Technical Paper THE IMPACT OF HYDRATION AND DEHYDRATION CONDITIONS ON THE PERMEABILITY OF LCC SYSTEMS

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1 Page : 1/8 THE IMPACT OF HYDRATION AND DEHYDRATION CONDITIONS ON THE PERMEABILITY OF LCC SYSTEMS Christoph Wöhrmeyer, Chris Parr, Jean-Michel Auvray, Michael Lievin, Eric Frier *, * Kerneos, France Presented at Unitecr 2007, Dresden, Germany

2 Page : 2/8 ABSTRACT One of the most critical steps in the usage chain of monolithics containing calcium aluminate binders is the initial dry out whether this is for precast pieces or large scale in situ sections. In addition the dry out is the step which requires the most time and represents the single largest part of the total downtime during most relining processes. In order to minimise downtime there is naturally a strong desire to reduce dry out times but this can only be considered if it can be done safely and without risk to the structure itself. A key concern is to limit the internal vapour pressure, arising from the water inside the concrete (free water and water combined in hydrates) to values less than the tensile strength of the concrete. Permeability of the castable is an indicator to evaluate the ease of dry out. This paper presents a study into the impact of initial hydration and subsequent de-hydration conditions on the permeability of a low cement model castable. This alumina spinel concrete contains high alumina cement as binder and reactive alumina as filler. Two different deflocculation systems based on Polycarboxylate Ethers (PCE) have been used. The impact of hydration time on castable permeability has been studied. Normal atmosphere hydration has been compared with hydrothermal hydration that can occur in very dense castable structures. During de-hydration in the dry-out process several phase changes occur. The knowledge about the dependence of the permeability on these phase formation, the temperature and the hydration conditions prior to the dry-out have been found as crucial to make dry-out safer and faster.

3 Page : 3/8 1 Introduction When pore pressure during the dry out process exceeds the tensile strength of the castable explosive spalling and destruction can be the consequence. Multiple studies have shown the importance of permeability as an input parameter to characterise the ease of moisture transfer in concretes. Of particular interest is the permeability in the temperature range up to 350 C. It is within this temperature range that most dehydration reactions of calcium aluminate binders occur under normal atmospheric pressure. The addition of polymeric fibres is a common tool to increase castable permeability without increasing significantly the porosity. Compared to fibrefree castables dry out time can be reduced without increased risk to explosive spalling. However the addition of polymer fibres is not always convenient since they are difficult to disperse homogeneously inside the dry mix and they often increase slightly the necessary amount of mixing water. Therefore a more detailed insight view of the parameter influencing the intrinsic castable permeability independently of the addition of polymer fibres is the objective of this work. Special focus will be given to the impact of deflocculation systems, curing times and hydrothermal conditions which can occur during dry-out of big monoliths.. 2 Materials and methods A deflocculated alumina spinel castable with a 70% alumina cement bond and reactive alumina as filler has been chosen as model system. Two different Polycarboxylate Ethers (PCE-1 and PCE-2) have been used in this model LCC system. Castable rheology has been measured using an vibration table (Amplitude 0,5 mm, vibration time 30 sec, conic mould with 50mm highed, 100mm lower and 70 mm upper diameter. Initial set time (or working time) is the moment when the castable doesn t flow anymore under vibration. The castable s temperature evolution under semi-adiabatic conditions has been followed by thermocouples. The ultrasonic method as described in (1) gives an in-situ image of the transfer of the fluid concrete into a hardened body. Green samples have been cured at 20 C and 100%rH prior to the strength measurements. Hydrate phases have been analysed using the Differential Scanning Calorimetry (DSC) and X-ray diffraction (XRD) including phase quantification by the Rietveld method. For the permeability measurement discs of 100 mm in diameter, 25 mm high, have been prepared by casting the concrete under vibration into a plastic mould. Curing follows in a climate chamber at 100%rH. A permeameter as described by Moore et al. (2) has been used to measure the air permeability. The sample is positioned above an evacuated chamber, while the other side of the sample is exposed to normal air pressure. The outer part of the sample disc has been sealed to prevent leakage. The evolution of the pressure in the chamber, which had been put under lower pressure by evacuation of the air at the beginning of the measurement, is measured. Due to the pressure difference between the two sides of the disc air flows through the sample to bring the pressure in the chamber back to atmospheric pressure. From the pressure change in the chamber over time the average permeability K has been calculated. Results are shown in mdarcy (1mD = 0, μm 2 ).

4 Page : 4/8 Tab. 1: Castable properties mm PCE 1 PCE 2 Tabular alumina Tabular alumina Tabular alumina 0,2-0,6 6 6 Tabular alumina 0-0,3 5 5 Sintered Spinel 0,5-1,0 9 9 Sintered Spinel 0-0,5 4 4 Sintered Spinel 0-0, Reactive Alumina d50 = 0, Calcium Aluminate Cement SECAR Polycarboxylate Ether PCE-1 0,04 Polycarboxylate Ether PCE-2 0,04 PP-Fibres 0-0,2 0-0,2 Water 5 4 Vibration Flow (mm) : 5 min min min Set time Exothermic peak maximum (h) Experimental results DEFLOCCULANTS AND LCC PERMEABILITY Deflocculants are used to reduce the water demand of low cement castables and consequently to reduce the porosity. Significant improvements have been achieved in recent years in the field of polycarboxylate ethers as defloccuants. However each improvement in this sector might create a potential risk during dry out if drying procedures are not adapted to the reduced porosity of these new LCC s. Two different polycarboxylate ethers have been used in this study. The replacement of polycarboxylate ether PCE-1 by PCE-2 changes first of all the fluidity of the concrete. High initial fluidity at lower water demand can be achieved. Furthermore, the flow of the castable with PCE-2 decays less rapidly than with PCE-1 which makes installation easier. More than 5 h difference in initial set time can be observed between the mixes. But as can be seen from the exothermal profiles, for PCE-1 the set time isn t in phase with a massive hydration of the calcium aluminate cement. The cement hydration appears to be blocked and impacted by the influence of PCE-1. Opposite to that the mix with PCE-2 starts to hydrate massively right after initial set has been observed. The ultrasonic profiles (Fig. 1) show quite clearly how differently the two PCE s influence the transformation from a liquid castable into a hardened body. The setting time observed for the mix with PCE-1 seems to be first of all a flocculation process and not a cement reaction. The polycarboxylate ether loses its activity as deflocculant and reactive alumina and cement particles coagulate and prevent any flow of the castable.

5 Page : 5/8 But at the same time PCE-1 retards strongly the hydration of the calcium aluminate cement. In case of PCE-2 hydration follows quickly after flocculation. PCE-1 due to the increased compaction and densification at constant water addition. 10,00 PCE-1 24H 4,5% H2O PCE-2 24H 4,5% H2O Velocity (m/sec) PCE-2 PCE-1 Permeability (mdarcy) 1,00 0,10 0,01 22 C 110 C 200 C 300 C Temperature ( C) Fig. 1: Ultrasonic profiles Time after water addition (h) Consequently strength build-up lasts much longer when PCE-1 is used also initial set time is shorter. 24h after concrete mixing the compressive strength is still below 10 MPa while with PCE-2 in the concrete massive cement hydration has occurred already and results in a compressive strength of more than 80 MPa. If the dry out process will be started 24h after mixing, the castable with PCE-1 might be at higher risk for structural damage by explosive spalling due to the low strength at this moment. On the other hand PCE-2 has a lower water demand and consequently a more dense structure which as also raise the question about the safety during dry out. The samples for the permeability tests have been cast with 4.5% water for both mixes. 24h after curing at room temperature and 100%rH the green bodies of both LCC s have extremely low permeability (below 0,01 mdarcy). Figure 2 shows that longer curing (72h) increases the permeability for castable with PCE-1 significantly while it remains still very low with PCE-2 at ambient temperature and after drying to 300 C. PCE-2 gives systematically lower permeability values than Permeability (mdarcy) 10,00 1,00 0,10 0,01 PCE-1 72H 22 C 110 C 200 C 300 C Temperature ( C) PCE-2 72H Fig. 2: Castable permeability after 24 and 72h curing at 22 C and 100%rH prior to dry out At constant fluidity (5% H2O for PCE-1 and 4% for PCE-2) the permeability is nearly 10 times lower for PCE-2. Dry-out of big blocks has to be conducted more carefully than with the higher water demand system based upon PCE-1. HYDROTHERMAL CONDITIONS Fast drying of massive monoliths can create a high internal pressure inside the humid structure. Calculations have shown (Fig 3) that the pressure can reach 10 bar or more. Little is known about the effect of pressure on the microstructure and permeability of the concrete. Therefore the samples for the permeability tests have been treated in an autoclave at 150 C and approximately 5 bar during 24h.

6 Page : 6/8 Inside the autoclave a water reservoir assures the hydrothermal conditions. Before the samples have been placed in the autoclave they have been treated by curing at 20 C and 100%rH during 24h. Then the hydration had to be stopped to make sure that all samples have the same curing time before they are treated all together in the autoclave. After the autoclave treatment the permeability of the samples has been measured again before they have been dried under normal atmospheric pressure between 110 and 350 C for a final permeability measurement. The evolution of the permeability in comparison to the samples without autoclave treatment has been determined. As can be seen in Fig 3 the autoclave treatment results in a reduction of permeability. The permeability of the samples treated at normal pressure show a significant increase in permeability between 110 and 150 C which can be explained by the presence of PP-fibres in this series of trials (Fig 3 & 4). At this temperature the polymer fibre starts to shrink and melts. They open channels through which the water can be evaporated from the concrete structure more easily. However this increase does not occur for the sample with the autoclave treatment. Since the material has seen already 150 C in the autoclave the PP-fibres are already molten. However permeability remains lower than without autoclave treatment. One reason for this might be the fact that under hydrothermal conditions the cement hydration is more complete and hydration can occur as well in the channels which have been formed before by shrinkage and melting of the PP-fibres. But further investigations are necessary to better understand why the effect of PP-fibres is eliminated in case of the autoclave treatment. Permeability (mdarcy) ,1 0,01 after drying at 110 C, normal pressure after autoclave treatment 150 C Temperature ( C) Fig. 3: Permeability of PCE-1 with 0,05% PP-fibres For the dry out of big monoliths where hydrothermal conditions can occur in the microstructure it would mean: The dry out does not only take more time because of the long transport distance for the water from the centre to the surface. It takes as well more time to transport the same water volume over the same distance due to the lower permeability as a consequence of the hydrothermal conditions. Another point to check was if under hydrothermal conditions the hydrate phases in the bond system might change. In order to get quantifiable amounts of hydrate phases with the XRD method, binder phase samples have been prepared consisting of 37% Spinel <0.09mm, 40.7% reactive alumina, 22.2% SECAR 71, PCE-1 and 14% H2O. These samples have been analysed by DSC as well to confirm the phase analyses from the XRD and to check at which temperatures the dehydration occurs. Under normal conditions the typical hydrates occur, mainly C3AH6 and AH3 (Gibbsite) beside some remaining un-hydrous cement phases, CA and CA2. Some traces of AH (Boehmite) might be present as well. The dehydration of C3AH6 happens all between 200 and 225 C while Gibbsite decomposes successively between 125 and 225 C (Fig 4).

7 Page : 7/8 Phase content (wt.%) Dry-out Temperature ( C) AH3 C3AH6 AH Fig. 4: Hydrate phase development during dry at normal atmosphere. Samples cured at 20 C at normal atmosphere prior to dry out Phase content (wt.%) AH3 C3AH6 AH Dry-out Temperature after autoclave treatment( C) Fig. 5: Hydrate phase development during dry at normal atmosphere. Samples treated in autoclave at 150 C prior to drying. The situation looks quite different for the sample treated in the autoclave (Fig 5). First of all the quantity of hydrates is higher. Hydration of the calcium aluminate cement is much more complete compared to the normal dry out conditions and includes all CA and CA2 phases of the cement. No un-hydrous CA or CA2 remains in the structure. Another significant difference is the fact that no AH3 is present under hydrothermal conditions. Instead of the tri hydrate Gibbsite, the mono hydrate Boehmite occurs. Different from AH3, AH is stable at much higher temperatures. In the DSC analyses it can be seen that it needs nearly 500 C to dehydrate the AH. Consequently big and very dense monoliths have to be dried longer, more carefully and up to higher temperatures to make sure that they are fully de-hydrated before they go into service. 4 Summary The safe dry out of highly densified refractory castables like Low Cement Castables with a high amount of filler needs special attention. Increasing the compaction by reducing the water demand can be achieved by the latest generation of polycarboxylate ethers. However this is accompanied by a reduction of castable permeability which indicates an increased risk to explosive spalling during dry out. It could be shown that by increasing the curing time the permeability could be increased in case of polycarboxylate PCE-1 while it doesn t change with PCE-2 since the hydration process is much faster in that case. It gives a significant higher strength level after 24h for PCE-2 and would give more resistance against high internal pore pressures during dry out. But this is at the same time compromised by a lower permeability. Another important aspect is the fact that under hydrothermal conditions as they can occur in big monoliths, Boehmite (AH) can occur instead of AH3 with the consequence that dry-out temperature has to be increased to 500 C where Boehmite dehydrates. A big monolith compared to a small piece of the same material has to be dried longer due to the longer distance of the water transport from the centre to the surface of the block. It has to be dried out more carefully because of the reduced permeability under hydrothermal conditions and it has to be heated to higher temperatures since Boehmite needs nearly 500 C before it releases its bonded water. The positive effect of polymer fibre addition on permeability is less significant when the castable has undergone the hydrothermal process. The reason for this has to be explored more in detail. However it is advisable to use polymer fibres in highly densified refractory castables to make the dry out process as safe as possible.

8 Page : 8/8 5 References [1] Abe H. The effect of permeability on the explosion of castable during drying. J Tech Ass Refract, Japan (2) [2] Moore RE, Smith JD, Sander TP. Dewatering monolithic refractory castables: experimental and practical experiences. Unitecr 1997, [3] Canon JM, Sander TP, Smith JD, Moore RE. Effect of organic fiber additions on permeability of refractory concrete. Unitecr 1997, [4] Meunier P, Ronsoux L: Permeability and dehydration of refractory castables. Unitecr 2005 [5] Parr C, Wöhrmeyer C, Touzo B, Bell D. The role of calcium aluminate cement during the installation and dry out of high purity alumina castables. Alafar 2001 [6] Salomao R, Pandolfelli VC, Bittencourt LRM. Advances on the understanding of the role of polymeric fibres as drying additives for refractory castables