Thermal activation and mechanical properties of high alumina coal gangue as auxiliary cementitious admixture

In order to use high alumina coal gangue as auxiliary cementitious admixture via a simple and convenient thermal activation technique, the thermal transformation, mineral phase transformation and structure changes of coal gangue at calcining temperatures of 500–1000 °C were analyzed using x-ray diffraction (XRD), differential thermal Analysis (DTA), thermogravimetry (TG), infrared analysis (IR) and scanning electron microscope (SEM). The mechanical properties of cement mortar with 30% coal gangue auxiliary cementitious admixture were also measured to determine the optimal calcining temperature. As calcining temperature was increased, the coal gangue experienced the following transformations: carbon combustion, dehydroxylation, metakaolin transformation and mullite transformation. The cement mortar with coal gangue auxiliary cementitious admixture calcined at 700 °C presented the highest 28-d flexural and compressive strength, increasing by 8.27% and 11.85% respectively as compared with the benchmark cement mortar. The maximum dosage of coal gangue auxiliary cementitious admixture in cement mortar was further identified to be less than 30% by mechanical properties testing. The activity of high alumina coal gangues at different calcining temperatures was explained from the view points of hydration degree and products. The present investigation can provide a useful reference to utilize high alumina coal gangue as auxiliary cementitious admixture by means of a simple thermal activation at 700 °C.


Introduction
Coal gangue is an industrial solid waste generated from coal mining and washing, which accounts for approximately 15%-20% of coal production in countries with developed coal resources, such as China, Australia and India [1,2].Coal gangue has become the largest solid waste in China, since a total of 700 million tons of coal gangue has been accumulated in numerous coal mines across China [3].The massive accumulation of coal gangue not only brings about safety hazards such as spontaneous combustion and landslides, but also causes a series of ecological and environmental problems such as water pollution and soil pollution [4].Therefore, it is an important and urgent matter for the sustainable development of the coal industry to use coal gangue reasonably, efficiently and in multiple ways.
Many investigations have showed that coal gangue often contains a large amount of clay minerals with pozzolanic activity after being stimulated under certain conditions, so the activated coal gangue can react with calcium hydroxide (Ca(OH) 2 ) to generate cementitious substances during the hydration process of Portland cement [5,6].The products generated from well activated coal gangue can increase the density and mechanical properties of mortar structure via consumption of Ca(OH) 2 to promote the hydration process of C 3 S and other hydraulic components in Portland cement.Therefore, coal gangue can partially replace Portland cement in construction applications by acting as an auxiliary cementitious admixture.It is of great importance to determine an appropriate activation condition for utilization of coal gangue as auxiliary cementitious admixture.Currently, several technical methods for activation of coal gangue have been proposed, such as mechanical, thermal, chemical, microwave, and composite activations [7][8][9][10].Among them, thermal activation has been widely concerned by researchers due to its ease of operation and significant effectiveness.Cao et al [11,12], obtained the highest activity of coal gangue from Shiguai mining area in Baotou City, China, via calcining at 800 °C, and achieved increments of 11.85% and 16.82% in 28-d flexural and compressive strength when the calcined coal gangue was used to replace 15% of P.O42.5 cement in the mortar preparation.Liu et al [13], prepared auxiliary cementitious admixture by calcining the coal gangue from Cuipingshan mining area in Longyan City, China, at the optimal thermal activation temperature of 750 °C, but reached only 97.2% and 69.64% of 28-d compressive strength of cement mortar as the calcined coal gangue was used to replace 10% and 30% of P.O42.5 cement in the mortar preparation.A great number of investigations have revealed that there is a great difference in mineral composition between coal gangues produced in different coal mine regions, consequently the thermal-activated products also vary widely in pozzolanic activity and exert different influence on mortar strength.
High alumina coal gangue is a special type among various coal gangues, primarily discharged in the leading coal producing areas in China, such as Inner Mongolia Autonomous Region and Shanxi Province.High alumina coal gangue usually has a high content of Al 2 O 3 above 30 wt.%, and a molar ratio of Al 2 O 3 to SiO 2 larger than 0.46, which means that it is composed mainly of kaolinite.Up to now, although some research works have been done on thermal activation of a variety of coal gangues and usage of coal gangues as cement auxiliary cementitious admixture, many details in the thermal activation process of high alumina coal gangue have not been fully unveiled, and the effect of high alumina coal gangue as auxiliary cementitious admixture on mechanical properties needs to be further evaluated.
In this paper, the structure evolution of high alumina coal gangue during calcining was studied; and the influence of high alumina coal gangue as auxiliary cementitious admixture on the mechanical strength of mortar was analyzed.

Raw materials
The raw coal gangue used in this study was received from a coal mine in Datong City, Shanxi Province, China.The coal gangue was crushed, ground and sieved into a particle size range of 180-200 meshes before use.The chemical composition of coal gangue powder calcined at 800 °C was measured using borate melting sample preparation method, as listed in table 1. SiO 2 and Al 2 O 3 were the main components of coal gangue in which the content of Al 2 O 3 was as high as 45.11%, and molar ratio of Al 2 O 3 to SiO 2 was 0.51.The as-received coal gangue is apparently a typical type of high alumina coal gangue according to the classification of coal gangues [14].In addition, the as-received coal gangue had much lower contents of Fe 2 O 3 and CaO as compared with coal gangues produced in other coal mining regions, China.Figure 1 shows the x-ray diffraction (XRD) pattern of asreceived coal gangue.It can be seen that kaolinite ) and quartz were identified, and almost all diffraction peaks of kaolinite were detected, suggesting that the main constituent was kaolinite.
The cement used to prepare mortar was an ordinary Portland cement of P.O42.5 produced by Yangchun Cement Co., Ltd in Zhucheng City, Shandong Province, China.Its chemical composition is also included in table 1.The sand used to prepare mortar was ISO standard sand produced by Xiamen ISO standard Sand Co., Ltd in Xiamen City, China.

Thermal activation
The thermal activation process of high alumina coal gangue was as follows: 200 g of coal gangue powder were weighed and put in a stainless steel box with 200 mm length, 100 mm width and 50 mm height, then calcined at a desired temperature for 2 h in a resistance furnace.The calcining temperatures were 500, 600, 700, 800, 900, 1000 and 1100 °C, respectively.After calcining, coal gangue powder was cooled in air, and then collected in hermetic bags for subsequent analysis and preparation of cement-coal gangue auxiliary cementitious admixture mortar specimens.

Preparation of mortar specimens
According to the method for testing the strength of cement mortar (Chinese Standard GB/T 17671-2021), each group of mortar specimens was prepared using 450 ± 2 g of cement or cement with a certain amount of coal gangue auxiliary cementitious admixture, 1350 ± 5 g of standard sand and 225 ± 1 g of water.Before making the cement-coal gangue auxiliary cementitious admixture mortar specimens, the cement, coal gangue auxiliary cementitious admixture and sand were mixed evenly in a V-shaped mixer for 30 min.The mixture of cement, coal gangue auxiliary cementitious admixture and sand was poured into a cement mixer, and then the required amount of water was added and stirred with cement mixer for 5 min.The fresh paste was quickly poured into plastic moulds with dimensions of 40 × 40 × 160 mm and vibrated for 60 s on a ZT-96 vibrating table to remove residue air in the molds.The molds were covered with polyethylene film and cured at relative humidity of 95% and 20 °C for 24 h, and then the mortar specimens were demoulded and cured again in water for 7 days and 28 days, respectively.The flexural strength and compressive strength of mortar specimens were tested on a multifunctional mechanical testing machine.Figure 2 shows the mechanical testing setups for flexural strength and compressive strength of mortar specimens.During the flexural strength tests, the distance between the supports was 100 mm; the loading speed was 50 N/s.After flexural strength testing, the broken halves of mortar specimens were then used for compressive strength testing.During the compressive strength tests, the stressed area was 1600 mm 2 ; the loading speed was 2200 N/s.The maximum load was recorded as the specimen was fractured, from which the flexural strength R f and compressive strength R c were calculated according to equations (1) and (2), respectively.= where F f is the maximum flexural load imposed on the middle part of the specimen, L is the distance between the supports, b is the side length of the square section of the specimen, i.e. 40 mm, F c is the maximum compressive load, A is the compressed area.

Characterization methods
The thermogravimetric characteristics of coal gangue powders were analyzed using a HITACHI STA7300 synchronous thermogravimetric analyzer under two different conditions, i.e. air environment and Ar atmosphere.The constituent phases of the coal gangue calcined at temperatures of 500-1100 °C were determined by a Rigaku D/MAX2500 X-ray diffractometer.The structure transformations during thermal activation of coal gangue were analyzed using a TENSOR 27 infrared spectrometer.The microstructures and chemical compositions of the calcined coal gangue powder and fractured specimens were analyzed using a TESCAN VEGA3 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).

Structural evolution of coal gangue during thermal activation
In order to identify the constituent phases of the coal gangue after calcining, XRD analysis was conducted on the coal gangue powders calcined at different temperatures, as shown in figure 3.After calcining at 500 °C, the main constituent phases were still kaolinite and quartz, and the two strong diffraction peaks of kaolinite at 2θ = 12.37°a nd 24.90°remained significant, suggesting that kaolinite in coal gangue kept its original structure.However, after calcining at 600 °C, the diffraction peaks of kaolinite almost completely disappear, and a broadening peak emerged between 2θ = 15°and 2θ = 30°and became the most pronounced characteristic.Such a morphological change in diffraction peak implies that the crystalline kaolinite in coal gangue began decomposing and transformed into amorphous metakaolin (Al 2 O 3 •2SiO 2 ).The corresponding chemical reaction is expressed by equation (3).
At higher calcining temperatures of 700-900 °C, XRD patterns of the coal gangues still remained the broadening peak characteristic, namely metakaolin in coal gangue primarily maintained its structure in the temperature range of 600-900 °C.After calcining at 1000 °C, a great number of diffraction peaks of mullite (3Al 2 O 3 •2SiO 2 ) occurred and became much stronger at 1100 °C, suggesting that metakaolin transformed into mullite as expressed by equation (4).Since mullite has almost no pozzolanic activity due to its relatively complete crystalline structure [9,15,16], calcining at high temperatures such as 1000 and 1100 °C could have negative effect on the reactivity of coal gangue auxiliary cementitious admixture during hydration.
Differential thermal analysis (DTA) and thermal gravimetric analysis (TG) of coal gangue were conducted within a temperature range of 100 °C-1200 °C in argon atmosphere and air environment respectively, from which the temperature ranges for combustion of carbon in coal gangue and thermal transformation of kaolinite could be determined accurately.DTA curves as well as corresponding TG curves obtained in air environment and argon atmosphere are shown in figure 4. In argon atmosphere, three exothermic peaks and two endothermic peaks were observed on DTA curve (figure 4(a).The exothermic peaks at 635.7 °C and 689.2 °C corresponded to combustion reaction between carbon in coal gangue and residual oxygen in argon atmosphere, while the exothermic peak at 1002.1 °C apparently stood for mullite formation.The endothermic peak at 612.4 °C with an onset temperature at 515.6 °C corresponded to dehydration of kaolinite, while the endothermic peak at 667.6 °C stood for formation of metastable metakaolin, i.e. amorphous metakaolin.It was noted that endothermic peak standing for dehydration of kaolinite in the studied high alumina coal gangue occurred at a higher temperature of 612.4 °C rather than at a temperature within 510-560 °C for other types of coal gangues [17][18][19].This might be attributed to a high degree of crystallinity in kaolinite for the studied high alumina coal gangue.As can be seen from TG curve (figure 4(b)), the mass loss began at 518.1 °C and finished at 680.1 °C.The mass loss ratio within this stage was calculated to be 13.1%, which is very close to the theoretical mass loss ratio of 14.4% for kaolinite dehydration.These results further prove that kaolinite is the main component of as-received high alumina coal gangue, and the mass loss in argon atmosphere is mainly caused by dehydration of kaolinite.In the air environment, three exothermic peaks and one endothermic peak occurred on DTA curve (figure 4(c)).The exothermic peaks at 537.2 °C and 649.7 °C corresponded to combustion of carbon in coal gangue, while the exothermic peak at 1000.5 °C stood for mullite formation.The endothermic peak at 550.3 °C represented the dehydration of kaolinite.Apparently, the dehydration temperature of kaolinite in air is much lower than that in argon atmosphere, which could be attributed to the promotion effect of combustion of a large amount of carbon in the high alumina coal gangue on decomposition of kaolinite.Carbonaceous matter has been reported to increase decomposition rate of kaolinite and accelerate structure tranformation from scale-shaped lamellar to irregular and amorphous [20].As can be seen from TG curve (figure 4(d)), the mass loss began at 423.4 °C and finished at 692.4 °C, the total mass loss ratio was calculated to be about 38.7%.Such a large mass loss ratio indicates that raw high alumina coal gangue used in the present study contained a quite high carbon content of about 24%, which was much higher as compared with other types of coal gangues produced in China.The most rapid and significant mass loss thus happened in the temperature range of 594.1-692.4°C, namely the main combustion temperature range of carbon.This means that the volatilization matters and carbon in coal gangue are almost totally burn up as temperature exceeds 692.4 °C.Therefore, despite a higher temperature required for dehydration of kaolinite in high alumina coal gangue, the decomposition of kaolinite into metakaolin can be accomplished below 700 °C by means of calcining in air.
The change in the functional groups containing in coal gangue during calcining was analyzed using infrared spectroscopy, as shown in figure 5. Before calcining, the characteristic absorption peaks of hydroxyl groups and bands such as Si-O-Si, Si-O-Al and Al-OH in coal gangue were presented clearly.After calcining at 500 °C, the intensities of stretching vibration absorption peaks of hydroxyl groups at 3695, 3655 and 3620 cm −1 decreased, indicating the commence of dehydration of kaolinite.This temperature is close to the onset temperature of dehydration of kaolinite measured in argon atmosphere by DTA.In addition, the stretching vibration absorption peaks of Si-O-Si between 1000 and 1100 cm −1 weakened or disappeared, especially the intensity of absorption peak of Si-O-Si at 1035 cm −1 considerably decreased and shifted to 1068 cm −1 in a broaden peak.However, the absorption peak of Al-OH at 912 cm −1 and absorption peaks of Si-O-Al, Si-O-Si and Al-O between 431 cm −1 and 794 cm −1 still preserved their basic peak morphologies despite great decrease in intensity.These results suggest that kaolinite is subjected to structure transformation to a certain extent.After calcining at 600 °C, the absorption peaks of hydroxyl groups at 3695, 3655 and 3620 cm −1 almost totally disappeared.The stretching vibration absorption peaks of Si-O-Si between 1000 cm −1 and 1100 cm −1 was merged into a weak broaden peak at 1077 cm −1 , the vibration absorption peaks of Si-O-Al, Si-O-Si and Al-O between 431 cm −1 and 794 cm −1 were merged into two broaden peaks at 810 cm −1 and 452 cm −1 , which corresponded to vibration absorption peaks of Al-O and Si-O, respectively.After calcining at temperatures of 700-900 °C, IR spectra of coal gangues have a shape similar to that at 600 °C, but totally changed at temperatures of 1000 and 1100 °C due to the transformation form metakaolin to mullite.On the basis of above results, it can be concluded that as calcining temperature is increased above 600 °C, the structure of kaolinite is transformed due to the removal of hydroxyl groups and breakage of Si-O-Al bond between Si-O tetrahedrons and Al-O octahedrons, resulting in formation of amorphous metakaolin.The amorphous metakaolin has high reactivity and can take part in hydration process of cement.

Surface morphologies and microstructures of calcined coal gangue powders
In order to compare the change in surface morphology and structure between raw coal gangue and calcined coal gangues, raw coal gangue and coal gangue powders calcined at temperatures of 600-900 °C were examined by SEM technique, as shown in figure 6.The surface of raw coal gangue block was relatively clear and smooth (figure 6(a)), while a few small flakes were formed on the surfaces of ground coal gangue powders with a particle size range of 5-45 μm due to mechanical impact (figure 6(b)).After calcining at 600 °C, the coal gangue powders agglomerated into larger particles with a size range of 30-85 μm (figure 6(c)) on which a few of flaky products formed due to dehydration of kaolinite.The flaky products were identified to be metakaolin by subsequent EDS  analysis.As the calcining temperature was increased to 700 °C, the surfaces of coal gangue powders were covered by fine particles and irregular flakes of metakaolin (figure 6(d)), and the details of the morphological characteristics could be clearly observed in the magnification image (figure 6(e)), suggesting their low crystallization degree.At 800 °C, the morphology of metakaolin products evolved from irregular flaky into circular, and most of them were connected together (figure 6(f)), suggesting a increase in crystallization degree.At 900 °C, most part of the surfaces of coal gangue powders was exposed, and the other part of the surfaces was covered by a few of scattered small particles and flakes of metakaolin (figure 6(g)).At 1000 °C, regular small flakes were formed again on the surfaces of coal gangue powders due to mullite transformation (figure 6(h)).
The calcining products on the surfaces of coal gangue powders were further analyzed using EDS point scanning technique.Figure 7 shows the locations of selected typical points that were indicated by numbers on the surfaces of coal gangue powders calcined at temperatures of 600 °C-800 °C.The corresponding chemical compositions at the points are listed in table 2. In the case of uncalcined coal gangue powder, there was a great difference in content of carbon element among the points 141, 142 and 144, ranging from 11.90% to 68.94%.The point 141 with the lowest content of carbon element had 48.34%O, 19.82%Al and 19.93%Si, which was very close to the theoretical contents of O, Al and Si elements in kaolinite, i.e. 55.8%O, 20.9%Al and 21.7%Si, suggesting that the substance at the point was mainly composed by kaolinite.The points 142 and 144 had a rather high content of carbon element above 40%, the substances at the two points were thus to be consisted mainly of carbon.In the case of coal gangue powders calcined at temperatures of 600 °C-800 °C, the content of carbon element at the indicated points was found to be decreased considerable due to significant carbon combustion as identified by DTA/TG analysis, lower than 10.67%, while the contents of Al and Si elements increased to high levels of 22.30%-27.24%and 23.08%-26.93%respectively except for the point 117.The high levels of Al and Si elements contents were close to the theoretical contents of Al and Si elements in metakaolin, i.e. 25.22%Al and 24.32%Si, suggesting that the surface products at these points were mainly composed by metakaolin.

Mechanical strength of mortar with coal gangue auxiliary cementitious admixtures
In order to determine the appropriate temperature for calcining high alumina coal gangue, mechanical tests were conducted on mortar specimens with coal gangue auxiliary cementitious admixture replacing 30% of cement.Figure 8 shows the flexural strength and compressive strength of mortar specimens with coal gangue auxiliary cementitious admixtures calcined at different temperatures after 7 and 28 days curing.The pure cement mortar specimens had flexural strength of 5.80 MPa and 7.48 MPa and compressive strength of 33.73 MPa and 44.21 MPa after 7 days and 28 days curing, respectively.The 7-d and 28-d mechanical properties of mortar specimens with coal gangue auxiliary cementitious admixture varied in a similar trend with increasing calcining temperature, i.e. they all reached their maximums at 700 °C, and then decreased continuously until 1000 °C.As compared with pure cement mortar, the 7-d and 28-d flexural strength and compressive strength of mortar with coal gangue auxiliary cementitious admixture calcined at 700 °C were increased by 16.5%, 12.6%, 8.27% and 11.85%, respectively.Therefore, the appropriate temperature for calcining high alumina coal gangue was identified as 700 °C.Similarly, a type of high alumina coal gangue produced in Luliang City, Shanxi Province in China, was also reported to have the same appropriate calcining temperature [21], which was attributed to a significant promotion effect of calcination at 700 °C on formation of high-activity metakaolin products with high resolution rate of activated alumina and silica.
The effect of dosage of coal gangue auxiliary cementitious admixture calcined at 700 °C on the mechanical properties of mortar was also evaluated via mechanical testing.Figure 9 shows the 7-d and 28-d flexural strength and compressive strength of mortar with different dosages of coal gangue auxiliary cementitious admixtures.Apparently, when the coal gangue auxiliary cementitious admixture replaced less than 30% of cement, the 7-d and 28-d mechanical properties were more or less enhanced except for the 7-d flexural strength at 20% of  dosage, as compared with that of pure cement mortar.However, when the dosage exceeded 30%, both 7-d and 28-d mechanical properties decreased remarkably.In addition, it is noted that the enhancement effect is much more significant for coal gangue auxiliary cementitious admixture after 28 days curing.This could be related with the role of metakaolin in the development stages of cement hydration.In the early stage of cement hydration, metakaolin in the coal gangue auxiliary cementitious admixture merely plays a dilution role, but in the middle and late stages, metakaolin starts reacting with CH (Ca(OH) 2 ) that was produced during cement hydration process.The participation of metakaolin not only promotes further formation of calcium silicate hydrate (C-S-H) from cement but also formation of calcium aluminosilicate hydrate (C-A-S-H) deriving from coal gangue auxiliary cementitious admixture [22][23][24].However, when the dosage of coal gangue auxiliary cementitious admixture exceeds 30%, the amount of CH generated from cement hydration is not sufficient for the hydration reaction of metakaolin.Therefore, the coal gangue auxiliary cementitious admixture exerts a detrimental dilution effect on the mechanical properties of mortar specimens, and the overall mechanical properties deteriorate sharply.It is suggested that if the high alumina coal gangue calcined at 700 °C is used as auxiliary cementitious admixture, the dosage should not exceed 30%.

Fracture surfaces analysis of mortar specimens
The compressive fracture surfaces of mortar specimens with coal gangue auxiliary cementitious admixtures were analyzed by SEM and EDS techniques.Figure 10 shows the fracture surface and corresponding elemental mappings for the mortar specimen with coal gangue auxiliary cementitious admixture calcined at 600 °C.The surface presents typical fragile fracture morphology (figure 10(a)), and the smooth areas indicated by yellow circles correspond to the locations of sand according to the elemental mappings for Si and O (figures 10(b) and (c)).The mapping for Al element told the locations of big and small coal gangue particles as indicated by yellow arrows (figure 10(d)) as calcined coal gangue particles had high content of Al element, and Ca element was found absent at these locations (figure 10(e)), suggesting almost no involvement of these big and small particles of coal gangue in the hydration reaction and no contribution to the strengthening effect on mortar.S element was basically distributed uniformly on the fracture surface except for those locations of sand and coal gangue particles (figure 10(f)).
Figure 11 shows the fracture surface and elemental mappings of mortar specimen with coal gangue auxiliary cementitious admixture calcined at 700 °C.The surface exhibited a little slighter fragile fracture morphology according to the increased rough areas on the fracture surface (figure 11(a)), and the locations of sand identified by the elemental mappings for Si and O (figures 11(b) and (c)), were still smooth.However, the number and size of coal gangue particles without involvement in the hydration reaction, identified from the mappings for Al and Ca elements (figures 11(d) and (e)), were much less than that on the fracture surface in figure 9. S element was still distributed uniformly in C-S-H gel (figure 11 (f)).This means that the coal gangue calcined at this temperature is fully activated and the vast majority can participate in the cement hydration reaction process.
Another finding is that acicular ettringite (Aft (3CaO )) structure was found to be formed at localized positions on the fracture surfaces, especially in pores.Figure 12 shows the acicular ettringite on the fracture surfaces with coal gangue auxiliary cementitious admixtures coalcined at temperatures of 700 °C-1000 °C.It can be seen that the amount and size of acicular ettringite are increased with calcining temperature rises.This finding can be used to explain the reason that the highest mechanical properties of mortar with coal gangue auxiliary cementitious admixture occurs at the calcining temperature of 700 °C.From   The variations in amount and size of acicular ettringite in the mortars clearly show that the hydration of cement mortar with coal gangue auxiliary cementitious admixture deteriorates as the calcining temperature is higher than 700 °C.The reason could be attributed to the difference in activity of coal gangue auxiliary cementitious admixtures calcined at different temperatures.After calcined at 700 °C, coal gangue was transformed into a high activity of metakaolin with low crystallization degree as identified by SEM analysis, which results in formation of a certain amount of C 3 AH 6 from metakaolin, and thus promotes the transformation of Aft to AFm.Therefore, only a little amount of acicular ettringite was observed on the fracture surface.Other research works also found that metakaolin with high activity could promote the hydration process of Portland cement by formation of more C 3 AH 6 [25,26].However, after cialcined at higher temperatures of 800 °C-1000 °C, the crystallization degree of metakaolin was increased with calcining temperature rising, metakaolin with low activity was not only incapable of producing sufficient C 3 AH 6 for hydration process of Portland cement but also counterproductive to the transformation of Aft to AFm due to its obstruction in the mortar.This explains the phenomenon that more acicular ettringite are found in the numerous pores in mortar structures with coal gangue auxiliary cementitious admixtures calcined at higher temperatures.The presence of acicular ettringite brings about quite amount pores in mortar structure and finally deteriorates the mechanical properties.
In order to identify the hydrate products generating from coal gangue auxiliary cementitious admixture, the chemical composition analysis was conducted at several points on the fracture surface with coal gangue auxiliary cementitious admixture calcined at 700 °C.The locations of points are indicated by numbers in figure 14   generated by the reaction with coal gangue.Both C 4 AH 13 and C 2 ASH 8 are the metastable products of C 3 AH 6 , suggesting that coal gangue auxiliary cementitious admixture can provide sufficient C 3 AH 6 to promote the hydration of cement.The chemical compositions at other points essentially correspond to that of C 3 S hydrate formed on metakaolin substrate.On the basis of hydrate products formed on the fracture surface, it can be concluded that coal gangue auxiliary cementitious admixture calcined at 700 °C had a strong activity and thus react with cement to form C-A-H and C-A-S-H gel during hydration process of cement.

Conclusions
1.During calcining process, high alumina coal gangue composed mainly of kaolinite underwent a series of transformations at following onset temperatures: carbon combustion at 426.3 °C, dehydroxylation at 515.6 °C, metakaolin transformation at 600 °C and mullite transformation at 1000.5 °C.
2. The appropriate calcining temperature for high alumina coal gangue coal gangue was 700 °C, at which the flexural strength and compressive strength of cement mortar with 30% coal gangue auxiliary cementitious admixture reached their maximums.
3. The mechanical properties of cement-coal gangue mortar decreased considerably as the dosage of coal gangue auxiliary cementitious admixture calcined at 700 °C exceeded 30%.
4. After calcined at temperatures higher than 700 °C, coal gangue auxiliary cementitious admixture gave rise to a large amount of ettringite formed in cement mortar and exerted a negative effect on the mechanical properties.
5. This study develops a simple and convenient technology to produce a new type of auxiliary cementitious admixture via calcining high alumina coal gangue powders at 700 °C.The high alumina coal gangue auxiliary cementitious admixture can be used to replace 30% Portland cement for preparation of blended cement in civil building and infrastructure.At present, most of the studies are focused on the thermal transformation, structure and mechanical properties, the durability tests and experiments for preparation of concrete will be conducted to investigate the structural performance.

Figure 2 .
Figure 2. Mechanical testing setups for flexural strength (a) and compressive strength (b) of mortar specimens.

Figure 5 .
Figure 5. Infrared spectra of coal gangue calcined at different temperatures.

Figure 8 .
Figure 8. Mechanical strength of cement specimens with coal gangue calcined at different temperatures (a) flexural strength (b) compressive strength.

Figure 9 .
Figure 9. Mechanical strength of cement mortar specimens with different substitution amounts of coal gangue auxiliary cementitious admixture calcined at 700 °C: (a) flexural strength, (b) compressive strength.

Figure 13 .
Figure 13.Schematic diagram of Portland cement hydration heat.

Figure 14 .
Figure 14.Chemical compositional analysis of the fracture surface with coal gangue auxiliary cementitious admixture coalcined at 700 °C (a) positions of the points in the SEM image (b)element spectrum at point.13.

Table 1 .
Main chemical components of calcined coal gangue and cement (wt%).

Table 2 .
Chemical compositions of the selected points (wt%).
, and their corresponding chemical compositions are listed in table 3. The chemical composition at the point 13 is close to that of 4CaO•Al 2 O 3 •13H 2 O (C 4 AH 13 : 9.64%Al, 28.57%Ca, 57.14%O), while the chemical composition at point No.16 is approximately equal to that of 2CaO•Al 2 O 3 •SiO 2 •8H 2 O (C 2 ASH 8 : 12.92%Al, 6.70%Si, 19.14% Ca, 57.42%O).Due to the low Al content of Portland cement, C 4 AH 13 and C 2 ASH 8 were considered to be