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The effect of lead nitrate on the physical properties of cement pastes
CEMENTand CONCRETERESEARCH. Vol. I I , pp. 235-245, 1981. Printed in the USA. 0008-8846/81/020235-II$02.00/0 Copyright (c) 1981 Pergamon Press, Ltd.
THE EFFECT OF LEAD NITRATE ON THE PHYSICAL PROPERTIES OF CEMENT PASTES
N.McN. Alford, A.A. Rahman, N. Salih University of Oxford Department of Metallurgy and Science of Materials, Parks Road, Oxford OXl 3PH, England.
(Communicated by A.J. Majumdar) (Received Nov. 13, 1980) ABSTRACT Quantities of lead in the form of lead nitrate in solution have been added to cement. The effects of solution concentration and compaction of the pastes were investigated by means of strength testing and porosity analysis. Dimensional changes of paste beem~ were recorded and the presence of microcracking was detected by electron microscopy. It is shown that extensive gel formation at high Pb (NO 3) 2 concentrations causes deleterious cracking of the pastes.
Nous avons dtudi~, par mesure de r~sistance et 1'analyse de porosit6, les effets de concentration des solutions de nitrate de plomb et de compacit~ sur la ~ t e de ciment. Nons avons pu enregistr~ les variations dimensionelles ~chantillons de pate et mis en ~vidence, par microscopie electronique, la presence des microcasseurs. Nous avons montre~ qua la formation intensive de gel ~ des concentrations ~leve~s de Pb(N03) 2 conduit ~ des casseurs n~fastes de la pate. Introduction The presence of lead compounds has been observed to inhibit the rate of hardening of concrete and cement paste. Previous workers notably Lea (1), Midgely (2), Liaber (3), Tashiro (4), Tashiro et al. (5,6,7) have investigated the physical properties of ce_,~nt paste and concrete with additions of lead compounds. The strength behaviour of cements with lead additions is not clear Midgely (2) notes that at late ages {more than 7 days) the compressive strengths of mortar cubes made with lead-containing aggregates were as good as or better than cubes made with granite aggregate. Lieber (3) shows that after an initial retardation, Portland cement pastes containing between 0.i - 0.4% PbO are capable of producing strengths which are higher than those pastes without lead additions. The purpose of this paper is to investigate in closer detail the physical 235
236
Vol. I I , No. 2 N.McN. Alford, et al.
properties of cement pastes with lead additions by means of strength testing and by analysis of porosity and pore size distributions. An earlier paper (8) discussed the effect of lead nitrate on the chemical properties of cement pastes. Experimental Methods An ordinary Portland cement with a compound composition of C3S:59.6%, C2S:II.8%, C3A:8.3%, ChAF:8.8%, was mixed for one minute in a planetary motion mixer. A detailed chemical composition of this cement is given in ref. (8). Eight replicate beam~ were cast in perspex moulds I0 x i0 x 50ram. The w:c ratio used was 0.3. Lead was added by making up the pastes with aqueous solutions of Pb(NO3) 2 containing 0.15, 0.45, 1.5, 3.0, 6.0 and 12% Pb(NO3) 2 by weight of cement. A second series of beams was made with a w:c ratio of 0.25 and the paste compressed for 30 seconds i ~ e d i a t e l y after mixing at a pressure ol 140 MPa in a stainless steel compression mould. Both series were allowed to hydrate in a 100% relative humidity atmosphere at 50°C. Both sets of beams were tested when their hydration had reached 40-45% as measured by thermogravimetry. For the uncompacted beams this was i0 days and for the compressed beams, 28 days. The samples were tested for flexural strength on an Instron model i122 in a saturated surface dry condition. The distance between the outer supports was 40ram and the beams were centre-point loaded at 0.2ram rain-I . Total porosities were determined on completion of the strength tests using a water immersion method (9). The coefficient of variation for total porosity measurements was about 6% and for the strength tests about 12%. The samples' pore characteristics were analyzed by means of mercury intrusion porosimetry. After the strength testing the samples were oven dried at 105°C for 24 hours. The samples were then transferred to a desiccator and stored over silica gel until the moment of testing. This method of drying was used in preference to other methods because reproducibility appeared to be better than, for example, vacuum desiccation. Winslow and Diamond (i0) also opted for this method of drying in preference to D-drying and P-drying. A Carlo-Erba model 220 porosimeter with a pressure range of 0-196 MPa was used for the mercury intrusion porosimetry. The data were processed by means of a Fortran program written here to determine cumulative pore volume, pore radius, specific surface and the differential of volume intruded with respect to pore radius. Dimensional changes in the beams were measured by means of a dial gauge accurate to 2 micrometres. The length changes of the lead nitrate beams were normalized with respect to a series of control beams without lead nitrate additions. This took into account any length change not caused by the presence of lead nitrate, e.g. temperature variation. In fact the variation in lengths of the control beams was found to be less than 0.005%. The beams used for length change measurements were stored in a 100% relative humidity atmosphere at room temperature and were removed only for length measurement. Re sul ts Table 1 gives the total porosities, the volume of mercury intruded and unintruded for the compressed and cast beams at varying lead nitrate concentrations. Figure 1 shows the effect of lead nitrate concentration on porosity. It is seen that there is a general increase in porosity with lead nitrate concentration. There is, however, an anomaly in both the compressed and cast beams in that the increase in porosity is not smooth. There is a decrease in the total porosity at a concentration of 1.5% lead nitrate for the compacted
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237 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
TABLE 1 Porosities and Hg Volume Intruded for Cement Paste Beams Total Porosity %
% Pb (NO3) 2 0 .15 .45 1.5 3.0
33.36 32.61 29.89 30.37 31.53 29.82 34.73
compaction
6.0 12.0 0 •1 5
.45 1.5 3.0 6.0 12.0
Compacted at 140 MPa
Hg Vol intruded (% of total porosity) 34.56 35.17 35.99 35.59 29.49 32.26 31.76
15.54 16.54 18.09 16.24 17.95 19.57 23.38
22.91 19.23
17.52 21.74 22.13 18.55 14.84
beams and between 1.5 - 6% for the uncompacted beams. This anomaly is reflected inversely in figure 2 which shows the flexural strengths of the beams with respect to lead nitrate concentrations. Each point represents the mean of 8 samples tested and the variance is given by the error bars. There is an increase in strength at about 1.5% and 3.0% lead nitrate concentrations for the compacted and uncompacted beams respectively. These increases
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FIG. 2 Lead nitrate concentration vs flexural strength in strength are significantly different at the 5% confidence level with respect to strengths on either side of the strength increases. The reasons for the enhanced strengths occurring at two different lead nitrate concentrations may be explained by the different compaction pressures so that increased compaction of the paste reduces the lead nitrate concentration necessary to produce an increase in strength. With the more open pore structures of the cast beams the concentration at which the anomalous increase in strength occurs is at 3% Pb(N03) 2. Because of the reasonably fair inverse correlation between total porosity and strength it is tempting to ascribe variations in strength to variations in total porosity. We believe that this implied causal relationh~tween strength and total porosity is too simplistic and that the strength variation might be better explained by closer analysis of the nature and extent of gel formation in the presence of lead nitrate solutions. There are two possible explanations. The first rests on the propensity for the gel products to fill space more effectively at the 'op~-~m' lead nitrate concentration (i.e. where there is an increase in strength). The fact that the strengths are always higher with no admixture implies that the gel products formed in the presence of lead are intrinsically weak. Even so, filling of the interstitial space between the cement grains is important for strength development. Thus low concentrations of Pb(NO3) 2 presumably do not effectively fill space, whereas high concentrations simply flood the system with structurally weak gel. Thomas et al. (8) showed that the addition of Pb(NO3) 2 to cement caused the rapid precipitation of basic lead salts (probably incorporating both nitrate and sulphate) in the form of colloidal gel - the gel forming coatings around the cement grains and filling the interstitial space. Thomas et al. (8) used relatively small quantities of admixture (0.15 - 4.0% by weight of cement); Groves (ii) found that hydration of clinker slices in concentrated (16% wt) produced significantly increased gel formation. An alternative explanation may be that the gel formation is associated with volume changes in the cement paste which lead to microcracking. Figure 3 indicates that at a 12% lead nitrate concentration with a w:c ratio of 0.3 there is an expansion of the beams by about 0.125% over a period of 30 days.
Vol. I I , No. 2
239 Pb NITRATE, CEMENTPASTES, PHYSICAL PROPERTIES
(The mean of 4 be~mq measured for each Point, the error bars represent standard error of the mean.) ,tl
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FIG. 3 Beam expansion with 12% Pb (NO 3) 2 solution This expansion is certainly connected with microcracking and the micrographs in figures 4a and 4b show the extent of the microcracking on polished sections of two pastes.
FIG. 4a NO Pb(N03) 2 Scanning electron micrographs
FIG. 4b 12% Pb(N03) 2
A tentative analogy is drawn between the behaviour of the gel studied here and the alkali-silica gel observed in the alkali-silica reaction. In the case of alkali-silica reactivity a pessimum proportion is observed above and below which deleterious reactions are inhibited. The cause of cracking is thought by some workers to be due to expansive pressure produced by the gel product (12,13,14). There is clearly an increase in the gel product at a 12% lead nitrate concentration but more research is needed to determine the extent of gel product at lower concentrations. The gel itself was subjected to electron probe microanalysis using a Camebax electron microprobe.
240
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Figure 5 shows a clinker grain which has been traversed by the probe analysing for lead. Lead is found throughout the paste although there is a suggestion of an increase at the gel/clinker interface. Gel coatings of basic lead com nounds on the surfaces of cement grains have been observed in the TEM work of Groves (Ii) and Thomas et al
(8). A closer examination of the pore structure and development of the gel was made feasible by mercury intrusion porosimetry. Figure 6 and 7 show the FIG. 5 cumulative pore volume plotted against Line intensity for lead Pore radii of the specimens. Concentrations of 12%, 3%, 0.15% have been chosen as representative of high, intermediate and low Pb(NO3) 2 concentrations. The control, absent in Pb(N03)2, has also been plotted. Figures 8a and 8b, 9a and 9b show the differential of these curves. The curves have two main features in common. The first is that from the
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Vol. I I , No. 2
241 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
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Vol. I I , No. 2 N.McN. Alford, et al.
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Vol. I I , No. 2
243 Pb NITRATE, CEMENTPASTES, PHYSICAL PROPERTIES
cumulative plots in figures 6 and 7 the intrusion starts at approximately 75ru~ The second feature is that from the ~V/~R curves there appears to be a certain degree of bimodality. A more detailed analysis of figures 6 and 7 reveals that the pore range at which intrusion is greatest is wider for the uncompacted beams and there is a gentler increase in the slopes of the curves. The compacted beam_- are llmlted more severely to a narrower pore range at which intrusion is greatest and also the final stage of intrusion is a plateau region. There is no such plateau in the uncompacted beams and there is a suggestion that the slope continues to rise beyond 3.75nm. This tendency is particularly marked in the cast beams with Pb (NO3 )2 concentrations at 6% and 12% indicative of a well developed pore domain beyond the scope of the instrument i.e. <3.75nm. The differential plots shown in figures 8a, 8b, 9a and 9b were chosen to begin at 75nm which is where the change in the cumulative curve is greatest. Figures 8a and 8b are the %V/~R curves for the uncompacted beam=. From 0-3.0% lead nitrate the first mode is within the range 75-15nm, the second mode lies between 8-3.75nm. At higher lead nitrate concentrations the first mode lies between 25-3.75nm, the second mode remaining at 7.5-3.75nm. For the compacted beams the distribution is better defined at 40-15nm for the first mode and 7.5-3.75nm for the second mode although compaction depresses the second mode. At all concentrations the limits of the second mode are similar but its magnitude is strongly affected by solution concentration. By reference to figures 8a and 8b it is observed that as the concentration in solution of lead nitrate increases, the area under the ~V/~R curve is increased at the smaller pore radii. There is a type of balancing effect whereby the second mode is enhanced at the expense of the first. This is shown schematically in figure i0. low Pb(N03) 2 uncompacted
compacted
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FIG. 10 Soh-m~tic diagram of the differential porosimetry curves At low Pb(NO3) 2 and no compaction pressure the first mode is the principal mode peaking at about 40z~n. With high concentrations of Pb(NO3) 2 and no compaction pressure the second mode is the principal mode peaking at about 5nm. If pressure is applied to the paste these effects are largely over-
244
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g Pb(NOs), FIG. ii Effect of Pb(N03) on the specific surface of compacted and uncompacted pastes ridden and the principal mode lies at about 25nm. It is believed that the increase in porosity at finer pore radii with high lead nitrate additions is associated with the precipitation of gel, these effects diminishing when pressure is applied. Supporting evidence is given in figure ii which shows the specific surface of the pastes plotted against lead nitrate concentration. The specific surface was calculated on the basis of a cylindrical pore model and although the authors are aware of the deficiencies of the method in absolute terms, as a comparitive tool it emphasizes the important differences most effectively. Referring to figure ii it is seen that the specific surface of the compacted specimens remains fairly constant at around 3-4 m2g -I while uncompacted pastes show a marked increase in specific surface at high lead nitrate concentrations. It is believed that the uncompacted samples with their more open pore structure still have enough space available to accon~nodate the gel and that the gel has an intrinsically higher specific surface. The increase in solution concentration of lead nitrate promotes the development of the gel hence the increase in specific surface at high lead nitrate concentrations. Conclusions i. The presence of lead nitrate in cement pastes causes a general decrease in strength, with anomalies occurring at certain admixture concentrations. This may be explained in terms of the expansive properties of the gels as shown by the expansion experiment. 2. Total porosity is affected in a similar way although it is not believed that the total porosity is responsible for strength variations. Rather, we believe that strength and porosity variations are better explained by the nature of gel formation.
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245 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
3. In general, there is a bimodal pore distribution at the fine end of the porosity (<100nm). The magnitude of the two peaks is affected by Pb(NO3)2 concentration particularly in the uncompacted pastes. The compacted pastes are limited more severely to a narrower range of pore size and the effects of varying Pb(N03) 2 are not so marked. 4. Gel formation increases dram~tically with Pb (N03)2 concentration in uncompacted pastes as shown by specifice surface measurements. In compacted pastes the space available has been restricted and there is no such variation in specific surface. Acknowledgements The authors gratefully acknowledge the financial support of the Science Research Council and Imperial Chemical Industries Ltd. in this work. The authors also thank Dr. D.D. Double for useful discussions. References i. 2 3. 4 5. 6. 7. 8. 9. 10. ii. 12. 13. 14.
Lea, F.M. 'The chemistry of cement and concrete', Edward Arnold Pub.Ltd. 3rd edn. (1970). Midgely, H.G., Mag.Concr.Res. 22 (70) p.42 (1970). Lieber, W., Proc. 5th Int.Symp.Chem.Cement, Tokyo, V.II, p.444 (1968). Tashiro, C., Proc. 7th Int.Cong.Chem.Cement, Paris, V.II, II-37 (1980). Tashiro, C., Oba, J. and Akama, K., Cem. & Concr.Res. 9(3), p.303 (1979). Tashiro, C. and Oba, J., Cem. & Concr.Res. 2(2), p.253 (1979). Tashiro, C., Takahashi, H., Kanaya, M., Hirakida, I. and Yoshida, R., Cem. & Concr.Res., ~(3), p.283 (1977). Thomas, N.L., Jameson, D.A. and Double, D.D., in preparation. Alford, N.McN. and Poole, A.B., Cem. & Concr.Res. 10(2), p.263 (1980). Winslow, D.N. and Diamond, S., Journal of Materials, JMLSA, 5(3), p.564 (1970). Groves, G.W., in preparation. Stanton, T.E., Proc.Am.Soc.Civ.Eng., 66, p.1781 (1940). Hobbs, D.W., Mag.Concr.Res., 30(105), p.215 (1978). Krogh, H,, Symp. on alkali-aggregate reaction, protective measures, Reykjavik, Iceland, p.133 (1975).
Amendments
l/
Page 2, line 2 instead of "An earlier paper (8)" read
2/
"A forthcoming paper (8)"
Page 4, 5 lines from the bottom "... in concentrated
(16% wt) produced..."
should read "...in concentrated
(16% wt)Pb(NO3) 2
solution produced..."
THE EFFECT OF LEAD NITRATE ON THE PHYSICAL PROPERTIES OF CEMENT PASTES
N.McN. Alford, A.A. Rahman, N. Salih University of Oxford Department of Metallurgy and Science of Materials, Parks Road, Oxford OXl 3PH, England.
(Communicated by A.J. Majumdar) (Received Nov. 13, 1980) ABSTRACT Quantities of lead in the form of lead nitrate in solution have been added to cement. The effects of solution concentration and compaction of the pastes were investigated by means of strength testing and porosity analysis. Dimensional changes of paste beem~ were recorded and the presence of microcracking was detected by electron microscopy. It is shown that extensive gel formation at high Pb (NO 3) 2 concentrations causes deleterious cracking of the pastes.
Nous avons dtudi~, par mesure de r~sistance et 1'analyse de porosit6, les effets de concentration des solutions de nitrate de plomb et de compacit~ sur la ~ t e de ciment. Nons avons pu enregistr~ les variations dimensionelles ~chantillons de pate et mis en ~vidence, par microscopie electronique, la presence des microcasseurs. Nous avons montre~ qua la formation intensive de gel ~ des concentrations ~leve~s de Pb(N03) 2 conduit ~ des casseurs n~fastes de la pate. Introduction The presence of lead compounds has been observed to inhibit the rate of hardening of concrete and cement paste. Previous workers notably Lea (1), Midgely (2), Liaber (3), Tashiro (4), Tashiro et al. (5,6,7) have investigated the physical properties of ce_,~nt paste and concrete with additions of lead compounds. The strength behaviour of cements with lead additions is not clear Midgely (2) notes that at late ages {more than 7 days) the compressive strengths of mortar cubes made with lead-containing aggregates were as good as or better than cubes made with granite aggregate. Lieber (3) shows that after an initial retardation, Portland cement pastes containing between 0.i - 0.4% PbO are capable of producing strengths which are higher than those pastes without lead additions. The purpose of this paper is to investigate in closer detail the physical 235
236
Vol. I I , No. 2 N.McN. Alford, et al.
properties of cement pastes with lead additions by means of strength testing and by analysis of porosity and pore size distributions. An earlier paper (8) discussed the effect of lead nitrate on the chemical properties of cement pastes. Experimental Methods An ordinary Portland cement with a compound composition of C3S:59.6%, C2S:II.8%, C3A:8.3%, ChAF:8.8%, was mixed for one minute in a planetary motion mixer. A detailed chemical composition of this cement is given in ref. (8). Eight replicate beam~ were cast in perspex moulds I0 x i0 x 50ram. The w:c ratio used was 0.3. Lead was added by making up the pastes with aqueous solutions of Pb(NO3) 2 containing 0.15, 0.45, 1.5, 3.0, 6.0 and 12% Pb(NO3) 2 by weight of cement. A second series of beams was made with a w:c ratio of 0.25 and the paste compressed for 30 seconds i ~ e d i a t e l y after mixing at a pressure ol 140 MPa in a stainless steel compression mould. Both series were allowed to hydrate in a 100% relative humidity atmosphere at 50°C. Both sets of beams were tested when their hydration had reached 40-45% as measured by thermogravimetry. For the uncompacted beams this was i0 days and for the compressed beams, 28 days. The samples were tested for flexural strength on an Instron model i122 in a saturated surface dry condition. The distance between the outer supports was 40ram and the beams were centre-point loaded at 0.2ram rain-I . Total porosities were determined on completion of the strength tests using a water immersion method (9). The coefficient of variation for total porosity measurements was about 6% and for the strength tests about 12%. The samples' pore characteristics were analyzed by means of mercury intrusion porosimetry. After the strength testing the samples were oven dried at 105°C for 24 hours. The samples were then transferred to a desiccator and stored over silica gel until the moment of testing. This method of drying was used in preference to other methods because reproducibility appeared to be better than, for example, vacuum desiccation. Winslow and Diamond (i0) also opted for this method of drying in preference to D-drying and P-drying. A Carlo-Erba model 220 porosimeter with a pressure range of 0-196 MPa was used for the mercury intrusion porosimetry. The data were processed by means of a Fortran program written here to determine cumulative pore volume, pore radius, specific surface and the differential of volume intruded with respect to pore radius. Dimensional changes in the beams were measured by means of a dial gauge accurate to 2 micrometres. The length changes of the lead nitrate beams were normalized with respect to a series of control beams without lead nitrate additions. This took into account any length change not caused by the presence of lead nitrate, e.g. temperature variation. In fact the variation in lengths of the control beams was found to be less than 0.005%. The beams used for length change measurements were stored in a 100% relative humidity atmosphere at room temperature and were removed only for length measurement. Re sul ts Table 1 gives the total porosities, the volume of mercury intruded and unintruded for the compressed and cast beams at varying lead nitrate concentrations. Figure 1 shows the effect of lead nitrate concentration on porosity. It is seen that there is a general increase in porosity with lead nitrate concentration. There is, however, an anomaly in both the compressed and cast beams in that the increase in porosity is not smooth. There is a decrease in the total porosity at a concentration of 1.5% lead nitrate for the compacted
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237 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
TABLE 1 Porosities and Hg Volume Intruded for Cement Paste Beams Total Porosity %
% Pb (NO3) 2 0 .15 .45 1.5 3.0
33.36 32.61 29.89 30.37 31.53 29.82 34.73
compaction
6.0 12.0 0 •1 5
.45 1.5 3.0 6.0 12.0
Compacted at 140 MPa
Hg Vol intruded (% of total porosity) 34.56 35.17 35.99 35.59 29.49 32.26 31.76
15.54 16.54 18.09 16.24 17.95 19.57 23.38
22.91 19.23
17.52 21.74 22.13 18.55 14.84
beams and between 1.5 - 6% for the uncompacted beams. This anomaly is reflected inversely in figure 2 which shows the flexural strengths of the beams with respect to lead nitrate concentrations. Each point represents the mean of 8 samples tested and the variance is given by the error bars. There is an increase in strength at about 1.5% and 3.0% lead nitrate concentrations for the compacted and uncompacted beams respectively. These increases
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FIG. 2 Lead nitrate concentration vs flexural strength in strength are significantly different at the 5% confidence level with respect to strengths on either side of the strength increases. The reasons for the enhanced strengths occurring at two different lead nitrate concentrations may be explained by the different compaction pressures so that increased compaction of the paste reduces the lead nitrate concentration necessary to produce an increase in strength. With the more open pore structures of the cast beams the concentration at which the anomalous increase in strength occurs is at 3% Pb(N03) 2. Because of the reasonably fair inverse correlation between total porosity and strength it is tempting to ascribe variations in strength to variations in total porosity. We believe that this implied causal relationh~tween strength and total porosity is too simplistic and that the strength variation might be better explained by closer analysis of the nature and extent of gel formation in the presence of lead nitrate solutions. There are two possible explanations. The first rests on the propensity for the gel products to fill space more effectively at the 'op~-~m' lead nitrate concentration (i.e. where there is an increase in strength). The fact that the strengths are always higher with no admixture implies that the gel products formed in the presence of lead are intrinsically weak. Even so, filling of the interstitial space between the cement grains is important for strength development. Thus low concentrations of Pb(NO3) 2 presumably do not effectively fill space, whereas high concentrations simply flood the system with structurally weak gel. Thomas et al. (8) showed that the addition of Pb(NO3) 2 to cement caused the rapid precipitation of basic lead salts (probably incorporating both nitrate and sulphate) in the form of colloidal gel - the gel forming coatings around the cement grains and filling the interstitial space. Thomas et al. (8) used relatively small quantities of admixture (0.15 - 4.0% by weight of cement); Groves (ii) found that hydration of clinker slices in concentrated (16% wt) produced significantly increased gel formation. An alternative explanation may be that the gel formation is associated with volume changes in the cement paste which lead to microcracking. Figure 3 indicates that at a 12% lead nitrate concentration with a w:c ratio of 0.3 there is an expansion of the beams by about 0.125% over a period of 30 days.
Vol. I I , No. 2
239 Pb NITRATE, CEMENTPASTES, PHYSICAL PROPERTIES
(The mean of 4 be~mq measured for each Point, the error bars represent standard error of the mean.) ,tl
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FIG. 3 Beam expansion with 12% Pb (NO 3) 2 solution This expansion is certainly connected with microcracking and the micrographs in figures 4a and 4b show the extent of the microcracking on polished sections of two pastes.
FIG. 4a NO Pb(N03) 2 Scanning electron micrographs
FIG. 4b 12% Pb(N03) 2
A tentative analogy is drawn between the behaviour of the gel studied here and the alkali-silica gel observed in the alkali-silica reaction. In the case of alkali-silica reactivity a pessimum proportion is observed above and below which deleterious reactions are inhibited. The cause of cracking is thought by some workers to be due to expansive pressure produced by the gel product (12,13,14). There is clearly an increase in the gel product at a 12% lead nitrate concentration but more research is needed to determine the extent of gel product at lower concentrations. The gel itself was subjected to electron probe microanalysis using a Camebax electron microprobe.
240
Vol. I I , N.McN. Alford,
No. 2
et al.
Figure 5 shows a clinker grain which has been traversed by the probe analysing for lead. Lead is found throughout the paste although there is a suggestion of an increase at the gel/clinker interface. Gel coatings of basic lead com nounds on the surfaces of cement grains have been observed in the TEM work of Groves (Ii) and Thomas et al
(8). A closer examination of the pore structure and development of the gel was made feasible by mercury intrusion porosimetry. Figure 6 and 7 show the FIG. 5 cumulative pore volume plotted against Line intensity for lead Pore radii of the specimens. Concentrations of 12%, 3%, 0.15% have been chosen as representative of high, intermediate and low Pb(NO3) 2 concentrations. The control, absent in Pb(N03)2, has also been plotted. Figures 8a and 8b, 9a and 9b show the differential of these curves. The curves have two main features in common. The first is that from the
12 ~, P b ( N O 3 ) 2
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FIG. 6 Cumulative pore distribution for uncompacted beams
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J
pore
radius
~
J
(rim)
~'. P b ( N O 3 ) 2 ";or ~: u
-03
12 3 •I 5 0
-• l
FIG. 7 Cumulative pore distribution for compacted beams .ol u
7SO0
?SO
rS pore
radius
{nm}
~S
:*TS
Vol. I I , No. 2
241 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
~'. P=,(.o~). ""~
*45
.04.
.1 5 0
•
~ •
.03-
.02. n* ~.~ .Ol.
J
i
75
Pore
radius
FIG. ~V/~R
~
,
,
,
(.,.)
,
,.s
=,n
8a
uncompacted
beams
'X
A / \
-0]
.O0-
bNo .056
•
•
.04-
.03-
ID •02"
-Or-
' "
Pore
'
FIG. ~V/~R
'
radius
uncompacted
(..,)
'
8b beams
'
'
' ~s
~n
242
Vol. I I , No. 2 N.McN. Alford, et al.
~, PbCNO~)~
.03
--n---e---A--
"4S • IS 0
-02
.01
"o > "a
IS
Pore r a d i u s (.") FIG.
9a
~V/SR compacted beams
-03
.02
Fx\ j -:i o/o I~\ ° \
"°
A
-O1
7S
.
.
.
.
.
.
Por, rNiu, l.mb FIG. 9b ~V/~R compacted be~_-
?:S
$'Ri
Vol. I I , No. 2
243 Pb NITRATE, CEMENTPASTES, PHYSICAL PROPERTIES
cumulative plots in figures 6 and 7 the intrusion starts at approximately 75ru~ The second feature is that from the ~V/~R curves there appears to be a certain degree of bimodality. A more detailed analysis of figures 6 and 7 reveals that the pore range at which intrusion is greatest is wider for the uncompacted beams and there is a gentler increase in the slopes of the curves. The compacted beam_- are llmlted more severely to a narrower pore range at which intrusion is greatest and also the final stage of intrusion is a plateau region. There is no such plateau in the uncompacted beams and there is a suggestion that the slope continues to rise beyond 3.75nm. This tendency is particularly marked in the cast beams with Pb (NO3 )2 concentrations at 6% and 12% indicative of a well developed pore domain beyond the scope of the instrument i.e. <3.75nm. The differential plots shown in figures 8a, 8b, 9a and 9b were chosen to begin at 75nm which is where the change in the cumulative curve is greatest. Figures 8a and 8b are the %V/~R curves for the uncompacted beam=. From 0-3.0% lead nitrate the first mode is within the range 75-15nm, the second mode lies between 8-3.75nm. At higher lead nitrate concentrations the first mode lies between 25-3.75nm, the second mode remaining at 7.5-3.75nm. For the compacted beams the distribution is better defined at 40-15nm for the first mode and 7.5-3.75nm for the second mode although compaction depresses the second mode. At all concentrations the limits of the second mode are similar but its magnitude is strongly affected by solution concentration. By reference to figures 8a and 8b it is observed that as the concentration in solution of lead nitrate increases, the area under the ~V/~R curve is increased at the smaller pore radii. There is a type of balancing effect whereby the second mode is enhanced at the expense of the first. This is shown schematically in figure i0. low Pb(N03) 2 uncompacted
compacted
illgh Pb(N03)2 vncompa¢led
"o
75
00
SO Pore
40 radius
30
20
10
3'75
nm
FIG. 10 Soh-m~tic diagram of the differential porosimetry curves At low Pb(NO3) 2 and no compaction pressure the first mode is the principal mode peaking at about 40z~n. With high concentrations of Pb(NO3) 2 and no compaction pressure the second mode is the principal mode peaking at about 5nm. If pressure is applied to the paste these effects are largely over-
244
Vol. N.McN. A l f o r d ,
II,
No. 2
et a l .
20
7
15
o
U) 104
.~_
S
compacted
/
,
d
,/X
g Pb(NOs), FIG. ii Effect of Pb(N03) on the specific surface of compacted and uncompacted pastes ridden and the principal mode lies at about 25nm. It is believed that the increase in porosity at finer pore radii with high lead nitrate additions is associated with the precipitation of gel, these effects diminishing when pressure is applied. Supporting evidence is given in figure ii which shows the specific surface of the pastes plotted against lead nitrate concentration. The specific surface was calculated on the basis of a cylindrical pore model and although the authors are aware of the deficiencies of the method in absolute terms, as a comparitive tool it emphasizes the important differences most effectively. Referring to figure ii it is seen that the specific surface of the compacted specimens remains fairly constant at around 3-4 m2g -I while uncompacted pastes show a marked increase in specific surface at high lead nitrate concentrations. It is believed that the uncompacted samples with their more open pore structure still have enough space available to accon~nodate the gel and that the gel has an intrinsically higher specific surface. The increase in solution concentration of lead nitrate promotes the development of the gel hence the increase in specific surface at high lead nitrate concentrations. Conclusions i. The presence of lead nitrate in cement pastes causes a general decrease in strength, with anomalies occurring at certain admixture concentrations. This may be explained in terms of the expansive properties of the gels as shown by the expansion experiment. 2. Total porosity is affected in a similar way although it is not believed that the total porosity is responsible for strength variations. Rather, we believe that strength and porosity variations are better explained by the nature of gel formation.
Vol. I I , No. 2
245 Pb NITRATE, CEMENT PASTES, PHYSICAL PROPERTIES
3. In general, there is a bimodal pore distribution at the fine end of the porosity (<100nm). The magnitude of the two peaks is affected by Pb(NO3)2 concentration particularly in the uncompacted pastes. The compacted pastes are limited more severely to a narrower range of pore size and the effects of varying Pb(N03) 2 are not so marked. 4. Gel formation increases dram~tically with Pb (N03)2 concentration in uncompacted pastes as shown by specifice surface measurements. In compacted pastes the space available has been restricted and there is no such variation in specific surface. Acknowledgements The authors gratefully acknowledge the financial support of the Science Research Council and Imperial Chemical Industries Ltd. in this work. The authors also thank Dr. D.D. Double for useful discussions. References i. 2 3. 4 5. 6. 7. 8. 9. 10. ii. 12. 13. 14.
Lea, F.M. 'The chemistry of cement and concrete', Edward Arnold Pub.Ltd. 3rd edn. (1970). Midgely, H.G., Mag.Concr.Res. 22 (70) p.42 (1970). Lieber, W., Proc. 5th Int.Symp.Chem.Cement, Tokyo, V.II, p.444 (1968). Tashiro, C., Proc. 7th Int.Cong.Chem.Cement, Paris, V.II, II-37 (1980). Tashiro, C., Oba, J. and Akama, K., Cem. & Concr.Res. 9(3), p.303 (1979). Tashiro, C. and Oba, J., Cem. & Concr.Res. 2(2), p.253 (1979). Tashiro, C., Takahashi, H., Kanaya, M., Hirakida, I. and Yoshida, R., Cem. & Concr.Res., ~(3), p.283 (1977). Thomas, N.L., Jameson, D.A. and Double, D.D., in preparation. Alford, N.McN. and Poole, A.B., Cem. & Concr.Res. 10(2), p.263 (1980). Winslow, D.N. and Diamond, S., Journal of Materials, JMLSA, 5(3), p.564 (1970). Groves, G.W., in preparation. Stanton, T.E., Proc.Am.Soc.Civ.Eng., 66, p.1781 (1940). Hobbs, D.W., Mag.Concr.Res., 30(105), p.215 (1978). Krogh, H,, Symp. on alkali-aggregate reaction, protective measures, Reykjavik, Iceland, p.133 (1975).
Amendments
l/
Page 2, line 2 instead of "An earlier paper (8)" read
2/
"A forthcoming paper (8)"
Page 4, 5 lines from the bottom "... in concentrated
(16% wt) produced..."
should read "...in concentrated
(16% wt)Pb(NO3) 2
solution produced..."