Evolution of Phases and Micro Structure in Hydrothermally Cured Ultra-High Performance Concrete (UHPC)

C. Lehmann, P. Fontana, and U. Müller1

Abstract. Thermal curing of Ultra-High Performance Concrete (UHPC) has a strong influence on its mechanical properties. By applying a water vapor satura-tion pressure additional to the increased temperature the curing conditions are strongly enhanced and lead to a significant improvement of the degree of hydra-tion of the cement paste. Increasing temperature accelerates the formation of crys-talline calcium silicate hydrates by dehydrating the cement paste and ends in the formation of gyrolite, truscottite and xonotlite at 200 °C and 15 bars. Thereby the micro structure undergoes an obvious change. Cement paste consists of close net-worked crystal fibers with dimensions up to 1 μm. By filling cracks and small pores with crystalline C-S-H phases, flaws in the matrix are healed and generate a more homogeneous micro structure. Additionally, autoclaving encourages dissolu-tion processes at quartz grains, which produces a better cohesion between fillers and the fine crystalline cement paste. As a consequence, autoclaving enhances compressive and flexural strength significantly but, compared to simple heat treat-ment at 1 bar, with very low scatter of the test results.

1 Introduction

Concrete technology turns its attention more and more to materials with enhanced properties such as high strength and durability as well as increased ecological per-formance. One of the more recent research topics in the field of improving the concrete composition is Ultra-High Performance Concrete (UHPC). The excep-tional strength of UHPC of 150 MPa and more as well as its remarkably increased durability is mainly based on its dense micro structure which is a result of the high

C. Lehmann, P. Fontana, and U. Müller Federal Institute for Materials Research and Testing (BAM), Berlin, Germany e-mail: Christian.Lehmann@bam.de, Patrick.Fontana@bam.de, Urs.Mueller@bam.de

www.bam.de

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cement content, the very low water cement ratio, the use of highly reactive silica fume and the granulometric adjustment of fillers [4]. Due to the addition of highly effective plasticizers a good workability of the fresh concrete is maintained. Pre-vious studies showed that thermal curing of UHPC can have a strong influence on its mechanical properties [1,3,5]. One effect of heat treatment is the development of a denser micro texture with formation of crystalline calcium silicate hydrate (C-S-H) phases, what usually results in increased compressive strengths [2]. The enhancement of the curing conditions by additional application of water vapour saturation pressure may increase significantly the flexural strength too [3]. The main interest of our study was focussed in how the micro structure and phase equi-librium is changed by autoclaving UHPC and how this relates to the mechanical properties. In addition the reaction rate of the used fly ash was of particular inter-est, since a higher rate could help to reduce the cement and silica fume content significantly.

2 Experimental Setup

2.1 Materials and Curing Regimes

The mix design of the UHPC was based on commercial raw materials to achieve results with practical relevance. In addition to a white cement CEM I 42.5-R a mi-cro fly ash (median particle diameter 0.2 μm) and silica fume were used as reac-tive components. The chemical compositions of the cement and the fly ash are shown in Table 1. The maximum size of the quartz aggregate was 2 mm. In order to optimize the particle size distribution a quartz filler with a median size of 50 μm was added. The water cement ratio was 0.26. The total water binder ratio was 0.22. The composition of the UHPC is given in Table 2. The self-compacting properties of the fresh UHPC were adjusted using a polycarboxylate-based super-plasticizer. The slump-flow was 260 mm (small cone according to EN 1015-3).

First, the solid components were dry mixed in a high shear mixer to homoge-nize the material. Then, water and superplasticizer were added and the material was mixed thoroughly. The fresh concrete was casted in prismatic steel moulds (160 x 40 x 40 mm³) and demoulded after 1 day. Thereafter the specimens were cured under six different conditions (Table 3).

Table 1 Chemical composition of cement and fly ash measured by X-ray fluorescence analysis in Wt.-%

Material MgO Al2O3 SiO2 P2O5 SO3 K2O CaO MnO TiO2 Cr2O3 Fe2O3 CO2

Cement 0.60 4.73 20.73 - 3.14 0.95 66.42 - 0.11 0.06 0.38 2.88

Fly ash 1.00 18.77 58.47 0.66 0.41 1.63 2.49 0.02 0.70 - 3.60 12.22

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Table 2 Composition of UHPC

Material content

Cement (kg/m³) 745.0

Silica fume (kg/m³) 72.4

Fly ash (kg/m³) 74.5

Quartz filler (kg/m³) 243.1

Quartz aggregate 0-0.5 mm (kg/m³) 238.2

Quartz aggregate 0.5-1.0 mm (kg/m³) 357.1

Quartz aggregate 1.0-2.0 mm (kg/m³) 357.1

Water (kg/m³) 168.0

Superplasticizer (g) 53.5

Table 3 Curing conditions

Curing condition Curing time

Series 1 – reference 23 °C / 1 bar 6 days

Series 2 – heat treated 90 °C / 1 bar 2 days

Series 3 – heat treated 150 °C / 1 bar 2 days

Series 4 – heat treated 200 °C / 1 bar 2 days

Series 5 – autoclaved 150 °C / 5 bar 8 hours

Series 6 – autoclaved 200 °C / 15 bar 8 hours

After 7 days the specimens were dried at 40 °C and 40 mbar to stop the hydration.

2.2 Analytical Techniques

For phase-analysis a combination of several techniques was used to achieve reli-able results. Firstly the samples were analyzed using a Philips PW 1710 X-ray dif-fractometer with Cu Kα radiation. All samples were scanned over a 2θ-range from 3 to 65° using a step size of 0.02° and a measuring time of 4 s each step. Addi-tionally detailed scans followed over a 2θ-range from 3 to 20° and a step size of 0.005°. To optimize the identification of C-S-H phases, and in particular the puz-zolanic consumption of portlandite, differential thermo analysis and thermal gra-vimetry was used (Netzsch – STA 449 Jupiter). Finally, for precise chemical and textural analysis in micro- and nanometer range, a scanning electron microscope (Leo Gemini 1530 VP) was employed. On all series compressive strength and flexural strength were tested. Mercury intrusion porosimetry was performed to ex-amine the evolution of pore diameters under the different curing conditions.

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3 Results

3.1 Microstructure and Mechanical Properties

Heat curing or autoclaving the UHPC generated an obvious change in the micro structure. The hydration of clinker phases and the fly ash was improved. Simple heat treatment produced a slightly denser micro structure and a remarkable in-crease of pore volume with a median pore diameter of 12 nm. Autoclaved samples exhibited an obviously higher degree of hydration. The texture of autoclaved UHPC showed a homogeneous, dense cement paste which consisted of close net-worked crystal fibers with a length up to one micrometer in the specimen cured at 200 °C and 15 bars. Cracks and small pores were filled with crystalline C-S-H (Fig. 1). The pore volume decreased compared to the heat treated specimen and the median pore diameter was reduced to 5 nm.

Fig. 1 SEM image of UHPC autoclaved at 200 °C / 15 bars. A crack is filled with crystal-line C-S-H (black arrows). The white arrow points to a completely hydrated grain of fly ash, which indicates a high degree of puzzolanic reaction

The autoclaved series exhibited dissolution processes around quartz grains, which produced a better cohesion between fillers and the fine crystalline cement paste (Fig. 2).

The mechanical tests showed a general increase of compressive strength pro-portional to the curing temperature. Indeed, the results of the autoclaved samples showed a smaller scatter and reached a higher final strength. The flexural strength increased clearly by autoclaving, while it decreased slightly by heat curing.

Evolution of Phases and Micro Structure in Hydrothermally Cured UHPC 291

Fig. 2 SEM image of UHPC autoclaved at 200 °C / 15 bars. Dissolution rims on a grain of the quartz filler producing strong cohesion with crystalline cement paste (black arrows)

3.2 Microchemistry and Phase Composition

With increasing temperature the amorphous C-S-H phases in the sample changed to crystalline C-S-H phases. The level of dehydration was proportional to the cur-ing temperature. In simply heat treated samples the Si/Ca atom ratio increased slightly from 0.65 to 0.75. The C-S-H first crystallized to tobermorite and jennite (90 °C). Heat curing at 150 °C effected the formation of foshagite from jennite and quartz (1) or jennite and tobermorite (2). Additionally jennite decomposed to afwillite in a small amount (3).

OHfoshagitequartzjennite 235934 +→+ (1)

OHfoshagiteetobermoritjennite 23083 +→+ (2)

OHafwillitejennite 223 +→ (3)

The formation of xonotlite and gyrolite from tobermorite was visible at 200 °C (4). Afwillite and portlandite react to hillebrandite (5), which subsequently reacted with portlandite to jaffeite (6).

OHgyrolitexonotliteetobermorit 21848 ++→ (4)

OHitehillebrandeportlanditafwillite 222 +→+ (5)

OHjaffeiteeportlandititehillebrand 222 +→+ (6)

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Hydrothermal curing led to total absence of portlandite, which indicates a high degree of puzzolanic reaction of the fly ash. The Si/Ca atom ratio increased sig-nificantly from 0.75 to 1.1. The specimens autoclaved at 150 °C and 5 bars ex-hibited the same coexisting phases as the samples simply heat treated at the same temperature. Additionally afwillite and portlandite turned into hillebrandite (5) and jennite reacted with quartz to xonotlite (7). Finally tobermorite decomposed to xonotlite and gyrolite at 200 °C and 15 bars (4). Gyrolite itself reacted with quartz to the Si-rich truscottite (8). Also hillebrandite was stable in absence of portlandite.

OHxonotlitequartzjennite 219362 +→+ (7)

OHtitecottrusquartzgyrolite 2788247 +→+ (8)

4 Conclusions

The homogenous cement paste matrix, consisting of close networked C-S-H crys-tal fibers, which develop a more stable structure than amorphous C-S-H phases, and the “healing” of flaws by filling them with C-S-H crystals are the main rea-sons for the improved mechanical properties of autoclaved UHPC. In addition autoclaving results in increased cohesion between cement paste and fillers by partly dissolution of quartz grains, and distinctive reduction of pore sizes.

The puzzolanic reaction of fly ash is significantly accelerated, so autoclaving might be a useful tool to reduce the high cement and silica fume content in UHPC by using secondary cementitious materials.

The thermal treatment of C-S-H phases results generally in the development of crystalline phases. Hereby the water content of the phases decreases with an in-crease in temperature. Pure heat treatment at 1 bar leads to formation of foshagite and xonotlite and Ca-rich phases like jaffeite. However, portlandite is still present at 200 °C in this series but the amount is strongly reduced.

Due to autoclaving the reaction rate and the final degree of hydration of cemen-titious materials are substantially enhanced. Portlandite is not observed anymore in autoclaved samples and the Si/Ca atom ratios in the cement paste are higher than in simply heat treated samples what is shown by the formation of Si-rich truscottite. Furthermore xonotlite, hillebrandite and foshagite are detected as final phases.

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