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Effect of ultrafine fly ash on mechanical properties of high volume fly ash mortar
Construction and Building Materials 51 (2014) 278–286
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of ultrafine fly ash on mechanical properties of high volume fly ash mortar Steve W.M. Supit, Faiz U.A. Shaikh ⇑, Prabir K. Sarker Curtin University of Technology, Perth, Western Australia, Australia
h i g h l i g h t s The addition of ultra fine fly ash increased the 7-day compressive strength of HVFA mortars. The above addition also increased the 28-day compressive strength of HVFA mortars. Ultra fine fly ash is effective in consumption of C-H in high volume fly ash system.
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Article history: Received 15 August 2013 Received in revised form 22 October 2013 Accepted 2 November 2013 Available online 28 November 2013 Keywords: Ultrafine fly ash High volume fly ash Compressive strength XRD BSE
a b s t r a c t This paper presents the effect ultrafine fly ash (UFFA) on compressive strength development of mortars containing high volume class F fly ash as partial replacement of cement. The experimental works are divided into two parts. Part one is conducted in binary blended cement mortar where Portland cement (PC) type I is replaced by UFFA at level of 5%, 8%, 10%, 12% and 15% (by wt). In this part, cement mortar and high volume fly ash (HVFA) mortars containing 40%, 50%, 60% and 70% of class F fly ash are also prepared and used as control mortars. The UFFA level which exhibited highest compressive strength is then selected and used in part two where the effect of UFFA in high volume fly ash replacement is evaluated. The study reveals that the cement mortars with 8% UFFA of cement replacement exhibited higher compressive strength at 7 and 28 days than control mortars. There is also a great improvement on compressive strength of HVFA mortars, particularly at early age. The large surface area of the UFFA promotes the hydration process and enhances the microstructure of the cement mortars to yields better strength and mechanical properties. In this study, the microstructure and phase identification after 28 days are also presented based on backscattered electron (BSE) image and x-ray diffraction (XRD) analysis of paste samples. The results indicate the effectiveness of UFFA in producing high packing density and in accelerating the pozzolanic activity to produce more C-S-H gel by consuming calcium hydroxide (CH) in order to improve the mechanical properties of HVFA mortars. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Concrete is one of the most widely used construction materials in the world. As the consumption of concrete rises, the world production of cement is continuing and grew to a significant amount of 3.6 billion tonnes in 2011 [1]. Portland cement production, however, is a highly energy intensive process, and emits CO2 during calcination process which has a crucial effect on global warming. In this connection it is suggested that efforts must be taken on finding environmentally friendly concrete with high performance in strength and durability. For many years, the incorporation of fly ash as partial replacement of cement in concrete is a common practice. The quantity
⇑ Corresponding author. Tel.: +61 8 92669054; fax: +61 8 9266 2681. E-mail address: [email protected] (F.U.A. Shaikh). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.002
of fly ash to replace the cement for typical application is limited to 15–20% by mass of the total cementitious material [2]. It is accepted that as a by-product of industrial process, the utilization of fly ash has made some progress in addressing the challenges of sustainable construction. In addition, fly ash has pozzolanic activity which is attributed to the presence of SiO2 and Al2O3. It reacts with calcium hydroxide during cement hydration, to form additional Calcium Silicate Hydrate (CSH) and Calcium Aluminate Hydrate (CAH) which are effective in forming denser matrix leading to higher strength and better durability [2–4]. The use of high volume fly ash as partial replacement of cement in concrete has also been studied extensively. The main concern in this respect is determining whether or not cement can be replaced by fly ash above the limiting quantity of 15–20% by mass in concrete. Indeed, the small percentage is beneficial in optimizing workability and low cost but it may not improve durability to any considerable extent [5]. On the other hand, due to the
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Fig. 1. XRD analysis of PC, Class F-FA and UFFA.
Table 1 Phase abundance (weight%) of fly ash samples. Phase
Weight%
Hematite Maghemite-C Mullite Mullite Quartz Amorphous Content
Class f fly ash
Ultrafine fly ash
1.7 2.8 16.8 – 15.0 63
– 0.7 – 6.0 11.7 81
Table 2 Chemical composition and physical properties of materials. Chemical analysis
Cement (%)
Class F fly ash (%)
Ultrafine fly ash (%)
SiO2 Al2O3 Fe2O3 CaO MgO MnO K2O Na2O P2O5 TiO2 SO3
20.2 4.9 2.8 63.9 2.0 – – – – – 2.4
51.80 26.40 13.20 1.61 1.17 0.10 0.68 0.31 1.39 1.44 0.21
73.4 17.7 4.4 0.9 0.6 <0.1 1.03 0.11 0.2 0.7 0.2
Physical Properties Particle size Specific gravity Surface area (m2/g) Loss on ignition (%)
25–40% 6 7 lm 2.7–3.2 – 2.4
40% of 10 lm 2.6 – 0.5
Mean size 3.4 lm 2.0–2.55 2.51 0.6
properties of fly ash particles, a higher tendency for possessing some negative effects in terms of early age strength and durability properties can be expected [6,7]. Moreover, the optimization of high volume fly ash has raised many arguments and limitations regardless of the fact that the variation of constituent in fly ash such as alkalis, sulphates, lime and organics may affect the crystallization and slow down the pozzolanic reaction [8]. In order to overcome this deficiency, the incorporation of very small size pozzolanic materials such as silica fume in HVFA system has been established [9–11]. Finer materials are expected to be easily dissolved and accelerate the pozzolanic reaction to improve the strength characteristics of mortars and concretes. Among many ultrafine pozzolanic materials ultrafine fly ash (UFFA) is recently developed. UFFA is produced by a proprietary separation system with a mean particle diameter of 1–5 lm and
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contains 20% more amorphous silica than typical class F fly ash [12]. Generally, ultrafine fly ash (UFFA) is produced from pure class F fly ash by grinding, and separating the ultrafine particles through the air-classification process. The classification system is performed for the removal of coarse particles by size and weight to retain the finer ash fraction. In some cases, this system is beneficial not only in producing finer materials, but also in reducing the carbon content and minimizing the variability of constituents in typical fly ash [13]. The finer particle size, therefore, improves the morphology, mineralogy and chemical composition of materials. Moreover, when compared to cement production, the UFFA production does not require high energy-intensive process thus can result in a cost saving [14]. The other benefits include reducing consumption of natural resources and reducing CO2 emission to stabilize climate change. On the other hand, it has been reported that a reduction in the particle size of fly ash increases the amorphous SiO2 content and tends to decrease the amount of SO3 which can prevent the hydration reaction of harmful ions in concrete or mortar [15]. When the UFFA is used to replace cement content, there is strong indication that an enhancement of strength and higher long-term durability can be obtained. It is because of the smaller particle size of UFFA offers greater surface area for hydration and hence accelerates the pozzolanic reaction. So, the complete hydration reaction can be attained at earlier ages when compared to the ordinary class F fly ash. In addition, the smaller particle is effective to densify the pores structure and, it increases the particle packing affect to increase the density of concrete or mortar. However, the UFFA, when present at appreciably high levels, it tends to increase the water demand as a consequence of accelerated reaction under fineness and high surface area. Too much water can yield a low strength performance. Therefore, the typical dosage rates of UFFA are suggested to be ranged from 8% to 12% of the total binder content [16]. This dosage is lower than the percentage used for class F fly ash in blended cement concrete. In conventional concretes, class F fly ash typically comprises 20% to 30% by mass of cementitious material. This fact indicates that UFFA is sufficient to obtain the most economical concrete mix that satisfies the strength and durability requirements in conjunction with the application of class F fly ash and even the highest reactive pozzolanic material, e.g. silica fume. Based on previous experimental research some beneficial effects have been reported that UFFA was able to enhance the compressive strength when used as partial replacement of cement with low w/c ratio. Obla et al. [12] studied the effect of ultrafine fly ash in concrete on compressive strength and some durability properties. It was concluded that the concrete containing UFFA has higher strength and has a tendency to minimize alkali-silica reaction expansion. It was also found that the strength activity index at 7 and 28 days was 25% to 30% higher than unprocessed fly ash (class F fly ash). Chindaprasirt et al. [17] studied the effect of different fineness of fly ash on pore structure and microstructure of fly ash cement pastes. They reported that the blended cement paste containing finer fly ash particles exhibited a denser matrix and resulted in higher compressive strength and lower total porosity. Hossain et al. [18] observed the effect of UFFA compared to silica fume concrete. It was revealed in their study that the replacement of cement with 12% UFFA content improved cracking resistance when compared to conventional Portland cement concrete and silica fume concrete. Subramaniam et al. [19] carried out an experimental investigation on UFFA concrete with two replacement levels i.e. 8% and 12%. It was concluded that the compressive strength of 8% UFFA was slightly decreased at 1 day but did not hamper the long-term strength development. On the other hand, increasing the UFFA content to 12% resulted in an increase in the resistance of shrinkage cracking. Similarly, the results of the study by Choi et al. [7] showed that the compressive strength
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Table 3 Mixture proportions of binary and ternary blended cement mortar. Series
Mix designation
Cement (kg/m3)
Class F fly ash (kg/m3)
Ultrafine fly ash (kg/m3)
Sand (kg/m3)
Water (kg/m3)
Control 1
PC FA40 FA50 FA60 FA70
400 240 200 160 120
– 160 200 240 280
– – – – –
1100 1100 1100 1100 1100
160 160 160 160 160
2
UFFA5 UFFA8 UFFA10 UFFA12 UFFA15
380 368 360 352 340
– – – – –
20 32 40 48 60
1100 1100 1100 1100 1100
160 160 160 160 160
3
FA32.UFFA8 FA42.UFFA8 FA52.UFFA8 FA62.UFFA8
240 200 160 120
128 168 208 248
32 32 32 32
1100 1100 1100 1100
160 160 160 160
Fig. 2. Workability of different mixes.
Fig. 3. Effect of ultrafine fly ash on workability of different high volume fly ash mixes.
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Fig. 4. Compressive strength of Mortars containing PC, FA and UFFA.
2. Experimental details 2.1. Materials The cement used in this study was ASTM type I cement in accordance with the ASTM C150 [20] standard. The commercially available class F fly ash (FA) and ultrafine fly ash (UFFA) were used as partial replacement of cement. The characteristics of the raw materials based on XRD analysis are shown in Fig. 1. Table 1 summarizes the quantitative analysis of crystalline minerals of fly ash such as hematite, maghemite-c, mullite and quartz. It also shows that ultrafine fly ash has about 81% amorphous content, which is higher than the amount of amorphous content in class F fly ash. The chemical analysis and physical properties of all materials used are listed on Table 2. 2.2. Mixture proportions
Fig. 5. Strength activity index of mortars containing PC, FA and UFFA.
of concrete increased with increasing fineness of fly ash. The compressive strength was lower than control mixes before 7 days and higher after 14 days. From their investigations, it was clear that the properties improvements were mainly caused by the fineness of fly ash and became a main factor contributing to the compressive strength. However, although extensive research has been carried out on ultrafine fly ash blended cement, very few studies evaluated the effectiveness of ultrafine fly ash in compressive strength development of high volume fly ash blended cement binder. The combination of UFFA and class F fly ash is expected to improve the early age and long-term strength development, where UFFA will compensate the low early strength of concrete containing fly ash and class F fly ash will contribute to the long-term strength development due to its slow pozzolanic reaction. However, this hypothesis needs to be confirmed experimentally before its use in the concrete industry. The present paper, therefore, focused on investigating the performance of ultrafine fly ash in binary and ternary blended cement (HVFA + UFFA) binders based on the compressive strength development at 7 and 28 days. The microstructure of paste samples on each mix were analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques.
The experimental works have been divided into two parts, the first part investigates the effect of 5%, 8%, 10%, 12% and 15% (by wt.) replacement of cement by UFFA on the compressive strength of mortar. The effects of high fly ash contents e.g. 40%, 50%, 60% and 70% (by wt.) as partial replacement of cement on compressive strength development is also evaluated in this part. The UFFA content that exhibited highest compressive strength in the first part is used in the second part of this study, where the effect of UFFA on the compressive strength of mortars containing 40%, 50%, 60% and 70% Class F Fly Ash (by wt.) as partial replacement of cement are investigated. The mixture proportions are described in Table 3. 2.3. Methods All mortars were mixed in a Hobart mixer using water to binder ratio of 0.4 and sand/binder ratio of 2.75. The mortar samples of size 50 50 50 mm3 cube were cast and demoulded after 24 h. The specimens were cured in water until compressive strength measurements were carried out after 7 and 28 days. Compressive strength of mortar specimens were tested according to the requirements of ASTM C109 [21] using a loading rate of 0.7 MPa/s. At least three cubes were tested for each mix and curing time. The compressive strength results presented in the figures are obtained by averaging of three cubes results.
3. Results and discussion 3.1. Effect of ultrafine fly ash on workability of mortars The effect of ultrafine fly ash on the workability of cement mortar and mortars containing high contents of fly ash is shown in Figs. 2 and 3. The workability of mortar is measured in terms of flow diameter according to ASTM C1437 [22]. It can be seen in
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Fig. 6. Effect of 8% UFFA on compressive strength of mortars containing HVFA.
amount of UFFA (e.g. 8% in this study) in HVFA mortars slightly reduced their workability. The ultrafine fly ash content of 8% was selected since it achieved the highest compressive strength and is discussed in the next section. 3.2. Effect of ultrafine fly ash on compressive strength of cement mortar
Fig. 7. Strength activity index of HVFA mortars containing 8% UFFA.
Fig. 2 that the rate of increase of flow values is higher in the case of HVFA mortars than that of mortars containing UFFA. The lower workability of mortars containing UFFA than that of HVFA mortars is due to smaller particle size and higher surface area of UFFA. The effect of UFFA on the workability of mortars containing HVFA can be seen in Fig. 3. The figure shows that the addition of a particular
The compressive strengths of binary blended cement mortars measured at 7 and 28 days are presented in Fig. 4. It can be seen that the mortars containing UFFA exhibited higher compressive strength as compared to that of control mortar and mortars containing high fly ash contents. Among different UFFA contents, the highest compressive strength was achieved when the cement was replaced by 8% UFFA. The results show that the UFFA8 mortar had a compressive strength of 43 MPa, whereas the compressive strength of control cement mortar was 35 MPa at 28 days. It was also evident that the UFFA addition had a positive effect on strength development of mortar at 7 days. The results were also compared to the control cement mortar in order to illustrate the improvement in compressive strength due to FA and UFFA replacement. Fig. 5 shows the strength activity index defined as the ratio of the compressive strength of mortar containing either HVFA or UFFA to that of control cement mortar. Fig. 5 clearly shows that all HVFA mortars except that with 40% FA exhibited lower compressive strength at 7 and 28 days than control cement mortars. On the contrary, an opposite trend can be identified
Table 4 Phase abundance of paste samples based on quantitative XRD analysis. Phase
Dicalcium Silicate – C2S Tricalcium Silicate – C3S Calcite – CaCO3 Ettringite – Ca6Al2(SO4)3(OH)1226H2O Portlandite – Ca(OH)2 Quartz – SiO2 Amorphous content
Weight (%) PC
FA40
FA60
UFFA8
FA32.UF8
FA52.UF8
16.6 5.0 5.0 0.5 12.1 – 54
8.7 2.9 2.9 – 7.2 6.5 63
15.1 3.2 4.3 0.4 13.0 0.9 58
6.4 0.7 2.1 0.4 3.3 9.8 63
5.9 1.5 – – 2.9 9.4 66
8.9 1.7 3.3 1.4 5.6 5.6 60
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Fig. 8. Black scattered electron (BSE)images of polished surface of paste samples after 28 days of curing.
in the mortars containing different ultrafine fly ash contents. The addition of UFFA enhanced the compressive strength of cement mortar at 7 and 28 days where the greatest improvement was achieved in mortar containing 8% UFFA. The strength activity index of this binary blended mortar (UFFA8) is 27% at 7 days and 23% at 28 days and indicates the higher influence of UFFA at early ages (7 days) than later ages (28 days) (see Fig. 5). Similar trend is also observed for other UFFA contents. The same conclusion was also reached by Subramaniam et al. [19], where the compressive strength of UFFA mortars at early ages was higher than control mix and was comparable with the compressive strength of silica fume concrete. The reason behind this improvement is due to the small particle of UFFA which accelerates the pozzolanic reaction and fills the pores resulting in improved compressive strength (see Fig. 5). 3.3. Effect of ultrafine fly ash on compressive strength of HVFA mortar The main purpose of this study was to evaluate the effectiveness of UFFA on the development of compressive strength of HVFA systems both at 7 days (early age) and 28 days. The effect of ultra-
fine fly ash on the compressive strength development of high volume fly ash (HVFA) mortars at 7 and 28 days is discussed here. The ultrafine fly ash content of 8% was selected since it achieved the highest compressive strength in binary blended cement mortars in the first part. Fig. 6 shows the effect of 8% UFFA on the compressive strength of HVFA mortars at 7 and 28 days. The compressive strengths of HVFA mortars and control cement mortar without UFFA are also shown in the same figure for comparison purpose. Results show that the addition of 8% UFFA improved the 7 and 28 days compressive strengths of HVFA mortars for all fly ash contents. With regard to the effect of 8% UFFA on 7 days compressive strength of HVFA mortars, it can be seen that the improvement is observed for all fly ash contents and it ranged between 26% and 63%. On the other hand, the addition of 8% UFFA slightly improved the 28 days compressive strength of HVFA mortar containing 40% fly ash and significantly improved the 28 days compressive strength of mortars containing more than 50% fly ash. The most significant improvement is observed in mortar containing 50% fly ash whose 28 days compressive strength is improved by about 31% due to addition of 8% UFFA (see Fig. 7). The quantitative XRD analysis results, shown in Table 4, also support this observation
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Fig. 9. XRD analysis of different paste samples after 28 days of curing.
where the addition of 8% UFFA consumed about 60% calcium hydroxide (CH) and generated 5% more amorphous C-S-H content in the mortar containing 40% fly ash. On the other hand, in the mortar containing 60% fly ash the reduction in CH content is 57% and increase in amorphous content is 3.5%. This clearly shows the effectiveness of UFFA in consuming CH and hence generating more amorphous CSH in HVFA mortars.
3.4. Microstructural analysis of HVFA pastes containing UFFA Scanning Electron Microscopy (SEM) was conducted to analyze the microstructure of different pastes at 28 days (Fig. 8a–f). The polished section of hardened binary and ternary blended cement paste were prepared and imaged using Backscattered Electrons (BSE) signal. BSE is useful to identify the constituent phases within
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cement paste based on their brightness. Different hydration products can be identified based on descending brightness of un-hydrated cement particles, calcium hydroxide (CH), other hydration products, C–S–H and voids/cracks [23,24]. Therefore, the brightest (or white) parts can be classified as un-hydrated cement particles, the dark (or black) parts as voids/cracks and grey to dark grey parts can be classified as CH, other hydration products and C–S–H in the BSE image [24]. The BSE images of paste containing 40% and 60% fly ash are shown in Fig. 8b and c, respectively. Many white and black areas can be identified in the images representing un-hydrated products and voids/cracks, respectively. Many unreacted spherical shape fly ash particles are also seen in the BSE images. The grey to dark grey areas in those images are believed to be those of hydration products. On the other hand, few voids/cracks (limited black areas) and un-hydrated cement particles (white areas) are noticed in the sample containing 8% UFFA (see Fig. 9d). No such spherical shape unreacted fly ash particles are seen in that BSE image, indicating the consumption of UFFA in the hydration reaction. In the same sample large number of grey to dark grey areas representative of hydration products are also observed. The effect of addition of 8% UFFA in the paste containing 38% and 58% fly ash can also be seen by comparing the BSE images between Fig. 8b and e and between Fig. 8c and f. Clear indication of positive effect of UFFA in HVFA paste can be observed where few black areas and more grey to dark grey areas compared to those of paste containing 40% and 60% fly ash can be clearly seen. Areas with dark grey are also comparably higher than that in the paste containing 40% and 60% fly ash, indicating the consumption of CH by the UFFA and the formation of C–S–H. 3.5. X-ray diffraction analysis of HVFA pastes containing UFFA The hydration of different pastes was also analysed by X-ray diffraction (XRD) technique. In this study, a D8 Advance Diffractometer (Bruker-AXS) with a X-ray source of Cu Ka radiation (k = 1.5406 Å) was used. The scan step was 0.02° using a scanning rate of .5°/min and in the range 2h Cu Ka from 7° to 70°. The horizontal scale (diffraction angle) of a typical XRD pattern gives the crystal lattice spacing, and the vertical scale (peak height) gives the intensity of the diffracted ray. Fig. 9 shows the XRD patterns of cement paste and pastes containing HVFA, UFFA and combined HVFA and UFFA (Fig. 9a–f) while the quantitative results of XRD data are presented in Table 4. In Fig. 9, the XRD analysis indicates predominance of Portlandite (CH) and Calcium Silicate (C2S/C3S). In all samples Quartz (SiO2) is also identified while the unnamed peaks with high intensity is identified as internal standard (corundum phase) which has not been included in the phase abundance calculations. Moreover, Ettringite (Ca6Al2(SO4)3(OH)1226H2O) and Calcite (CaCO3) are present in small quantity. The CH has a strong peak located at 2theta angle of 18.05° and was selected to be the main indicator of hydration performance in all paste samples. As a product of cement hydration, CH is able to react with SiO2 in supplementary cementing system and form additional C-S-H to improve the mechanical properties of mortars. From the diffractogram peaks in Fig. 9a–d, it is apparent that the intensity peaks of CH were significantly decreased when cement was replaced with 8% UFFA. The intensity counts of CH at 2-theta angle of 18.05° in 8% UFFA paste dropped to 3179 from 5986 for pure cement paste and from 4319 and 5486 for pastes with 40% and 60% fly ash contents, respectively. Similarly, a positive performance was found in HVFA pastes containing 8% UFFA. The diffraction peak intensity of CH at 2-theta angle of 18.05° decreased from 4319 (in paste containing 40% FA) to 3078 of paste containing 32% FA and 8% UFFA. The intensity of CH in 60% FA paste also decreased from 5485 to 4973, about 9.3% reduction when 8% UFFA was added
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(see Fig. 9e and f). These results are confirmed with the quantitative X-ray diffraction analysis as listed in Table. 4. The percentage of CH in HVFA pastes with 8% UFFA addition was 2.9% and 5.6%, respectively for 40% and 60% of cement replacement. These amounts were 60% and 56% lower than the CH content in HVFA pastes without UFFA addition. Furthermore, the findings indicate that the consumption of CH in the pozzolanic reaction by the SiO2 of fly ash in HVFA with 8% UFFA paste is more effective in contributing the additional pozzolanic reaction products to strength enhancement.
4. Conclusions Based on 7 and 28 days compressive strength, SEM and XRD results on the effect of ultrafine fly ash in high volume fly ash mortars, the following conclusions can be drawn: 1. The ultrafine fly ash had a more pronounced effect on compressive strength of mortars, particularly at early ages. Cement mortar containing 8% UFFA as partial replacement of cement exhibited 27% improvement in compressive strength at 7 days as compared to control cement mortar. At 28 days, the compressive strength of 8% UFFA mortar increased by 23%. The findings imply that the strength development of mortars is largely determined by the use of ultrafine fly ash which has finer particle sizes instead of the coarser class F fly ash. This suggests that UFFA has a more pronounced effect on compressive strength, particularly at early days. However, the proper mixture proportions should be considered in order to achieve the effective improvement. 2. The addition of 8% UFFA accelerated the pozzolanic reaction and increased the compressive strength at 7 days of HVFA mortars containing 40%, 50%, 60% and 70% between 26% and 63%. At 28 days, the increase in compressive strength was between 2% and 31%. It is noted that UFFA compensates the low early strength of binary blended mortars containing cement and class f fly ash. Therefore, blended cement containing UFFA and class F fly ash in HVFA mortars offers great potential for use in concrete construction where both short and long term compressive strength are considered. 3. According to BSE image analysis, the incorporation of 8% UFFA, as a partial cement replacement of cement in HVFA pastes densified the microstructure which led to the improvement of compressive strength. It is suggested that the particle packing effect of UFFA plays an important role in reducing the volume of pores. 4. The XRD analysis showed that the ultrafine fly ash replacement of cement is effective in increasing the CH consumption level of HVFA pastes and hence production of additional CSH gels. It is attributed to the higher of amorphous SiO2 content of UFFA.
Acknowledgement Authors acknowledge fly ash Australia for donating class F fly ash and ultrafine fly ash in this study. References [1] Global carbon capture and storage Institute
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of ultrafine fly ash on mechanical properties of high volume fly ash mortar Steve W.M. Supit, Faiz U.A. Shaikh ⇑, Prabir K. Sarker Curtin University of Technology, Perth, Western Australia, Australia
h i g h l i g h t s The addition of ultra fine fly ash increased the 7-day compressive strength of HVFA mortars. The above addition also increased the 28-day compressive strength of HVFA mortars. Ultra fine fly ash is effective in consumption of C-H in high volume fly ash system.
a r t i c l e
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Article history: Received 15 August 2013 Received in revised form 22 October 2013 Accepted 2 November 2013 Available online 28 November 2013 Keywords: Ultrafine fly ash High volume fly ash Compressive strength XRD BSE
a b s t r a c t This paper presents the effect ultrafine fly ash (UFFA) on compressive strength development of mortars containing high volume class F fly ash as partial replacement of cement. The experimental works are divided into two parts. Part one is conducted in binary blended cement mortar where Portland cement (PC) type I is replaced by UFFA at level of 5%, 8%, 10%, 12% and 15% (by wt). In this part, cement mortar and high volume fly ash (HVFA) mortars containing 40%, 50%, 60% and 70% of class F fly ash are also prepared and used as control mortars. The UFFA level which exhibited highest compressive strength is then selected and used in part two where the effect of UFFA in high volume fly ash replacement is evaluated. The study reveals that the cement mortars with 8% UFFA of cement replacement exhibited higher compressive strength at 7 and 28 days than control mortars. There is also a great improvement on compressive strength of HVFA mortars, particularly at early age. The large surface area of the UFFA promotes the hydration process and enhances the microstructure of the cement mortars to yields better strength and mechanical properties. In this study, the microstructure and phase identification after 28 days are also presented based on backscattered electron (BSE) image and x-ray diffraction (XRD) analysis of paste samples. The results indicate the effectiveness of UFFA in producing high packing density and in accelerating the pozzolanic activity to produce more C-S-H gel by consuming calcium hydroxide (CH) in order to improve the mechanical properties of HVFA mortars. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Concrete is one of the most widely used construction materials in the world. As the consumption of concrete rises, the world production of cement is continuing and grew to a significant amount of 3.6 billion tonnes in 2011 [1]. Portland cement production, however, is a highly energy intensive process, and emits CO2 during calcination process which has a crucial effect on global warming. In this connection it is suggested that efforts must be taken on finding environmentally friendly concrete with high performance in strength and durability. For many years, the incorporation of fly ash as partial replacement of cement in concrete is a common practice. The quantity
⇑ Corresponding author. Tel.: +61 8 92669054; fax: +61 8 9266 2681. E-mail address: [email protected] (F.U.A. Shaikh). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.002
of fly ash to replace the cement for typical application is limited to 15–20% by mass of the total cementitious material [2]. It is accepted that as a by-product of industrial process, the utilization of fly ash has made some progress in addressing the challenges of sustainable construction. In addition, fly ash has pozzolanic activity which is attributed to the presence of SiO2 and Al2O3. It reacts with calcium hydroxide during cement hydration, to form additional Calcium Silicate Hydrate (CSH) and Calcium Aluminate Hydrate (CAH) which are effective in forming denser matrix leading to higher strength and better durability [2–4]. The use of high volume fly ash as partial replacement of cement in concrete has also been studied extensively. The main concern in this respect is determining whether or not cement can be replaced by fly ash above the limiting quantity of 15–20% by mass in concrete. Indeed, the small percentage is beneficial in optimizing workability and low cost but it may not improve durability to any considerable extent [5]. On the other hand, due to the
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Fig. 1. XRD analysis of PC, Class F-FA and UFFA.
Table 1 Phase abundance (weight%) of fly ash samples. Phase
Weight%
Hematite Maghemite-C Mullite Mullite Quartz Amorphous Content
Class f fly ash
Ultrafine fly ash
1.7 2.8 16.8 – 15.0 63
– 0.7 – 6.0 11.7 81
Table 2 Chemical composition and physical properties of materials. Chemical analysis
Cement (%)
Class F fly ash (%)
Ultrafine fly ash (%)
SiO2 Al2O3 Fe2O3 CaO MgO MnO K2O Na2O P2O5 TiO2 SO3
20.2 4.9 2.8 63.9 2.0 – – – – – 2.4
51.80 26.40 13.20 1.61 1.17 0.10 0.68 0.31 1.39 1.44 0.21
73.4 17.7 4.4 0.9 0.6 <0.1 1.03 0.11 0.2 0.7 0.2
Physical Properties Particle size Specific gravity Surface area (m2/g) Loss on ignition (%)
25–40% 6 7 lm 2.7–3.2 – 2.4
40% of 10 lm 2.6 – 0.5
Mean size 3.4 lm 2.0–2.55 2.51 0.6
properties of fly ash particles, a higher tendency for possessing some negative effects in terms of early age strength and durability properties can be expected [6,7]. Moreover, the optimization of high volume fly ash has raised many arguments and limitations regardless of the fact that the variation of constituent in fly ash such as alkalis, sulphates, lime and organics may affect the crystallization and slow down the pozzolanic reaction [8]. In order to overcome this deficiency, the incorporation of very small size pozzolanic materials such as silica fume in HVFA system has been established [9–11]. Finer materials are expected to be easily dissolved and accelerate the pozzolanic reaction to improve the strength characteristics of mortars and concretes. Among many ultrafine pozzolanic materials ultrafine fly ash (UFFA) is recently developed. UFFA is produced by a proprietary separation system with a mean particle diameter of 1–5 lm and
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contains 20% more amorphous silica than typical class F fly ash [12]. Generally, ultrafine fly ash (UFFA) is produced from pure class F fly ash by grinding, and separating the ultrafine particles through the air-classification process. The classification system is performed for the removal of coarse particles by size and weight to retain the finer ash fraction. In some cases, this system is beneficial not only in producing finer materials, but also in reducing the carbon content and minimizing the variability of constituents in typical fly ash [13]. The finer particle size, therefore, improves the morphology, mineralogy and chemical composition of materials. Moreover, when compared to cement production, the UFFA production does not require high energy-intensive process thus can result in a cost saving [14]. The other benefits include reducing consumption of natural resources and reducing CO2 emission to stabilize climate change. On the other hand, it has been reported that a reduction in the particle size of fly ash increases the amorphous SiO2 content and tends to decrease the amount of SO3 which can prevent the hydration reaction of harmful ions in concrete or mortar [15]. When the UFFA is used to replace cement content, there is strong indication that an enhancement of strength and higher long-term durability can be obtained. It is because of the smaller particle size of UFFA offers greater surface area for hydration and hence accelerates the pozzolanic reaction. So, the complete hydration reaction can be attained at earlier ages when compared to the ordinary class F fly ash. In addition, the smaller particle is effective to densify the pores structure and, it increases the particle packing affect to increase the density of concrete or mortar. However, the UFFA, when present at appreciably high levels, it tends to increase the water demand as a consequence of accelerated reaction under fineness and high surface area. Too much water can yield a low strength performance. Therefore, the typical dosage rates of UFFA are suggested to be ranged from 8% to 12% of the total binder content [16]. This dosage is lower than the percentage used for class F fly ash in blended cement concrete. In conventional concretes, class F fly ash typically comprises 20% to 30% by mass of cementitious material. This fact indicates that UFFA is sufficient to obtain the most economical concrete mix that satisfies the strength and durability requirements in conjunction with the application of class F fly ash and even the highest reactive pozzolanic material, e.g. silica fume. Based on previous experimental research some beneficial effects have been reported that UFFA was able to enhance the compressive strength when used as partial replacement of cement with low w/c ratio. Obla et al. [12] studied the effect of ultrafine fly ash in concrete on compressive strength and some durability properties. It was concluded that the concrete containing UFFA has higher strength and has a tendency to minimize alkali-silica reaction expansion. It was also found that the strength activity index at 7 and 28 days was 25% to 30% higher than unprocessed fly ash (class F fly ash). Chindaprasirt et al. [17] studied the effect of different fineness of fly ash on pore structure and microstructure of fly ash cement pastes. They reported that the blended cement paste containing finer fly ash particles exhibited a denser matrix and resulted in higher compressive strength and lower total porosity. Hossain et al. [18] observed the effect of UFFA compared to silica fume concrete. It was revealed in their study that the replacement of cement with 12% UFFA content improved cracking resistance when compared to conventional Portland cement concrete and silica fume concrete. Subramaniam et al. [19] carried out an experimental investigation on UFFA concrete with two replacement levels i.e. 8% and 12%. It was concluded that the compressive strength of 8% UFFA was slightly decreased at 1 day but did not hamper the long-term strength development. On the other hand, increasing the UFFA content to 12% resulted in an increase in the resistance of shrinkage cracking. Similarly, the results of the study by Choi et al. [7] showed that the compressive strength
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Table 3 Mixture proportions of binary and ternary blended cement mortar. Series
Mix designation
Cement (kg/m3)
Class F fly ash (kg/m3)
Ultrafine fly ash (kg/m3)
Sand (kg/m3)
Water (kg/m3)
Control 1
PC FA40 FA50 FA60 FA70
400 240 200 160 120
– 160 200 240 280
– – – – –
1100 1100 1100 1100 1100
160 160 160 160 160
2
UFFA5 UFFA8 UFFA10 UFFA12 UFFA15
380 368 360 352 340
– – – – –
20 32 40 48 60
1100 1100 1100 1100 1100
160 160 160 160 160
3
FA32.UFFA8 FA42.UFFA8 FA52.UFFA8 FA62.UFFA8
240 200 160 120
128 168 208 248
32 32 32 32
1100 1100 1100 1100
160 160 160 160
Fig. 2. Workability of different mixes.
Fig. 3. Effect of ultrafine fly ash on workability of different high volume fly ash mixes.
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Fig. 4. Compressive strength of Mortars containing PC, FA and UFFA.
2. Experimental details 2.1. Materials The cement used in this study was ASTM type I cement in accordance with the ASTM C150 [20] standard. The commercially available class F fly ash (FA) and ultrafine fly ash (UFFA) were used as partial replacement of cement. The characteristics of the raw materials based on XRD analysis are shown in Fig. 1. Table 1 summarizes the quantitative analysis of crystalline minerals of fly ash such as hematite, maghemite-c, mullite and quartz. It also shows that ultrafine fly ash has about 81% amorphous content, which is higher than the amount of amorphous content in class F fly ash. The chemical analysis and physical properties of all materials used are listed on Table 2. 2.2. Mixture proportions
Fig. 5. Strength activity index of mortars containing PC, FA and UFFA.
of concrete increased with increasing fineness of fly ash. The compressive strength was lower than control mixes before 7 days and higher after 14 days. From their investigations, it was clear that the properties improvements were mainly caused by the fineness of fly ash and became a main factor contributing to the compressive strength. However, although extensive research has been carried out on ultrafine fly ash blended cement, very few studies evaluated the effectiveness of ultrafine fly ash in compressive strength development of high volume fly ash blended cement binder. The combination of UFFA and class F fly ash is expected to improve the early age and long-term strength development, where UFFA will compensate the low early strength of concrete containing fly ash and class F fly ash will contribute to the long-term strength development due to its slow pozzolanic reaction. However, this hypothesis needs to be confirmed experimentally before its use in the concrete industry. The present paper, therefore, focused on investigating the performance of ultrafine fly ash in binary and ternary blended cement (HVFA + UFFA) binders based on the compressive strength development at 7 and 28 days. The microstructure of paste samples on each mix were analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques.
The experimental works have been divided into two parts, the first part investigates the effect of 5%, 8%, 10%, 12% and 15% (by wt.) replacement of cement by UFFA on the compressive strength of mortar. The effects of high fly ash contents e.g. 40%, 50%, 60% and 70% (by wt.) as partial replacement of cement on compressive strength development is also evaluated in this part. The UFFA content that exhibited highest compressive strength in the first part is used in the second part of this study, where the effect of UFFA on the compressive strength of mortars containing 40%, 50%, 60% and 70% Class F Fly Ash (by wt.) as partial replacement of cement are investigated. The mixture proportions are described in Table 3. 2.3. Methods All mortars were mixed in a Hobart mixer using water to binder ratio of 0.4 and sand/binder ratio of 2.75. The mortar samples of size 50 50 50 mm3 cube were cast and demoulded after 24 h. The specimens were cured in water until compressive strength measurements were carried out after 7 and 28 days. Compressive strength of mortar specimens were tested according to the requirements of ASTM C109 [21] using a loading rate of 0.7 MPa/s. At least three cubes were tested for each mix and curing time. The compressive strength results presented in the figures are obtained by averaging of three cubes results.
3. Results and discussion 3.1. Effect of ultrafine fly ash on workability of mortars The effect of ultrafine fly ash on the workability of cement mortar and mortars containing high contents of fly ash is shown in Figs. 2 and 3. The workability of mortar is measured in terms of flow diameter according to ASTM C1437 [22]. It can be seen in
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Fig. 6. Effect of 8% UFFA on compressive strength of mortars containing HVFA.
amount of UFFA (e.g. 8% in this study) in HVFA mortars slightly reduced their workability. The ultrafine fly ash content of 8% was selected since it achieved the highest compressive strength and is discussed in the next section. 3.2. Effect of ultrafine fly ash on compressive strength of cement mortar
Fig. 7. Strength activity index of HVFA mortars containing 8% UFFA.
Fig. 2 that the rate of increase of flow values is higher in the case of HVFA mortars than that of mortars containing UFFA. The lower workability of mortars containing UFFA than that of HVFA mortars is due to smaller particle size and higher surface area of UFFA. The effect of UFFA on the workability of mortars containing HVFA can be seen in Fig. 3. The figure shows that the addition of a particular
The compressive strengths of binary blended cement mortars measured at 7 and 28 days are presented in Fig. 4. It can be seen that the mortars containing UFFA exhibited higher compressive strength as compared to that of control mortar and mortars containing high fly ash contents. Among different UFFA contents, the highest compressive strength was achieved when the cement was replaced by 8% UFFA. The results show that the UFFA8 mortar had a compressive strength of 43 MPa, whereas the compressive strength of control cement mortar was 35 MPa at 28 days. It was also evident that the UFFA addition had a positive effect on strength development of mortar at 7 days. The results were also compared to the control cement mortar in order to illustrate the improvement in compressive strength due to FA and UFFA replacement. Fig. 5 shows the strength activity index defined as the ratio of the compressive strength of mortar containing either HVFA or UFFA to that of control cement mortar. Fig. 5 clearly shows that all HVFA mortars except that with 40% FA exhibited lower compressive strength at 7 and 28 days than control cement mortars. On the contrary, an opposite trend can be identified
Table 4 Phase abundance of paste samples based on quantitative XRD analysis. Phase
Dicalcium Silicate – C2S Tricalcium Silicate – C3S Calcite – CaCO3 Ettringite – Ca6Al2(SO4)3(OH)1226H2O Portlandite – Ca(OH)2 Quartz – SiO2 Amorphous content
Weight (%) PC
FA40
FA60
UFFA8
FA32.UF8
FA52.UF8
16.6 5.0 5.0 0.5 12.1 – 54
8.7 2.9 2.9 – 7.2 6.5 63
15.1 3.2 4.3 0.4 13.0 0.9 58
6.4 0.7 2.1 0.4 3.3 9.8 63
5.9 1.5 – – 2.9 9.4 66
8.9 1.7 3.3 1.4 5.6 5.6 60
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Fig. 8. Black scattered electron (BSE)images of polished surface of paste samples after 28 days of curing.
in the mortars containing different ultrafine fly ash contents. The addition of UFFA enhanced the compressive strength of cement mortar at 7 and 28 days where the greatest improvement was achieved in mortar containing 8% UFFA. The strength activity index of this binary blended mortar (UFFA8) is 27% at 7 days and 23% at 28 days and indicates the higher influence of UFFA at early ages (7 days) than later ages (28 days) (see Fig. 5). Similar trend is also observed for other UFFA contents. The same conclusion was also reached by Subramaniam et al. [19], where the compressive strength of UFFA mortars at early ages was higher than control mix and was comparable with the compressive strength of silica fume concrete. The reason behind this improvement is due to the small particle of UFFA which accelerates the pozzolanic reaction and fills the pores resulting in improved compressive strength (see Fig. 5). 3.3. Effect of ultrafine fly ash on compressive strength of HVFA mortar The main purpose of this study was to evaluate the effectiveness of UFFA on the development of compressive strength of HVFA systems both at 7 days (early age) and 28 days. The effect of ultra-
fine fly ash on the compressive strength development of high volume fly ash (HVFA) mortars at 7 and 28 days is discussed here. The ultrafine fly ash content of 8% was selected since it achieved the highest compressive strength in binary blended cement mortars in the first part. Fig. 6 shows the effect of 8% UFFA on the compressive strength of HVFA mortars at 7 and 28 days. The compressive strengths of HVFA mortars and control cement mortar without UFFA are also shown in the same figure for comparison purpose. Results show that the addition of 8% UFFA improved the 7 and 28 days compressive strengths of HVFA mortars for all fly ash contents. With regard to the effect of 8% UFFA on 7 days compressive strength of HVFA mortars, it can be seen that the improvement is observed for all fly ash contents and it ranged between 26% and 63%. On the other hand, the addition of 8% UFFA slightly improved the 28 days compressive strength of HVFA mortar containing 40% fly ash and significantly improved the 28 days compressive strength of mortars containing more than 50% fly ash. The most significant improvement is observed in mortar containing 50% fly ash whose 28 days compressive strength is improved by about 31% due to addition of 8% UFFA (see Fig. 7). The quantitative XRD analysis results, shown in Table 4, also support this observation
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Fig. 9. XRD analysis of different paste samples after 28 days of curing.
where the addition of 8% UFFA consumed about 60% calcium hydroxide (CH) and generated 5% more amorphous C-S-H content in the mortar containing 40% fly ash. On the other hand, in the mortar containing 60% fly ash the reduction in CH content is 57% and increase in amorphous content is 3.5%. This clearly shows the effectiveness of UFFA in consuming CH and hence generating more amorphous CSH in HVFA mortars.
3.4. Microstructural analysis of HVFA pastes containing UFFA Scanning Electron Microscopy (SEM) was conducted to analyze the microstructure of different pastes at 28 days (Fig. 8a–f). The polished section of hardened binary and ternary blended cement paste were prepared and imaged using Backscattered Electrons (BSE) signal. BSE is useful to identify the constituent phases within
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cement paste based on their brightness. Different hydration products can be identified based on descending brightness of un-hydrated cement particles, calcium hydroxide (CH), other hydration products, C–S–H and voids/cracks [23,24]. Therefore, the brightest (or white) parts can be classified as un-hydrated cement particles, the dark (or black) parts as voids/cracks and grey to dark grey parts can be classified as CH, other hydration products and C–S–H in the BSE image [24]. The BSE images of paste containing 40% and 60% fly ash are shown in Fig. 8b and c, respectively. Many white and black areas can be identified in the images representing un-hydrated products and voids/cracks, respectively. Many unreacted spherical shape fly ash particles are also seen in the BSE images. The grey to dark grey areas in those images are believed to be those of hydration products. On the other hand, few voids/cracks (limited black areas) and un-hydrated cement particles (white areas) are noticed in the sample containing 8% UFFA (see Fig. 9d). No such spherical shape unreacted fly ash particles are seen in that BSE image, indicating the consumption of UFFA in the hydration reaction. In the same sample large number of grey to dark grey areas representative of hydration products are also observed. The effect of addition of 8% UFFA in the paste containing 38% and 58% fly ash can also be seen by comparing the BSE images between Fig. 8b and e and between Fig. 8c and f. Clear indication of positive effect of UFFA in HVFA paste can be observed where few black areas and more grey to dark grey areas compared to those of paste containing 40% and 60% fly ash can be clearly seen. Areas with dark grey are also comparably higher than that in the paste containing 40% and 60% fly ash, indicating the consumption of CH by the UFFA and the formation of C–S–H. 3.5. X-ray diffraction analysis of HVFA pastes containing UFFA The hydration of different pastes was also analysed by X-ray diffraction (XRD) technique. In this study, a D8 Advance Diffractometer (Bruker-AXS) with a X-ray source of Cu Ka radiation (k = 1.5406 Å) was used. The scan step was 0.02° using a scanning rate of .5°/min and in the range 2h Cu Ka from 7° to 70°. The horizontal scale (diffraction angle) of a typical XRD pattern gives the crystal lattice spacing, and the vertical scale (peak height) gives the intensity of the diffracted ray. Fig. 9 shows the XRD patterns of cement paste and pastes containing HVFA, UFFA and combined HVFA and UFFA (Fig. 9a–f) while the quantitative results of XRD data are presented in Table 4. In Fig. 9, the XRD analysis indicates predominance of Portlandite (CH) and Calcium Silicate (C2S/C3S). In all samples Quartz (SiO2) is also identified while the unnamed peaks with high intensity is identified as internal standard (corundum phase) which has not been included in the phase abundance calculations. Moreover, Ettringite (Ca6Al2(SO4)3(OH)1226H2O) and Calcite (CaCO3) are present in small quantity. The CH has a strong peak located at 2theta angle of 18.05° and was selected to be the main indicator of hydration performance in all paste samples. As a product of cement hydration, CH is able to react with SiO2 in supplementary cementing system and form additional C-S-H to improve the mechanical properties of mortars. From the diffractogram peaks in Fig. 9a–d, it is apparent that the intensity peaks of CH were significantly decreased when cement was replaced with 8% UFFA. The intensity counts of CH at 2-theta angle of 18.05° in 8% UFFA paste dropped to 3179 from 5986 for pure cement paste and from 4319 and 5486 for pastes with 40% and 60% fly ash contents, respectively. Similarly, a positive performance was found in HVFA pastes containing 8% UFFA. The diffraction peak intensity of CH at 2-theta angle of 18.05° decreased from 4319 (in paste containing 40% FA) to 3078 of paste containing 32% FA and 8% UFFA. The intensity of CH in 60% FA paste also decreased from 5485 to 4973, about 9.3% reduction when 8% UFFA was added
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(see Fig. 9e and f). These results are confirmed with the quantitative X-ray diffraction analysis as listed in Table. 4. The percentage of CH in HVFA pastes with 8% UFFA addition was 2.9% and 5.6%, respectively for 40% and 60% of cement replacement. These amounts were 60% and 56% lower than the CH content in HVFA pastes without UFFA addition. Furthermore, the findings indicate that the consumption of CH in the pozzolanic reaction by the SiO2 of fly ash in HVFA with 8% UFFA paste is more effective in contributing the additional pozzolanic reaction products to strength enhancement.
4. Conclusions Based on 7 and 28 days compressive strength, SEM and XRD results on the effect of ultrafine fly ash in high volume fly ash mortars, the following conclusions can be drawn: 1. The ultrafine fly ash had a more pronounced effect on compressive strength of mortars, particularly at early ages. Cement mortar containing 8% UFFA as partial replacement of cement exhibited 27% improvement in compressive strength at 7 days as compared to control cement mortar. At 28 days, the compressive strength of 8% UFFA mortar increased by 23%. The findings imply that the strength development of mortars is largely determined by the use of ultrafine fly ash which has finer particle sizes instead of the coarser class F fly ash. This suggests that UFFA has a more pronounced effect on compressive strength, particularly at early days. However, the proper mixture proportions should be considered in order to achieve the effective improvement. 2. The addition of 8% UFFA accelerated the pozzolanic reaction and increased the compressive strength at 7 days of HVFA mortars containing 40%, 50%, 60% and 70% between 26% and 63%. At 28 days, the increase in compressive strength was between 2% and 31%. It is noted that UFFA compensates the low early strength of binary blended mortars containing cement and class f fly ash. Therefore, blended cement containing UFFA and class F fly ash in HVFA mortars offers great potential for use in concrete construction where both short and long term compressive strength are considered. 3. According to BSE image analysis, the incorporation of 8% UFFA, as a partial cement replacement of cement in HVFA pastes densified the microstructure which led to the improvement of compressive strength. It is suggested that the particle packing effect of UFFA plays an important role in reducing the volume of pores. 4. The XRD analysis showed that the ultrafine fly ash replacement of cement is effective in increasing the CH consumption level of HVFA pastes and hence production of additional CSH gels. It is attributed to the higher of amorphous SiO2 content of UFFA.
Acknowledgement Authors acknowledge fly ash Australia for donating class F fly ash and ultrafine fly ash in this study. References [1] Global carbon capture and storage Institute
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