ALKALI SILICA REACTION MITIGATING PROPERTIES OF TERNARY BLENDED CEMENT WITH CALCINED CLAY AND LIMESTONE.

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1 ALKALI SILICA REACTION MITIGATING PROPERTIES OF TERNARY BLENDED CEMENT WITH CALCINED CLAY AND LIMESTONE. Aurélie R. Favier, Cyrille F. Dunant, Karen L. Scrivener EPFL-STI-IMX LMC, Station12, CH-1015 Lausanne, Switzerland, ABSTRACT Supplementary cementitious materials (SCMs) are widely used in concrete either in blended cements or added separately in the concrete mixer. SCMs such as calcined clays, slags or fly ashes are widely used to partially substitute plain Portland cement (PC). A particurlarly promising blend is a blend with a high level of substitution by widely available SCMs such low grade calcined clay and limestone. The use of such materials, where no additional clinkering process is involved leads to a significant reduction in CO2 emissions per ton of material. Further, blended systems have numerous well established benefits in terms of durability. ASR is the most important such issue not related to reinforcing steel. Prevention of this phenomenon is critical as sources of non-reactive aggregates are increasingly scarce. The most economical path to ASR resistant concrete is through ternary blends. Since the reaction occurs between alkalis in pore solution and reactive silica, most mitigation methods rely on lowering the alkalinity of the solution through Supplementary Cementitious Materials (SCMs). The effectiveness of SCMs in mitigating ASR is attributable to pore refinement, alkali binding by secondary hydration due to replacing part of the Portland cement, but mainly to the inhibition of silica dissolution when Al ions provided by the SCM are present in the solution. Due to yet uncommon usage in the field, the performances and mechanisms which underlie the properties of such blends are still not wholly understood. In this study, we demonstrate the performance of blends with high level of replacement (reaching 50%) of cement with limestone and calcined clay. The use of these two SCMs at such high level of replacement promise improvement of the resistance to expansion compared to PC in environmentally friendly blends. Key-words: Alkali silica reaction, blended cement, limestone, calcined clay INTRODUCTION Concrete structures are often subject to degradation due to the alkali silica reaction (ASR). ASR is a reaction between amorphous silica from some aggregates and the alkalis from the pore solution of the cement paste. This results in an expansive gel which induces cracks until a complete deterioration of the concrete. The use of supplementary cementitious materials to control the expansion is well established and a number of reviews have been published[1 5]. However, the role of SCMs and their mechanisms in mitigating ASR remain unclear. Some researchers have shown that mineral additions lead to a reduction in the concentration of alkali-hydroxides in the pore solution of concrete, the amount of reduction is being directly linked to the SCM replacement levels. The reduction was thought to be the principal factor in the mitigating properties, however, field experience showed expansions in very low alkali systems. It has been also suggested that the presence of alumina in SCMs contributed some way to prevent the release of alkali back to the pore solution. However, Chappex et al [6] showed that this effect is extremely small. Alumina is an inhibitor of silica dissolution [7]. Based on these observations, the use of alumina-rich SCMs, such as metakaolin should be favoured for the purpose of mitigating ASR effects. 1

2 Previous studies have shown that a replacement of 45% of clinker by a combination of calcined clay and limestone is possible and allows good strength. In this study, we investigate the performance of such blends against ASR. MATERIALS AND METHODS Materials Two different binders are studied, a ternary blended cement with a substitution of 30% by calcined clay and 15% of limestone noted LCC B and a CEM I noted PC. Their composition is given Table 1. The alkali content is adjusted to be identical by adding NaOH in water during the mix. We also used two types of aggregates. The aggregates used were a highly reactive North American river aggregate (Jobe) and a French ground sand (assumed non-reactive noted Quartz) with the same particle size distribution given in Figure 1. Mortars bars were cast with the same Water/Binder ratio of 0.46, the amount of plasticizer was adjusted to obtain a workable mixture. Some mortar bars were cast with inset metal measurement studs to monitor expansion. The aggregate to cement ratio was 3. The mortars bars were first cured for 28 days at 20 C and 95 %RH. They were then put in alkaline solution containing 0.32 mol/l NaOH as it is close to the pore solution and at 38 C to accelerate the expansion. Table 1 Chemical composition of cements in % % SiO 2 Al 2O 3 CaO Fe 2O 3 Na 2O eq LCC B PC Figure 1 Particle size distribution of both sand aggregates Methods 2

3 Expansion was measured regularly following ASTM C b[8] at 38 C. Depending on the results of expansion, mechanical properties such as flexural strength, compressive strength and elastic modulus were measured. Then, at these times, samples were taken for SEM analysis on a Bruker AXS. A high resolution montage of 144 pictures at a magnification of 800 was acquired to analyse a representative amount of aggregates. The amount of degradation in the aggregates was quantified by the image analysis method developed by Dunant [9]. First, the aggregate particles are identified by a combination of grey level and shape. Then the dark areas, corresponding to cracks and gel in the aggregates are thresholded and measured. RESULTS Accelerated expansion tests Expansion curves for the various blends and the two aggregates are shown in Figure 2. Due to the difference of alkalinity, the expansions have to be compared with care. Naturally, the blended cement LCC B has a lower rate of alkali. In order to be comparable and representative, sodium hydroxide is added to the mixing water in the alkali-deficient system. It is seen that the highly reactive North American aggregate expands more and at a higher rate than the Quartz aggregate for all systems as shown in Figure 2a. It is also visible in Figure 2b that the system containing alumina (LCCB) shows lower expansion over time. To reflect the expansion results, the system LCC B has a behaviour very close to a conventional PC in the presence of non-reactive sand. For these system LCC B with reactive aggregate and PC with non-reactive sand, we observed the competition between shrinkage and expansion and before 120 days, shrinkage controls these systems. a) b) Figure 2 Expansion as a function of time after immersion in Na OH solution a) for two different sands with PC b) for two different cements with Jobe sand. The dotted line indicates the limit of innocuous behaviour on concrete at one year according to ASTM C b[8] Effect on mechanical properties Figure 3 shows the evolution of mechanical properties, the values at 28 days would be the reference values as mortar bars were not yet immersed in NaOH solution. 3

4 a) b) c) Figure 3 a) Compressive strength b) flexural strength c) elastic modulus of mortars with Jobe sand as a function of time. Between 28 days and 4 months the LCC B mix experiences neither compressive nor flexural strength loss (Figure 3a & 3b), rather a slight gain is observed. Despite the visible external cracks, the PC mix shows only a slight decrease in compressive strength. The loss in flexural strength and the decrease of elastic modulus are much more important for this PC mix than for the blended system. These results are in good agreement with the expansion measurements. Previous studies have also confirmed that tensile strength and elastic modulus are more sensitive to ASR cracking [10,11]. Furthermore, the PC system 4

5 demonstrated a distinctive behavior that after a drop in elastic modulus a recovery occurs at the later ages. However, the initial loss and later recovery is not seen in the flexural strength values as it was reported for both flexural strength and elastic modulus values in Ahmed et al. and Bektas et al. [11,12]. The phenomenon has been explained as on-going hydration dominating over the effect of ASR. In summary, mechanical testing demonstrates the beneficial effect of calcined clay and limestone in mitigating ASR and associated deterioration. However, mechanical tests give information only on the macroscopic behaviour of our systems and do not fully capture the state of deterioration. For this, microscopic observations were necessary. Damage quantification by SEM image analysis SEM observations have confirmed the visual conditions of the specimens. The aggregates in the PC system were heavily damaged (Figure 4 a). For the same age, aggregates present in the blended system LCC B seem less damaged (Figure 4 b). a) b) Figure 4 Jobe aggregate state over time on a) PC matrix b) LCC B matrix. Ref is the state at 3d without exposure to NaOH solution. The total damage, including the cracks and the gel pockets measured by image analysis, is shown in Figure 5. The results show a clear effect of calcined clay and limestone as SCMs on the damages of the aggregates. The rate of deterioration is much faster in the case of PC system. 5

6 Figure 5 Reacted fraction analysis over time of NA sand at 3 days before immersion and then after immersion in NaOH solution. DISCUSSION AND CONCLUSIONS In this study, we shown that ternary blend cement with calcined clays and limestone can help to mitigate the ASR. These results are not surprising considering the work of Chappex et al. [6,7,13], highlighting the positive effect of alumina in mitigating ASR. The values of the expansion do not cross the limit recommended by the standard [8]. However, this norm is defined for concrete consisting of poly dispersed aggregates after 1 year of exposure. The sands of this study are mono-sized and it has already been shown [14 16] that the size of aggregates plays an important role on the expansion rate after 100 days. Moreover, this study emphasizes two interesting points: - After one month, the expansion stabilizes despite the increase of damage within aggregates observed and quantified by image analysis. This suggests that a spate of expansion can be expected in the longer term. - The flexural strength and elastic modulus are more representative indicators than the compressive strength. It is true that the flexural strength is not the most essential property but from a practical point of view, a drop in flexural strength will affect the durability, cracks being preferential paths to external attacks. To conclude, the blended system containing aluminosilicate clays is a promising cement to formulate ASR-resistant concretes. However, it remains to show whether the presence of limestone in our system also plays an important role as it also consume aluminium to form carboaluminates phases [17]. REFERENCES 1. Chen, H.; Soles, J. A.; Malhotra, V. M. Investigations of supplementary cementing materials for reducing alkali-aggregate reactions. Cem. Concr. Compos. 1993, 15,

7 2. Duchesne, J.; Bérubé, M.-A. Long-term effectiveness of supplementary cementing materials against alkali silica reaction. Cem. Concr. Res. 2001, 31, Kandasamy, S.; Shehata, M. H. The capacity of ternary blends containing slag and high-calcium fly ash to mitigate alkali silica reaction. Cem. Concr. Compos. 2014, 49, Shehata, M. H.; Thomas, M. D.. Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali silica reaction in concrete. Cem. Concr. Res. 2002, 32, Thomas, M. The effect of supplementary cementing materials on alkali-silica reaction: A review. Cem. Concr. Res. 2011, 41, Chappex, T.; Scrivener, K. Alkali fixation of C S H in blended cement pastes and its relation to alkali silica reaction. Cem. Concr. Res. 2012, 42, Chappex, T.; Scrivener, K. L. The influence of aluminium on the dissolution of amorphous silica and its relation to alkali silica reaction. Cem. Concr. Res. 2012, 42, ASTM C b Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction; ASTM International, Dunant, C. Experimental and Modelling Study of the Alkali-Silica Reaction in Concrete, EPFL, Swamy, R. N.; Al-Asali, M. M. Engineering Properties of Concrete Affected by Alkali-Silica Reaction. ACI Mater. J. 1988, Ahmed, T.; Burley, E.; Rigden, S.; Abu-Tair, A. I. The effect of alkali reactivity on the mechanical properties of concrete. Constr. Build. Mater. 2003, 17, Bektas, F.; Wang, K. Performance of ground clay brick in ASR-affected concrete: Effects on expansion, mechanical properties and ASR gel chemistry. Cem. Concr. Compos. 2012, 34, Chappex, T.; Scrivener, K. L. The Effect of Aluminum in Solution on the Dissolution of Amorphous Silica and its Relation to Cementitious Systems. J. Am. Ceram. Soc. 2013, 96, Dunant, C. F.; Scrivener, K. L. Effects of aggregate size on alkali silica-reaction induced expansion. Cem. Concr. Res. 2012, 42, Ramyar, K.; Topal, A.; Andiç, Ö. Effects of aggregate size and angularity on alkali silica reaction. Cem. Concr. Res. 2005, 35, Zhang, C.; Wang, A.; Tang, M.; Wu, B.; Zhang, N. Influence of aggregate size and aggregate size grading on ASR expansion. Cem. Concr. Res. 1999, 29, Antoni, M.; Rossen, J.; Martirena, F.; Scrivener, K. Cement substitution by a combination of metakaolin and limestone. Cem. Concr. Res. 2012, 42, AKNOWLEDGMENTS The Swiss Development and Cooperation agency is thanked for funding the project of Limestone Calcined Clay cement. 7