationshave been realized by taking advantage of the expansive nature of ettringiteformation (e.g. shrinkage compensating concrete).


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In the recent years, expansive cements and agents found us e in the projects on

self-healing of concrete. Use of calcium aluminate based materials as healingagent has been investigated by a number of researchers. Hosoda et al [3]andKishi et al [4] studied the use of cementitous expansive agents as cementreplacement material and their potential as self -healing agent. Their results

revealed that a portion of replacement agent can remain unreacted in the systemand can heal an eventual crack formation upon meeting with dormant orstreaming water [5]. They also pointed out that using carbonates would increasethe healing ability, which is possibly due to the formation of carbonate analog ofettringite ([Ca3Al(OH)6 .12H2O]2 .(CO3)3 . 2H2O).

The agent generally used for ettringite production is a ternary mineralmixture of hauyne (4CaO.3Al 2O3.SO4), lime (CaO), and anhydrate (CaSO 4)minerals. Instead of hauyne, several other calcium aluminates such as3CaO.Al2O3 (a.k.a. C3A), CaO.Al 2O3, 4CaO.Al2O3.Fe2O3 can also be used.

However, performance/price ratio might be the decisive parameter. Regardlesswhat the involved production costs can be, C 3A was reported to produce thehighest expansion among all common calcium aluminates [6]. In conventional

production techniques C 3A forms at around 1500 C, however recent

developments reduced this temperature down to 1050 C, providing a cost

efficient alternative [7].

On the Encapsulation of Calcium Aluminate B ased AgentsCurrent techniques have proved that cracks up to 0.4mm can be

autogenously healed by calcium aluminate based agents when used as cementreplacement material [5]. However using these agents might pose certain risk

regarding uncontrolled expansio n and cracking. As an alternative, in the newproject, it will be investigated to find out whether or not encapsulating the

ettringite-producing agents would provide technical advantages.There are several challenges against suitabl e encapsulation of expansive

agents. The encapsulation method must be a non -aqueous type in order to avoidearly hydration of the agents. An ideal encapsulation material should be

inorganic, insoluble and resistant against high pH of cement paste. Furthermore,it must be mechanically stable enough to remain intact during the concreteproduction.

As stated earlier, the healing agent is a ternary blend of different mineralsin powder form. Therefore, as a first step, granulation of the agents should be

realized prior to the enc apsulation stage. Need for non -aqueous processthroughout the entire sequence must be strictly followed, which unfortunatelyleaves extremely limited number of options. One of these options is a techniquecalled freeze granulation, which is based on sprayi ng (in this case non -aqueous)

suspensions into liquid nitrogen and consequently drying the granules by freeze -drying. It has been reported that the size of the granules can be optimized furtherwith this technique [8]. A more challenging second phase of th e project isencapsulating the granules preferably by an inorganic coating that has a lower E -

modulus in comparison with the surrounding cement paste. This is to ensurecrack propagation towards the capsules and to expose the healing agents to an


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eventual water canal. This behavior can be simulated by computational fracture

models (e.g. [9]) and the results can be used in pursuit of finding properencapsulation materials.

An illustration of the concept for stages of calcium aluminate based agentencapsulation and its possible use in concrete is given in Figure 2.

References1. Jonkers, H.M., et al., Application of bacteria as self -healing agent for the development of

sustainable concrete Ecological Engineering, 2009. doi:10.1016/j.ecoleng.2008.12.036

Article in Press.2. Damidot, D. and F.P. Glasser, Sulphate attack on concrete: prediction of the aft stability

from phase equilibria , in International Congress on the Chemistry of Cement . 1992: New

Delhi. p. 316 321.3. Hosoda, A., et al., Self Healing of Crack and Water Permeability of Expansive Concrete ,

in First International Conference on Self Healing Materials . 2007, Springer: Noordwijk

aan Zee.4. Kishi, T., et al., Self-Healing Behaviou r by Cementitous Recrystallization of Cracked

Concrete Incorporating Expansive Agent , in First International Conference on SelfHealing2007, Springer: Noordwijk aan Zee.

5. Hosoda, A., et al., Healing Properties of Self Healing Concrete with Water Passing

Through Crack, in 8th International Symposium on Utilization of High -Strength and High-Performance Concrete . 2008: Tokyo, Japan. p. 218 -224.

6. Odler, I. and J. Colan -Subauste, Investigations on cement expansion associated with

ettringite formation. Cement and Concrete Research, 1999. 29(5): p. 731 -735.7. Tas, C., Chemical Preparation of the Binary Compounds in the Calcia Alumina System

by Self -Propagating Combustion Synthesis. J. Am. Ceram. Soc, 1998. 81(11): p. 2853 -

63.8. Rundgren, K., O. Lyckfeldt, an d M. Sjstedt, Improving Powders with Freeze Granulation.

Ceramic Industry, 2003. April: p. 40 -44.9. Schlangen, E. and J.G.M. Vanmier, Lattice Model for Simulating Fracture of Concrete.

Numerical Models in Fracture Mechanics of Concrete, 1993: p. 195 -205.

Figure 2. Concept for stages of calcium aluminate based agent encapsulationand its use in concrete.

icshm extended - o. copuroglu (1).pdf

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