2.1. Raw Materials and Mix Proportions of Concrete
In this study, the cement used was Conch brand P·O 42.5 ordinary Portland cement produced by Liquan Conch Cement Plant in Xianyang City, Shaanxi Province, China, with its physical and mechanical properties as shown in
Table 1.
The silica fume originated from the northern part of Shaanxi Province. The Grade I fly ash used was produced by Shaanxi Weihe Power Plant. Using the Bettersize 2600 laser particle size analyzer, the volume-based average particle size of silica fume and fly ash were measured as 12.35 μm and 12.71 μm, respectively. The particle size distribution is presented in
Figure 1, and the physical properties of fly ash and silica fume are shown in
Table 2, and the main chemical components of cement, fly ash and silica fume are shown in
Table 3.
Natural river sand with a fineness modulus of 2.66 was used as fine aggregate, with an apparent density of 2630 kg/m3 and a bulk density of 1480 kg/m3. Limestone with continuous grading between 5 mm and 20 mm was used as coarse aggregate, with an apparent density of 2835 kg/m3 and a bulk density of 1720 kg/m3.
The superplasticizer applied was the Q8086 powder high performance water reducing agent produced by Shaanxi Qinfen Building Materials Co., Ltd., (Weinan, China) with a water reducing rate of 33%, air content of 2.0%, and solid content of 40%. The water reducer comprised 0.5% of the weight of the cementitious material.
The water used was ordinary tap water from Xi’an, Shaanxi Province, China.
In the experiment, five different composite cementitious material system concretes were prepared, while the external admixture method was used to keep the amount of cement unchanged. The control group without silica fume was designated as group A. In the other four groups, 25%, 50%, 75%, and 100% of fly ash were replaced with silica fume, which were numbered B, C, D, and E, respectively. The water–binder ratio was 0.48, and the mix proportions of concrete were as shown in
Table 4.
2.2. Sample Preparation and Experimental Methods
(1) Prior to preparing the concrete specimens, the raw materials were weighed according to the pre-determined mixing ratios and placed in the test pan. At the same time, the superplasticizer was poured into the water to fully dissolve, the twin-shaft concrete mixer was cleaned, and the mortar with the same water-binder ratio was prepared. The remaining slurry was poured into the wall-hanging slurry in the mixer. Next, coarse aggregate, cement, silica fume, fly ash, and fine aggregate were added in sequence and stirred for 60 s. Finally, the water mixed with superplasticizer was evenly poured into the mixer and stirred for 120 s.
(2) After unloading, the slump of fresh concrete was measured according to the Chinese standard “Standard for Test Methods for the Performance of Ordinary Concrete Mixtures” (GB/T50080-2016), and the HC-7L direct-reading concrete air content tester was employed to measure the air content of fresh concrete.
(3) The remaining concrete mixture was put into a 100 mm × 100 mm × 100 mm cube mold, placed on a vibrating table for vibrating compaction, and vibrated until the surface continued to produce slurry. Each group of A, B, C, D, E made 28 specimens, totaling 140 specimens.
(4) The concrete specimens were demolded 24 h after production, and 25 specimens of each mixing ratio group were naturally cured at a curing temperature of 20 ± 2 °C and relative humidity of 95%. The WAW-1000KN universal testing machine was used to test the compressive strength of concrete cubes with different amounts of substituted silica fume at curing ages of 1 d, 3 d, 7 d, 14 d, and 28 d.
(5) The remaining three specimens in each group of mix proportions were wrapped with a polyethylene film to prevent the entry and evaporation of water, as shown in
Figure 2. LF-NMR microstructure analysis and imaging was performed by the model MacroMR12-150H-I produced by Suzhou Niumag Analytical Instrument Corporation, as shown in
Figure 3. LF-NMR aims to analyze the composition of a test sample based on the nuclear magnetic resonance characteristics of the hydrogen nucleus, low field nuclear magnetic resonance spectroscopy was carried out on the concrete specimens at the curing ages of 1 d, 3 d, 7 d, 14 d, and 28 d, the CPMG pulse sequence data were collected, and perform inversion calculations through
T2 inversion software. Thus, a spectrum composed of the LF-NMR signal amplitude and relaxation time of different types of water in the concrete test block is obtained [
26].
In the hardening process of concrete, cement hydration consumes part of the physically bound water, thereby generating chemically bound water. Jehng measured the apparent transverse relaxation time of the chemically bound water in cement paste to be 12 μs [
27]. It is generally assumed that physically bound water in concrete mainly includes three forms: adsorbed water, pore water and free water [
28], as shown in
Figure 4.
The
T2 spectrum includes three relaxation peaks, which are defined as peak 1, peak 2, and peak 3 from left to right, which correspond to the LF-NMR signals of hydrogen nuclei in adsorbed water, pore water, and free water, respectively [
29]. The change in the peak area of the
T2 spectrum reflects the change in the contents of adsorbed water, pore water and free water in the filler slurry.
According to the volume and signal of the sample, a unit volume nuclear magnetic signal can be obtained, as shown in Equation (1). Then, the total water content of the tested sample can be obtained according to the calibration formula in Equation (2) [
30].
where
is the unit volume nuclear magnetic signal,
x is the total water content,
V and
A are the volume and signal of the sample, respectively.
(6) The JSM-7610F field emission scanning electron microscope produced by JEOL was utilized to test the surface micromorphology of 28-day-old concrete. Before the test, the sample was sprayed with gold and vacuumed for inspection by the scanning electron microscope imaging system.