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Vol. 8. Issue 5.
Pages 4757-4765 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4757-4765 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.08.022
Open Access
Investigation of compressive strength and microstructures of activated cement free binder from fly ash - calcium carbide residue mixture
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Saofee Dueramaea, Weerachart Tangchirapatb,
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weerachart.tan@kmutt.ac.th

Corresponding author.
, Piti Sukontasukkulc, Prinya Chindaprasirtd,e, Chai Jaturapitakkulb
a Department of Civil Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, Sathorn, Bangkok 10120, Thailand
b Department of Civil Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Thung Khru, Bangkok 10140, Thailand
c Construction and Building Materials Research Center, Department of Civil Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand
d Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand
e Academy of Science, The Royal Society of Thailand, Dusit, Bangkok 10300, Thailand
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Tables (4)
Table 1. Physical properties of the materials.
Table 2. Chemical compositions of materials.
Table 3. Mix proportion of mortars.
Table 4. Mix proportion of pastes.
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Abstract

This paper investigates compressive strength and microstructural properties of activated cement free binder from fly ash (FA) - calcium carbide residue (CC). Different activation methods to enhance the strength of the FA-CC mixture were used including the following: 1) curing at elevated temperature of 60°C, 2) adding 1% NaOH by weight of binder and 3) combining both 1% NaOH by weight of binder and curing at temperature of 60°C. The compressive strengths of mortars with different activation techniques were determined at 3, 7, 28, and 90 days. Microstructural properties of activated cement free binder from the FA-CC mixture, that is, XRD patterns, SEM microscopy, EDS analysis, and degree of reaction were examined. Results from this study revealed that the activated cement free binder from FA-CC had components of Ca, Al, and Si and indicated CSH and CASH likewise the end products of pozzolanic reaction. Use of activation techniques had significantly affected on the microstructure in cementing system. The activated cement free binder from FA-CC mixture with addition of 1% NaOH and curing at 60°C increased binder strength and promoted the formation of CSH, CASH, and some amount of NASH phase or hybrid C(N)ASH.

Keywords:
Fly ash
Calcium carbide residue
Compressive strength
Microstructural properties
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1Introduction

Portland cement is a primary material used in construction projects. Portland cement production process has a lot of undesirable impacts to the environment since the production process has a high energy demand to burn cement clinker. Additionally, the process releases huge amount of CO2 gas into the air (every 1ton of cement product releases 0.9ton of CO2) [1]. It is now established that a new cementing is required to replace OPC, resulting in reduced environmental damage. Previous studies had demonstrated that a mixture made from fly ash (FA) and calcium carbide residue (CC) could be used as a new cement free binder [2,3], and the optimum ratio of FA to CC was 70:30 by weight to generate the highest compressive strength [2].

The mixture of FA and CC is an alternative binder, as SiO2 and A2lO3 from FA react with Ca(OH)2 from CC to become cementitious material. Somna et al. [4] found that the chemical reaction between FA and CC is similar to pozzolanic reaction and one of final products is CSH structure. Although the mixture of FA and CC had the potential to be used as a new cement free binder, the strength development was still slow as compared to OPC, particularly at the early stage [2,3]. To improve the mechanical properties of FA-CC mixture, Namarak et al. [5] found that the curing at high temperature can accelerate pozzolanic reaction and improve compressive strength of FA-CC mortars.

Chemical components of FA and CC are aluminum, silica, and calcium, which are similar to alkali activated material (AAM). Thus, alkali solutions such as NaOH and KOH can be used to improve the hardened properties of FA-CC binder. Regarding AAM, the alkali activator involves the leaching of silica (Si4+) and alumina (Al3+) in the material, enabling reaction with calcium source to form CSH products [6]. Additionally, the alkali activator solution affected the dissolution of calcium and participation of alumina and led to the CASH phase [7,8]. The product of AAM can possibly generate NASH phase, in which Na reacts with product in these systems, as a coexistence of CSH, CASH and NASH phases [9].

Therefore, these experiments have attempted to improve a new binder made from FA-CC mixture by activation process, from which three different methods were used for a cement free binder from the FA-CC mixture, including the following: 1) curing at temperature of 60°C, 2) adding 1% NaOH by weight of binder, and 3) combining both adding 1% NaOH by weight of binder and curing at temperature of 60°C. This research studied the compressive strength of activated cement free binder from FA-CC mixture. Microstructural properties in terms of XRD patterns, SEM microscopy, and EDS analysis, and the degree of reaction were also studied. Moreover, this study was targeted to improve the understanding and additional knowledge to build confidence of this new cement free binder made from the FA–CC mixture, especially the microstructural properties of the new binder have not been reported.

2Experimental program2.1Materials

FA is derived from burning lignite coal by a pulverized combustion process in order to generate the electricity in power plants. The power plant station was located in Lampang province, the northern part of Thailand.

CC is an industrial waste from the acetylene gas creation in Samutsakorn province, Thailand. Since CC acquired directly from manufactory had high moisture content, thus it was dried in an oven at 100°C for 24h before being used in this study.

FA to CC ratio of 70:30 by weight was mixed together and utilized as a cement free binder in this experiment, as supported by the previous study [2]. Then, the FA-CC mixture was ground together until the particles were remained on a 45μm sieve of 1.1% by weight (represent as GFC). GFC had an average particle size of 2.93 microns and had a relative density of 2.73 (see Table 1).

Table 1.

Physical properties of the materials.

Material  Relative density  Remained ona Sieve No. 325 (45μm sieve) (%)  Average particlesize: d50 (micron) 
Cement (OPC)  3.15  20.0  14.60 
Ground FA-CC (GFC)  2.73  1.1  2.93 
2.2Chemical composition of materials

Chemical properties of OPC, FA, CC, and GFC were identified by XRF analysis and are reported in Table 2. FA had a sum content of SiO2, Al2O3 and Fe2O3 of 76.1%; thus, it was indicated as Class F fly ash in agreement with ASTM C618 [10]. For CC used in this experiment, the main chemical of CC was 56.5% of CaO.

Table 2.

Chemical compositions of materials.

Chemical composition (%)  Cement (OPC)  FA  CC  GFC 
Silicon Dioxide (SiO220.9  41.9  4.3  29.0 
Aluminum Oxide (Al2O34.8  21.5  0.4  13.6 
Iron Oxide (Fe2O33.4  12.7  0.9  7.6 
Calcium Oxide (CaO)  65.4  13.9  56.5  32.6 
Sulfur Trioxide (SO32.7  0.6  0.1  0.5 
Magnesium Oxide (MgO)  1.3  2.6  1.7  1.9 
Sodium Oxide (Na2O)  0.3  2.7  –  2.1 
Potassium Oxide (K2O)  0.4  2.5  –  1.7 
Loss on Ignition (LOI)  0.9  0.7  36.1  10.1 

GFC had a mainly composition of 32.6% of CaO. GFC had a sum content of SiO2, Al2O3, and Fe2O3 of 50.2% and had LOI of 10.1% by weight The high degree of LOI in GFC can be caused by Ca(OH)2 in CC being disintegrated as CaO and H2O at a temperature of 550°C, while that of LOI of the CC was investigated at high temperature of 750±50°C [11].

2.3Mix proportions

The mix ratios of mortar in this experiment are listed in Table 3. All mortar mixtures had a water to binder ratio of 0.25. A ratio of binder to sand was kept constant at 1:2.75 by weight according to ASTM C109 [12]. Polymer-based superplasticizer (type F) was added to increase mortar flow between 105 and 115%. Three different activation methods used in mortars and pastes made with FA-CC mixture were investigated and the obtained result was compared with GFC mortar (mortar without activation) and OPC mortar having the same W/B ratio. The activation methods used in this study were 1) GFC-1N mortar—adding 1% NaOH by weight of binder and cured in tap water at room temperature (25–30°C), 2) GFC-60H mortar—curing at temperature of 60°C for 24h after removal from the mold and then submerged in water at room temperature, and 3) GFC-1N-60H mortar—adding 1% NaOH by weight of binder and curing mortar for 24h at temperature of 60°C after removal from the mold and then submerged in water at room temperature.

Table 3.

Mix proportion of mortars.

MixMix proportion (by weight)W/BFlow(%)
OPC  FA  CC  Sand  Water  NaOH  SPa 
CT  100  –  –  275  24.2  1.82  0.25  106 
GFC  –  70  30  275  23.8  2.54  0.25  111 
GFC-60H  –  70  30  275  23.8  2.54  0.25  112 
GFC-1N  –  70  30  275  23.5  3.65  0.25  109 
GFC-1N-60H  –  70  30  275  23.5  3.65  0.25  110 
a

The water in the superplasticizer is approximately 50% by weight.

Note that, GFC-60H and GFC-1N-60H mortars were wrapped with a plastic sheet to protect the evaporation of free water after the specimens were removed from the mold. The specimens were cured for 24h at temperature of 60°C, and the plastic sheet was removed. Then, the specimens were cured in water until the testing age.

Additionally, FA-CC pastes were prepared and were investigated by XRD, SEM, EDS analysis, and degree of reaction. The mix proportions of FA-CC pastes, that is, W/B ratio and activation techniques are shown in Table 4. Moreover, FA-CC pastes were mixed without superplasticizer.

Table 4.

Mix proportion of pastes.

MixMix proportion (by weight)W/B
FA  CC  Water  NaOH 
GFC  70  30  25.0  0.25 
GFC-60H  70  30  25.0  0.25 
GFC-1N  70  30  25.0  0.25 
GFC-1N-60H  70  30  25.0  0.25 
3Details of test3.1Compressive strength of mortar

The mortar samples were removed from the mold at 24h after casting and submersed in tap water till reaching the testing age. The compressive strength was examined from mortars at the ages of 3, 7, 28, and 90 days. Five mortar specimens with sizes of 50×50×50mm3 were tested to investigate the compressive strength.

3.2X-ray diffraction (XRD)

The XRD analysis was used to verify the crystallite phases of the binder. XRD analysis using an X-ray diffractometer scanning from 10–80° 2θ with a 0.02 step size was used to analyze activated FA-CC pastes at the age of 28 days.

3.3Scanning electron microscopy (SEM)/Energy dispersive spectroscopy (EDS)

The SEM and EDS analyses were used to examine microstructure characterization of FA-CC pastes at the age of 28 days. SEM was used to obtain SEM photographs of FA-CC pastes, and EDS analysis was used to verify the elements within the microstructure.

3.4Degree of reaction

The degree of reaction (DR) of FA-CC pastes at 28 days was investigated based on the method of dissolution of ground powder with acid and base solutions, including 2N HCl (hydrochloric acid) and 3% Na2CO3 (sodium carbonate) solution [13]. The samples were ground until the particles of powder were passed through a 150μm sieve (No. 100). Three grams of ground FA-CC pastes were liquefied by 30 cm3 of 2N HCl in a beaker and kept in water bath having a temperature of 60°C for 20min. Then, the liquid sample was filtered and cleansed with distilled water to remove the HCl solution. For final filtration, acetone was used to remove moisture prior to dry the sample at a temperature of 70°C for 2h. The residue of the sample was liquefied by 30cm3 of 3% Na2CO3 and kept in a water bath at a temperature of 80°C for 20min. The liquid phase was filtered and cleansed with distilled water and acetone before drying. DR was calculated as Eq. (1)[14,15].

where Wsample=weight of sample (g).

Wresidue=weight of dried residue (g).

LOI=loss of ignition of sample according to ASTM C114 [16].

4Results and discussion4.1Compressive strength of FA-CC mortars

Compressive strengths of FA-CC mortars with different activation methods compared to OPC mortar are summarized in Fig. 1. The compressive strengths of OPC mortar at 3, 7, 28, and 90 days were 45.5, 50.7, 56.8, and 63.4MPa, respectively. For mortar using ground FA-CC binder without any activated technique (GFC mortar), it had compressive strengths of 6.7, 22.3, 34.2, and 41.2 at 3, 7, 28, and 90 days, respectively. The strength development of GFC mortar was slow, particularly at the early age of 3 days, and GFC mortar had the early age compressive strength ranging from 15 to 44% of OPC mortar. This behavior was due to the strength of GFC mortar based only on the pozzolanic product between the reaction of FA and CC. Thus, the reaction between FA and CC was very slow and took longer time than the hydration reaction of cement.

Fig. 1.

Compressive strength of mortar and age.

(0.16MB).

For GFC-60H mortar, it was clearly that the GFC-60H mortar under curing at temperature of 60°C had rapidly increased in compressive strength at the early age but gradually increased its strength at the later age. GFC-60H mortar had compressive strengths of 26.4 and 27.4MPa at 3 and 7 days, and increased to 32.4 and 38.0MPa at 28 and 90 days, respectively. For the elevated curing temperature, the structural formation of pozzolanic reaction products had a random spread, and were less compacted and caused to a slow development of strength at later ages [5,17].

Considering the effect of a NaOH activator by comparing the GFC mortar (without NaOH addition) and GFC-1N mortar (adding 1% NaOH), it was found that GFC-1N mortar enhanced the strength development at both early and later ages. At 3, 7, 28, and 90 days, GFC-1N mortar had compressive strengths of 18.5, 31.8, 43.1, and 50.3MPa, respectively. The enlargement of compressive strength of mortar is attributed to NaOH activator leaching silica (Si4+) and alumina (Al3+) in FA particles, thereby increasing the reaction with Ca(OH)2 from CC [18]. As seen in Fig. 1, GFC-1N mortar had strength development lower than GFC-60H until 7 days and then the compressive strengths of GFC-1N mortar were in the opposite direction. Although NaOH solution could improve dissolution of FA to react with Ca(OH)2, the process to generate CSH and/or CASH phase at ambient temperature took more time than that in the elevated temperature [19,20].

In addition, GFC-1N-60H mortar had the greatest compressive strength compared to the other activation techniques at all ages. GFC-1N-60H mortar had the compressive strengths of 38.0, 41.5, 47.5, and 51.1MPa at 3, 7, 28, and 90 days, respectively. Increasing the curing temperature has a higher potential to increase the amount of Na and increases the degree of reaction. The products of pozzolanic reaction of GFC-1N-60H mortar that precipitated occurring at 60°C lead to the increasing of the strength of the mortar. Furthermore, the amounts of Na incorporation in CSH gel contributed to NCSH phase in the system [21,22]. This was confirmed by XRD and SEM/EDS analysis that the formation in GFC-1N-60H mortar was a hybrid C(N)ASH or the combination of CASH and NASH.

4.2X-ray diffraction (XRD) patterns of FA-CC pastes

XRD patterns of powder materials are shown in Fig. 2, the peaks of SiO2 (quartz), Al2O3, Fe2O3 (hematite) and CaO (lime) are found in FA. For CC used in this experiment, Ca(CO)3 and Ca(OH)2 were the major peaks in CC. The patterns of XRD of FA-CC powder (70% of FA and 30% of CC mixture) showed the presence of SiO2, Al2O3, Fe2O3, CaO, and Ca(OH)2 similar to that were detected from FA and CC. The broad hump in XRD patterns between 16 and 37° 2θ, which was the result of FA-CC powder, indicated the amount of amorphous phase. Moreover, the peaks of the XRD patterns of each material was related to the chemical compositions, which was indicated by XRF analysis.

Fig. 2.

X-ray diffraction (XRD) patterns of the powder materials.

(0.26MB).

Fig. 3 exhibits XRD patterns of activated FA-CC pastes with different activation methods. The results of XRD patterns of all activated FA-CC pastes indicated the main product of CSH, as indicated by the peak at 29.5° 2θ. Additionally, the product of CASH in activated FA-CC pastes was also obtained, which was a similar product of the pozzolanic reaction. CSH and CASH formations were generated from CaO bond with SiO4 tetrahedron and AlO4 tetrahedron to form CSH and CASH [23,24]. Moreover, the CaO content in the raw materials had a significant impact on CASH occurring in activated FA-CC pastes. High Ca2+ ion promotes the synthesis of CASH in the matrix. García-Lodeiro et al. [25] proposed that with the high Ca level in the material presence of Al in a substitute CSH system, CSH was developed and contributed to CASH due to Ca rich formulation in binder and stabilized CASH form in systems. According to Myers et al. [26], the formation rate of CASH increases at the elevated temperature through the reaction between Al with CSH. Calcium silicate and aluminosilicate were major products in the combination matrix and resulted in good physical properties of activated FA-CC pastes.

Fig. 3.

X-ray diffraction (XRD) patterns of activated FA-CC pastes at 28 days. (3a) GFC paste. (3b) GFC-60H paste. (3c) GFC-1N paste. (3d) GFC-1N-60H paste.

(0.44MB).

In addition, the main products of activated FA-CC pastes with the addition of NaOH activator (GFC-1N and GFC-1N-60H pastes) were not limited to CSH and CASH. These systems may form combinations of CASH and NASH or hybrid C(N)ASH. The forming of an additional C(N)ASH phase resulting from a partial substitution of NASH in the system was acquired from incorporation of aluminosilicate and sodium in the system [9]. Note that the small amount of Na element in GFC-1N and GFC-1N-60H pastes were detected by EDS analysis. This behavior is explained in more details in the next section on EDS analysis.

4.3Scanning electron microscopy and EDS analyses of FA-CC pastes

Fig. 4 illustrates the SEM images at 28 days of different activated FA-CC pastes. It can be observed from Fig. 4a that the GFC paste without activation technique had an incomplete reaction or remained un-reacted with FA particles within the matrix since the reaction between FA and CC was mild. This characteristic can cause a decrease in the strength of the paste. Fig. 4b shows the characteristic of GFC-60H paste under cured at 60°C and indicates that the elevated curing temperature significantly improved the formation of pozzolanic reaction products. Thus, the matrix of GFC-60H paste was more consolidated and denser than GFC paste. In Fig. 4c, the micrographs of GFC-1N paste are also shown to be denser and more homogenous than GFC paste. It was seen that some surfaces of fly ash particle in the matrixes were dissolved by NaOH solution. The alkali solution could leach Si and Al from FA to react with Ca(OH)2[27], thereby increasing the density of the paste. For GFC-1N-60H paste with adding 1% of NaOH into the binder and curing at temperature of 60°C (Fig. 4d), this method appeared to improve homogeneity and denseness of the structure. The matrix of GFC-1N-60H paste had crosslinking similar to a reticulate morphology. The elevated temperature was a significant factor in accelerating dissolution of NaOH solution into OH ion and Na+ ion and resulted in increasing the reaction of activated FA-CC pastes [28]. Moreover, cation Na+ could react with Ca, Al, and Si subsequently polymerized the products (NASH and/or C(N)ASH) at high temperature condition [18,28]. Thus, the different products were responsible for the different microstructural matrix of GFC-1N-60H paste.

Fig. 4.

SEM Morphology and Energy Dispersive X-ray Spectrometry (EDS) of FA-CC pastes at 28 days.

(1.16MB).

EDS results were obtained from selection points within the SEM region of the activated FA-CC pastes. Figs. 4a and 4b plot the elements of GFC and GFC-60H pastes, respectively. It was found that the major components of both GFC and GFC-60H pastes were Ca, Al, and Si and indicated the forming of CSH and CASH compounds such as a pozzolanic product. CSH and CASH were obtained from CaO, SiO2, and CaO-Al2O3[26,29].

For GFC-1N and GFC-1N-60H pastes (Figs. 4c and 4d), it was observed that the main elements were Al, Si, Ca and a small amount of Na. Comparing the EDS results of GFC-1N and GFC-1N-60H pastes, it was seen that the Na element increased with the curing temperature. The atomic of Na in GFC-1N-60H paste under curing at temperature of 60°C was 2.34%, while GFC-1N paste without temperature curing had atomic of Na of 1.62%. Therefore, the high curing temperature accelerated the dissolution of Al3+ and Si4+ from fly ash. Additionally, the structural matrix of GFC-1N-60H paste may not be obtained from only a pozzolanic product. It was also attributed from hybrid C(N)ASH or combination of CASH and NASH, as described by previous researchers [21,30–32]. This result is consistent with the observed from the microstructure, which shows a denser microstructure.

4.4Degree of reaction of FA-CC pastes

The degree of reaction (DR) of activated FA-CC pastes samples are shown in Fig. 5. It was found that all of the activation techniques could improve the degree of reaction of cement free binder made from FA-CC mixture. The DR values of GFC, GFC-60H, GFC-1N, and GFC-1N-60H pastes were 63, 65, 69, and 72%, respectively. The result of DR was related to reactive phase of the binder. Thus, these activation techniques, that is, adding of NaOH, curing at elevated temperature, and a combination of adding NaOH and curing at elevated temperature could significantly improve the reactive process and led to the increase of strength of FA-CC mortars. DR was confirmed to be the result of compressive strength of mortar made with activated cement free binder from the FA-CC mixture since the degree of reaction increased with the increased of compressive strength.

Fig. 5.

Degree of reaction of FA-CC pastes at 28 days.

(0.05MB).
5Conclusions

The results can be concluded as follows:

  • 1

    All activation techniques used in this study could be improved the compressive strength of mortar made with FA-CC binder. Combining both 1% NaOH and curing at temperature of 60°C gave the highest compressive strength of FA-CC mortars.

  • 2

    All activated FA-CC pastes indicated that the mainly cementing products were CSH and CASH likewise the end products of pozzolanic reaction.

  • 3

    Adding 1% NaOH by weight of binder into pastes made with FA-CC binders could contribute a hybrid C(N)ASH or the combination of CASH and NASH, which was a partial substitution NASH in the system acquired from the combination of aluminosilicate and Na+ in the system resulting in the increase of compressive strength of FA-CC mortars.

  • 4

    The degree of reaction of FA-CC pastes was increased by all activation techniques, and the increase in compressive strength of activated FA-CC mortars was directly associated with the results of degree of reaction.

Acknowledgements

This study was supported by the Petchra Pra Jom Klao Ph.D. Scholarship, King Mongkut's University of Technology Thonburi. The authors also gratefully acknowledged the Thailand Research Fund (TRF) under the TRF Research Career Development Grant No. RSA6280032 and the TRF Distinguished Research Professor Grant No. DPG6180002.

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Copyright © 2019. The Authors
Journal of Materials Research and Technology

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