Utilising unprocessed low-lime coal fly ash in foamed concrete
M.R. Jones*, A. McCarthy
Concrete Technology Unit, Department of Civil Engineering, University of Dundee, Dundee, Scotland DD1 4HN UK Received 19 November 2003; received in revised form 21 September 2004; accepted 21 September 2004
Available online 15 December 2004
Abstract
This paper describes an extensive laboratory-based investigation into the use of unprocessed, run-of-station, low-lime fly ash in foamed concrete, as a replacement for sand. Foamed concrete with plastic densities ranging between 1000 and 1400 kg/m3and cube strengths from 1 to 10 N/mm2were tested. It is shown that by using this type of fly ash in this way can significantly enhance many of the properties of foamed concrete, including rheology and compressive strength development, whilst providing almost complete immunity to sulfate attack. Given the high carbon content of this type of fly ash, however, it was found that there was a need to increase greatly the amount of foam required to achieve the specified design plastic density. However, given the relatively low cost of foam production, this is not likely to have significant implications for the use of material.
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Keywords:Run-of-station; Low-lime fly ash; Foamed concrete; Filler/sand
1. Introduction
Low emission and flue gas desulfurisation systems fitted to modern power stations have significantly changed the characteristics of fly ash arising from coal combustion. In particular, this has led to higher carbon contents and a coarser particle size distribution and, thereby, less pozzolanic reactivity when used in cement and concrete, which remains one of the most important end uses for the material. In response, extensive research and development effort is being directed towards post- production processing, such as carbon, coarse particles and ammonia removal, activation etc to enhance the as- produced fly ash. However, these methodologies necess- arily add embodied energy to the material and contribute to greenhouse gas emission, let alone the additional cost processing entails. Furthermore, there are at present difficulties with storing and utilising the phases removed during processing.
Against these background issues, this paper addresses the use of unprocessed, run-of-station (low-lime) fly ash, arising from bituminous coal combustion in foamed concrete.
Foamed concrete (sometimes referred to as aerated concrete, but should not be confused with cellular concrete blocks) is a mixture of cement, filler (typically a fine sand) and preformed foam. This gives it unique properties, such as low density 400–1600 kg/m3, and a flowing and self- compacting rheology. Although its relatively low strength, typically 1–15 N/mm2 (although up to 30 N/mm2 is possible) does not lend itself directly to highly-stressed structural applications, the use of foamed concrete has expanded rapidly worldwide. It is used in a wide range of applications from house foundations and fire protection, utilising its high thermal insulation capacity, to geotechni- cal, highway, bridge abutment and backfill uses.
The research work reported here specifically dealt with the potential of using this type of fly ash as a replacement for sand and how this affected the rheological, strength development and permeation/durability properties. Not only does this allow unprocessed ash to be used, it reduces the dependence on primary aggregate resources and helps develop a more sustainable approach to concrete.
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doi:10.1016/j.fuel.2004.09.030
* Corresponding author. Tel.:C44 1382 344343; fax:C44 1382 344816.
E-mail addresses: [email protected] (M.R. Jones), [email protected] (A. McCarthy).
2. Experimental programme
2.1. Materials, mix proportions and specimen preparation The constituent materials used in the laboratory to produce foamed concrete comprised (i) Portland cement (PC, conforming to BS 12 42.5N and CEM I to EN 197-1) at a fixed content of 300 kg/m3, (ii) natural sand, fine aggregate (conforming to BS 882 Grade F, with particles greater than 2.36 mm removed by sieving), (iii) coarse fly ash (designated here FAcoarse, with a 45mm sieve retention of 26.0% and conforming to BS 3892-2, BS EN 450 and ASTM C 618-94a Class F) and (iv) free water to give a water/cement ratio (w/c) of 0.50. The surfactant used for the production of the preformed foam was of commercially available synthetic type.
The mix proportions of the 1000, 1200 and 1400 kg/m3 plastic density foamed concretes, which are summarised in Table 1, were calculated by equating the design plastic density value to the sum of solids (cement and fine aggregate) and water in the mix. However, when FAcoarse was used as replacement of sand, this was considered within the w/c ratio to ensure there was sufficient free water available to ‘wet’ the large surface area of the FAcoarse particles[1].
The preformed foam (with a density of 50 kg/m3) was prepared from a 6% aqueous surfactant solution in a dry system generator, which is typical of the type used by industry. The base mix, i.e. PC, water and filler, and preformed foam were combined in a rotary drum (free- falling action) mixer, following the mixing sequence reported by Kearsley[2]and Jones et al.[3]. Actual plastic densities within G50 kg/m3 of the design value were accepted. The mix was sampled in accordance with BS EN 12350-1 and placed in steel moulds lined with plastic, kitchen-type ‘cling’ film, to prevent interaction with mould release oil. The specimens were then covered with cling film for 24 h and, following demoulding, were sealed-cured (i.e.
wrapped in cling film and stored in sealed plastic bags at 20G28C) until testing.
2.2. Test procedures
Consistence measurements comprised assessment of spreadability according to the Brewer test for controlled low-strength material[4], BS 4551-1ast obtaining only the initial spread, without vibration and slump flow (i.e. spread of collapse slump in two directions [5]) test methods.
‘Flowability’ was assessed with a modified Marsh cone (in terms of orifice diameter and volume of efflux), as described by Jones et al.[3].
The components of the rheological behaviour of foamed concretes were assessed with a RVT Brookfield laboratory viscometer using T-bar spindles and a Helipath stand. This enabled measurement of torque on fresh material each time, since the trace of the rotating spindle was a helical path, thereby avoiding the known ‘channelling’ effect. Readings on the torque dial were taken after the spindle had rotated for a given time at each speed increment.
The mix stability and resistance to segregation of the mixes in the fresh state was assessed (i) by comparing the calculated and actual quantities of foam required to achieve a plastic density within 50 kg/m3of the design value and (ii) visual observation during mixing and placement. The additional amount of foam required was calculated from Eq. (1). Given that the foam generator produces, for a given volume of surfactant solution, foam with 25 times the volume of the solution, the contribution of foam collapse to additional water in the mix is as follows:
VsolutionZVadditional foam!1000=Vfoam produced by 1lof solution
(1) where
VsolutionZ1.06!Vwater, given that 60 g surfactant are diluted in 1lwater
Vadditional foamZMadditional foam/Dfoam DfoamZ50 kg/m3
Vfoam produced by 1lof solutionZ25l
Segregation measurements on hardened concrete were made by quantifying the difference in oven-dry densities
Table 1
Mix proportions of the 1000, 1200 and 1400 kg/m3foamed concretes used to prepare the test specimens Design plastic
densitya, (kg/m3)
Cement type Fine aggregate type
Mix proportions (kg/m3)
PC Sand FAcoarse Waterb Foam
1000 PC Sand 300 550 – 150 24.9
FAcoarse 300 – 367 333 18.5
1200 PC Sand 300 750 – 150 21.1
FAcoarse 300 – 500 400 12.8
1400 PC Sand 300 950 – 150 17.7
FAcoarse 300 – 633 467 7.2
a These plastic densities were selected after consultation with industry colleagues and reflect typical range of values. Similarly, it was found that this PC content was the minimum used by industry.
b The FAcoarsewas considered in the w/c ratio of 0.50 to ensure there was sufficient water to ‘wet’ the large surface area of the fine (smaller than 125mm) fly ash particles compared to the relatively coarse sand, where this was not necessary.
between two 25 mm slices taken from the top and bottom of a standard 100 mm diameter and 300 mm length cylinder.
100 mm cube (sealed-cured) strength was measured to BS 1881-116 at 1, 7, 14, 28, 56 and 180 days, and an average of two reported tested at each test age.
Water sorptivity was measured according to the method developed by Hall [6]. The oven-dried 100 mm cube specimens were placed on a 3 mm thick plastic mesh in a container filled with water to a height of 5 mm above the base of the specimen and their change in weight (% of initial weight) measured at designated time intervals. Sorptivity (S) was obtained as the slope of the line of the graph of cumulative water absorption per unit area of the inflow surface (i, measured in mm) against square root of time (t, measured in minutes), where i was calculated from Eq. (2).
iZDw
Ar; (2)
where
DwZincrease in weight with time (g) AZcross-sectional area (mm2)
rZdensity of water (i.e. 1000 kg/m3)
The performance of foamed concrete in aggressive chemical environments was examined in terms of length change and chemical analysis. The former was assessed using 75!75!225 mm prisms with Demec studs at 150 mm apart, while the latter was carried out on 10 mm ground slices of 40!40!200 mm prisms with XRD equipment at 3–6082q, with a step size of 0.182q. Length change measurements were made at designated time intervals, while the XRD analyses were carried out after 6 months exposure. The Design Sulfate (DS) exposure classes were as specified in BRE SD1-1 [7], with the sulfate content achieved by a combination of gypsum anhydrite with epsomite (50/50% at DS2 and 30/70% at DS4, respectively).
3. Consistence
The consistence indices of the 1000, 1200 and 1400 kg/m3foamed concretes, which comprised measure- ments of ‘spreadability’ and ‘flowability’, are summarised in Table 2, while the relationships between these are examined inFigs. 1 and 2.
3.1. Spread
As can be seen in Table 2, the spread values ranged between 115 and 280 mm, between 85 and 210 mm and between 405 and 650 mm according to the Brewer, BS 4551-1 and slump flow methodologies, respectively. The performance ranking of all density concretes remained constant throughout all spread measurements (reflecting a good relationship between the fundamental characteristics measured by the different spread tests), with the minimum and maximum values observed on the 1400 kg/m3concretes with sand and FAcoarsefine aggregates, respectively.
For a given plastic density, the spreads obtained on FAcoarse concretes were up to 2.5 times greater than those noted on the sand fine aggregate mixes. The enhanced consistence of the FAcoarseconcretes compared to sand is likely to be due to a combination of factors, the ‘ball-bearing effect’ of FA particles due to their spherical morphology[8];
improved packing of the solid phase and adsorption of mix water on to the FAcoarse particles reducing inter-particle friction [1]. As regards the latter, an increase in the mix water will reduce the yield stress value of concrete[9], and, in turn, improve spread[5].
Given that the volume of air in the 1000–1400 kg/m3 density foamed concretes can account for up to half the total unit volume, it would be expected to have a significant effect on its fresh properties. FromTable 2it is apparent that, for a given cement and water content (as is the case with the sand foamed concretes), the spread value decreased with increas- ing density. As a result, in order for all sand foamed concretes to achieve the 200 mm spread benchmark required for flowing properties[4], higher quantities of mix water would be necessary for 1400 kg/m3mix compared to 1000 kg/m3 mix. However, the reverse trends were noted for the FAcoarse
Table 2
Summary of consistence indices for the 1000, 1200 and 1400 kg/m3foamed concretes
Test mix Spread (mm) Flow time (s)
Plastic density, (kg/m3) Fine aggregate type Brewer Test BS 4551-1testa Slump flow
1000 Sand 175 100 430 200
FAcoarse 245 160 570 15
1200 Sand 145 95 410 180
FAcoarse 275 185 605 10
1400 Sand 115 85 405 190
FAcoarse 280 210 650 15
a Initial spread, without vibration.
mixes, i.e. there were greater spreads at higher densities. The decreasing consistence at lower densities is probably due to the reduced self-weight and greater cohesion resulting from the higher air contents[10].
Given the wide range of different spread test method- ologies available, the relationship between these on results obtained on the foamed concretes was examined, as shown inFig. 1. Regression analysis of the data proved that good
Fig. 2. Relationship between spread and flow time measurements for the range of foamed concretes.
Fig. 1. Relationship between spread measurements according to Brewer [4], BS 4551-1 (initial spread, without vibration) and slump flow tests.
correlation (R2Z0.97 and R2Z0.95) existed between all spread values, and thus, the BS 4551-1 spread and slump flow can be predicted accurately from the Brewer spread measurement using Eqs. (3) and (4), respectively:
BS 45511 spreadZ42:757 e0:0054!Brewer spread; mm (3) Slump flow spreadZ273:67e0:003!Brewer spread; mm (4) In addition, it was found that the 200 mm spread require- ment for flowing concrete according to the Brewer test method corresponded to minimum spreads of 130 and 500 mm for the BS 4551-1 spread and slump flow test methods, respectively.
3.2. Flowability out of a modified Marsh cone
The Marsh cone efflux times, which are an indication of plastic viscosity [5], ranged between 10 and 200 s. As expected from the spread measurements, enhanced consist- ence (i.e. lower flow time) was noted when sand was fully replaced by FAcoarse. Indeed, the flow of the FAcoarse concretes was rapid and continuous, with efflux times less than 60 s, which, according to the Dundee ranking method, is Class 1 [3]. There were significant differences in flow times between the sand (180–200 s) and FAcoarse(10–15 s) mixes, again due to the reduction in internal friction with the replacement of sand by fly ash. The small variation of flow times for the 1000, 1200 and 1400 kg/m3 concretes for a given fine aggregate type, suggests that there is little effect of plastic density itself on flow time.
A comparison of the Brewer spread and flow time is given inFig. 2. As expected, for the range of design plastic densities considered, at a given cement content and w/c ratio, greater spread values (hence lower yield stresses) corresponded to shorter flow times (hence reduced plastic viscosity), thereby indicating a relationship between these two properties on foamed concrete. However, for the limited number of concretes examined and the scatter in the data, no correlation between plastic viscosity and yield stress could be derived.
4. Rheology
Measurements of foamed concrete rheology were made with a Brookfield RVT viscometer on the more fluid FAcoarseconcretes. As expected from the literature[11], the shape of the speed-torque curve during increasing (upcurve) and decreasing (downcurve) speed increments, when plotted on a nonlogarithmic scale, demonstrated that foamed concrete is thixotropic (due to the build up of a structure within the material). For a given rotational speed, the torque measurements differed for the two sections of the curves (hysteresis effect), indicating that the viscosity at any
particular rate of shear depended on the amount of previous shearing (thinning and thickening) it had undergone.
For the three plastic densities considered, the relationship between rotational speed and viscometer (torque) readings on a log–log scale, obtained with the same T-spindle (‘C’) during decreasing (downcurve) rotational speed increments is given inFig. 3. The torque reading at 0 rpm is indicative of the apparent yield value, which reflects the minimum force required to initiate concrete movement. The apparent plastic viscosity is a function of the reciprocal of the slope of the rheology curve, which for thixotropic materials approximates a line[12].
As can be seen inFig. 3, the ranking of apparent yield measurements was 1200, 1400 and 1000 kg/m3 density foamed concretes in increasing order of values, which broadly matched that of spread measurements discussed above. Indeed, the 1000 kg/m3 mix, which exhibited the highest apparent yield stress, also measured the smallest spread, confirming the link between spreadability and yield stress of concrete[13]. The greater resistance to initiation of flow on the lower densities can also be attributed to the reduced self-weight of the material, higher air content (resulting in increased cohesion; [10]) and lower water content, compared to the 1200 and 1400 kg/m3concretes.
The differences in the slope of all three lines inFig. 3for the range of foamed concrete densities were small, indicating similar apparent plastic viscosities. This was also reflected in the flow time measurements, which were very short and approximately the same and are indicative of mix viscosity trends[14].
The relationship between rotational speed and visc- ometer (torque) readings on sand concretes was examined with the same T-bar spindle. This was not as linear as that of the FAcoarsemixes, probably due to more work required to complete the structural breakdown, hence never reaching Bingham state by the time of the downcurve [11]. As a result, analysis of the hysteresis loops was difficult and determination of their apparent rheological parameters was not carried out. However, it is expected, given the findings of Dhir et al. [15]on 1400–1800 kg/m3foamed concretes with higher cement contents, that the sand foamed concretes would exhibit higher yield values than those with FAcoarse due to the much larger and more angular shaped sand particles with smaller surface area.
Although the graphical method of rheological data analysis enabled comparative rankings between apparent yield values and apparent plastic viscosities of FAcoarse foamed concretes, the T-bar spindles with the Helipath stand do not have directly definable shear rate and shear stress values[12]and, therefore, the actual yield values (t0) and plastic viscosities (h) could not be established.
Rheological measurements were also attempted with interrupted helical impellers, using scaled down sizes of the ones successfully used by Domone et al.[16]with the two- point workability test. However, these were not successful, since the impellers appeared to cause a pronounced
‘channelling’ effect, with foam bubbles rising to the surface within the area of spindle rotation. The use of beakers with diameter similar to that of the diameter of the helical impellers might have minimised the channelling by reducing the stationary region[17], although it is anticipated that the bubble formation on the surface might still occur.
The use of dynamic test methods (oscillatory shear, which modify the strain rate) may be more appropriate for rheological measurements on foamed concrete, as these provide considerable information about viscoelastic proper- ties of (colloidal) suspensions[18].
5. Volumetric stability—resistance to segregation The mix stability and resistance to segregation of the foamed concretes in the fresh state, assessed in terms of foam loss during mixing, is examined inTable 3. Given that
no superplasticising, viscosity modifying or other chemical admixture was used in the foamed concretes, the stability and resistance to segregation depended solely on the mix constituents, namely quantity of water and presence of fines.
The comparisons between calculated and actual foam quantities required to achieve a plastic density within G50 kg/m3 of the design value, indicated that the mix proportioning method for foamed concrete developed at Dundee University[15]accurately predicted the amount of foam required with sand fine aggregate (actual/calculated foam ratios were near 1.0). The good overall stability of the sand mixes, also noted visually during mixing (good cohesion, uniform incorporation of foam) and placement (no bleeding), indicated that the corresponding sand base mixes were sufficiently ‘wet’ to provide a stable environ- ment for the foam.
On the other hand, in the majority of cases with FAcoarse, the actual amount of foam required was up to three times
Table 3
Comparison between calculated and actual foam quantities and effect on actual free water content Plastic density
(kg/m3)
Fine aggregate type
Calc. water (kg/m3)
Calc. foam (kg/m3)
Actual foam (kg/m3)
Actual/calc.
foam
Actual watera, (kg/m3)
Actual w/c ratio
1000 Sand 150.0 23.5 23.2 0.987 149.8 0.50
FAcoarse 332.5 18.5 20.9 1.130 334.3 0.50
1200 Sand 150.0 20.8 29.5 1.418 156.6 0.52
FAcoarse 405.4 12.3 30.8 2.504 419.8 0.53
1400 Sand 150.0 18.7 20.4 1.091 151.3 0.50
FAcoarse 471.0 6.8 21.9 3.221 482.4 0.52
a Quantities calculated from Eq. (1).
Fig. 3. Rheological parameters of FAcoarsefine aggregate concretes.
higher, due to significant foam collapse in the fresh state.
This can be attributed to adverse effects resulting from the consistence of the base mix (i.e. too wet) and residual carbon content [19] in the FAcoarse. This foam collapse resulted in additional ‘free’ water content (by up to 15 kg/m3), which corresponded to an increase in w/c ratio by up to 0.03. A visual assessment of the FAcoarse mixes concluded that these also exhibited limited amount of bleeding, despite the presence of fines, which are often used to offset the problem. However, these observations suggest that, with fly ash, there is scope for reducing the w/c ratio of the mixes (hence potentially achieving even greater compressive strengths), without compromising their rheo- logical attributes.
Segregation measurements carried out on hardened foamed concrete, quantified as difference in oven-dry densities between two 25 mm thick slices taken from the top and bottom of a 300 mm length cylinder, indicated that all sand and FAcoarseconcretes were stable (i.e. density did not vary more than G50 kg/m3 compared to the mean value).
6. Compressive strength
The sealed-cured 100 mm cube compressive strengths (up to 56 days) of the mixes are given inFig. 4. Although the early age strengths (1 day measurements) were very similar (around 1.0 N/mm2) for both sand and FAcoarseconcretes, the 28 day values varied significantly with both density and fine aggregate type. More specifically, the 28 day strengths of the sand foamed concretes at 1000, 1200 and 1400 kg/m3 were 1.0, 1.5 and 2.0 N/mm2, respectively, while the corresponding strengths of the FAcoarse concretes were more than three times higher (i.e. 3.9, 5.3 and 7.3 N/mm2).
For either aggregate type, compressive strength increased with increasing plastic densities, as expected, due to the lower pore volume[20–22].
The replacement of sand with FAcoarsehad a significant effect on compressive strength development. Indeed, the strengths of FAcoarseconcretes were up to six times higher than those of corresponding sand specimens, with differ- ences becoming increasingly larger with increasing test ages due to the relatively slow nature of the pozzolanic activity of the fly ash[20,21,23]. More specifically, while the strengths of the sand mixes remained fairly constant beyond 28 days, those of FAcoarseconcretes at 56 and 180 days were up to 1.7 and 2.5 times higher than the 28 day values, respectively.
In addition, for a given foamed concrete density, the sand mixes contain slightly higher quantities of air compared with FAcoarse, due to differences in their bulk densities (2630 kg/m3for sand and 2270 kg/m3for FAcoarse), which would also contribute to comparatively lower strengths.
However, for a given PC content, there probably exists a maximum FAcoarse content, beyond which there is no particular advantage in increasing the proportion of fly ash, due to the resulting reduction in the Ca/Si ratio and insufficient Ca(OH)2 for cement hydration[24]. This also has implications for an embedded carbon steel since it may not be fully passivated in this type of concrete.
7. Sorptivity
The one-dimensional sorptivity of foamed concrete was studied in order to determine its resistance to ingress of water and any dissolved aggressive ions, if the material were to be placed in the ground. The sorptivity indices calculated for the range of foamed concretes examined are summarised
Fig. 4. Compressive strength development of PC foamed concretes at 1000, 1200 and 1400 kg/m3plastic densities.
inTable 4, while the influence of constituents and strength on the values is examined inFig. 5.
As can be seen inTable 4, for a given plastic density, the FAcoarseconcretes exhibited higher sorptivity than the sand mixes, with the greatest differences between the two fine aggregate type concretes noted at increasing densities. This trend was observed since the only phase in the foamed concrete matrix capable of sorbing water (assuming discreet, unconnected bubbles, which would obstruct the flow of water in the same way as coarse aggregate particles [25]) is the paste, comprising PC, FAcoarseand water, and the greater volumes of paste are present in the FAcoarse
concretes and at the higher densities. Indeed, there is a good relationship (R2Z0.89) between the fines (PC, FAcoarse) content in the paste and the calculated sorptivities, which is demonstrated inFig. 5and Eq. (5).
SorptivityZ0:0006!ðFines contentÞK0:1023; mm=min1=2 (5) Given the higher compressive strengths resulting from the greater volume of reactive/binding material with FAcoarse, the relationship between sorptivity and compressive strength (sealed-cured 100 mm cube) at 28 days was examined. As can be seen in Fig. 5, there is a clear relationship between the two properties, which can be
approximated (R2Z0.83) using Eq. (6).
SorptivityZ0:0595!ð28 Day Cube StrengthÞ
C0:0001; mm=min1=2 (6) Although the sorptivities of the sand fine aggregate concretes were very similar at all plastic densities (due to the constant volume of paste in the matrix), sorptivities of the FAcoarse concretes tended to increase with density, in line with observations of lightweight mortars [26]. In addition, the greater volume of sorbing paste at the higher densities would be expected to lead to higher sorptivity.
In order to assess whether the sorptivity calculations, reported as increase in mass per unit of dry mass, were sensitive to differences in plastic density (unlike normal weight concrete, where densities are very similar), the results were also expressed as increase in mass per unit volume of concrete. However, the result trends remained the same, in contrast to observations of Kearsley and Wain- wright[25]on water absorption of foamed concretes, where the amount of water absorbed was very similar at all densities. However, this may be attributed to differences in the permeation mechanisms.
Overall, liquid transport through porous media is complex and differentiating between permeation mechan- isms is difficult, given also the evolving concrete pore structure with time, particularly with the fly ash mixes[27].
8. Resistance to aggressive chemical environments Given that foamed concrete is frequently used in backfill, foundations and other groundworks often in ‘brownfield’
sites, its resistance to aggressive chemical environments was investigated. Linear expansion strains were measured on 1000 and 1400 kg/m3 concretes exposed to Design Sulfate Class 2 (DS2) and Class 4 (DS4) chemical solutions with pHO5.5 and these are compared with expansions of specimens stored in water in Fig. 6. In addition, XRD
Fig. 5. Influence of fines content and 28 day sealed-cured 100 mm cube compressive strength on sorptivity.
Table 4
Sorptivity indices of the 1000, 1200 and 1400 kg/m3 sand and FAcoarse
foamed concretes Plastic density (kg/m3)
Fine aggregate type
Sorptivity (mm/min1/2)
FAcoarse/sand sorptivity ratio
1000 Sand 0.101 3.3
FAcoarse 0.332
1200 Sand 0.075 3.7
FAcoarse 0.281
1400 Sand 0.074 6.8
FAcoarse 0.501
analyses of the concretes after 6 months exposure in DS4 solution are compared with those of prisms stored in water inFig. 7.
As can be seen inFig. 6, linear expansion increased with time, however, the measurements during one year exposure appeared ‘erratic’ and the performance ranking of the different fine aggregate type concretes did not remain constant throughout the testing period. More specifically, the 1000 and 1400 kg/m3PC foamed concretes with sand fine aggregate exhibited the greatest expansion at the early stages (within 4 weeks of exposure) in all solutions, but the rate of length increase with time levelled off thereafter, while that of the FAcoarsefine aggregate mixes continued to increase, achieving the highest values (up to 600mstrain) by the end of the exposure period reported. Although the differences between sand and fly ash concrete for a given chemical environment did not exceed 300mstrain, these trends tend to reflect the relative sorption rates. In some cases, the expansion recorded after 50 weeks exposure exceeded 500mstrain, beyond which some visual damage from sulfate reactions would be anticipated [28,29].
However, there was no such evidence. This may be due to the fact that the approximate ‘net’ expansion caused by the chemical environment (and not due to ‘swelling’ from immersion in water) was less than 300mstrain. This, combined with lack of visual evidence of chemical attack, suggests that the expansions noted on the 1000 and 1400 kg/m3concretes during the first 12 months of exposure are not due to sulfate and/or acid reactions. Therefore, foamed concrete has a good resistance to aggressive chemical attack at least up to 12 months.
When comparing the performance in terms of plastic densities, the 1000 kg/m3 foamed concretes exhibited slightly greater length expansion (up to 200mstrain difference at the end of the test period) than the corresponding 1400 kg/m3specimens. These differences in length with changes in density may be attributed to larger pores and more interconnected microstructure of the lower density mixes, allowing the ingress of greater quantities of solution in these specimens, when totally immersed.
In addition to length measurements, specimens were taken for XRD analyses to detect mainly the presence of
Fig. 6. Resistance of 1000 and 1400 kg/m3foamed concretes to aggressive chemical environments.
sulfate products, i.e. gypsum (calcium sulfate—CSH 2), ettringite (calcium sulfo-aluminate hydrate—C6AS3H32) and monosulfate (calcium aluminum sulfate hydrate—
C4ASH 11). The XRD patterns obtained are given in
Fig. 7. As expected, the intensity of the majority of peaks was greater on the DS4 specimens than those in water. All concretes exhibited traces of gypsum (G, mainly at 20.88 2q), while the majority showed minimal traces of ettringite
Fig. 7. XRD patterns of 1000 and 1400 kg/m3 foamed concretes subjected to DS4 exposure with pHO5.5 and reference (H2O) solution (E, Ettringite; G, Gypsum; MS, Monosulfate; P, Portlandite; Q, Quartz; L, Larnite; M, Mullite).
(E, principally at 9.082q). The significantly greater intensity in peaks of gypsum compared with those of ettringite, suggest that the dominant chemical reactions in the foamed concrete specimens took place between magnesium sulfate, calcium silicate hydrates and crystalline Ca(OH)2 in the hydrated cement and calcium aluminate hydrates resulting in the formation of gypsum, rather than reaction between calcium sulfate and calcium aluminate hydrates producing ettringite[30].
These observations support the high resistance to attack noted in the limited expansion tests, but it is recognised that further longer-term testing is necessary before crucial conclusions can be drawn. One interesting point was that there was little or no Portlandite (P, commonly known as Ca(OH)2) in the FAcoarseconcretes, indicating that all was consumed by the pozzolanic reactions.
9. Conclusions
Overall, the replacement of sand with unprocessed run- of-station fly ash had a significant beneficial effect on fresh foamed concrete properties. Indeed, the FAcoarse mixes exhibited enhanced consistence (greater spreadability and flowability out of a modified Marsh cone) and rheology (reduced apparent yield) compared with the sand concretes, due to differences in the fine aggregate particle shape and size. However, the FAcoarseconcretes required up to three times more foam than the calculated quantity to achieve the design plastic density. This was due to foam instability, possibly due to the highly fluid consistency of the base mix and the adverse effects of the high residual carbon in the ash.
However, the slightly lower mix stability of the FAcoarse concretes could potentially be overcome by using lower w/c ratios than the sand mixes.
The use of fly ash in foamed concrete also significantly benefited compressive strength development, particularly after 28 days. At a given age, the FAcoarseconcretes were up to 6 times stronger than those of equivalent sand concretes.
From 28 days to 180 days, the fly ash mixes increased in sealed-cured compressive strength by up to 2.5 times.
Although the FAcoarseconcretes exhibited slightly higher sorptivities than those of the corresponding sand mixes (due to the higher volumes of sorbing paste in the former).
However, this did not have an adverse effect on its performance in aggressive chemical solutions, and the early indications are that this type of concrete could be almost immune from attack.
Acknowledgements
The Authors would like to acknowledge the support, in undertaking this projects of the following organisations:
Department of the Environment, Transport and the Regions, Eastern Electricity Ltd, Foamcrete Ltd, MBT Admixtures,
Propump Engineering Ltd, Ready-mixed Concrete Bureau and Scottish Power Ash Sales.
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