Technical Paper AGEING OF ALUMINOUS CEMENT IN LCC. by Alain MATHIEU, Jean-Pierre BAYOUX, Jean-Pierre FALASCHI, Michel VIALLE

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1 Page : 1/8 AGEING OF ALUMINOUS CEMENT IN LCC by Alain MATHIEU, Jean-Pierre BAYOUX, Jean-Pierre FALASCHI, Michel VIALLE presented at the UNITECR congress, Japan; 1995.

2 Page : 2/8 Abstract LCC formulations are tuned to the right properties not only by adjusting the particles size distribution of the castable but also by finding the right admixtures to defloculate the finest particles and regulate the hydration of the mortar according to the reactivity of the cement. When the cement ages its reactivity will change and along with it the properties of the LCC. This paper proposes various techniques to get a better insight into the ageing of the cement. As a complement, mechanical tests will show how the information and the ageing properties of LCC mixes can be modified by the use of pre-aged cement.

3 Page : 3/8 1 Introduction 2 Characterization of the ageing of the cement A calcium aluminate cement, which is a laboratory 7 % Al2O3 cement sample, was used for the formulation of LCC concretes. This Secar 71 was either a cement used straight after grinding or the same cement aged in the laboratory. Both cements were characterized using various laboratory techniques. Conductimetry of stirred suspensions gives information on dissolution and precipitation reactions. CO2/H2O loss on ignition analyser gives information on carbonation and hydration of the sample when aged. Infra red spectroscopy is a complementary technique to the previous one. Inverse gas chromatography gives information on modification of specific surface du to carbonation / hydration reactions during the aging process. The fresh and aged cements were used in LCC formulations and mechanical properties are presented in comparison focusing mainly on working time, 6 hours and 24 hours strengths. 7% Al2O3 cement 2-1 The cement, ageing conditions The cement is the 7 % Al2O3 produced by sintering in the laboratory. Its characteristics are given in the following table (Figure 1). This cement is produced and the clinker is ground to a fineness of 45 m 2 /kg. Chemical composition % Ins SiO 2 TiO 2 Al 2 O3 Fe 2 O 3 CaO MgO LO1 TOTAL 9,,3,1 71,2,1 27,25,2,3 99,45 7% Al2O3 cement XRD mineralogical composition (impulse) C 12 A 7 CA CA 2 C 4 A 3 S A a Figure 1 - Characteristics of the cement The cement was aged in a thin layer at 2 C and 5 % relative humidity. Samples were taken at different age and tested in standard European mortars to determine the evolution of flow and set.

4 Page : 4/8 2-2 Characterization of the influence of ageing on flow and set The results are presented in the following graphs FLOW VALUE min 15 min 6 min Figure 2 - Influence of ageing on flow of EN mortars SETTING TIME Initial Set Final Se Figure 3 - Influence of ageing on initial and final set of EN mortars From these graphs it becomes very clear that ageing at 2 C and 5 % relative humidity modifies the hydration properties of the cement. On fresh cement the difference between flow at min and flow at 6 min is large (Figure 2) whereas after 7 to 14 ageing the difference becomes very little. The cement has a lower initial flow but its fluidity stays nearly constant for 1 hour with as a consequence a much better flow at 6 min. When the cement is aged for a long time the overall fluidity is largely improved. From these measurements it seems that the ageing is a 2 steps process. In the following characterization most of the tests will be carried out on 7 aged sample. 3 Proposal for various techniques to characterize the ageing of cement 3-1 Conductivity of stirred suspensions This technique consists of measuring the electrical conductivity of a stirred suspension of cement in water at 2 C. The water / cement ratio is 5. Conductivity is a very good picture of the ionic concentration in water. The conductivity of the suspension gives information on the dissolution / nucleation / precipitation steps of the hydration reaction of cement in water. The results presented (Figure 4) show how the conductivity varies when the cement ages. For the fresh cement, dissolution is very fast (conductivity increases very sharply) and then level off at a rather high conductivity. The oversaturation is high and massive precipitation associated with the conductivity drop occurs rather quickly. When cement ages dissolution is much slower and over-saturation is much less (level of conductivity plateau) nucleation is much longer (length of conductivity plateau). As a result the hydration is much slower. The longer the ageing the latter the massive precipitation of hydrates. Concerning the evolution of the set (Figure 3) the evolution is at the beginning, up to 7 rather quick, then it tends to level off.

5 Page : 5/8 2,5 2 1,5 1,5 CONDUCTIVITY : ( T=2 C, W/C=5) Age d 7 d 28 d Figure 4 - Influence of ageing of conductivity of cement suspensions. From this technique it is evidenced that ageing leads to a decrease of the reactivity of the cement. 3-2 Water and CO2 analysis During ageing the cement most probably reacts with H2O and CO2 from the atmosphere. Using a LECCO LOI analyser we have followed the evolution of H2O % and CO2 % during ageing. The % H2O increase from O to 28 is very little (Figure 5). The % CO2 increase is slightly higher and it is before 7 that he variation is the highest. min Comparing the bands in the 3-38 cm -1 area the following comments can be made (Figure 6). There is more bulk water in the aged sample as indicated by the broad feature around 33 cm -1. Some tentative identifications have been made. The 3343 cm -1 peak for the fresh sample suggests the presence of C2AH5. The 3527 and 3468 cm -1 peak for the aged sample may indicate that CAH4 is present. ageing 7 ageing 1 day,16,14,12,1,8,6,4,2 Evolution of % H2 and CO H2O CO2 Figure 5 - Evolution of % H2O, % CO2 during ageing. In order to complement this information we have done some IRFT transformations on both samples. The sample was analysed in KBr pellet. The traces are difficult to interpret since the variations in water and CO2 content are very little. Figure 6 - IRFT curves for different stages of ageing. 3-3 Inverse gas chromatography This technique is a very powerful technique to investigate the surface of ground powders. The product to be analyzed is the stationary phase of the gas chromatography column and various solvents are injected in the column to investigate the surface properties. Particularly inverse gas chromatography permits to determine the specific surface of a powder by measuring the adsorption isotherm of a series of tracer. By doing so, one can calculate the amount of an adsorbed mono-layer and knowing the surface of the adsorbed molecule the specific surface can be determined.

6 Page : 6/8 For the measurements 1/4 of an inch diameter and 2 cm long column were prepared by filling them with 4 to 5 g of cement. Before being analyzed, they were stabilized overnight under the at 4 C. Different amounts of octane were injected into the column and the chromatograms were recorded (Figure 7). Non-aged cement 3-4 Partial conclusion From the various techniques proposed here above it can be said that due to surface reactions with CO2 and H2O the reactivity of the cement decreases due to ageing. Hydrates seems to form on the surface of the cement grain as seen by IR spectrometry and measured by inverse gas chromatography. In the second part of this paper we will show how the ageing can modify the mechanical properties of LCC formulation and their stability towards ageing. Pic de l'air Aged cement Figure 7 - Inverse gas chromatography diagrams for fresh and aged cement. The difference between the fresh and the aged cement is significant. The retention curves have given us the following results. For the fresh samples the Specific Surface is 164 m 2 /kg whereas for the aged sample it is much higher around 225 m 2 /kg. Most probably this increase of specific surface is due to the formation / modification of surface hydrates during the ageing. 4 Effect of ageing on LCC formulation 4-1 LCC formulation In order to get a good LCC which flows correctly and sets in a reasonable time one need to have, first a good grading of the particles form coarse to fine and to adjust the additives package in order to defloculate the fines and regulate the hydration and set. Concerning the LCC, the following formulation was used (Figure 8). Formulation ALCOA Tab Alumina Pechiney P152 SB F silica Cement Elkem 7% Al cement Admix pack. Water LCC 22, % 9,9 % 18,7 % 2,5 % 9 % 1 % 1 % 5 %,1 % 5 % Figure 8 - The reference LCC formulation. The additive package used in the formulation contains various additives like citric acid (CA), sodium bicarbonate (SB), tri-polyphosphate (TPP) and Darwan 811D (DW). The total package contains a mix of these various additives representing up to,1 % of the total LCC mix.

7 Page : 7/8 4-2 Effect of ageing on the additive demand When the cement ages the amount of additives necessary to adjust the working time at 6 minutes varies largely. The older the cement the less additive is required (Figure 9) to get a working time of 6 min. The additive demand could be a very good mean to control the ageing of the cement. %,3,25,2,15,1,5 1 Days Additive Demand TPP BS AC DW Figure 9 - Effect of ageing on the additive demand. AC BS TPP DW 4-3 Effect of ageing of cement on the ageing properties of LCC formulation LCC formulations were prepared either with the fresh cement or with the cement after ageing for 7 at 2 C - 5 % HR. Both formulations were tuned to a working time of 6 minutes by the use of the additives. The amount of additives are given in Figure 9. For the fresh cement it consists of a mix of 4 additives whereas in the case of the 7 aged cement only the tripolyphosphate and the Darwan 811D were required. Both formulations were left to age at 2 C and 5 % HR. Working time as well as compressive strengths at 6 hours and 24 hours were measured at different date at 2 C. There are striking differences between both cement (Figure 1-11) % Al2O3 cement / Working time ageing d ageing 7d Figure 9 - Effect of ageing on the working time. MPa % Al2O3 cement C6H ageing d ageing 7d Figure 1 - Effect of ageing on the compressive strength 6 h. MPa % Al2O3 cement C24H ageing d ageing 7d Figure 11 - Effect of ageing on the compressive strength 24 h. The working time of the LCC formula prepared with the fresh cement increases a lot when the formula ages. Along with this increase of the working time the 6 hours strengths decrease very sharply. After 7 ageing the 6 hours strengths are down to. In the case of the pre-aged cement, the LCC formula made with it, is much less sensitive to ageing. The working time increases slightly and the compressive 6 hours strength remain at a

8 Page : 8/8 reasonable level of 4 MPa even when the formula has been aged for 28. The 24 hours compressive strengths are not modified in both cases (Figure 11). These results clearly evidence the advantage of using a pre-stabilized cement for he preparation of LCC formulas. These formulas will remain stable for a longer time which is of big advantage for both supplier and customer. 5 Conclusion On a laboratory 7 % Al2O3 cement various techniques have been used to follow the ageing of the cement when it is stored at 2 C and 5 % relative humidity. These techniques which need to be repeated on other samples show that the cement during its ageing reacts with humidity and CO2 from the atmosphere. Hydrates have been identified by unfrared spectrometry and an increase of specific surface area was measured by inverse gas chromatography confirming this hydrate formation. When the cement is aged it enables to prepare LCC formulation without using to much additives to regulate the fluidity and the set. Such formulas are by far more stable against ageing than those made with a reactive fresh cement.