The Effects of 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 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 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 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 . 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, (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, (Na ) and + (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. As

the CBH6 crystal layers covering the surface of cement particles dissolve, hydration reaction will be rapid and the Vol. 17, Ë'á 7, 20/0 The Effects of Boron Compounds on the Properties ofCementitious Composites

chemical cycle mentioned above will be renewed. If adequate pore water that would sustain hydration is present in the environment, these reversible reactions will continue until the end of cement hydration (Figure Id).

As the dissolvable B2O3 concentration in the pore solution increases, the dissolvability of CaO increases at the beginning. However, after a period of time, concentrations of Ca2+ cations and OH'anions in the pore solution decrease to form CBH6. As a result, CBH6 rapidly covers the surface of cement grains and the hydration stops. As B2O3 concentration increases, the dissolvability of CBH6 is also retarded. This phenomenon weakens the bonds forming among the cement grains. Therefore, as the setting time of cement paste extends after the hydration reaction depending on the B2O3 concentration, strength of the cement matrix weakens.

B2O3 concentration is the most important factor affecting the product quality (fire resistance, neutron shielding, antibacterial medium etc.) of the cementitious composite containing boron compound. In such studies, the effective

B2O3 concentration should be determined primarily depending on the purpose. The concentration depends on the B2O3 percentage of the boron compound. As B2O3 concentration of the compound decreases, hardening retardation and strength loss of cementitious composites will improve proportionally. However, some benefits of B2O3 to cement composites such as fire resistance, neutron shielding, antibacterial medium etc. may decrease in turn.

In this research, the effects of boron trioxide/cement dosage (B2O3/c) ratio on the initial-finish setting times, pH, electrical conductivity and temperature values of fresh cement mortar were investigated in detail. The formation of

CBH6 barrier layers on surfaces of the cement grains should be delayed for attaining optimum setting time and the strength value at the purpose-oriented B2O3/c ratio. For this purpose, the effects of using B[OH]3 as a boron source by different methods on cement hydration were investigated. B[OII]3 was added as a powder material into mortar mixtures in the direct usage method and as impregnated to LWA in the indirect method. Furthermore, the effects of B[OH]3 usage at different B2O3/c ratios in the direct and indirect methods on 3, 7, 28, 90-day compressive strengths and 28-day flexural strength of hardened concrete were analyzed. The obtained results were compared with Portland cement (control) and BAB cement concrete samples. In addition, the equations for estimating initial and final setting times of fresh mortar, the compressive strengths and 28-days flexural strength of hardened concrete depending on the boron trioxide/cement ratios were derived by analyzing of experimental data. Finally, the effect of B2O3/c ratio on the hydration and micro-structure of concrete were examined through thin section and SEM images of the concrete samples.

3. MATERIAL AND METHODS

3.1. Raw materials

B(OH)3 obtained from Eti Mine Works, BAB cement from G ltas Cement Factory and B2O3 derived from B(OH)3 in laboratory environment were used in this study. The types of binders, aggregates, boron compounds used in this study and also the meanings of the symbols belonging to 12 different concrete samples and the usage methods of boron compounds are given in Table 1.

Dry-powder B(OH)3 were directly added to G-l, BO and BS samples at boric acid / cement ratios of 0.5%, 1.0%,

1.5% and 2.0% by weight. For G-II samples, B(OH)3 solutions were prepared at boric acid/water ratios of 5%, 10%,

15% and 20% by weight. 1-2 mm grain-sized LWA was kept in the prepared B(OH)3 solutions for a period of 24 hours. LWA was removed from the solution after reaching saturated-dry surface conditions. The amount of LWA added to mortar mixtures was calculated by Equation 1. In addition, the physical and chemical properties of LWA are given in Table 2. Metin Davraz Science and Engineering of Composite Materials

Table 1 The symbols of concrete samples, types of cement, aggregate and boron compounds, the ratios

of BA/c and B2O3/c, usage methods of boron compounds

Sample Groups BA/c B20j/c Binder Aggregate Borate Usage Ratio5 Ratio6 Type1·2 Type3'4 Type Method Symbol Explanation (%) (%)

Cl Control samples CEM I L - - - 0 -

B5 Samples added B[OH]3 CEM I L - B[OH]3 0.5 0.289 Direct U

Q. BIO" Samples added B[OH]3 CEM I L - B[OH]3 1.0 0.577 Direct u1 B15 Samples added B[OH]3 CEM1 L - B[OH]3 1.5 0.866 Direct

B20 Samples added B[OH]3 CEM I L - B[OH]3 2.0 1.154 Direct

BO Samples added B2O3 CEM I L - B203 - 1.143 Direct

Samples used boron BAB BAB L - - 1.120 Direct cement

C2 Control samples CEM I L + A.S. - - 0 -

g- BS5 Samples added B[OH]3 CEM I L + A.S. B[OH]3 0.5 0.289 Indirect u BS10 Samples added B[OH] CEM I L + A.S. B[OH] 1.0 0.577 Indirect Q. 3 3 2 0 BS15 Samples added B[OH]3 CEM I L + A.S. B[OH]3 1.5 0.866 Indirect

BS20 Samples added B[OH]3 CEM I L + A.S. B[OH]3 2.0 1.154 Indirect

1. Using cement type CEM 142.5 R 4. A.S. : Amorphous silica (

2. BAB : Boron modified active belite cement 5. BA/C : B[OH]3 /Cement ratio (by weight)

3. L: (Crashed) limestone 6. B2O3/c : B2O3/Cement ratio (by weight) Vol. Ð. No. l, 2010 The Effects of Boron Compounds on the Properties qfCementilious Composites

Table 2 Physical and chemical properties of LWA (amorphous silica)

Physical Properties of Amorphous Silica

Specific Apparent Oven-Dried Saturated Dry-Surface Water absorption Gravity Grain Density Grain Density (at 24 hours) 3 3 3 3 ps(g/cm ) pa(g/cm ) prd(g/cm ) prd(g/cm ) WA24 (by weight, %)

2.360 2.227 0.988 1.545 56.307

Chemical Properties of Amorphous Silica

Components % Components %

SiO2 92.48 CaO 0.31

A1203 2.6 Na20 1.08

TiO2 1.34 K2O 0.04

Fe2O3 0.09 S03 0.09

MgO 0.00 Loss on Ignition 1.85

Physical Properties of Amorphous Silica

Specific Apparent Oven-Dried Saturated Dry-Surface Water absorption Gravity Density Grain Density Grain Density (at 24 hours) 3 3 3 3 ps(g/cm ) p.(g/cm ) prd(g/cm ) prd(g/cm ) WA24 (by weight, %)

2.360 2.227 0.988 1.545 56.307

Chemical Properties of Amorphous Silica

Components % Components %

SiO2 92.48 CaO 0.31

A1203 2.6 Na2O 1.08

Ti02 1.34 K2O 0.04

Fe2O3 0.09 SO3 0.09

MgO 0.00 Loss on Ignition 1.85 Metin Davraz Science and Engineering of Composite Materials

- "1B[OH]3 / (WA24h X %w/w) (1)

"ILWA : Amount of saturated dry-surface LWA added to mortar mixture, g niB[OHj3 : Amount of B(OH)3 that should be added to mortar mixture, g

W ALwA-24h : 24-hour water absorption of LWA, (by weight %) %w/w : Dissolving material amount/ total solution amount, g/g

If 2 mol of B(OH)3 is heated to approximately 600°C;

2B(OH)3 + heat -> B2O3 + 3H2O

1 mol of B2O3 and 3 mol of water forms during the reaction. As l g of B(OH)3 heated, approximately 0.577g of B2O3 is obtained. For BO samples, B2O3 amount was calculated according to Equation 2.

MB203 = Cx0.02x 0.577 (2)

MB2O3 : Amount of B2O3 included in BO mortar mixtures (g) C : Cement dosage (g)

In BAB samples, the dosage of BAB cement used was chosen equivalent to OPC which was added as a binder. Physical and chemical properties of CEM I 42.R Portland cement and BAB cement that were used as binders are given in Table 3 717, 187. In order to increase the workability of mortar mixtures in low water/cement (w/c) ratios, super plasticizer (poly carboxylic acid) was used at proportion of 1.5% cement by weight.

3.2. Design of concrete mixtures The high dosage of cement was chosen in the design of concrete mixtures for the purpose of observing the effects of boron compounds. The w/c ratio, cement dosage, total amount of aggregate by volume and amount of super plasticizer by weight were held constant in all concrete mixture designs. Mixture proportions of concrete samples are given in Table 4.

3.3. Preparation, Curing and Testing of Concrete Samples

In order to determine the effects of B(OH)3 on initial and final setting time of cement paste, mortar mixtures were prepared at 6 different BO/C ratios between 0.00% - 1.154%. The initial and final setting times of fresh mortar samples were measured by automatic Vicat apparatus as per TS EN 196-3 /19/. The experiments were repeated three times and the arithmetic average of the three initial and finish setting times was calculated for each B2O3 ratio. Mortars whose mixture designs are displayed in Table 4 were cast in 100 mm cube, 40x40x160 mm prisms and vibrated for 30 seconds, before being kept in a curing cabinet until testing at 23 ± 1°C and 100% relative humidity (RH). 3, 7, 28 and 90-day compressive strengths of five cubic samples and, 28-day flexural strengths of three prism samples from each group were tested 720,217. Vol. 17, No. 1,2010 The Effects of Boron Compounds on the Properties ofCementitious Composites

Table 3 Physical and chemical properties of OPC and BAB cement

Chemical Properties of Clinker Physical Properties of Cement

Components (%) BAB OPC BAB OPC

SiO2 20.37 20.52 Volumetric expansion (mm) 0 1

A1;O3 4.45 4.00 Fineness (90μ) 0.1 0.10

Fe2O3 3.27 3.45 Fineness (200μ) 1.8 1.10

CaO 58.19 64.28 Specific surface area (cm2/g) 3560 3340

MgO 4.70 1.63 Initial set (min.) 220 185

SO3 3.08 2.53 Final set (min.) 265 240

3 Na,O+K2O 1.50 1.35 Specific gravity (g/cm ) 2.98 3.12

B203 1.12 0.00 Flexural strength (at 2 days) 2.5 4.5

CaO (Free) 0.63 1.81 Flexural strength (at 7 days) 4.1 5.8

L.O.I. 4.02 2.72 Flexural strength (at 28 days) 6.0 7.2

Clinker Phases (%) Compressive strength (at 2 days) 11.7 27.1

C3S - 56.66 Compressive strength (at 7 days) 23.2 39.3

C2S 66.23 17.65 Compressive strength (at 28 days) 38.6 51.0

C3A 7.86 6.33 Other Properties of Cement

C4AF 14.01 12.03 cr 0.000 0.006 Metin Davraz Science and Engineering of Composite Materials

Table 4 Mixture proportions of concrete samples

A. Mixture Water Cement Aggregate Plasticizer Boron Compound B 0 /c w/c Silica 2 3 No (1) (kg) (kg) (kg) Types (%) (kg)

Cl 0.35 245 700 1424 0.00 10.5 - 0.00

Â5 0.35 245 700 1417 0.00 10.5 B[OH]3 powder 0.289

a. uο ΒΙÏ 0.35 245 700 1410 0.00 10.5 B[OH]3 powder 0.577 U

BIS 0.35 245 700 1404 0.00 10.5 B[OHJ3 powder 0.866

Â20 0.35 245 700 1397 0.00 10.5 B[OH]3 powder 1.154

BO 0.35 245 700 1412 - 10.5 B2O3 powder 1.143

BAB 0.35 245 700 1397 - 10.5 B203 1.120

C2 0.35 245 700 1203 0.00 10.5 - 0.00

5% B[OH] BSS 0.35 245 700 1203 125 10.5 3 0.289 solution NN »* á. 10%B[OH] BS10 0.35 245 700 1203 125 10.5 3 0.577 1 solution U 15%B[OH] BS15 0.35 245 700 1203 125 10.5 3 0.866 solution

20%B[OH] BS20 0.35 245 700 1203 125 10.5 3 1.154 solution

pH, EC and T are the most important parameters demonstrating the hydration processes of cementitious mortars. CEM I 42.5 R type of Portland cement is a cement type with early high strength. Hydration of Portland cement is relatively fast compared to some other types of cements. At the beginning, temperature of fresh mortar increases rapidly due to the high level of heat released from the reaction of aluminate (C3A) and alite (C3S) with water. In addition, it could be expected that the pH and EC values increase depending on the rising concentration of Ca(OH)2 in pore water. Especially, the decrease of T and EC values can be attributed to slowing down or stopping of hydration. For the purpose Vol. 17, No 1. 2010 The Effects of Boron Compounds on the Properties ofCementitious Composites of determining this relationship, the time-dependant changes in the pH, EC and T values for each fresh mortar sample were measured by pH-EC meter apparatus (Elmetron-Multifunction 401). Furthermore, thin sections and SEM samples were also prepared from the 50 mm cube concrete samples. Hydration processes were analyzed through the microscopic images of thin sections taken by optical microscope (Nikon-Pol). Micro-cracks in cement pastes were investigated from SEM images. By evaluating the findings of the research, the effects of boron compounds on cement hydration and concrete

physico-mechanical properties with respect to B2O3/c ratios and usage methods of the boron compound have been discussed.

4. RESULTS AND DISCUSSION

4.1. Initial and finish setting times of fresh mortars

The initial and final setting times of six fresh mortar samples possessing different B2O3/c ratios are given in Figure 2.

550

ο Initial Setting Time

á Final Setting Time

150 0,000 0,200 0,400 0,600 0,800 1,000 1,200

B2(Vc ratio (%, by weight)

Fig. 2: Initial and final setting times of fresh mortars having different B2O3/c ratios

As B2O3/c ratios of fresh mortars increase, their initial (t|) and final (t2) setting times increase too. While the initial and final setting times of control mortars are 180 and 245 minutes, respectively, the same durations for mortars

possessing a B2O3/c ratio of 1.154 are 336 and 490 minutes, respectively.

The relationship between B2O3/c ratios and initial-final setting times of mortars using CEM I-42.5-R type Portland cement are given in Table 5. Table 5 The functions developed for estimating the initial and final setting times

of fresh mortars with respect to B2O3/c ratio

Functions R2

2 Initial selling time t, = [-63.675 (B203/c) + 273.87 (B2O3/c) + 267.3] 0.976

2 Finish setting time t2 = [-80.988 (B2O3/c) +217.63 (BjOa/c) + 188.78] 0.981

10 hletin Davraz Science and Engineering of Composite Materials

4.2. The changes in pH, EC and T values of fresh mortars according to B2O.}/c ratio The pH, EC and T values of fresh mortars were measured by pH-EC meter. The findings are presented in Figure 3 as a graphical illustration.

12.0

11.8 ^ ·"· .-ÆÃ.— BAB- n β " OPC ... : _rt 500 —· _ _ ,., IJ11.6· ξΐ1 6 ^ S~ u'577 "^ 11.4 Â 114 2^ ^- — — ~~ ° ' ——0.866 —^ΛΐΑ — 112 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min.) Time (min.) 2.0 2.0 ...>·"· Uro p iû: ·\1. Ê0 · C/5 55 ..^^^•-""^ _ _^~— BAB ·— — in ^ 1 0' • jil ^0.577 ' 0 • ^— ' <= è5.0· C./ i 0 50 LU tu — 1.154-^ c 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min.) Time (min.) 6en0

PC OPC" ^40 ...-° ,40 B..·· Cο C >-20 1 "-20-

0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min.) Time (mm^ fin .·- ,40 ...."PPC Ç 20 H 20·

0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min.) Time (min.)

PPC" ,40- V X '- ~.i -BO— ^j H20 r"" " I

0 100 200 300 400 500 600 Time (min.)

Fig. 3: Time-dependent variations in pH, EC and T values of fresh mortar samples

If the measurement results of fresh mortars are evaluated as a whole, the correlations of pH-EC-T values with the hydration process can be seen clearly. In the normal OPC mortar without any boron content, the cement components such as CaO, K2O and Na2O etc. reacted rapidly with mixing water and the EC value approximately rose to 1800 mS/cm due to the rapid increase in concentrations of ions such as Ca+2, Na+, K+ and OH" etc. in the pore solution. Furthermore, the boost in OH* anions also raised the pH value of pore solution to 11.9. During the time interval of 250 to 600 minutes, the temperature of normal mortar increased to approximately 52 C° levels due to occurrence of the final

11 Vol. Ð. No. 1.2010 The Effects of Boron Compounds on the Properties ofCementitious Composites setting. These three parameters showed that the cement hydration reaction took place relatively rapidly in the normal OPC mortars without any boron content. However, this situation is quite different for mortars containing boron compound (B2O3/c > 0.289%). As B2O3/c ratio increased, pH- EC-T values of the pore solution simultaneously decreased. B(OH)3 dissolved quickly within the alkaline pore solution and it reacted with OH" anions to form B(OH)4" compounds. As a result of this situation, pH value of the pore solution decreased. In addition, EC values of pore 2+ solution decreased depending on the B2O3/c ratio since B(OH)4" reacted with Ca to form CBH6. The fresh mortar mixtures including boron compounds have low temperatures compared to normal OPC mortar. This situation demonstrates that CBH6 covering the surfaces of cement grains stopped the hydration of these mortars completely or at least partially. The occasional and minor rises and falls in EC values can be attributed to the restarting of the hydration reaction by partial dissolution of CBH6 molecules and then stopping due to newly formed CBH6. Maximum temperature of BAB cement and all mortars including boron compound during the first 600 minutes of hydration is between 28 and

30°C. The maximum EC values of fresh mortars with B2O3/c ratios of 0.289, 0.577, 0.866 and 1.154% were measured as 1642, 1127,765 and 708 mS/cm, respectively. Furthermore, the maximum EC value of fresh mortar containing BAB cement was measured as 1256 mS/cm. Similarly, the maximum pH values of fresh mortars with B2O3/c ratios of 0.289, 0.577, 0.866 and 1.154% were measured as 11.70, 11.56, 11.40 and 11.31, respectively. BAB cement has the highest pH level (11.81) among all mortar samples.

4.3. The effect of boron compounds on the microstructure of cement paste

When the microscope images given in Figure 4 arc examined, structure of the hardened cement paste (CP) and aggregate particles (A) of Cl and C2 samples can be seen clearly. In addition, a significant part of cement paste of B20

Fig. 4: Microscopic images of thin sections of Cl, C2, B20 and BS20 samples

12 Metin Davraz Science and Engineering of Composite Materials sample is covered with hydration rings (HR). The hydration rings of BS 20 sample are more extensive than those of B 20 sample (Figure 4). The hydration rings of B20 and BS20 samples are a clear indication of the ongoing hydration reaction inside the cement paste structure due to high B2O3 ratios of samples. SEM images belonging to Cl and C2 samples are given in Figure 5. Micro-cracks are clearly observed in the cement paste of Cl samples without any boron content. The micro-cracks observed in B20 sample which has a high B2O3/c are shorter and thinner compared to Cl sample. This situation can be associated with the lower hydration heats of the samples containing boron compounds compared to the control samples.

Ar addu Unwersrtv EHT = 20.00 W 1QO|im Anadclu Urwersrt» EHT = 20.00 kV ΊΟΟμτη Material Sei SEng. yyp - β£) rwn Material Sei i.Er>9- WO = 8.0 rw Dale - 1 9 .iun !OOP Maa = 50D X Oat? 15 Jun OT9 M*ci= SODX

Fig. 5: SEM images of cement pastes of Cl and B20 samples

4.4. The effects ratio and the usage method of boron compound on the compressive strength of concrete

Three, seven, twentyeight and ninety-day compressive strengths based on B2O3/c ratio of concrete samples cured at 100% relative humidity are presented in Figure 6 as a graphical illustration. The compressive strengths of concrete samples are evaluated as follows: 3-day compressive strengths

For B2Oj/c ratios of > 0.30%, the compressive strengths of G-I are lower than those of G-II. The strengths of G-I and G-H with B2O3/c ratios greater than 0.80% are 1 and 19 MPa, respectively. The compressive strength of BAB samples with a B2O3/c ratio of 1.12% is 26.5 MPa on average, whereas the B2O3/c ratios of G-I and G-II samples which possess equivalent compressive strength to BAB samples are 0.45 and 0.65%, respectively. 7-day compressive strengths

For B2(Vc ratios of > 0.50%, the compressive strengths of G-I are lower than those of G-II. The strengths of G-I and G-II with B2O3/c ratios greater than 0.98% are approximately 1 and 23 MPa, respectively. The compressive strength of BAB samples with a B2O3/c ratio of 1.12% is 35.6 MPa on average, whereas the B2(yc ratios of G-I and G-II samples which possess equivalent compressive strength to BAB samples are 0.56 and 0.62%, respectively. Similarly, the strength of BO samples with a B2Oyc ratio of 1.143% is 20.12 MPa on average, whereas the B2O3/c ratios of G-I and G-II sample's possessing equivalent compressive strength to BO samples are 0.71 and > 1.143%, respectively. 28-day compressive strengths

Except for B2O3/c ratios between 0.31 and 0.70%, the compressive strengths of G-I are higher than those of G-II •with other ratios. The strengths of G-I and G-II with a BjO^c ratio of 1.154% are approximately 54 and 41 MPa,

13 Vol. 17, No. 1,2010 The Effects of Boron Compounds on the Properties ofCementitious Composites

respectively, whereas, the compressive strength of BAB samples with a B2O3/c ratio of 1.12% is 62.8 MPa on average, whereas the B;O3/c ratios of G-l and G-II possessing equivalent compressive strength to BAB samples are 0.32 and

0.55%, respectively. Similarly, the strength of BO samples with a B2O3/c ratio of 1.143% is 78.7 MPa on average. The strengths of BO samples are higher than those of G-l and G-II. 90-day compressive strengths

For B2O3/c ratios greater than 0.50%, the compressive strengths of G-I are higher than those of G-II with other ratios.

The strengths of G-I and G-II having a B2O3/c ratio of 1.154% are approximately 78.0 and 58.5 MPa, respectively. The

compressive strength of BAB samples with a B2O3/c ratio of 1.12% is 84.0 MPa on average, whereas the B2O3/c ratios of G-I and G-II possessing equivalent compressive strength to BAB samples are 0.03 and 0.27%, respectively. Similarly,

the strength of BO samples with a B2O3/c ratio of 1.143% is 83 MPa on average.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 BjOj/c ratio. % (by weight)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 100 1.10 1.20 B;03/c ratio. % (by weight)

Ï 0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 BA/c ratio. % (by weight)

° 0.00 0.10 0.20 0.30 0.40 0.50 Ο.βÏ 0.70 0.80 0.90 1.00 1.10 120 8iOj/c ratio, % (by weiqht)

Fig. 6: The 3, 7,28 and 90-day compressive strengths of samples with respect to B2O3/c ratios

14 Metin Davraz Science and Engineering of Composite Materials

Additionally, the functions that were developed to estimate 3, 7, 28 and 90-day compressive strengths of G-I and

G-II samples comprising CEM I type Portland cement with respect to different B2O3/c ratios are displayed in Table 6. Table 6 The functions developed for estimating 3,7,28 and 90-day compressive strengths

of G-I and G-II samples with respect to B2O3/c ratio

Group for 3-day R2

3 2 I fck-3 = [136.47 (B203/c) - 222.07 (B2O3/c) + 38.1 8 (B2O3/c) + 43.47 ] 0.990

3 2 II fck-j = [62.63 (B2O3/c) - 141.28 (B2O3/c) + 48.87 (B^c) + 36.25 ] 0.981 for 7-day

3 2 I fck.7 = [158.41 (B203/c) - 299.82 (B2Oj/c) + 92.04 (B2O3/c) + 49.56 ] 0.943

3 2 II fck-7 = [78.61 (BjOj/c) - 142.87 (B2O3/c) + 38.25 (B2O3/c) + 47.55 ] 0.997 for 28-day

3 2 I fck-28 = [158.41 (B2O3/c) - 299.82 (B2O3/c) + 92.04 (B2O3/c) + 49.56 ] 0.943

3 2 II fok-28 = [78.61 (B2O3/c) - 142.87 (B2O3/c) + 38.25 (B2O3/c) + 47.55 ] 0.997 for 90-day

3 2 I fck-9o = [-43.42 (B2O3/c) + 93.08 (B2O3/c) - 68.78 (B2O3/c) + 76.65 ] 0.917

3 2 II fck-9o = [121.97 (B203/c) - 255.65 (B2O3/c) + 1 19.38 (B2O3/c) + 54.52 ] 0.806

Furthermore, the ratios of compressive strengths of samples containing B2O3 to the strength of control samples were :alculated as well (Table 7). Table 7 The ratios of compressive strengths of G-I, G-II, BO and BAB samples to the strengths of control samples at different curing ages Compressive strength ratios at different curing ages (%) Sample (Compressive strength of sample with boron 1 00 / Compressive B 03/c No 2 strength of control sample) 3 -day 7- day 28 - day 90 - day Kl 0.00 100 100 100 100 B5 0.289 91 108 84 92 BIO 0.577 41 65 78 88 »•4 Ü B15 0.866 4 14 77 88 B20 1.154 4 6 72 90 BAB 1.120 62 70 83 97 BO 1.143 5 39 104 97 K2 0.00 100 100 100 100 BS5 0.289 109 102 126 115 6 BS10 0.577 80 78 110 96 BS15 0.866 36 52 81 73 BS20 1.154 2 47 70 77

15 Vol 17,Ko 1,2010 The Effects of Boron Compounds on the Properties ofCementitious Composites

Compared to the control sample (Cl), BAB samples have exhibited the highest strength improvement among all

samples containing B2O3. In terms of 3, 7 and 28-day compressive strengths, the usage of boron compounds according to indirect method is more effective compared to the direct method. However, 90-day strengths of G-l samples in direct method are higher than those of G-II samples in indirect method. In terms of 7 and 28-day strengths, the usage of boron trioxide is more effective compared to boric acid. When 3, 7, 28 and 90-day compressive strengths are taken into consideration, the optimum B2O3/c ratio is found to be < 0.577%

4.4. The effects of B2Oyc ratio and the usage method of boron compound on the flexural strength of concrete

28-day flexural strengths with respect to B2O3/c ratio of prismatic concrete samples cured at 100% relative humidity are presented in Figure 7 as a graphical illustration. As B2O3/c ratio increased, the flexural strengths of concrete samples decreased. The flexural strengths of G-I samples are higher than those of G-II samples for all B2O3/c ratios.

n Grouo Ä Grouo

02 04 06 08 1 0 1 2 B,0,/c ratio % (bv

Fig. 7: The flexural strengths of concrete samples based on B2O3/c ratio

In addition, the equations that were developed to estimate 28-day flexural strengths of G-I and G-II samples with respect to B2(Vc ratio are displayed in Table 8 below.

Table 8 The functions developed for the estimating 28-day flexural strengths

of G-I and G-II samples based on B2O3/c ratio

Group Functions R2 2 I fcf-28 = [0.720 (B2O3/c) - 1.772 (B2O3/c) + 0.929 2 II fcf-28 = [0.428 (B203/c) - 1.534 (B2O3/c) + 0.966

5. CONCLUSIONS

This study was conducted to assess the effects of boron compounds and to compare usage methods as an admixture material within the fresh and hardened concretes. The result of this study can be summarized as below:

- As B2O3/c ratio increased, initial and final setting times of fresh mortars increased.

- As the B2O3/c ratio increased, their pH-EC-T values decreased. However, although the B2O3/c ratio of BAB samples was approximately 1.20%, their pH and EC values were very close to those of control samples.

- Samples containing boric acid or boron trioxide at B2O3/c ratio of > 0.866, reached the final strength at approximately 90 days. At the same time, a significant drop in the hydration heat reduces the formation of micro- cracks in cement paste.

16 Metin Davraz Science and Engineering of Composite Materials

- In term of compressive strengths of samples, the optimum B2O3/c ratio of boron compounds such as boric acid for direct and indirect usage methods are < 0.289 and < 0.577, respectively.

- For the maximum B2O3/c ratio, the highest 3, 7 and 28-day compressive strengths were obtained from BAB cements

- As BaO3/c ratio increased, the flexural strength of samples decreased because of the extended hydration time. The

28-day flexural strengths of samples with a B2O3/c ratio of 1.154% decreased by approximately 12.5%.

REFERENCES

l.Bell S., Coveney P.V., "Molecular modelling of mechanism of action of borate retarders on hydrating cement at high temperature", Molecular Simulation, Informa Ltd., pp. 331- 356, U.K., 1998 2.Kula I., Olgun A., Sevinc V., Erdogan Y., "An investigation on the use of tincal ore waste, fly ash and coal bottom ash as Portland cement replacement materials", Cem. Concr. Res. 32, pp. 227- 232,2002 3.Targan S., Olgun A., Erdogan Y., Sevin9 V., "Influence of natural pozzolan, colemanite ore waste, bottom ash, and fly ash on the properties of Portland cement", Cem. Concr. Res. 33, pp. 1175-1182,2003 4. Saghk A., SUmer Ï., Τυης Å., Kocabeyler M.F., Qelik R.S., "Boron modified active belite (BAB) cement and its applicability for DSI projects", DSI technical bulletin Vol.105, Ankara, 2009. 5. Volkman D.E., Bussolini P.L. "Comparison of fine particle colemanite and boron in concrete for time-strength relationship", JTE, Vol. 20, Issue 1, USA, 1992. 6. Demir D., Kele§ G., "Radiation transmission of concrete including boron waste for 59.54 and 80.99 keV gamma rays", NIM-B 245, p. 501-504,2006 7. BOREN, "Bor ve ςϊηιεηΐο", http://www.eie.gov.tr/duyurular/EV/EV_etkinlik/2008_bildiriler/04-OTURUM_ve ve_AR-GE70403.pdf 8. Celik A.G., "Seliilozik izolasyon malzcmesi ve uygulama alanlan", 27. Enerji Verimliligi Haftasi Konferansi ve Fuan, Ankara, 2008 9. Bothe J. V., Brown P.W., "Kinetics of tricalcium aluminate hydration in the presence of boric acid and calcium hydroxide", J. Am. Ceram. Soc., 82, p. 1882-88,1999 10. Taylor, H.F.W., "Cement chemistry (2nd Edition)", Thomas Telford publishing, London, 1997 11. Conner, J.R., "Chemical fixation and solidification of hazardous wastes", Van Nostrand Reinhold, N.Y., 1990 12.Zhou, II. and Colombo, P., "Solidification of radioactive waste in a cement/lime mixture, waste management, pp. 164-168,1984 13. Jeffrey, J., Garner, L. and House, W., Cement as a stabilization media", Waste Management Vol. 2, pp. 359-63, 1991 H.Csetenyi, L.J. and Glasser, P.P., "Borate substituted ettringites", Mat. Res. Soc. Symp. Proc. Vol. 294, MRS Publ., Pittsburgh, PA, USA, pp. 273-278, 1993 IS.Csetenyi, L.J. and Glasser, P.P., "Borate retardation of cement set and phase relations in the system Na2O-CaO-B2O3-H2O", Adv. Cem. Res. Vol. 7 No. 25, pp. 13-19, 1995 16. Casabonne Masonnave, J.M., "Fixation of Radioelements Coming From Nuclear Wastes in Form of Insoluble Compounds or By Exchange Reactions in Concrete (in French)", PhD. Thesis, University of Dijon, France, 1987 17. "Analyses Results of Monthly (November) BAB Cement in GOLTAS Cement Co. Operation", Isparta, 2007 18. Analyses Results of Monthly (April) Cement in GOLTAS Cement Co. Operation", Isparta, 2009 19. TS EN 196-3, "Methods of testing cement-part 3: Determination of setting time and soundness", TSE, Ankara, 2003 20. TS EN 12390-3, "Testing hardened concrete-Part 3: Compressive strength of test specimens", TSE, Ankara, 2002 21. TS EN 12390-5, "Testing hardened concrete - Part 5: Flexural strength of test specimens", TSE, Ankara, 2002

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