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Alkali aggregate reactivity in dense concretes containing synthetic or porous natural aggregate
CEMENT and CONCRETE RESEARCH. Vol. 19, pp. 278-288, 1989. Printed in the USA 0008-8846/89. $3.00+00. Pergamon Press plc.
ALKALI AGGREGATE REACTIVITY IN DENSE CONCRETES CONTAINING SYNTHETIC OR POROUS NATURAL AGGREGATE
by R J Collins Building Research Establishment, Garston, Herts, UK
(Communicated by C.D. Pomeroy) (Received July 12, 1988)
ABSTRACT Studies have been made into the relative importance of pore solution dilution, accommodation of gel pressures and potential reactivity of porous aggregate in the suppression of alkali aggregate expansion in concretes containing a pessimum proportion of Thames Valley sand. The results also indicate that the alkalinity of the pore solution may be a more direct measure of susceptibility to reaction than the Na~O equivalent content of the concrete. Slow reactivity in some sintered aggregates has been monitored for up to 10 years and the relatively low expansions are discussed in terms of fluxing impurities in the glassy phases. Introduction In a previous paper (I) it has been reported that the use of porous aggregate can be beneficial to the suppression of alkali-silica reaction (ASR). Very much reduced expansions were found when these aggregates replaced a dense limestone in a reactive mix containing Thames Valley sand. There was no conclusive proof that any gel due to ASR was accommodated within porous aggregate and it was considered more probable that the reaction was reduced through the dilution of alkali metal concentrations in the pore water by water absorbed in the aggregates, and/or (in some cases) by the porous aggregates themselves being susceptible to alkali attack. The work reported in this paper was carried out to provide further evidence and to investigate more fully the susceptibility of synthetic aggregates to alkali aggregate reaction. Experimental (i)
Constant total water/cement ratio
If the main effect of porous coarse aggregate in relation to ASR is to cause dilution of alkali then similar expansions should be obtained for concretes of the same total water/cement ratio, irrespective of the type of coarse aggegate present. Table I compares expansion results for constant total water/cement ratio with results for constant free water/cement ratio. 278
Vol. 19, No. 2
279 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
In order to compact the concretes at a constant total water/cement ratio a superplasticiser was used. The total alkali content (Na20 * 0.658 K20) of the superplastlciser if available would increase the Na20 equivalent of the concrete containing 700 kg/m ~ of cement by 0.4 k g / m 3 at the recommended dosage rate of I litre per 50 kg of cement. Control prisms cast with the low porosity coarse aggregate RLI did not however show any signficant difference in expansion when superplasticiser was used.
TABLE I.
EFFECT OF INITIAL WATER CONTENT ON EXPANSION DUE TO ASR Expansion measurements for concrete prisms (200 x 75 x 75 mm) after I year's storage at 38oC and 100% relative humidity. (700 kg/m 3 0 P C with Na20 equivalent 6.7 kg/m3; Coarse aggregate /Thames Valley sand : 70/30 by volume)
TOTAL W/C = 0.325 Coarse Superplasticiser Aggregate dosage I litre/ Identity Description FREE water/cement - 0.325 50 kg cement No of No superplasticiser (*double dosage) (and 24 hr Coarse absorption Aggregate Expansion at Expansion at Total I year (%) value ) I year (%) water/ prism prism cement prism!prism I 2 average ratio I 2 iaverage RLI (0.5%) Carbonlf. limestone
0.34
0.320 O.304
0.312
0.332
0.305
0.319
R4A (4.1%)iJurassic limestone
0.39
0.053 0.073
0.063
!0.169
0.149
0.159
RL3 (16%)
Lytag
0.42
0.018 0.010
0.014
0.026
0.027
0.027
RL3 (16%)
Lytag
0.023*
0.023*
Expansion of the concrete containing the porous aggregate R4A was very much increased at the lower water content indicating that the dilution theory is operative. However the expansion did not approach the value obtained for the low porosity control aggregate RLI. This would suggest that there is also relief of gel pressure by the porous aggregate possibly encouraged by stronger concrete at reduced free water/cement ratio. Examination of the concrete in thin section revealed the formation of significant quantities of gel for both RLI and R4A although less for R4A. There was only slight evidence of gel impregnation of the aggregate R4A. The fine-grained micrite matrix of some aggregate particles contained gel, but the majority of the porosity in oolites was unaffected. It is possible that gel pressures can be released merely by the existence of voids in the aggregate without growth of the gel into the pores necessarily occurring. Any alkali reactivity in the limestone coarse aggregate can be virtually excluded as it is 96% CaCO, and examination in thin section reveals impurities only of quartz and no highly reactive minerals such as opal. Expansion of the concrete containing the sintered fly ash coarse aggregate RL3 was not very much increased at the lower water content and examination
280
Vol.
19, No. 2
R.J. Collins
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Vol. 19, No. 2
281 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
in thin section did not reveal the presence of any gel or physical distress within the concrete. Because of the absence of any gel the low expansions observed are probably due to the potential reactivity of the coarse aggregate(2) which results in competition for the available alkali. Such an effect is seen when the pessimum proportion of reactive aggregate is exceeded. (ii)
Pore solution alkalinity or equivalent alkali content?
Many of the results in this and the preceding paper (I) are attributed at least in part to changes in pore solution alkalinity. A direct study of pore fluid squeezed out of the concrete could thus be beneficial but this was not attempted as there are practical difficulties for concrete with water/cement ratios below about 0.40. However, a recent review of pore solution studies (3) indicates that for concrete made with ordinary Portland cement (OPC) and no cement replacement materials, the hydroxyl ion concentration [OH-] is not far different from [OH-] calculated from Na20 equivalent divided by the initial water content of the concrete. Although the loss of pore water by hydration reactions might be expected to concentrate alkalinity, there is also uptake of alkali by the hydration products which approximately balances the concentration effect for most OPC concretes (4). The simplified calculated value is given in Table 2 for concretes containing a non-reactive coarse aggregate and a Thames Valley sand. ASR expansion of the concrete (measured at I year) reduces as [OH-] reduces except for one of the mixes containing 5.8 kg/m 3 Na20 equivalent for which a very much lower figure was obtained. Overall it appears that under these conditions [OH-] of about 800 millimoles per litre or more is required to produce a deleterious expansion of greater than 0.05%. These observations do not overrule the importance of the Na20 equivalent content of concretes in determining the susceptibility of concrete to ASR both because of the exception noted above and the fact that at a constant total water content Na20 equivalent content is linearly related to calculated [OH-]. (lii)
Suppression of expansion by potentially reactive aggregates
In (i) above it was noted that potential for ASR in a sintered fly ash coarse aggregate (Lytag) had suppressed the expansion that might have been expected for a high alkali mix containing Thames Valley sand as 30% of the aggregate. Table 3 shows how this occurs even when only small proportions of an inert coarse aggregate are substituted by Lytag. Aggregate pore volume and the theoretical concentration of alkali in the pore solution are well in the range shown in Table 2 to be associated with ASR expansion. This is similar to results obtained previously for some sandstone aggregates (I). Substitution of potentially reactive coarse aggregates in these mixes is considered to have the effect of reducing alkali concentrations In the pore water below that required for Thames Valley sand to show an expansive reaction. The above results do not indicate whether a pesslmum proportion of reactive aggregate had been exceeded and indeed whether or not the coarse aggregates themselves could give an expansive reaction. Thus a further series of concrete prisms were cast wlth inert limestone coarse and fine aggregate i n which small proportions of potentially reactive aggregates were substituted. The results are given in Table 4 for Lytag and for two
-.--
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20
30
40
50
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0
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60
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80
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90
100 fine)
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for replacement
aggregate reactive aggregate
I% by volume) of dense inert limestone
Expansions
Replacement
Figure I
0
0
Thames Valley flint gravel
0
w
*
0”
)--’
Vol. 19, No. 2
283 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
TABLE 3.
EFFECT OF LrrAG ON ASR EXPANSION Expansion measurements for concrete prisms (200 x 75 x 75 mm) after I year's storage at 38°C and 100% relative humidity (700 kg/m' OPC with Na=O equivalent 6.7 kg/m'; free water/cement ratio = 0.325; coarse aggregate/Thames Valley sand : 70/30 by volume)
Replacement of Carboniferous lime[OH-] pore s t o n e coarse Expansion at I year (%) aggregate RL1 by solution concentration Lytag RL3 (% by volume ) mmole/l* prism 1 prism 2 average 0
920
0.320
0.304
0.312
10
890
0.103
0.013
0.058
18.5
875
0.011
0.004
0.008
5O
810
0.001
o.003
0.002
* theoretical concentration calculated as detailed in Table 2.
EXPANSIONS AT DIFFERENT PROPORTIONS OF COARSE AGGREGATE
TABLE 4.
Average expansion measurements for duplicate concrete prisms (200 x 75 x 75 mm) after I year's storage at 38°C and 100% relative humidity. (700 kg/m' OPC wlth Na=O equivalent 6.7 kg/m'; free water/cement ratio = 0.325; coarse aggregate/Carbonlferous limestone fine aggregate : 70/30 by volume)
Expansion at I year (%) for replacement coarse aggregate:
Replacement o f Carboniferous lllmestone coarse
aggregate RLI (% by volume)
Lytag RL3
Sandstone R11 Sandstone R40 (Old Lawrence Rock) (Upper Haslingden Flags) (Absorption 2.0%) (Absorption 3.2%)
3
0.OO7
0.004
0.007
7
-0.005
-0.001
0.009
15
0.o13
0.002
0.010
30
0.013
0.001
0.015
0. 003
0.01 7
4O 100
0.012
0.014
284
Vol. 19, No. 2 R.J. Collins
fine-grained Carboniferous sandstone aggregates. Similar but very slight non-deleterious expansions were obtained for Lytag and sandstone R40 and virtually no expansion for sandstone R11. Results were also obtained for Thames Valley flint gravel coarse aggregate and are used in figure I to illustrate the difference between expansively reactive and potentially reactive aggregates. Results for R40 in Table 4 are omitted from figure I for clarity, but some further results for high proportions of the sandstones R11 and R40 are also included. The expansion of these latter prisms is accompanied by slight cracking and observation in thin section shows a fairly low level of gel formation typical of alkali silicate reaction. There has been little further expansion in these prisms between I and 3 years. (iv)
Further results for synthetic aggregates
The observation that Lytag can suppress the expansion that might have been expected in a concrete containing a pessimum proportion of Thames Valley sand may also be seen for other synthetic aggregates (Table 5). Because of the moderately high porosity of these synthetic aggregates it is possible that the suppression is as much due to dilution of pore water or accommodation of gel pressures within aggregate pores as it is to potentially reactive aggregate. However, when the synthetic coarse aggregate is used with crushed synthetic aggregate fines or inert crushed limestone fines there is also a small but non-deleterious expansion. This is in contrast to the results obtained for Lytag which showed no significant expansion when Lytag fines or limestone fines were used (I). Slight expansion attributed to alkali aggregate reaction has been reported for other sintered and expanded aggregates (I, 8). Recent measurements on long-term specimens (Tables 6 and 7) indicate that there is also a continuing but very slow expansion. In a few cases this has reached or slightly exceeded 0.05% which is generally considered as an approximate threshold for the initiation of cracking. It is interesting to note that the aggregates prepared by intergrinding up to a 30% proportion of pyrex glass with colliery spoil before firing have not retained the high reactivity of pyrex glass and do not show significantly more expansion than the aggregate prepared using colliery spoil on its own. The evidence so far is that sintered and expanded aggregates are not as reactive as many natural aggregates containing free silica. Recent work on the reactivity of some synthetic glass aggregates (9) supplies a possible reason for this in terms of the requirement for sufficient silica and not too much impurity (CaO, AI~O 3) to be reactive. Glassy phases in slntered and expanded aggregates will generally be fairly impure as it is the impurities which act as fluxing materials, making the slntering temperature lower than the overall melting temperature or slagging temperature. This theory appears to offer a reasonable explanation for the higher expansion observed with aggregate from spoil C7 in relation to that from C4 (Table 7). The two aggregates have almost identical physical properties and firing temperatures but reference to chemical analyses (10) shows that C4 has more fluxing materials than C7. The effect of firing temperature on the reactivity of aggregates from spoil C9, however, is less easily explained in these terms. Perhaps this is simply determined by the difference in density which results in more reactive material per unit volume for aggregate fired at the lower temperature.
Vol. 19, No. 2
285 A L K A L I A G G R E G A T E REACTION,
TABLE 5.
EXPANSION,
PORE SOLUTIONS
AAR CONCRETE PRISM TESTS WITH SYNTHETIC AGGREGATES 700 kg/m 3 0 P C with Na=O equivalent ratio 0.325; coarse a g g r e g a t e / f i n e
Coarse Aggregate - raw material, process conditions, relative density (~) and 24 hour water absorption value (wa)
6.7 kg/m3; free w a t e r / c e m e n t aggregate 70/30 by volume.
Fine Aggregate*
Average expansion (%) of duplicate 200 x 75 x 75 mm prisms stored at 38°C and 100% relative humidity
28d
3m
9m
1y
I .5y
Colliery spoil - Newdigate tip Dense aggregate process (5) and rotary kiln p - 1.70, wa = 12%
C
TV RLI
0.0O5 O.006 0.008
0.012 0.012 0.011
0.028 0.026 0.021
0.030 0.027 0.024
0.030 0.027 0.024
Mine taillngs - Abernant colliery Laboratory pre-production trial firing (6) p = 1.63, wa = 14%
C TV RL1
0.002 0.010 0.005
0.007 0.012 0.008
O.013 0.022 0.014
0.015 0.021 0.011
0.017 0.021 0.O16
* C - crushed fines from synthetic coarse aggregate TV - Thames Valley sand RLI - crushed Carboniferous limestone
TABLE 6.
ASTM C227 MORTAR BAR TESTS WITH SYNTHETIC AGGREGATES NaOH added to give NazO equivalent of 1.5% with respect to cement
Crushed and graded synthetic aggregate
BRE synthetic dense aggregate from ¢olllery spoil (ref 5)
spoil C9/1200°C spoil C9/1250°C CD/glass = 90/10 C9/glass = 70/30
Colliery spoil C10/1300°C shaft kiln Colliery spoil CIO/1100oC shaft kiln* Dredged silt S7/1060=C ( r e f 7)
Expansion (%) after storage at 380C and 100% humidity (average for 4 mortar bars) .J 28d 3m 6m ly 5y 10.5y 0.007 0.009 0.013 0.013
0.020 0.017 0.023 0.025
0.029 0.022 0.029 0.032
0.038 0.025 0.036 0.041
0.056 0.026 0.050 0.053
0.060 0.030 0.063 0.068
0.018 0.075 0.010
O.O2a 0.096 0.016
0.032 0.102 0.023
0.033 0.113 0.029
0.043 0.145 0.035
0.050 0.161 0.040
* expansion caused mainly by excessive sulphates in the underburnt aggregate (3.0% total sulphur as SO,: 1.8% water soluble SO,)
C o n c l u s i o ns
C1)
Pore solution alkalinity appears to be a more direct indicator of ASR expansion in concretes c o n t a i ning a typical reactive fine aggregate (Thames Valley sand) than the Na=O equivalent of the concrete. However for concretes with the same total water content, the c a l c u l a t e d alkalinity of the pore solution will be directly related to the NazO equivalent content of the concrete.
286
Vol. 19, No. 2 R.J. Collins
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Vol. 19, No. 2
287 ALKALI AGGREGATE REACTION, EXPANSION,
PORE SOLUTIONS
(ll)
The primary effect of porous aggregates on susceptibility to ASR is dilution of the pore solution. Some accommodation of gel within the aggregate has been observed but appears in the present work to be of much lesser importance for suppressing ASR expansion. However it is probable that the mere existence of voids in the aggregate will be sufficient to reduce swelling pressures, without growth of gel into the pores necessarily occurring.
(ill)
Some sandstones and synthetic aggregates show slight alkali-aggregate reactivity which gives relatively low expansions and can suppress alkali silica reaction involving other aggregates in the concrete. The reaction observed in sandstone produced only a small quantity of gel and may be alkali silicate reaction. For synthetic aggregates the low expansion may be due to a reduced reactivity in glassy phases which contain impurity introduced by fluxing materials. Acknowledgement
The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director. References I.
R J Collins and P D Bareham. "Alkali-sillca reaction: suppression of expansion using porous aggregate". Cement and Concrete Research 17, 89-96 (1987).
2.
I Sims and S Bladon. "An exploratory assessment of the alkall-reactlvlty potential of sintered pfa in concrete preliminary report" Proc 2nd International Conference on Ash Technology and Marketing, London (1984).
.
4.
P J Nixon and C L Page. "Pore solution chemistry and alkali aggregate reaction". Katherine and Bryant Mather International Conference on Concrete Durability (ed. John M Scanlon) ACI SP-IO0 Vol. 2, 1833-1862 (1986). H F W Taylor. "A method for predicting alkali ion concentrations in cement pore solutions". Advances in Cement Research !, 5-17 (1987). R J Collins. "Manufacture of aggregate from colliery spoil". Proc Symposium on the Reclamation, Treatment and Utilisation of Coal Mining Wastes, paper 41, Durham (1984).
5.
.
L F Parks. "Production of lightweight aggregate from washery railings at Abernant Colliery". Proc Symposium on the Reclamation, Treatment and Utilisation of Coal Mining Wastes, paper 40, Durham (1984).
7.
R J Collins. "Dredged silt as a raw material for the construction industry". Resource Recovery and Conservation ~, 337-362 (1980).
8.
S Urhan.
"Alkali silica and pozzolanic reactions in concrete.
288
Vol. 19, No. 2 R.J. Collins
Part 2: observations on expanded perlite aggregate concretes". Cement and Concrete Research 17, 465-477 (1987). .
Tang Ming-Shu, Xu Zhong-Zi and Han Su-Fen. "Alkali reactivity of glass aggregate". Durability of Building Materials ~, 377-385 (1987).
10.
R J Collins. "A method for measuring the mineralogical variation of spoils from British collieries". Clay Minerals 11, 31-50 (1976).
ALKALI AGGREGATE REACTIVITY IN DENSE CONCRETES CONTAINING SYNTHETIC OR POROUS NATURAL AGGREGATE
by R J Collins Building Research Establishment, Garston, Herts, UK
(Communicated by C.D. Pomeroy) (Received July 12, 1988)
ABSTRACT Studies have been made into the relative importance of pore solution dilution, accommodation of gel pressures and potential reactivity of porous aggregate in the suppression of alkali aggregate expansion in concretes containing a pessimum proportion of Thames Valley sand. The results also indicate that the alkalinity of the pore solution may be a more direct measure of susceptibility to reaction than the Na~O equivalent content of the concrete. Slow reactivity in some sintered aggregates has been monitored for up to 10 years and the relatively low expansions are discussed in terms of fluxing impurities in the glassy phases. Introduction In a previous paper (I) it has been reported that the use of porous aggregate can be beneficial to the suppression of alkali-silica reaction (ASR). Very much reduced expansions were found when these aggregates replaced a dense limestone in a reactive mix containing Thames Valley sand. There was no conclusive proof that any gel due to ASR was accommodated within porous aggregate and it was considered more probable that the reaction was reduced through the dilution of alkali metal concentrations in the pore water by water absorbed in the aggregates, and/or (in some cases) by the porous aggregates themselves being susceptible to alkali attack. The work reported in this paper was carried out to provide further evidence and to investigate more fully the susceptibility of synthetic aggregates to alkali aggregate reaction. Experimental (i)
Constant total water/cement ratio
If the main effect of porous coarse aggregate in relation to ASR is to cause dilution of alkali then similar expansions should be obtained for concretes of the same total water/cement ratio, irrespective of the type of coarse aggegate present. Table I compares expansion results for constant total water/cement ratio with results for constant free water/cement ratio. 278
Vol. 19, No. 2
279 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
In order to compact the concretes at a constant total water/cement ratio a superplasticiser was used. The total alkali content (Na20 * 0.658 K20) of the superplastlciser if available would increase the Na20 equivalent of the concrete containing 700 kg/m ~ of cement by 0.4 k g / m 3 at the recommended dosage rate of I litre per 50 kg of cement. Control prisms cast with the low porosity coarse aggregate RLI did not however show any signficant difference in expansion when superplasticiser was used.
TABLE I.
EFFECT OF INITIAL WATER CONTENT ON EXPANSION DUE TO ASR Expansion measurements for concrete prisms (200 x 75 x 75 mm) after I year's storage at 38oC and 100% relative humidity. (700 kg/m 3 0 P C with Na20 equivalent 6.7 kg/m3; Coarse aggregate /Thames Valley sand : 70/30 by volume)
TOTAL W/C = 0.325 Coarse Superplasticiser Aggregate dosage I litre/ Identity Description FREE water/cement - 0.325 50 kg cement No of No superplasticiser (*double dosage) (and 24 hr Coarse absorption Aggregate Expansion at Expansion at Total I year (%) value ) I year (%) water/ prism prism cement prism!prism I 2 average ratio I 2 iaverage RLI (0.5%) Carbonlf. limestone
0.34
0.320 O.304
0.312
0.332
0.305
0.319
R4A (4.1%)iJurassic limestone
0.39
0.053 0.073
0.063
!0.169
0.149
0.159
RL3 (16%)
Lytag
0.42
0.018 0.010
0.014
0.026
0.027
0.027
RL3 (16%)
Lytag
0.023*
0.023*
Expansion of the concrete containing the porous aggregate R4A was very much increased at the lower water content indicating that the dilution theory is operative. However the expansion did not approach the value obtained for the low porosity control aggregate RLI. This would suggest that there is also relief of gel pressure by the porous aggregate possibly encouraged by stronger concrete at reduced free water/cement ratio. Examination of the concrete in thin section revealed the formation of significant quantities of gel for both RLI and R4A although less for R4A. There was only slight evidence of gel impregnation of the aggregate R4A. The fine-grained micrite matrix of some aggregate particles contained gel, but the majority of the porosity in oolites was unaffected. It is possible that gel pressures can be released merely by the existence of voids in the aggregate without growth of the gel into the pores necessarily occurring. Any alkali reactivity in the limestone coarse aggregate can be virtually excluded as it is 96% CaCO, and examination in thin section reveals impurities only of quartz and no highly reactive minerals such as opal. Expansion of the concrete containing the sintered fly ash coarse aggregate RL3 was not very much increased at the lower water content and examination
280
Vol.
19, No. 2
R.J. Collins
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Vol. 19, No. 2
281 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
in thin section did not reveal the presence of any gel or physical distress within the concrete. Because of the absence of any gel the low expansions observed are probably due to the potential reactivity of the coarse aggregate(2) which results in competition for the available alkali. Such an effect is seen when the pessimum proportion of reactive aggregate is exceeded. (ii)
Pore solution alkalinity or equivalent alkali content?
Many of the results in this and the preceding paper (I) are attributed at least in part to changes in pore solution alkalinity. A direct study of pore fluid squeezed out of the concrete could thus be beneficial but this was not attempted as there are practical difficulties for concrete with water/cement ratios below about 0.40. However, a recent review of pore solution studies (3) indicates that for concrete made with ordinary Portland cement (OPC) and no cement replacement materials, the hydroxyl ion concentration [OH-] is not far different from [OH-] calculated from Na20 equivalent divided by the initial water content of the concrete. Although the loss of pore water by hydration reactions might be expected to concentrate alkalinity, there is also uptake of alkali by the hydration products which approximately balances the concentration effect for most OPC concretes (4). The simplified calculated value is given in Table 2 for concretes containing a non-reactive coarse aggregate and a Thames Valley sand. ASR expansion of the concrete (measured at I year) reduces as [OH-] reduces except for one of the mixes containing 5.8 kg/m 3 Na20 equivalent for which a very much lower figure was obtained. Overall it appears that under these conditions [OH-] of about 800 millimoles per litre or more is required to produce a deleterious expansion of greater than 0.05%. These observations do not overrule the importance of the Na20 equivalent content of concretes in determining the susceptibility of concrete to ASR both because of the exception noted above and the fact that at a constant total water content Na20 equivalent content is linearly related to calculated [OH-]. (lii)
Suppression of expansion by potentially reactive aggregates
In (i) above it was noted that potential for ASR in a sintered fly ash coarse aggregate (Lytag) had suppressed the expansion that might have been expected for a high alkali mix containing Thames Valley sand as 30% of the aggregate. Table 3 shows how this occurs even when only small proportions of an inert coarse aggregate are substituted by Lytag. Aggregate pore volume and the theoretical concentration of alkali in the pore solution are well in the range shown in Table 2 to be associated with ASR expansion. This is similar to results obtained previously for some sandstone aggregates (I). Substitution of potentially reactive coarse aggregates in these mixes is considered to have the effect of reducing alkali concentrations In the pore water below that required for Thames Valley sand to show an expansive reaction. The above results do not indicate whether a pesslmum proportion of reactive aggregate had been exceeded and indeed whether or not the coarse aggregates themselves could give an expansive reaction. Thus a further series of concrete prisms were cast wlth inert limestone coarse and fine aggregate i n which small proportions of potentially reactive aggregates were substituted. The results are given in Table 4 for Lytag and for two
-.--
10
20
30
40
50
Replacement of coarse aggregate only
0
a
60
at dil’l‘erent proportions
of potentially
aggregate
70
’
RLl
(coarse
80
and
90
100 fine)
Replacement of coarse and fine aggregate
Thames Valley flint gravel Thames Valley coarse and Leighton Buzzard sand (inert) Thames Valley coarse and fine Lytag Lytag coarse and fine Sandstone Rl 1 With crushed sandstone Rll I340 I R45B (Warley Wise Grit) As inert fine aggregate Rl 1 Coarse and fine R40 Coarse and fine
for replacement
aggregate reactive aggregate
I% by volume) of dense inert limestone
Expansions
Replacement
Figure I
0
0
Thames Valley flint gravel
0
w
*
0”
)--’
Vol. 19, No. 2
283 ALKALI AGGREGATE REACTION, EXPANSION, PORE SOLUTIONS
TABLE 3.
EFFECT OF LrrAG ON ASR EXPANSION Expansion measurements for concrete prisms (200 x 75 x 75 mm) after I year's storage at 38°C and 100% relative humidity (700 kg/m' OPC with Na=O equivalent 6.7 kg/m'; free water/cement ratio = 0.325; coarse aggregate/Thames Valley sand : 70/30 by volume)
Replacement of Carboniferous lime[OH-] pore s t o n e coarse Expansion at I year (%) aggregate RL1 by solution concentration Lytag RL3 (% by volume ) mmole/l* prism 1 prism 2 average 0
920
0.320
0.304
0.312
10
890
0.103
0.013
0.058
18.5
875
0.011
0.004
0.008
5O
810
0.001
o.003
0.002
* theoretical concentration calculated as detailed in Table 2.
EXPANSIONS AT DIFFERENT PROPORTIONS OF COARSE AGGREGATE
TABLE 4.
Average expansion measurements for duplicate concrete prisms (200 x 75 x 75 mm) after I year's storage at 38°C and 100% relative humidity. (700 kg/m' OPC wlth Na=O equivalent 6.7 kg/m'; free water/cement ratio = 0.325; coarse aggregate/Carbonlferous limestone fine aggregate : 70/30 by volume)
Expansion at I year (%) for replacement coarse aggregate:
Replacement o f Carboniferous lllmestone coarse
aggregate RLI (% by volume)
Lytag RL3
Sandstone R11 Sandstone R40 (Old Lawrence Rock) (Upper Haslingden Flags) (Absorption 2.0%) (Absorption 3.2%)
3
0.OO7
0.004
0.007
7
-0.005
-0.001
0.009
15
0.o13
0.002
0.010
30
0.013
0.001
0.015
0. 003
0.01 7
4O 100
0.012
0.014
284
Vol. 19, No. 2 R.J. Collins
fine-grained Carboniferous sandstone aggregates. Similar but very slight non-deleterious expansions were obtained for Lytag and sandstone R40 and virtually no expansion for sandstone R11. Results were also obtained for Thames Valley flint gravel coarse aggregate and are used in figure I to illustrate the difference between expansively reactive and potentially reactive aggregates. Results for R40 in Table 4 are omitted from figure I for clarity, but some further results for high proportions of the sandstones R11 and R40 are also included. The expansion of these latter prisms is accompanied by slight cracking and observation in thin section shows a fairly low level of gel formation typical of alkali silicate reaction. There has been little further expansion in these prisms between I and 3 years. (iv)
Further results for synthetic aggregates
The observation that Lytag can suppress the expansion that might have been expected in a concrete containing a pessimum proportion of Thames Valley sand may also be seen for other synthetic aggregates (Table 5). Because of the moderately high porosity of these synthetic aggregates it is possible that the suppression is as much due to dilution of pore water or accommodation of gel pressures within aggregate pores as it is to potentially reactive aggregate. However, when the synthetic coarse aggregate is used with crushed synthetic aggregate fines or inert crushed limestone fines there is also a small but non-deleterious expansion. This is in contrast to the results obtained for Lytag which showed no significant expansion when Lytag fines or limestone fines were used (I). Slight expansion attributed to alkali aggregate reaction has been reported for other sintered and expanded aggregates (I, 8). Recent measurements on long-term specimens (Tables 6 and 7) indicate that there is also a continuing but very slow expansion. In a few cases this has reached or slightly exceeded 0.05% which is generally considered as an approximate threshold for the initiation of cracking. It is interesting to note that the aggregates prepared by intergrinding up to a 30% proportion of pyrex glass with colliery spoil before firing have not retained the high reactivity of pyrex glass and do not show significantly more expansion than the aggregate prepared using colliery spoil on its own. The evidence so far is that sintered and expanded aggregates are not as reactive as many natural aggregates containing free silica. Recent work on the reactivity of some synthetic glass aggregates (9) supplies a possible reason for this in terms of the requirement for sufficient silica and not too much impurity (CaO, AI~O 3) to be reactive. Glassy phases in slntered and expanded aggregates will generally be fairly impure as it is the impurities which act as fluxing materials, making the slntering temperature lower than the overall melting temperature or slagging temperature. This theory appears to offer a reasonable explanation for the higher expansion observed with aggregate from spoil C7 in relation to that from C4 (Table 7). The two aggregates have almost identical physical properties and firing temperatures but reference to chemical analyses (10) shows that C4 has more fluxing materials than C7. The effect of firing temperature on the reactivity of aggregates from spoil C9, however, is less easily explained in these terms. Perhaps this is simply determined by the difference in density which results in more reactive material per unit volume for aggregate fired at the lower temperature.
Vol. 19, No. 2
285 A L K A L I A G G R E G A T E REACTION,
TABLE 5.
EXPANSION,
PORE SOLUTIONS
AAR CONCRETE PRISM TESTS WITH SYNTHETIC AGGREGATES 700 kg/m 3 0 P C with Na=O equivalent ratio 0.325; coarse a g g r e g a t e / f i n e
Coarse Aggregate - raw material, process conditions, relative density (~) and 24 hour water absorption value (wa)
6.7 kg/m3; free w a t e r / c e m e n t aggregate 70/30 by volume.
Fine Aggregate*
Average expansion (%) of duplicate 200 x 75 x 75 mm prisms stored at 38°C and 100% relative humidity
28d
3m
9m
1y
I .5y
Colliery spoil - Newdigate tip Dense aggregate process (5) and rotary kiln p - 1.70, wa = 12%
C
TV RLI
0.0O5 O.006 0.008
0.012 0.012 0.011
0.028 0.026 0.021
0.030 0.027 0.024
0.030 0.027 0.024
Mine taillngs - Abernant colliery Laboratory pre-production trial firing (6) p = 1.63, wa = 14%
C TV RL1
0.002 0.010 0.005
0.007 0.012 0.008
O.013 0.022 0.014
0.015 0.021 0.011
0.017 0.021 0.O16
* C - crushed fines from synthetic coarse aggregate TV - Thames Valley sand RLI - crushed Carboniferous limestone
TABLE 6.
ASTM C227 MORTAR BAR TESTS WITH SYNTHETIC AGGREGATES NaOH added to give NazO equivalent of 1.5% with respect to cement
Crushed and graded synthetic aggregate
BRE synthetic dense aggregate from ¢olllery spoil (ref 5)
spoil C9/1200°C spoil C9/1250°C CD/glass = 90/10 C9/glass = 70/30
Colliery spoil C10/1300°C shaft kiln Colliery spoil CIO/1100oC shaft kiln* Dredged silt S7/1060=C ( r e f 7)
Expansion (%) after storage at 380C and 100% humidity (average for 4 mortar bars) .J 28d 3m 6m ly 5y 10.5y 0.007 0.009 0.013 0.013
0.020 0.017 0.023 0.025
0.029 0.022 0.029 0.032
0.038 0.025 0.036 0.041
0.056 0.026 0.050 0.053
0.060 0.030 0.063 0.068
0.018 0.075 0.010
O.O2a 0.096 0.016
0.032 0.102 0.023
0.033 0.113 0.029
0.043 0.145 0.035
0.050 0.161 0.040
* expansion caused mainly by excessive sulphates in the underburnt aggregate (3.0% total sulphur as SO,: 1.8% water soluble SO,)
C o n c l u s i o ns
C1)
Pore solution alkalinity appears to be a more direct indicator of ASR expansion in concretes c o n t a i ning a typical reactive fine aggregate (Thames Valley sand) than the Na=O equivalent of the concrete. However for concretes with the same total water content, the c a l c u l a t e d alkalinity of the pore solution will be directly related to the NazO equivalent content of the concrete.
286
Vol. 19, No. 2 R.J. Collins
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Vol. 19, No. 2
287 ALKALI AGGREGATE REACTION, EXPANSION,
PORE SOLUTIONS
(ll)
The primary effect of porous aggregates on susceptibility to ASR is dilution of the pore solution. Some accommodation of gel within the aggregate has been observed but appears in the present work to be of much lesser importance for suppressing ASR expansion. However it is probable that the mere existence of voids in the aggregate will be sufficient to reduce swelling pressures, without growth of gel into the pores necessarily occurring.
(ill)
Some sandstones and synthetic aggregates show slight alkali-aggregate reactivity which gives relatively low expansions and can suppress alkali silica reaction involving other aggregates in the concrete. The reaction observed in sandstone produced only a small quantity of gel and may be alkali silicate reaction. For synthetic aggregates the low expansion may be due to a reduced reactivity in glassy phases which contain impurity introduced by fluxing materials. Acknowledgement
The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director. References I.
R J Collins and P D Bareham. "Alkali-sillca reaction: suppression of expansion using porous aggregate". Cement and Concrete Research 17, 89-96 (1987).
2.
I Sims and S Bladon. "An exploratory assessment of the alkall-reactlvlty potential of sintered pfa in concrete preliminary report" Proc 2nd International Conference on Ash Technology and Marketing, London (1984).
.
4.
P J Nixon and C L Page. "Pore solution chemistry and alkali aggregate reaction". Katherine and Bryant Mather International Conference on Concrete Durability (ed. John M Scanlon) ACI SP-IO0 Vol. 2, 1833-1862 (1986). H F W Taylor. "A method for predicting alkali ion concentrations in cement pore solutions". Advances in Cement Research !, 5-17 (1987). R J Collins. "Manufacture of aggregate from colliery spoil". Proc Symposium on the Reclamation, Treatment and Utilisation of Coal Mining Wastes, paper 41, Durham (1984).
5.
.
L F Parks. "Production of lightweight aggregate from washery railings at Abernant Colliery". Proc Symposium on the Reclamation, Treatment and Utilisation of Coal Mining Wastes, paper 40, Durham (1984).
7.
R J Collins. "Dredged silt as a raw material for the construction industry". Resource Recovery and Conservation ~, 337-362 (1980).
8.
S Urhan.
"Alkali silica and pozzolanic reactions in concrete.
288
Vol. 19, No. 2 R.J. Collins
Part 2: observations on expanded perlite aggregate concretes". Cement and Concrete Research 17, 465-477 (1987). .
Tang Ming-Shu, Xu Zhong-Zi and Han Su-Fen. "Alkali reactivity of glass aggregate". Durability of Building Materials ~, 377-385 (1987).
10.
R J Collins. "A method for measuring the mineralogical variation of spoils from British collieries". Clay Minerals 11, 31-50 (1976).