WASTE MANAGEMENT, Vol. 9, pp. 13-17, 1989 Printed in the USA. All rights reserved.

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0956-053X/89 $3.00 + .00 Copyright © 1989 Pergamon Press plc

EVALUATION OF CEMENT MATRICES FOR THE IMMOBILIZATION OF POLYCHLORINATED BENZENES

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Muhammad Riaz* Pakistan Atomic Energy Commission, P.O. Box 1114, lslamabad, Pakistan

Edmondo Zamorani** Commission of the European Communities, Joint Research Centre--lspra Establishment, 21020 lspra (Va), Italy

ABSTRACT. Measurements of physical properties and water leaching behaviour of cement blending variable amounts of pentachlorobenzene are described. The results indicate a slight decrease (48 to 38 MPa) in compressive strength when increasing amounts (0 to 13%) of pentachlorobenzene were blended in cement. The leach tests carried out at 50°C in distilled water indicate a very low concentration of pentachlorobenzene (about 900 ng in 400 mL) in the leachate. The results of previous investigations concerning immobilization of 1,3,5-trichlorobenzene have been compared with those of pentachiorobenzene. This indicate the feasibility of using Ordinary Portland Cement as a good confinement matrix for the immobilization of 1,3,5-trichiorobenzene, pentachlorobenzene and polychlorinated benzenes in general.

INTRODUCTION

In continuation of the previous investigations con- cerning the immobilization of hazardous and toxic organic compounds in cement (1,2), measurement of physical properties, blending variable amounts of pentachlorobenzene in cement and the leaching be- haviour in water at 50°C were studied.

The chlorobenzenes are often used as interme- diates in the manufacture of certain dyestuffs and many pesticides. They are also formed from the deg- radation of other organochlorine compounds (3). Pentachlorobenzene, which represents industrial waste, was selected from the chlorinated benzenes on the basis of its high aquatic toxicity level (4). It is difficult to test all the chlorinated benzenes in terms of solidification in a cement matrix and the most suitable approach would be to test a few compounds for physical properties and leaching behaviour under simulated accidental conditions and to correlate the results. Two compounds selected were 1,3,5-tri- chlorobenzene and pentachlorobenzene. The results

REVISED 22 SEPTEMBER 1988; ACCEPTED 30 SEPTEMBER 1988. *Currently at the JRC Ispra, under a research grant from DG

XII, Commission of the European Communities, Brussels. **To whom correspondence should be addressed. Acknowledgement - - We thank Dr. S. Facchetti, Head of the

Radiochemistry Division, and Dr. De Pauw for useful discussions and guidance in this work.

concerning the immobilization of 1,3,5-TCB in ce- ment have been reported elsewhere (1).

The advantage of using a cement matrix as a co- finement of hazardous waste are well established by the works carried out mainly in the nuclear field. Apart from its low cost and stability against damage and fire, Ordinary Portland Cement can be consid- ered a very useful host material, especially in the case of industrial organic sludges where other gen- erally accepted techniques such as incineration can- not be applied. The important feature in using ce- ment as a waste confinement matrix is given by its ability to retain the immobilized waste against the water drainage.

One of the problems linked with organic waste immobilization in cement is the set retardation due to hydration retarding effects. In view of the lack of experimental data, the effect of an organic com- pound on cement rheology cannot be predicted pre- cisely. In any case, a relation between acceleration/ retardation effects on physical properties of cement containing reference organic waste would be inter- esting to postulate.

Some important parameters concerning the im- mobilization of chemical waste, influencing the phys- ical and chemical properties of cement matrix have to be investigated. Properties such as apparent (bulk) density (AD), pycnometric density (PD), pore vol-

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14 M. RIAZ AND E. ZAMORANi

ume (Vp), compressive strength (CS), pore size dis- tribution, and leaching resistance to water on cement samples containing pentachlorobenzene have been studied. To correlate the physical properties, samples of neat cement and cement containing reference waste were prepared at constant water-to-cement (W/C) ratio and under the same set of conditions.

E X P E R I M E N T A L S EC TI ON

Sample Preparation Ordinary Portland Cement 325, pentachlorobenzene and water were mixed in a predetermined ratio. An easily workable paste was obtained at a water-to- cement ratio of 0.35. Cylindrical polystyrene con- tainers of internal diameter 20 mm and 60 mm height were filled with the paste. The samples were cured for 3 days at room temperature and for 11 days at 60°C and 98% relative humidity, as previously described (5). After the curing period, the plastic moulds were removed and the samples were cut on a parallel blade cutting machine to obtain a final sample height of 35 mm.

Analytical Procedure Pentachlorobenzene from aqueous leachate was ex- tracted using a procedure previously applied to de- termine the amount of 1,3,5-tr ichlorobenzene. Leachate samples of 200 mL were drawn through a SUPELCO (USA) SPE LC-18 cartridge (0.5 g, 6 mL). Acetone was used to desorb pentachloroben- zene from the LC-18 cartridge. Analyses were per- formed by using a Carlo-Erba gas chromatograph equipped with an SP 2330 capillary column con- nected to a Zab-2F mass spectrometer and a VG data system. Details of the analytical procedure are de- scribed elsewhere (1).

Measurement of Physical Properties In order to evaluate the influence of pentachloro- benzene on cement matrix, physical properties such as apparent/bulk density (AD) and compressive strength (CS) were measured on wet cylindrical spec- imens of dimension H = 35 mm, D = 20 mm. Pyc- nometric density (PD), pore volume (Vp) and pore size distribution were measured on sample granules (2.83-3.36 mm) dried overnight and maintained in a silica gel container. The pycnometric density was determined by filling the pores with distilled water in pycnometric bottles under vacuum.

A hydraulic press from "Scientific Instrument" (Milano, Italy) was used in the measurement of com- pressive strength. The cumulative pore volume and pore size distribution were determined by a modified AG 60 Mercury Intrusion Porosimeter (MIP) from Carlo-Erba (Milano, Italy) at a maximum pressure of 100 MPa. In calculating the pore radii, a mercury

TABLE 1 Physical Properties Measured on Neat Cement

and Cement Samples Blended with Pentachlorobenzene at W/C = 0.35

SO/C AD PD Vj,(MIP)

Neat cement 2.05 2.496 0.089 O. 05 2.01 2.482 O. 089 O. 10 2.01 2.450 O. 111 0.20 2.03 2.373 0.116

W/C = water-to-cement ratio by weight. SO/C = Reference solid organic waste-to-cement ratio by weight. AD = apparent (bulk) density (g.mL ~). PD = pycnometric density (g.mL-E). Vp(MIP) = volume of pores determined by Mercury Intrusion Porosimetry (mL.g- ') .

contact angle of 141 ° was assumed. All the physical properties were measured on neat cement samples and cement samples containing reference organic waste, according to the procedure described previ- ously (5,6). The results are reported in Table 1, and Figs. 1 and 2. A comparison of the values of com- pressive strength, measured on cement containing 1,3,5-TCB from previous investigations and pen- tachlorobenzene from present work, is given in Fig. 1.

Leach Tests Static leach tests on cement samples (H = 35 mm, O -- 20 mm) containing pentachlorobenzene were carried out at 50°C in closed containers at a solid surface-to-liquid volume ratio S/VL = 7 m ~. The cement samples used for leaching experiments con- tained pentachlorobenzene corresponding to a ref- erence solid organic waste-to-cement ratio by weight (SO~C) = 0.20. Several polyethylene bottles con- taining one cement sample each were filled with 400 mL of distilled water. At the end of the leaching period, the cement sample was removed from the

! sg

-r- 40

z 30

O9

, . 20

~ lO r r

0 0 o

Neat Cement

~ ° ~ ° ~ o 1 ~ Pentachlorobenzene [ ] ~ (W/C = 0.35)

1,3,5-Tric hlorobenzene (W/C = 0.40)

0.05 0.10 0.15 0.20

SO/C " - "

FIGURE 1. Variation of compressive strength (CS) vs. Reference Solid Organic Waste-to-Cement Ratio (SO~C).

EVALUATION OF CEMENT MATRICES

PORE DISTRIBUTION: W/C 035-107 EXPERIMENTS: P297

100

8O E ~ 60 O ,_1

~ 4o a

20

0 I I

EO 2 5

I

1

E1 2 5 E2 2 5 E3 2 5 E4 R (nm)

ILl

-- I O

1.1,.I n -

O Q_

PORE DISTRIBUTION: W/C035 SO/C020-082 EXPERIMENTS: P25t

100

80 ~ tud~

60 _J

(b) 0 :~ 4o

0 20 / ~ , ~ _ _

o ' ' 5 s EO 2 5 E1 E2 E3 E4 R (nm)

FIGURE 2. Cumulative percent of pore volume and pore size distribution vs. pore radius (R) for (a) neat cement; (b) cement containing pentachlorobenzene at S O / C = 0.20.

container. The leachate was cooled to room tem- perature and analyzed to determine pentachloroben- zene, calcium and pH. The results of the leaching of pentachlorobenzene and calcium are reported in Ta- ble 2 and Fig. 3, respectively. The temperature of 50°C for leaching experiments was selected by as- suming a possible scenario for waste confining re-

T A B L E 2 Leached Amounts in 400 mL Water of

Pentachlorobenzene Immobil ized in Cement at SOl(? = 0.20, W/C = 0.35 and T --- 50 *(2

Leaching time Amount leached (days) pH (ng)

1 11.68 400 3 11.90 550

14 12,15 850 30 12,25 980 59 12.30 910 98 12.20 850

120 12.28 880

Average amount of pentachlorobenzene per ce- ment sample = 2.86 g. Average amount of calcium per cement sample = 6.23 g.

10 -1

15

! (~ 10-;

10-3 101 102 103

TIME (DAYS)

FIGURE 3. Experimental calcium leached fraction (C/Co) vs. time (days) from cement samples containing: Q 1,3,5-Trichloro- benzene, [] Pentachlorobenzene, at S O / C = 0.20, T, = 50 °C.

pository and to compare the leaching results previ- ously measured at this temperature.

D I S C U S S I O N OF T H E RESULTS

Physical Properties As the internal structure of cement is dependent on the initial water-to-cement ratio, preparation and curing conditions, a uniform method was adopted to prepare and test the neat cement samples and the samples containing waste. This method facilitated the correlation of the modified physical properties of the cement matrix when variable amounts of waste were blended with it. A rapid set observed during the preparation of the samples in the previous studies (1,2) was retarded by adjusting the water-to-cement ratio to obtain a fluid and workable paste. For the preparation of neat cement specimens, a water con- tent corresponding to W/C -- 0.35 is considered the best condition in terms of obtaining a homogeneous workable paste, best strength of cured samples and minimum pore volume. In the preparation of cement samples with pentachlorobenzene, no set accelerat- ing/retarding effect was observed and, consequently, ratio W/C = 0.35 was used.

A gradual decrease in compressive strength with increasing ratio SO/C is observed (Fig. 1) as com- pared to neat cement. It appears from the results of the compressive strength values for waste cement samples that pentachlorobenzene and 1,3,5-trichlo- robenzene, reported in the plot of Fig. 1, present an adverse strengthening effect on cement which is pro- portional to the amount immobilized.

A slight increase in pore volume is observed (Ta- ble 1) with increasing SO/C ratio which also corre- sponds to a decrease in pycnometric density and pore size distribution. For neat cement (Fig. 2a), maxi- mum pores are represented in the range of diameter 18-31 nm. There is a slight shift in peak towards the formation of pores of diameter 20-37 nm with the

16 M. RIAZ AND E. ZAMORANI

addition of waste in cement at a ratio SO/C = 0.20 (Fig. 2b).

Leach Tests The amount of pentachlorobenzene released from the cement matrix increases in the first about 30 days of leaching period and then is more or less constant, suggesting that it follows a trend similar to that pre- viously described for 1,3,5-TCB. In the case of leach- ing of 1,3,5-TCB in water, a great increase is ob- served in the leachate during the initial period of 22 days and after this period it is almost constant, in- dicating that release of 1,3,5-TCB is dependent on cement matrix dissolution. In this case, it also seems as if the release of pentachlorobenzene follows the same mechanism of cement matrix dissolution. The constant value of pentachlorobenzene release indi- cates that it is well confined in the cement matrix. The maximum amount leached out in 120 days leach- ing period is 980 ng, which represents a very low fraction release in view of the 2.86 g pentachloro- benzene blended per cement specimen.

The cement matrix decomposition is determined by the kinetic release of calcium, which is the main cement constituent. The effect of external water is to dissolve calcium hydroxide, which is formed dur- ing the hydration of cement along with the calcium silicate hydrate (C-S-H) gel. The experimental re- sults of calcium release from cement samples con- taining pentachlorobenzene have been applied to de- termine its kinetic release in the leachate media.

An empirical relationship has been applied in which the dependence of leached fraction C/Co versus time t is given by C/Co = Bt ~. In this relationship the value B represents leached fraction at t = 1 day. The results indicate that the release of calcium follows a time exponent value n -- 0.5 for the initial period and then n = 0.25 (Fig. 3), indicating a diffusion mechanism similar to that described in previous stud- ies (1,7). Furthermore, leaching studies are best understood taking into account information on the pore structure of the matrix from which leaching is to be carried out. Normally the leachability of a sub- stance from the matrix is dependent not only on the pore size but also on the interconnectivity of pores on a path from the sample to the bulk of the leachate solution. The pores inaccessible to MIP are also in- accessible to the leachate (8), indicating that parallel examination of leaching data with MIP scans is use- ful. This is also in agreement with our studies con- cerning 1,3,5-TCB and pentachlorobenzene. The formation of smaller pores with the addition of 1,3,5- TCB in cement matrix corresponds to a lower cal- cium fraction release as compared to the calcium release fraction from the pentachlorobenzene ce- ment samples, where there is a little shift of peak towards the formation of pores of slightly bigger di- ameter.

CONCLUSIONS

The present investigations concerning the changes in physical characteristics and leaching behaviour of the cement matrix containing reference organic waste indicate that Ordinary Portland Cement represents a good host material for pentachlorobenzene. A gradual decrease in compressive strength is observed with successive additions of pentachlorobenzene as compared to neat cement specimens. The lowest value of compressive strength obtained is about 38 MPa at a ratio SO/C = 0.20. There is not a great influence on pore diameter and pore size distribution. In the present work no shrinkage or internal defects were noticed in cement samples when reference organic waste were added.

The solubility of pentachlorobenzene in water at 25°C as reported in the literature (9) is 0.831 mg.L- and the maximum amount released in water is much lower than the solubility value. Taking into account the almost constant release of pentachlorobenzene over a long period, it could be assumed that its re- lease is independent of time and dependent on ce- ment matrix destruction/dissolution. The kinetics of cement degradation measured by calcium release and by assuming C/Co = 1 (for total matrix dissolution) in the empirical relationship C/Co = Bt", gives a matrix degradation rate of about 1.15 × 10- ~5 m.sec- (3.65 × 10-6 cm.yr -l) as compared to 5.07 × 10 -16 m.sec -~ (1.6 x 10 -6 cm.yr -~) reported (10) for neat cement. This variation in the results of cement matrix degradation might be attributed to the formation of pores of slightly bigger diameter which then cause a slight increase in the initial calcium release fraction.

While correlating the leaching results of penta- chlorobenzene, solidified in cement, with those of 1,3,5-TCB, it was noticed that the trend of calcium and waste release in water is similar in both cases. The rate of matrix dissolution/destruction in the case of 1,3,5-TCB (3.65 × 10 -6 cm.yr -1) is similar to that of pentachiorobenzene.

The results of physical properties and leaching experiments indicate the feasibility of using Ordinary Portland Cement as a good confinement matrix for the immobilization of 1,3,5-TCB and pentachloro- benzene. From the outcome of the present work it is considered safe to mention that other polychlori- nated benzenes will have similar properties when blended with cement. There could be a slight vari- ation in the leachability trend and compressive strength values, but the overall effects may be similar to those of 1,3,5-TCB and pentachlorobenzene.

R E F E R E N C E S 1. Riaz, M. and Zamorani, E. A study of the effects of 1,3,5-

trichlorobenzene solidified in cement. Cem. Concr. Res. (in press).

2. Riaz, M. and Zamorani, E. A study of the degradation of cement matrices containing 2,4,5-trichlorophenol. EUR 11619

EVALUATION OF CEMENT MATRICES 17

EN, Commission of the European Communities, Luxem- bourg (1988).

3. Sitting, M. Handbook of Toxic and Hazardous Chemicals. Noyes publication, Park Ridge, NJ (1981).

4. Wong, P. T. S., Chau, Y. K., Rhamey, J. S., and Docker, M. Relationship between water solubility of chlorobenzenes and their effects on a freshwater green alga. Chemosphere, 13:991 (1984).

5. Zamorani, E. and Mazzocchi, N. Preparation and testing of hydrated cement samples for immobilization of simulated me- dium active waste. European Appl. Res. Rept., Nucl. Sci. Technol. 5:741 (1984).

6. Zamorani, E. and Blanchard, H. Pore volume and pore size distribution of cement samples measured by a modified mer- cury intrusion porosimeter. EUR 11069 EN, Commission of the European Communities, Luxembourg (1987).

7. Zamorani, E. Water corrosion and release mechanism of ce- ment matrix incorporating simulated medium-level waste. Nuclear Technology 77:313 (1987).

8. Glasser, E P., Rahman, A. A., Macphee, D., McCulloch, C. E., and Angus, M. J. Immobilization of radioactive waste in cement-based matrices. DOE/RW/84.159 Department of Chemistry, University of Aberdeen, Scotland (1984).

9. Miller, M. M., Ghodbane, S., Wasik, S. P., Tewari, Y. B., and Martire, D. E. Aqueous solubilities, octanol/water partition coefficients, and entropies of melting of chlorin- ated benzenes and biphenyls. J. Chem. Eng. Data 29- 184 (1984).

10. Zamorani, E., Lanza, E, Serrini, G., and Blanchard, H. Water leachability of cement forms for medium-level waste immobilization. Nucl. Chem. Waste Manage. 6:197 (1986).