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The Effects of Boron Compounds on the Properties of Cementitious Composites

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The Effects of Boron Compounds on the Properties of Cementitious Composites The Effects of Boron Compounds on the Properties of Cementitious Composites Metin Davraz Department of Construction, Senirkent Vocational School, Süleyman Demirel University, 32600, Senirkent/Isparta, Turkey Tel.: +90-246-2111740. E-mail address:[email protected], [email protected] ABSTRACT The retarding and stopping effects of boron compounds at even very low concentrations on the process of cement hydration are well known phenomena. Due to this property, boron compounds are used in concrete and other cementitious composites as set retarding additives. In addition, the possibilities of usage for boron compounds in cement composites for different purposes are still being investigated by many research studies. In this research, the effects of boron compounds on initial-final setting times, pH level, electrical conductivity and temperature values of fresh cement mortar and the related retarding mechanism have been discussed in detail. Furthermore, effects of the usage methods of boric acid at different concentrations on 3, 7, 28 and 90-day compressive strengths and 28-day flexural strength of hardened concrete samples have also been investigated. The obtained results were compared with the concrete control samples comprising Portland cement and BAB cement. According to the results of this study, the indirect usage method of boric acid is more advantageous compared to the direct method with respect to 3, 7 and 28-day compressive strengths of the concrete samples. Regarding the maximum boron trioxide/cement ratio, the highest compressive strength values were obtained from the concrete samples comprising BAB cement. The equations for estimating the initial and final setting times of fresh mortar with respect to boron trioxide/cement ratios and the compressive and 28-days flexural strengths of the hardened concrete with respect to curing ages have been derived through analysis of the experimental data. Keywords: Boric acid, BAB cement, B2O3/c ratio, usage method, cementitious composites. 1. INTRODUCTION Boron compounds such as boric acid (B[OH]3) and borates extend the period of cement hydration. Therefore, boron compounds are used in concrete as efficient set-retarding additive materials, similar to hydroxylated carboxylic acid, lignins, sugar and some phosphate compounds. Borates have been used regularly in oil-well cementing operations both as conventional retarders and auxiliary admixtures for viscosity at high temperatures III. Furthermore, radioactive wastes of nuclear power plants are mixed with high borate concentration solutions and then solidified by cement to be stored underground. However, the major strength problems arising during the hardening process have not been resolved to date. Excluding these two specific fields, the usage of boron compounds with inorganic binders such as cement, slaked lime and gypsum could not become widespread because of the problems concerning hardening and the related strength so far. Vol. Ð. No. l, 2010 The Effects of Boron Compounds on the Properties qfCementitioiis Composites In recent years, many researches have been carried out for the usability of boron wastes in cement production. Kula et al. 121, have stated that the use of tincal ore waste gives rise to an improvement in the properties of Portland cement (OPC) at 1% replacement level. Although it retards setting time, it presents an opportunity as a cement replacement material up to 5% by weight (wt) of the cement 121. According to Targan et al. /3/, the 90-day compressive strength of concrete samples produced from 81-96% Portland clinker, 4% colemanite waste and 5-15% natural pozzolan materials turned out to be equivalent to 90% of the strength level of control concrete samples. They have also stated that the 28-day strength levels of concrete samples constituting of higher pozzolan content cements including colemanite waste were higher than those of the concrete samples with low pozzolan content /3/. In year 2006, production of boron modified active belite (BAB) cement from colemanite ore took place in G ltas Cement Factory (Turkey). In a research carried out on this cement type, it was stated that this product could be successfully used especially in concretes for dam construction due to its very low hydration temperature with no alite (C3S) phase. In addition, the final strength of concrete made by BAB cement was much higher than that of concrete made by OPC /4/. As several properties such as shrinkage and fire resistance of concretes and other cementitious composites containing boron compounds at convenient concentrations are improved, these products may also acquire new qualities such as radiation impermeability and antibacterial effect. In particular researches studying this subject, Volkman and Bussoline have stated that the addition of the boron compounds into concrete leads to absorption of neutrons beams and radiation of low energy Gamma rays, therefore providing an effective radiation protection 151. However, they have also emphasized that adding boron compounds into concrete mixtures seriously retards the setting time and decreases the concrete strength 151. On the other hand, Demir and Kele§ 161 have prepared concrete samples containing borogypsum and colemanite concentrator wastes. They then tested the differences in gamma rays transition energies for both the normal and boron waste added samples of concrete, observing that concrete samples including boron wastes provided effective protection against radioactive radiations 161. National Boron Research Institution (BOREN) ÐΙ stated that ÂËÂ cement's neutron absorbing capacity is 20% higher compared to OPC. Celik has also stated that cellulose insulation materials doped with approximately 20% B(OH)3 or borax exhibited 57% higher fire resistance in addition to possessing an eradication level of 99.8% against micro-organisms and insects /8/. The improvements and additional features provided by the addition of boron compounds to concretes and other cementitious composites are very closely related to B2O3 concentrations of the boron compound admixtures. Nevertheless, it is a well known and intensively researched phenomenon that as the concentration of B2O3 increases, the cement hydration slows down or even comes to a halt, which results in extension of setting period. 2. THE EFFECTS OF BORON COMPOUNDS ON CEMENT HYDRATION AND THE RETARDING MECHANISM Soluble borates such as B(OI I)3 prevent the flash setting of cement paste and retard the hardening of concrete or cementitious composite. There are only a few researches explaining the effects of soluble borates on the process of cement hydration and hardening /9/. In the studies by Taylor /10/, borates were described very briefly as compounds that retard cement hydration, possibly by means of a precipitation mechanism. In Conner's research /I I/, borates were listed as short-term setting retarders that disrupt the cement matrix. In neither of the studies was any explanation was provided about the mechanisms taking place during the hardening process. Zhou and Colombo /12/ used masonry cement, consisting of 50% (m/m) Portland type I cement and slaked lime (Ca[OH]2). By using this type of cement, they were able to immobilize up to 15% (m/m) dry boric acid (based on total mass of binder, water and dry waste). When 5 to 15% (m/m) Metin Davraz Science and Engineering of Composite Materials boric acid was added, the compressive strength was found to be at 18 to. 50% of the strength level in plain masonry cement I\2I. Jeffrey et al. /13/ found that it was quite difficult to stabilize any liquid boric acid waste with a dissolved boron concentration exceeding 12% (m/m) and containing additional lime for cement and pH control. Csetenyi and Glasser /14/ investigated the extent to which borate can be rendered insoluble by inclusion into ettringite mineral where it replaces the sulphates. They determined the range of borates in ettringites where borates replaced different proportions of sulphates and obtained products that were stable across a broad range of physicochemical conditions and had very low solubility. Therefore, they explained that the ettringite may function as a borate trap in cementitious systems /14/. Csetenyi and Glasser /I5/ investigated the effects of borate in a Na2O-CaO-B2O3- H2O system. Twentyone similar composites were prepared and then reacted in sealed containers. Solid phases were characterized and the aqueous phase was analyzed chemically for Ca, Na and B. Thermodynamic modeling of solubility relations was also carried out. Combining these two methods, they were able to determine the equilibrium characteristics of the system /15/. Taking the related researches into consideration, the effects of boron compounds on cement hydration could be summarized as below: - During the hydration reaction, calcium oxide (CaO) reacted with water (H2O) to produce calcium hydroxide (Ca[OH]2) (Figure la). - During this reaction, water in pores rapidly transforms to an alkaline solution. Concentrations of calcium cations 2+ (Ca ) and hydroxide anions (OH") in alkaline solution increase and B(OH)3 rapidly dissolves. B(OH)3 ions in mixture react with OH" ions and they form the tetra hydroxyborate (B[OH].»~) compound (Figure Ib). Afterwards, Ca* cations react with B(OH)4" 1161. Mix water 2+ Ca + 2[B(OH)4"] + 2H2O <=> Ca[B(OH)4]2. 2H2O Fig. I: A representation of the effects of boron compounds on cement hydration The precipitated calcium di borate (CBH6) compound partially or fully covers the surface of cement grains. Hydration reaction of cement particulates partially or fully covered with an impermeable CBH6 layer either completely stops or fairly retards. This phenomenon causes coagulation of cement paste and also formation of flash setting (Figure Ic). 2+ + - Due to the formations of CH and CBH6 in pore solution, Ca concentration decreases. However, sodium (Na ) and + potassium (K ) cations occur in pore solution when the alkalis (NaO, K2O etc.) are freed as a result of cement hydration, and OH" anions increase again in parallel to this situation. Depending on the increase in OH" anions, pH 2+ value df the pore solution increases again and CBH6 can dissolve once more to form Ca cations in a short time.
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