1. School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan 232001, China 2. School of Materials Science and Engineering, Southeast University, Nanjing 211189, China 3. Anhui Province Engineering Laboratory of Advanced Building Materials, Anhui Jianzhu University, Hefei 230022, China
The composite slag and fly ash were used as the main precursors to design and prepare a novel type of lime and gypsum-activated low carbon cementitious material (LCM), to decrease the environmental loading of the cement industry. The evolution rules of mechanical properties of LCM mixtures with different mass ratios and the hydration characteristics of LCM were analyzed via quantitative X-ray diffraction (QXRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), mercury intrusion porosimetry (MIP) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) method. Results showed that the hydration products of LCM were mainly composed of ettringite, C-(A)-S-H gel with a low Ca/Si atomic ratio, and a small amount of AFm-CO3. A large amount of ettringite formed in the early stage of hydration, causing the heat flow curve of LCM to exhibit a new exothermic peak during the decelerating period of hydration. After long-term hydration, rod-like ettringite crystals were embedded into C-(A)-S-H gel as the skeleton, forming a dense paste, and the pore structure of LCM had a high tortuosity and low connectivity. The mechanical performance of LCM at various ages was significantly increased by optimizing the mixture proportion. The compressive strength of C3 mortar at 28 days could reach 53.5 MPa, which represented an increase of 37.9% compared to the control group, and the strength continued to increase over time. A small amount of Portland cement was introduced into the LCM, which effectively increased the alkalinity of the liquid phase, promoted the hydration reaction of the mineral admixtures and increased the overall hydration rate of LCM, thereby improving the mechanical properties of LCM in the mid-to-late stages.
Fig.2Quantitative analysis results of XRD patterns of LCM samples
Fig.3FTIR spectra of LCM samples
Fig.4TGA curves of LCM samples after 90 days
样品
wB/%
氢氧化钙(TGA)
氢氧化钙(QXRD)
化学结合水
C1-90 d
3.9
3.7
13.8
C2-90 d
4.2
4.1
13.5
C3-90 d
6.5
6.7
14.7
C4-90 d
10.8
11.7
15.4
C5-90 d
9.7
10.3
15.7
Tab.3Content of Ca(OH)2 and chemically bound water in LCM samples at 90 days
Fig.5SEM images of microstructure of C3 samples at 3 and 90 days
Fig.6Result of MIP test of LCM samples at 90 days
样品
P/%
Pe/%
η/%
τ
C1-0.3
21.5
6.1
28.4
7.2
C3-0.3
15.8
4.6
29.3
7.0
C5-0.3
13.4
4.1
30.4
6.9
C3-0.5
31.3
14.9
47.7
4.6
PC-0.5
27.6
14.8
53.6
4.0
Tab.4Parameters of pore structure of LCM samples
Fig.7Hydration rate and cumulative heat of hydration of LCM samples
Fig.8Influence of mass fraction of gypsum on heat of hydration of LCM
Fig.9pH value of pore solution from LCM fresh and hardened paste
Fig.10Evolution rule of mechanical strength of LCM samples
M/(kg·kg?1)
原材料
LCM
水泥熟料
0.880
C1
0.12
消石灰
0.750
C2
0.17
矿渣粉
0.083
C3
0.21
粉煤灰
0.008
C4
0.24
石膏
0.200
C5
0.29
硅酸盐水泥
0.850
C6
0.20
—
—
C7
0.21
Tab.5Carbon emission of raw materials and LCM samples
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