icshm extended - k. sisomphon.pdf

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Self-healing blast furnace slag mortar subjected tocarbonation and frost salt scalingK. Sisomphon*, O. opurolu, A.L.A. Fraaij

Faculty of Civil Engineering and Geosciences, Delft University of Technology,

Stevinweg 1, 2628 CN Delft, The Netherlands* Contact author. Telephone: +31 (0)15 27 81325, E-mail address: [email protected]

1. Introduction

Blast furnace slag cement (BFSC) is the most important product of the cementindustry in northern European countries. From both environmental and economicalpoints of view, blast-furnace slag (BFS) is a very attractive mineral admixture to usein concrete, particularly in low-heat concrete for massive structures or in highperformance concrete. Even though using BFS in concrete has several benefits, ithas been found that concrete containing a high amount of BFS has a poor resistanceagainst carbonation due to its significantly low Ca(OH)2 content. The carbonationcauses micro-cracking on the surface and reduces the micro-mechanical propertiesand durability of the BFSC concrete drastically. Frost salt scaling is considered asone of the most dangerous treats on BFSC concrete infrastructure. Due to concretesurface disintegration, also other attacks on the structure have more chance, whichcan result in a dramatic decrease in durability.

Sodium monofluorophosphate (Na-MFP) treatment is one of the corrosion inhibitionmethods for reinforced concrete widely in use [1-3]. However, it has been recentlyreported that Na-MFP can also improve surface durability of BFSC concrete. Inprevious studies [4-5], it was reported that the application of Na-MFP treatmentimproves the frost salt scaling resistance of carbonated BFSC pastes. The 10% Na-MFP solution was used as a surface post-treatment compound on carbonated BFSCpaste in the study. In general, the results show that the treatment modifies themineralogical structure and improves the resistance of carbonated BFSC pasteagainst frost salt attack. The target of this study is to investigate the possibility of aself-healing action in the BFSC concrete structures which can be potentially affectedby carbonation and frost salt scaling. The main objective is to research theeffectiveness of using Na-MFP as a self healing agent in the cementitous system,which would enable the self-healing of the carbonation microstructure autogenouslywithout any requirement of external application further.

2. Experimental investigation

The expanded clay lightweight aggregate (LWA) which has a maximum size of 4 mmwas used as healing media. To prepare the encapsulated lightweight aggregate(ELWA), the air-dried LWA was soaked into the 40% Na-MFP solution under vacuumpressure. After 30 minutes impregnation, the soaked LWA was filtrated by a thinfilter paper to remove the excess compound solution. The wetted LWA wasmeasured the weight, and hand-mixed with cement powder. The cement-to-wettedLWA ratio of 0.40 was controlled throughout the study. Two types of cement namelyordinary Portland cement (OPC 42,5 R) and BFSC (CEM III /B) were used as coatinglayer. After 24 hours, the prepared ELWAs were cured at 100%RH for 28 days. The

mortars prepared from these ELWAs were designated as E-OPCand E-BFS,respectively.


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To produce mortar mixtures, a CEM III /B 42,5 N HSR LH from ENCI cementmanufacturer was used. The mortars with w/c ratio of 0.45 were prepared. Thecontrol mixture was designed with the sand-to-cement ratio of 2.5 by weight. Theother mixtures with presences of ELWA were formulated to maintain the volumefraction of aggregate phase similar to the control mix. The specimens were cast in

plastic containers with 55 mm diameter to a height of about 50 mm. Thereafter, thespecimens were pre-conditioned at 50-55% RH until an age of 28 days. After that,the circumference surface and the bottom area were coated with low viscosity epoxy.The top trowel surface was only the area which was exposed to carbon dioxideenvironment. The accelerated carbonation which provides the carbon dioxideconcentration of 3% by volume, with 65% RH was set up for the experiment. Afterthe specimens have been carbonated for 14 days, the frost salt scaling test byconforming the ASTM C672 freezing-thawing cycle consisting of 17 h in -20oCfreezing and then 7 h at 25

oC was conducted. The scaled material was collected by

using a filter paper after each cycle. Eventually, it was oven-dried at 105oC andweighted after 24 hours drying. The microstructure of mortars was studied by using

environmental scanning electron microscope (ESEM):Philips XL30.The detail ofpreparation method was described in previous works[4-5].

3. Results and discussion

Fig. 1 presents the frost salt scaling result of mortars up to 7-cycle exposure. It wasclearly seen that the control mortar with normal sand was severely attacked. In theexperiment, it has been observed that the surface of specimen with normal sand wascompletely disintegrated after 2 cycles. However, for the mortars with ELWA, thespecimens showed better resistance against the frost salt attack. The conditions ofmortars after one cycle scaling were shown in Fig. 2. In comparison, the E-OPC

mortar performed slightly better than the E-BFS mortar. This would be due to aformation of calcium carbonate which is a product of Ca(OH)2 carbonation. Thephotomicrographs of carbonated BFSC mortars are presented in Fig. 3. Generally,dark areas show the porosity, while the unreacted slag particle could be seen as lightgrey areas. Unreacted clinker particles are even lighter. In Fig. 3(a), it is obviouslyseen at 500 magnification that carbonation induces severely shrinkage which leadsto coarsening pore in the matrix phase. Fig. 3(b) shows a photomicrograph of thesand mortar in non-carbonated zone which has much denser microstructure forcomparison. After carbonation attack, the pore coarsening would be due to thedecomposition of C-S-H phases and formation of porous silica gel.

(a) (b)

(c)Fig.1 Frost scaling of mortars Fig.2 Condition of mortars after 1-cycle scaling(a) Sand mortar; (b) E-OPC; (c) E-BFS


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Unlike the carbonation on calcium hydroxide (CH), carbonation on hydrated phasesdoes not show any significant problem on alkalinity reduction. However the reactionyields the reduction in volume of the matrix which is normally called carbonationshrinkage. Due to a low CH content in the BFSC paste, carbonation process inducesto a coarser and even weaker microstructure compared with the OPC matrix [5].

However, it was obviously seen in Fig. 3(c) and (d) that the utilization of ELWAdramatically decreases the total porosity of the carbonated matrix. The pore sizewas significantly refined with presences of ELWA. After carbonation, the porositycoarsening would take place not only at the matrix zone, but also on the coatinglayer. This phenomenon would induce a carbonation triggered mechanism whichallows the encapsulated Na-MPF released to the matrix after the coating paste layerhas been attacked by CO2. The influence of carbonation attack on the pore structureof coating layer and interfacial transition zone (ITZ) can be obviously seen in Fig. 4.

(a) Carbonated sand mortar(b) Non-carbonated control

mortar(c) Carbonated E-OPC (d) Carbonated E-BFS

Fig.3 - Photomicrographs of mortars

(a) Non-carbonated E-OPC (b) Carbonated E-OPC

(c) Non-carbonated E-BFS (d) Carbonated E-BFS

Fig.4 - Photomicrographs of ITZ

ELWA

Matrix

Coating

Matrix

ELWA

Coating

ELWA

Matrix

Coating

ELWA

Matrix

Coating


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The EDX analysis was performed on polished specimens to investigate the leachingof encapsulated healing compound. At 500x magnification, the concentration of

phosphorous in the matrix area at the position about 500 m apart from the LWA

surface was determined. The concentration was measured at 20 different randomlyselected paste locations through the area. The result indicated that the ELWA

released a significant amount of Na-MFP. There is no noticeable different betweenthe mortars with OPC and BFSC coating layer. The phosphorous concentration ofabout 13-17% by weight can be detected in carbonated area, while the slightly lowerconcentration (7-11% by weight) was observed in the non-carbonated area. Thisresult would confirm the triggered mechanism of encapsulation system. Theencapsulated compound was released after the matrix and coating layer has beenattacked by carbon dioxide. It would be remarked that the phosphorousconcentration determined from the control mortar was only about 2-3% by weight.

4. ConclusionA self-healing system using encapsulated lightweight aggregate for carbonated blast

furnace slag mixtures subjected to frost scaling attack was proposed. The resultsfrom freezing and thawing test show that the mortars with encapsulated lightweightaggregate revealed an excellent frost salt durability. The photomicrographs showedan improvement of pore structure of the carbonated matrix particularly on theinterfacial transition zone between sand and matrix. After carbonation attack, thepore structure of coating layer was coarsened which allows the encapsulatedcompound to be released to the matrix. From EDX results, phosphorous leachedfrom ELWA to the matrix can be observed. For the mortar with ELWA coated withOPC, the healing mechanism would be due to the combination of treatment by Na-MFP solution and also calcium hydroxide supplied from the coating Portland cementpaste layer.

Acknowledgement

Authors would like to thank SenterNovem for the financial support granted for theproject: SHM0616.

References

1. Ngala VT, Page CL, Page MM, Corrosion inhibitor systems for remedial treatmentof reinforced concrete: Part 2. Sodium monofluorophosphate, Corrosion Science45 (2003) 1523-1537.

2. Alonso C, Adrade C, Argiz C, Malric B, Na2PO3F as inhibitor of corrodingreinforcement in carbonated concrete, Cement and Concrete Research 26 (1996)405-415.

3. Andrade C, Aloso C, Acha M, Malric B, Preliminary testing of concrete Na2PO3Fas a curative corrosion inhibitor for steel reinforcements in concrete, Cement andConcrete Research 22 (1992) 869-881.

4. opurolu O, Fraaij ALA, Bijen JMJM, Effect of sodium monofluorophosphatetreatment on microstructure and frost salt scaling durability of slag cement paste,Cement and Concrete Research 36 (2006) 1475-1482.

5. Sisomphon K, opurolu O, Fraaij ALA, Transport properties and frost saltscaling resistance of carbonated blast-furnace slag mortars after sodium

monofluorophosphate treatment, to be published inProc.the 2

nd

InternationalRILEM Workshop on Concrete Durability and Service Life Planning, Haifa, Israel,2009.

icshm extended - k. sisomphon.pdf

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