Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete

ELSEVIER Plh S0016-2361 (96)00193-7

Fuel Vol. 76, No. 8, pp. 761-765, 1997 © 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0016-2361/97 $17.00+0.00

Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete

Elizabeth Freeman, Yu-Ming Gao, Robert Hurt and Eric Suuberg Division of Engineering, Brown University, Providence, RI 02912, USA E- Mail. Robert_Hurt@bro wn. edu (Received 25 January 1996; revised 25 September 1996)

The most important commercial outlet for coal ash is as a partial replacement for Portland cement in the concrete industry. High levels of unburnt carbon can render ash samples unsuitable for this high-value market by interfering with the action of air-entraining admixtures, which are specialty surfactants used to stabilize air bubbles in concrete mixtures. An initial laboratory investigation was carried out to identify the fundamental interaction mechanisms between fly ash and air-entraining admixtures. The results indicate that the interaction is time-dependent and occurs to a degree that correlates only crudely with the amount of carbon present. Measurements made on a variety of model additives suggest that admixtures are preferentially adsorbed from the aqueous phase on non-microporous carbonaceous surfaces. © 1997 Elsevier Science Ltd.

(Keywords: fly ash; carbon; concrete mixes)

Despite the ongoing development of new products incorporating fly ash from coal combustion, the most important high-value outlet for ash continues to be in the concrete industry, as a partial replacement for Portland cement 1. However, high levels of unburnt carbon can make fly ash unsuitable for this high-value market 2. The ASTM has established specifications for the use of Class F ash (ASTM C 618-89a) which limit loss-on- ignition (LOI) to 6wt%. Some regions have indepen- dently instituted more stringent specifications 3. Most of the research on concrete applications of fly ash has focused on the inorganic (major) component of fly ash- - its particle size, morphology and chemical composition. The organic portion of fly ash is routinely characterized only by the ASTM LOI test, which is an approximate measure of the amount of carbon present and provides no information on the form or properties of the carbon. Although one researcher has questioned the adequacy of the simple LOI criterion 4, it has often been regarded in practice as sufficient to identify those fly ash streams unsuitable for concrete applications.

In recent years, however, the widespread installation of low-NOx burner systems has led to increases in fly ash carbon content at many sites 2'5'6. Many utilities are currently seeking outlets for higher-carbon ash streams and thus there is substantial motivation to critically re-evaluate the simple ASTM LOI specification. In addition, recent field experience (P. Calvert, personal communication, 1995) suggests that, for certain ash streams, unburnt carbon can cause problems even at levels <3wt%. There have been almost no fundamental studies of the specific interactions between fly ash and concrete air-entraining admixtures that could help

explain these observations. For these reasons, an initial systematic investigation of the behaviour of carbon- containing fly ash streams in concrete was undertaken with the goal of identifying the fundamental role played by carbonaceous solids.

EFFECTS OF CARBON ON FLY ASH CONCRETE

The effects of carbonaceous solids on fly ash concrete are known to include (1) discolouration, which is aestheti- cally unacceptable in certain applications, (2) poor air entrainment behaviour and (3) mixture segregation. Most of the concern has centred on the air entrainment effect. In many concrete formulations, air bubbles are intentionally entrained into the mixture through mechanical agitation and stabilized by the addition of specialized surfactants known as air-entraining admixtures (AEAs). The presence of small air bubbles (<0.25mm) increases the workability and cohesion of the mix, and allows a reduction of water content that ultimately improves the mechanical properties of the set concrete, especially its resistance to freeze-thaw cycles 7. The proper dosage of AEA is critical for achieving the desired amount of entrained air (typically 3-6vo1.%) and for stabilizing the air bubbles during mixing and transportation to the job site.

There are a variety of commercial AEAs on the market, some derived from natural sources (e.g. neutralized wood resins) and others, increasingly, synthetic materials. Most are proprietary mixtures, whose exact composition is known only by the specialty chemical companies who manufacture and market them. A typical

Fuel 1997 Volume 76 Number 8 761

Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete: E. Freeman et al.

resin-derived AEA may consist of a mixture of multiring organic acids that have been neutralized to introduce an ionic group at one location. These compounds have a dual hydrophilic-hydrophobic nature and are surface- active in the aqueous medium of concrete mixtures. AEAs stabilize air bubbles by collecting at the air-water interface with their hydrophobic end protruding into the bubble interior and their hydrophilic end oriented into the aqueous phase and possibly adsorbed on the surface of cement particles 7. The bridging action of the AEAs stabilizes the air bubbles and provides a network structure incorporating the cement particles and the bubble phase, which increases the mixture cohesion 7.

The degree of interaction between an AEA and a given fly ash sample can be estimated using the foam index test (ref. 4; also K. Nelson, personal communication, 1994), a laboratory procedure in which a small concrete-ash mixture is titrated with AEA solution until stable bubbles are observed on the surface. The amount of AEA required is reported as the 'foam index', a high value indicating that large amounts of AEA will be required to achieve the desired volume percentage of air. Ultimately, the actual performance must be confirmed by direct quantitative measurement of the volume percentage of entrained air in a concrete mixture and observation of its stability over time.

In general, high-carbon ash streams require the use of large amounts of AEA, and when this is coupled with the inevitable variations in ash properties encountered in the field, the result is often large fluctuations in the volume percentage of air in concrete, which in turn leads to variable and unacceptable mechanical properties 4. It is postulated that carbon interferes with the air entrainment process by adsorbing the AEAs and rendering them inactive for bubble stabilization.

EXPERIMENTAL

A modified version of the foam index test was used to investigate the interactions between commercial AEAs (Daravair 1000 and Darex II*) and a variety of model carbonaceous materials and carbon-containing fly ash samples from the field. The samples included two commercial activated carbons (Norit A, Fisher C-177), a pelletized carbon black (Huber Inc.), a bituminous coal (Bruceton), and commercial fly ash samples with LOI values varying from 1.5 to 16wt%. The foam index test was modified to accommodate smaller fly ash samples; the procedure was as follows: 8 g of Portland cement and 2g of fly ash were placed in a wide-mouth 350mL jar. Water was added (25 mL) and the jar shaken for 1 min. A tenfold dilution of Daravair-1000 was used as the titrant, added one to five drops at a time. Between each addition the jar was shaken vigorously for 15 s and the lid was removed to observe the liquid surface. The end point was realized when a constant foam was maintained on the surface for at least 45 s. The same procedure was used in the tests involving model carbonaceous additives, which were introduced (substituting for fly ash) in amounts calculated to simulate the loading of unburnt carbon in the fly ash tests.

For each solid sample, loss-on-ignition (LOI) was determined as an approximate measure of carbon

* Trademarks of W. R. Grace & Co.

20 p,m

Figure 1 Typical unburnt carbon particles in the fly ash samples examined. Most of the carbon particles exhibit a fused, vesicular morphology with extensive macropore networks

content and total surface area was determined by single-point N 2 BET measurement (at a reduced pressure Pr -- 0.3). For each of the fly ash samples, surface areas were measured before and after air oxidation at 700°C (1 h hold time) to differentiate between the surface areas associated with the combustible (organic) and incom- bustible (inorganic) portions of the samples. Particle morphologies in the fly ash samples were examined by optical and scanning electron microscopy.

RESULTS AND DISCUSSION

Microscopic examination revealed that the inorganic portion of the fly ash samples consists predominantly of glassy spheres. Most of the carbon present is in the form of distinct, fused particles with an extensive macro- porous vesicular structure (partially oxidized coke), an example of which is seen in Figure 1. Other studies have revealed similar fly ash morphologies 8'9.

The quantitative data are summarized in Table 1. LOI values ranged from 1.5 to 16wt% for the four fly ash samples examined. Ash surface areas ranged from 1.18 to l l.2m2g -1, primarily reflecting the wide variation in carbon content. Air oxidation was observed to reduce the ash surface area substantially: the combustible organic fraction was responsible for most of the N 2 BET area in all but one sample (FA2 with only 2 wt% LOI). Table 1 also presents N 2 BET surface areas for the model carbonaceous additives.

Table 1 also provides ratios of organic surface area (total area minus inorganic area) normalized by the measured LOI values. These ratios are estimates of the mass-specific surface areas of the carbonaceous portions of the samples, and as such can be regarded as properties of the carbon materials. This specific carbonaceous surface area is seen to vary from 16 to 134m2g -1 for the fly ash samples, values that are generally lower than the carbon dioxide surface areas recently reported for a set of eight residual carbon extracts from commercial fly

9 ash samples . The difference is most likely related to differences in technique; the low adsorption temperature used in the N 2 BET test (77 K) often renders a portion of

762 Fuel 1997 Volume 76 Number 8

Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete." E. Freeman e t al.

Table 1 Data summary

Sample

Total Inorganic Modified Specific carbonaceous LOI surface area ~ surface area b foam index surface area ~

(wt%) (m 2 g-l) (m 2 g 1) (mL) (m 2 g-l)

Portland cement - - 0.12 -

Fly ash 1 (FA1) 2.0 1.2 0.88 0.20 16

Fly ash 2 (FA2) 3.4 1.9 0.66 0.56 37

Fly ash 3 (FA3) 16.0 I1 0.80 1.67 65

Fly ash 4 (FA4) 1.5 2.7 0.66 0.78 134

FA2 oxidized 0.0 0.66 0.66 0.16

Pellitized carbon black >99 120 - 0.23 120

Bruceton coal 95 8.4 - 0.15 9.9

Activated carbon (Fisher) >99 - 1.08 -

Activated carbon (Norit) >99 720 1.15 720

a N2 BET method b Measured after air oxidation of the raw samples at 700°C

Calculated as (total area - inorganic area)/LOI, and reported per unit mass of carbonaceous matter

the internal surface area inaccessible, due to slow restricted diffusion in carbon micropores. Across the whole data set (including the model additives), the specific carbonaceous surface area varied from 9.9m2g -1 (for coal) to 720m2g q (for activated carbon). Clearly the nature and properties of the carbon materials used in this study varied widely.

,4EA interactions The raw foam index results for Daravair 1000 are

summarized in Table 1. Qualitatively similar results were obtained for Darex II. In initial experiments using only Portland cement, very little AEA was required to stabilize the surface air bubbles (~0.12mL). When carbon-containing fly ash or carbonaceous solids were added, the AEA requirement increased substantially and varied greatly from one sample to another. Figure 2 plots the foam index measurements versus the amount of carbonaceous material present in the tests (presented as an equivalent LOI). Foam index generally increases with increased carbon loading, but no unique relation exists. The dashed line indicates typical behaviour for fly ash samples FA1-FA3, a subset of samples taken under well-controlled combustion conditions from a single industrial source. In contrast, fly ash 4 shows a very high activity toward AEAs despite its low LOI (1.5wt%). This sample was reported to represent poorly controlled conditions in the field.

The model carbonaceous additives show a wide variation in behaviour. The activated carbons show a much greater interaction with AEA per unit weight than typical fly ash carbons (in FA1-FA3) , while the bituminous coal and pelletized carbon black are much less active than fly ash carbons. Making a direct comparison at an equivalent LOI of 3.4 wt%, the order of activity is:

activated carbons > typical fly ash carbons

> pelletized carbon black > coal

An identical order is observed when the materials are ranked by surface area, suggesting that the AEA interaction is primarily a surface phenomenon.

Figure 3 plots the same data set versus the total BET surface area present in the concrete mixture. Surface area provides a somewhat better unification of the data set than LOI, but the relation is still not unique. In

1.6 t . . . . ~ . . . . ~ . . . . I ~ 1 I I

1.4 .............................. i ............................. i ............................... . . . . . . . . . . . . . . . . . . . . . . . . . . . x ~ i f i

1 .?~ ................................ activated ............ J -----i ............................. --= ~ ~ . ~ carbons /

1 t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . .

,? z , o . e : l ........................ i .................. ~ . . . ..................... ~ ........................

o . d . . . . " .................. . . . . . . . . . . . . . . . . . . . . . . . . . . , ............................

0.4~ ................ ~,~ .......................... i ....................... i ............... 0.2~.. , . ~ / pelletized carbon ' black

~ . ~ A ~ i _ b i t u m i n o u s coal i

0 5 I 0 1S 20

Carbon Loading in Foam Index Text Mixture (presented as equivalent % LOI)

Figure 2 Effect of various ash samples and model carbonaceous additives on air bubble stability in simulated concrete mixtures. Squares, fly ash samples from commercial power stations; triangles, model carbonaceous additives. Note that the results for the fly ash samples are plotted versus LOI; results for the model carbonaceous solids are plotted versus LOI of a hypothetical ash sample containing the same carbon mass as the additive. In this way, the abscissa represents the total carbon loading in the test mixture, allowing direct comparisons of the specific activity of the residual carbons and model carbonaceous additives

~. . . . . . . . . 1 1 1 1 1 i J i k ! i i t i i

i

/ activated E m ~'~ , carbon.~.~ 0 u_ , 1 ....................... ~...-¢ ................................... . ......................................

~ , 7 i t i

~5 =mr o o .s ......... / i ................................. i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . :~

V ! pelletized carbon black

0 . . . . . . . . . . . . ; . . . . . . . . 10 20 30 40 50

Total surface area in FI tes t mixture, m 2

Figure 3 The role of total carbonaceous surface area on air bubble stability in concrete mixtures. Squares, fly ash samples from commercial power stations; triangles, model carbonaceous additives. The x-axis represents the total carbonaceous surface area (in m 2 absolute) present in the cement mixture during the foam index test

F u e l 1 9 9 7 V o l u m e 7 6 N u m b e r 8 763


particular, the activated carbon has an anomalously low activity on an area basis, suggesting that this high-area material (720 m 2 g-l) contains much surface that is not effective in AEA adsorption. High surface areas in commercial activated carbons are achieved through the development of micro- and mesopores, and the foam indices indicate that much of this fine porosity does not participate in AEA adsorption. This is not surprising in the light of the small size of micropores (<2 nm), which are comparable with the molecular size of the AEAs.

Overall, the results indicate that AEA adsorption is related to easily accessible carbon surface area, con- sisting of the external surfaces of the carbonaceous particles and some contribution from internal surfaces lying in larger pores. Figure 4 illustrates the likely adsorption sites for AEAs on carbon solids in fly ash. Attachment of the hydrophobic ends of AEA molecules to carbon surfaces renders them unavailable for attachment to air bubbles. Because the external surfaces of particles are easily accessible, the AEA affinity of a given sample may be expected to increase with decreasing carbon particle size. It may also increase with increasing macro- or mesoporosity. The microporous matrix in the char particle interior (see Figure 4) is largely inactive.

Finally, Figure 5 shows that AEA-carbon interactions are time-dependent: the amount of AEA required to stabilize air bubbles increases over the first 2h after mixture preparation, especially for FA2. From 2 to 4 h a plateau region is observed, after which the AEA volume increases again. This latter increase is believed to reflect the hydration reactions, which thickened the suspen- sions noticeably at times >4 h. This time dependence is not captured in the simple foam index test, but can be significant for concrete applications in the field; of particular concern is the effect observed in the first 1-2 h. The mechanism responsible for the time dependence is not known, but may be related to the slow diffusion of AEA to less accessible adsorption sites in the particle interior.

CONCLUSIONS

Initial laboratory testing on a variety of fly ash samples and carbonaceous additives has yielded insight into the role of carbonaceous solids in concrete mixtures. Carbons adsorb air-entraining admixtures from aqueous solution, thus rendering them ineffective as surfactants for stabilization of entrained air bubbles. The degree of interaction with AEAs correlates only crudely with the amount of carbon present; different carbon materials show widely differing degrees of interaction, and the rank order of activity suggests an adsorption process occurring on non-microporous carbon surfaces. A somewhat better correlation is observed with N2 BET surface area, but this correlation is also not adequate, because the area measurement includes at least a portion of carbon microporosity which is believed to be ineffective in AEA sorption.

The results of this work confirm the fundamental insufficiency of the simple LOI test, which is a measure of the amount of carbon present irrespective of its properties. In the light of the wide variety of coal types and combustion systems in use, it is not surprising that problems have been reported in the field with the use of certain fly ash samples of relatively low LOI. Conversely, it is expected that some fly ash samples will be suitable

fine carbon coarse carbon particles particles

. . . . oporous matrix © (inactive)

0

AEA adsorption s i tes

Figure 4 Likely adsorption sites for AEAs on carbon solids

-~ 0.8-

0.7-

0.6-

0.s- " ( 0 . 4 " I . I J , <

"6 0.3: o) 0 . 2 -

o . , i O :

> 0

.............. [ I hydration effbct$ i

.................. ! . . . . . . . . . . . . . . . . . . i..........-:.:~- F A Z.

.................... i ............. .......... i ................... 4

........... ~-. . i .................. i .................. 4 ................... i ................. . - t ! i i i

i i i i i ................................... ~ - F - A ' i ............................. T ................... I ........................................ i ................. i .................................. !

~ . ~ _ . . ~ . . . . . . .1il...-..~..~_..~ ...................... + ...................

, -,~ ~ L _ p c ~ = E ~ I ' [ , , , i , , , ( , , , I , , , I , , , (

2 4 6 8 10

Time (hrs)

F i g u r e 5 Effect of time on the amount of air-entraining admixture (Daravair 1000) necessary for bubble stability. Absolute titration volumes in these time-dependent experiments differ from those observed in foam index testing, due to the smaller jar volume used here (120mL) and the smaller liquid free surface

for concrete applications at higher levels of unburnt carbon than allowed in the governing specifications. More fundamental work is needed in this area: (1) to precisely identify the active area for AEA adsorption; (2) to develop suitable techniques for its direct measurement; (3) to understand the role of surface chemistry, especially the number and nature of oxygen-containing functional groups on carbon surfaces; and (4) to docu- ment further the variations in field sample behaviour. The limited database on the sorptive properties of ash 1° and residual carbon 8 should be expanded.

ACKNOWLEDGEMENTS

The authors would like to acknowledge financial support from the Electric Power Research Institute and the substantial technical contributions of Peter Calvert at New England Power Co. The authors are also grateful for the contributions of Nader Sabanegh at Brown University and for information on the foam index test supplied by Ken Nelson at W. R. Grace.

REFERENCES

Tyson, S. and Blackstock, T., Paper presented at the Unburned Carbonaceous Material on Utility Fly Ash Conference. US DOC PETC, Pittsburgh, 1995.

764 Fuel 1997 Volume 76 Number 8


2 Makansi, J., Power, 1994, (Aug.), 37. 3 Hower, J. C., Wild, G. D. and Graham, U. M., in Pro-

ceedings, l l th International Symposium on Use and Man- agement of Coal Combustion By-Products. EPRI TR- 104657-V2, Electric Power Research Institute, 1995, p. 62-1.

4 Helmuth, R., Fly Ash in Cement and Concrete. Portland Cement Association, 1987.

5 Hurt, R. H. and Gibbins, J. R., Fuel 1995, 74(4), 471. 6 Hower, J. C. and Roble, T. L., Journal of Coal Quality,

1993, 12(1), 22.

7 Rixom, M. R. and Mailvaganam, N. P., Chemical Admixtures for Concrete. Spon, London, 1986.

8 Hurt, R. H., Davis, K. A., Yang, N. Y. C., Headley, T. R. and Mitchell, G. D., Fuel 1995, 74(9), 1297.

9 Roble, T. J., Hower, J. C., Graham, U. M. and Rath- bone, R. F., in Proceedings of the Twelfth Annual Inter- national Pittsburgh Coal Conference. University of Pittsburgh, 1995, p. 121.

10 Banerjee, K., Cheremisinoff, P. N. and Cheng, S. L., Environmental Science and Technology, 1995, 29(9), 2243.

Fuel 1997 Volume 76 Number 8 765

Documents

Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete