The influence of barium sulphate and barium carbonate on the Portland cement

Procedia Engineering 151 ( 2016 ) 42 – 49 1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICEBMP 2016 doi: 10.1016/j.proeng.2016.07.358 ScienceDirect Available online at www.sciencedirect.com International Conference on Ecology and new Building materials and products, ICEBMP 2016 The influence of barium sulphate and barium carbonate on the Portland cement Anežka Zezulováa,*, Theodor StanČka, Tomáš Opravilb aResearch Institute for Building Materials, HnČvkovského 30/65, Brno 617 00, Czech republic bFaculty of Chemistry, Brno University of Technology, PurkyĖova 118, Brno 612 00, Czech republic Abstract Shielding of different types of radiation should be studied these days when there is a great development in the field of nuclear science. Barium ions can shield different type of radiation, therefore, by incorporation of barium ions into the system of Portland clinker, a binder with shielding properties can be made. The work is focused on influence of barium ions on the formation of Portland clinker and its properties. As a source of barium, barium sulphate and barium carbonate were added to raw meal. Clinkers and cements with a different amount of barium were prepared and the effect of barium on the clinker and cement properties respectively was studied. By Rietveld analysis (using X-Ray diffraction) and microscopic point counting method, the ratio of clinker phases was studied. With increasing amount of barium, the alite : belite ratio was decreasing and the amount of free lime was increasing. Sulphate decelerates the decreasing ratio and causes grow of alite crystals to great dimensions. The presence of barium in different phases was observed by SEM with EDS. The highest amount of barium was found in belite and clinker melt. In clinker melt, conglomerations of barium were formed. Furthermore, a difference in formation temperature of clinker phases was observed and some elementary tests were done on prepared cements according to the European standards. © 2016 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of ICEBMP 2016. Keywords: Portland clinker; alite; belite; barium oxide; Portland cement; raw meal; barium carbonate; barium sulphate * Corresponding author. Tel.: +420-730-519-704; fax: +420-543-216-029. E-mail address: [email protected] © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICEBMP 2016 43 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 1. Introduction Portland cement is a very widely used hydraulic binder all over the world mostly in construction. Appropriate batching and mixing with water and aggregate causes high strength and long-term volume stability after hardening. Properties of this mortar or concrete depend mostly on clinker phase composition which is influenced by foreign ions in raw meal, among other things. This work is focused on the influence of barium compounds on the formation of Portland clinker and Portland cement respectively. Cement with a content of barium could be used as a binder resistant to different types of radiation. Nowadays, other shielding materials are common such as leaded glass or a plate of lead. However, for some applications lead is not the best choice. To protect a whole building or to build a shelter resistant to radiation would be easier and less destroyable to use concrete with barium. A small research on clinker with a content of barium has shown the effect of barium ions on clinker composition. If barium ions are a part of raw meal, barium incorporates into crystal lattice of alite, belite and interstitial phase. Some authors found no barium in ferrite, only in tricalcium aluminate [1]. Due to a different crystal lattice of clinker minerals, different amount of barium ions can substitute calcium ions or only incorporate into the lattice. Belite lattice can incorporate more foreign ions than alite lattice. The ratio of barium in alite and barium in belite remains the same at different percentage of barium in clinker [2]. Juel et al. [2] studied presence of BaO in clinker minerals and they found that the highest content is present in belite and melt, in alite there is only a small amount. The amount of BaO increases in every mineral linearly with increasing content of BaO in the whole clinker. Ba2+ ions substitute Ca2+ ions and an appropriate amount of free lime is formed. However, after exceeding a certain concentration of BaO, barium ions inhibit crystallization of alite or decompose alite in favour of belite and free lime. Some authors were looking for a miscibility limit of barium in alite, Kurdowski and Wollast [3] determined it as 2.96 wt. % at 1600 °C. Appendino and Montorsi [4] tried another way by finding composition of compound formed at 1450 °C (Ca1,86Ba0,14SiO4), and they established that tricalcium silicate is not capable of dissolving quantities of BaO far in excess of 1 mol % (1.99 wt. %). Katyal et al. [1] prepared samples of pure alite and its solid solutions with different amount of BaO. Free lime increases slowly with increasing amount of barium in these samples up to a certain amount of BaO, after 1.85 wt. % free lime starts to increase rapidly. By recalculation of wt. % to mol %, it is clear that Ba2+ ions substitute Ca2+ ions up to around 1.85 wt. % of BaO in the sample. At higher values of BaO, alite formation is inhibited or it causes alite decomposition with formation of free lime and belite. Addition of barium induces changes in crystal structure. Barium ions are much bigger than calcium ones (1.43 Å and 1.06 Å respectively.), thus dimensions of crystal lattice are changed. 0.1 – 4 wt. % of BaO in alite stabilizes other polymorphic modifications, consecutively T1, T2 a M1 [1]. In belite structure, high temperature modifications are more stable, firstly Į’ (ĮL’ and ĮH’) then Į [2]. A small amount of BaO raises strength of concrete after 28 days due to higher reactivity of C2S. With 0.3 wt. % BaO, the strength increases by 20 %, 0.5 wt. % causes higher strength by 10 % and with 0.7 wt. % BaO, there is no change in reactivity [5]. BaO exists only as a part of a compound, in this work, BaCO3 and BaSO4 is used. Therefore, the clinker is influenced by the whole compound. Pure barium carbonate decomposes around 1098 °C [6], BaO is probably present in mineral formation separately without CO2. Pure BaSO4 melts first around 1580 °C, BaSO4 influences probably mineral formation as a whole compound or with an effect of SO3 [7]. SO3 in raw meal regulates modification and percentage of alite in clinker. Usually, alite is present in industrial clinker in M1 or M3 modification or its mixture. The presence of SO3 stabilizes M1 modification and with a low alkali content, alite : belite ratio decreases and crystal size of alite grows [2,8,9]. Substitution of silica ions for sulphate ions is four or five times easier in belite than alite and it is accompanied by two aluminium ions. AlO45and SO42substitute SiO44-. With higher amount of sulphate ions, content of belite increases so as incorporation of aluminium ions [10]. Simultaneously, amount of C3A is reduced [10,11]. The effect of SO3 can be compensated by MgO or alkalis (Na2O and K2O) [8,9,10]. The difference in mineral ratio has an effect also on cement quality. Cement with low alite, tricalcium aluminate and alkali content gains its strength slower, however, it has better sulphate resistance [10,11]. 44 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 In SO3 presence, viscosity of clinker melt and surface tension are reduced – ability for movement is better but calcium oxide dissolves slowly and it is immediately transported far away. Oversaturation of calcium oxide is necessary for alite crystallization but it occurs later at sufficient high concentration of CaO. Because of low viscosity, alite crystallizes then very quickly and reaches great dimensions [8,12]. The size of alite crystals depends on weight percentage of SO3, some authors found a dependence but only for a small percentage of SO3 [10]. BaCO3 can be used also as an addition to cement to inhibit sulphate attack. Sulphates entering the pore network of the specimen are fixed in the form of barite by BaCO3 presence while calcite from portlandite and barium carbonate is formed. The reaction goes according to equation (1) [6].   o  2OHNaCaCOBaSOCa(OH)BaCO 34 OHSONa 23 2 2 4x (1) In addition, calcite volume is greater than the volume of the reagents by 11 %, mortar porosity is reduced and thereby hindering aggressive agent access. BaCO3 reduces the rate of cement hydration [6]. 2. Materials and methods Clinker samples were prepared from primary raw meal that was calculated to get a reference clinker with 60 – 65 wt. % of alite, 20 wt. % of clinker melt and with dominative C4AF in melt. Raw materials were limestone (from ýertovy Schody), SUK (specially ground quartz), Al(OH)3, Fe2O3 and as alkalis Na2CO3 and K2CO3 were used. As a source of BaO, BaCO3 and BaSO4 (per analysis) were chosen. One raw meal was prepared with no barium and 12 raw meals with two different sources and gradual amount of BaO – from 0 to 5 wt. %. A list of clinkers prepared from these raw meals is in Table 1. Table 1. Samples of clinker with their content and source of BaO. Source of BaO Sample name BaO (wt. %) Source of BaO Sample name BaO (wt. %) – S0 0.0 BaCO3: 0.5C 0.5 BaSO4: 0.5S 0.5 1C 1.0 1S 1.0 2C 2.0 2S 2.0 3C 3.0 3S 3.0 4C 4.0 4S 4.0 5C 5.0 5S 5.0 2.1. Equilibrium burning process 13 different raw meals were burnt by an equilibrium burning process. Principle of this process is elimination of free lime and getting homogenous structure where it is easier to determine the ratio of clinker minerals and therefore, suitability of primary raw meal. The process consists of two burnings. The first one is with temperature rate 10 °C/min, isothermal stamina at 1100 °C for four hours and fast cooling. Burnt material is milled and burnt again with temperature rate 10 °C/min, isothermal stamina at 1450 °C for four hours and fast cooling. The prepared clinker is appropriate for microscopic point counting method if it is required [13,14]. The ratio of clinker phases was evaluated by microscopic point counting method and controlled by XRD Rietveld analysis (X-ray diffraction). The first method involves grinding and polishing of the clinker, the section is etched by vapour of acetic acid to get different interference colours of clinker phases. The prepared sample is placed in semi-automatic table and 2000 points are distinguished in reflected light by petrographic polarizing microscope. 45 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 2.2. Common burning process Samples burnt by equilibrium burning process were studied and six of them were chosen for common burning process – S0, 1C, 2C, 4C, 2S, and 4S. Lime saturation factor (LSF) of the sample 4C and 4S was recalculated to get a clinker with sufficiently low content of free lime (max. 2 wt. %). The appropriateness of these samples is commented in results and discussion. Common burning process consists of temperature rate 10 °C/min, isothermal stamina at 1450 °C for two hours and fast cooling. This type of burning process is very similar to burning in rotary kiln. Structure is not as homogenous as in the case of equilibrium burning and there can be a small percentage of unreacted free lime. Polished coloured sections were prepared again and they were studied in reflected light by petrographic polarizing microscope. Mineralogical composition was controlled by XRD Rietveld analysis. 2.3. Preparation of cements Three samples (4C and 4S with recalculated LSF and S0) were chosen for preparation of cements – C-S0, C-4C and C-4S. Raw meals were prepared in amount of 3 kg and burnt by common burning process. Clinkers were crushed, milled together with 5 wt. % of gypsum to get specific area of 400 m2/kg. Elementary testing of cements was done mainly according to EN 196-1 and EN 196-3 and it concluded determination of normal consistency (according to Vicat), initial and final setting time (according to Vicat), soundness (Le Châtelier apparatus), compression and flexural strength after 2, 7, 28 and 90 days, specific surface (Lea and Nurse method according to British standard B. S. 12:1958) and density (according to ýSN 72 2113). 3. Results and discussion 3.1. Equilibrium burning process By equilibrium burning process, 13 clinkers with different amount and source of BaO were burnt. Mineral composition was evaluated by microscopic point counting method and by XRD Rietveld analysis. The ratio of minerals in each clinker determined by microscopic point counting method is shown in Fig. 1. The ratio of clinker phases remains almost the same up to around 2 or 3 wt. % of BaO in case of BaCO3 and 3 or 4 wt. % in case of BaSO4. Up to a certain amount, barium ions probably replace calcium ones in clinker phases and appropriate mol % of free lime is formed. After that, barium inhibits alite formation or it causes alite decomposition to belite and free lime. At 4 wt. % of BaO with BaCO3, nearly no alite is formed in the clinker. Clinker minerals were determined also by XRD and the ratio was calculated by Rietveld analysis. However, results from Rietveld analysis are not reliable using higher amount of BaO, Ba2+ ions are much bigger than Ca2+ so they probably change dimensions of crystal lattice. By observing the samples on petrographic polarizing microscope, approximate ratio of clinker phases and the difference in crystal size can be seen (Fig. 2). With higher amount of BaSO4, the amount of alite crystals decreases and their sizes increase. The presence of SO3 leads to lower viscosity and surface tension of melt [12]. Oversaturation of calcium oxide occurs much later, then alite crystals grow quickly and reach great dimensions [8]. Comparing clinker without SO3 (Fig. 2a) and with 2.6 wt. % of SO3 (i.e. 5 wt. % BaO with BaSO4, Fig. 2b), alite crystals are several times bigger with SO3. Pictures were taken in reflected light and the surface was coloured by a vapour of acetic acid. 3.2. SEM with EDS Scanning electron microscope with energy-dispersive spectrometer (SEM with EDS) was used to evaluate the amount of barium in clinker phases and in different samples. By correlative microscopy, two pictures of the same area of clinker 3C were taken and compared. By optical microscopy, clinker phases are distinguished and by SEM with EDS, presence of barium can be displayed, see Fig. 3. It can be seen that barium ions are incorporated 46 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 mainly into belite and clinker melt, there is only a small amount of barium in alite and probably no barium in free CaO. Fig. 1. Phase composition of clinkers with BaCO3 and BaSO4. a b Fig. 2. (a) Microstructure of clinker S0, blue crystals – alite, light brown grains – belite, light areas between grains – melt, black areas – open pores; (b) Microstructure of clinker 5S, blue crystals – alite, light brown grains – belite, multi-coloured small grains – free lime, light areas between grains – melt, black areas – open pores. Presence of barium in main clinker phases – alite and belite – was measured in detail and it is shown in Fig. 4a. It can be seen that barium concentrates in belite but the ratio of barium in alite and barium in belite remains almost the same. The difference between barium in alite and belite is caused by a different crystal lattice. C2S can contain more foreign ions than C3S. 3.3. Thermo-gravimetric and differential thermal analysis of clinker phases (TG-DTA) Temperature of the main clinker processes was monitored for five samples by TG-DTA. Influence of increasing BaO on formation of belite (exothermic reaction) and clinker melt (endothermic reaction) was observed. The influence is shown in Fig. 4b. It can be seen that increasing amount decreases the temperature of both processes. More significant change occurs using carbonate. The change in carbonate and sulphate is probably caused by different decomposition temperature of the two compounds. Pure BaCO3 decomposes before formation of clinker 47 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 melt around 1098 °C [6], therefore, BaO can affect the formation separately. Pure BaSO4 melts first around 1580 °C [7], it is probably present as a whole compound during formation of belite and crystallization of alite. ab Fig. 3. (a) Microstructure of clinker 3C, grey crystals – alite, brown grains – belite, blue grains – free lime, light areas between grains – melt, black areas – open pores; (b) Clinker 3C – mapping of barium in the same area as (a), evaluated by EDS, green dots – barium. a b Fig. 4. (a) BaO in alite and belite evaluated by EDS; (b) Temperature of clinker phases formation of 5 samples evaluated by TG-DTA. 3.4. Common burning process and preparation of cements Some clinkers prepared by common burning contain a small amount of free CaO as it was expected but none of them has exceeded 2 wt. %. Clinker 4C contained only belite, melt and no lime because of LSF reduction, see Fig. 5a. The structure of all samples was particularly homogenous even by using this type of burning. Higher amount of SO3 increased size of alite crystals, whereas free lime was present near big alite crystals and sometimes inside them, see Fig. 5b. Alite crystals grow in direction to belite and to free lime, therefore, free lime inside alite signs that the crystal did not manage to finish crystallization. Clinkers S0, 4C and 4S were used for preparation of cements – C-S0, C-4C and C-4S. Specific surface, density, normal consistency, soundness and initial and final setting time are shown in Table 2. Fig. 6 represents compression and flexural strength of cements. 48 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 ab Fig. 5. (a) Ratio of clinker phases in samples used for cement preparation; (b) Microstructure of clinker 4S, light blue crystals – alite, dark brown or dark blue grains – belite, small multi-coloured grains – free lime, light areas between grains – melt, dark large areas – open pores. a b Fig. 6. Samples of 3 different cements after 2, 7, 28 and 90 days (a) compression strength; (b) flexural strength. Table 2. Elementary testing of cements. Name of a test C-S0 C-4C C-4S Specific surface (m2/kg) 403 399 394 Density (kg/m3) 3131 3286 3226 Normal consistency (%) 28.3 29.3 28.3 Soundness (mm) 1.0 3.7 0.7 Initial set (h : min) 2:00 0:20 2:40 Final set (h : min) 3:30 0:40 3:50 Elementary testing of cements has shown that barium effects density of cement, barium atoms are heavier than others, therefore the density of samples with barium is higher. Normal consistency remains approximately the same because of cement milling to similar specific surface. Soundness does not reach the value of 15.0 mm, cements comply with standard EN 197-1. Lower alite : belite ratio leads to reduction of cement strength, furthermore, properties of cement paste can be negatively influenced also by higher amount of barium. Then C-4S does not reach 49 Anežka Zezulová et al. / Procedia Engineering 151 ( 2016 ) 42 – 49 the strength of C-S0 even in 90 days, C-4C consists only of belite and melt, therefore, its strength stays relatively low. 4. Conclusions This article is focused on the influence of barium compounds on Portland cement. Samples of clinker with different amount (0 – 5 wt. %) of BaO were prepared using BaCO3 or BaSO4 and some of them were used for cement preparation. Several analyses were done to evaluate changes in clinker and cement. It was found that Ba2+ ions substitute Ca2+ but only up to a certain percentage of BaO in the sample, a higher amount inhibits alite formation or leads to alite decomposition to belite and free lime. Presence of SO3 also influences size of alite crystals, alite crystals are several times bigger in clinker with 2.6 wt. % of SO3 using BaSO4 (i.e. 5 wt. % BaO). A high temperature modification of belite Į’-C2S is stabilized by increasing amount of BaO. Presence of barium in clinker phases was analysed by SEM with EDS. The highest amount of barium was found in clinker melt where aggregates are formed and in belite. There is only a small amount of barium in alite and probably no barium in free lime. The great difference of barium content in alite and belite is due to their crystal lattice. Ratio of barium in alite and barium in belite remains the same with different amount of BaO in the whole sample. With increasing amount of BaO, formation temperature of clinker phases is decreasing. Curve of TG-DTA shows bigger differences in the case of carbonate which occurs probably due to different decomposition temperature. BaCO3 decomposes before formation of clinker phases, BaO influence the formation separately. BaSO4 acts as a whole compound. Prepared cements underwent elementary testing. Density is higher because of heavier barium, normal consistency, specific surface and soundness are normal. Reduction of cement strength occurs due to low alite : belite ratio and higher amount of barium. Cement C-4C (4 wt. % of BaO, source BaCO3, lower LSF) consists only of belite and melt, therefore, its strength stays relatively very low. Despite some differences from industrial Portland cement, the sample C-4S (4 wt. % of BaO, source BaSO4, lower LSF) contains enough barium, nearly no free CaO and clinker phases are in an appropriate ratio. The sample C-4C differs in mineral composition, it consists only of belite and clinker melt, its strength is not high enough but it can be used as cladding. Testing of these samples will continue with analysis of shielding ability. Acknowledgements This research was done within the project No. 16-08959J, financed by the Czech Science Foundation. References [1] N.K. Katyal, S.C. Ahluwalia, R. Parkash, Effect of barium on the formation of tricalcium silicate, Cement Concrete Res. 29 (1999) 1857– 1862. [2] I. Juel, E. 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