Resistance of Cement Paste and Its Relationship to Strength under the Corrosive Action of Ambient Media ()
1. Introduction
To determine the hydrous state and properties of cement paste by response of electrical features is both effective and fast. For example, the AC impedance of the paste can be used to determine both early-stage and middlestage hydration mechanisms [1-3]. AC impedance spectroscopy allows non-destructive monitoring of the dispersity of fibers in fiber-reinforced electrically conductive cement-based composites [4]. The combined use of timedomain reflectometry (TDR) and AC-impedance spectroscopy can improve the response of fresh fiber reinforced cement-based composites to impedance [5]. The eddy current method is a very good approach to the study of early contractions in concrete [6]. The galvanostatic method can be used to measure the electrical properties and microstructure of cement-based composites [7]. The resistance of hydrated cement paste can reflect the hardening process and texture [8,9]. Hardened hydrated cement paste is a relatively condensed perforated system that can be damaged by freeze-thaw cycles, fresh water, seawater, and other corrosive media, all of which can reduce its lifetime. The mechanical strength and microstructure of the paste are closely related to changes in its resistance. In this way, changes in paste resistance in external electric fields can be used to evaluate the endurance of the material [10]. This study focused on investigating the resistance of the new PALC and PC under the effects of chemical corrosion and freeze-thaw cycles [11]. Its aim is to foster understanding of the rules behind changes in paste resistance changes the action of corrosive media at the micro-structural level and to correlate changes in the micro-structure of the paste to changes in its mechanical strength under the influence of corrosive media via resistance properties. This will provide a basis for the development of fast, non-destructive methods for the assessment of changes in paste quality.
2. Experiments
2.1. Materials
This study used PALC and PC. Artificial seawater was used as chemical corrosive media (see Table 1 for composition) and MgSO4 solution at a mass concentration of
Table 1. Chemical composition of artificial seawater (g/l).
5% in water. Chemically pure agents were used in all solution preparations: NaCl, Na2SO4, CaCO3, MgNO3, MgCl2, CaCl2, and MgSO4.
2.2. Sample Preparation and Testing Methods
PALC was prepared by our laboratory. PC was manufactured by Shandong Cement Factory using a rotary kiln. Fineness was controlled by filtering 4 w/% sieve residue through 80 μm sieve. PC and PALC were molded to 20 × 20 × 20 mm net paste samples. After 28 days of standard curing, they were moved to artificial seawater, moved to 5% MgSO4 solution, or subjected to freeze-thaw cycles. After the samples were soaked for the designated lengths of time or subjected to designated numbers of freezethaw cycles, a HP4191 radiofrequency impedance analyzer (Agilent Technologies, US) was used to measure the resistances of cement net pastes under different conditions. The testing frequency was 837.8 Hz.
3. Results and Analysis
3.1. Resistance of Cement Paste after Freeze-Thaw Cycles
Figures 1 and 2 illustrate changes in the resistance and the compressive strength of the paste samples over time after freeze-thaw cycles. The changes in the resistance of cement paste are mainly influenced by two factors. First, as the cement continues to hydrate, the paste structure becomes condensed and the amount free water is reduced, leading to increased resistance. Second, after the freezethaw cycles, the capillary water in the cement net paste freezes and expands, producing micro-fractures. These micro-fractures grow as the number of freeze-thaw cycles increases, permitting further invasion of liquid media. This increases the concentration of transportable ion impurities and decreases resistance decrease. At this point, the dispersion of the hydrated gel can usually compensate for the decreased resistance.
The parameters measured at the end of the 28 days of standard curing were used as values at cycle 0. When the number of freeze-thaw cycles increased to 50, the resistances of both types of cement net paste showed a decreasing trend (Figures 1 and 2). However, the resistances reached peak values at cycle 100, which corresponded to peak values in the compressive strengths. This suggested that during the period from cycle 50 to cycle 100, the paste structure condensed, and hydration played a dominant role in this process. After the 100th cycle, the resistances showed a decreasing trend, and so did the strengths,
Figure 1. Resistance of cement paste subjected to freezethaw cycles.
Figure 2. Compressive strength of cement paste subjected to freeze-thaw cycles.
especially that of the PC sample. This suggested substantial increase and expansion of the micro-fracture in the paste due to freeze-thaw. During all freeze-thaw cycles, the resistance of the PALC paste was consistently 15 - 20 times higher than that of the PC paste (Figure 1). The same trend was observed for compressive strength (Figure 2). Such results suggest that the structure of the PALC paste became condensed with a relatively small number of fractures in PALC paste, which expanded slowly. These fractures caused the high tolerance of PALC to freeze-thaw cycles. At cycle 150, there was still no substantial decrease in strength.
Resistance of cement paste Figure 3 shows an XRD analysis of the hydrated paste before and after the freezethaw cycles. As illustrated in Figure 3(a), for PC, the diffraction peaks of hydration products calcium hydroxide and ettringite increased after several freeze-thaw cycles, whereas the diffraction peaks of clinker compounds C2S and C3S continued to decrease as the process continued. Figure 3(b) shows that, for PALC, after several freezethaw cycles, except for the clinker compounds of d values 0.374 nm and 0.296 nm showed a decrease in the diffracttion peak. Compounds with d values of 0.280 nm showed a decrease the diffraction peak but also a broadening of the peak base. The diffraction intensities of hydration products with d values of 1.419 nm and 0.718 nm increased
Figure 3. XRD of cement hydrated paste subjected to freezethaw cycles.
after several freeze-thaw cycles. This suggested that, during the freeze-thaw cycles, although the hydration of the cement paste was limited during the freezing period, the clinker compounds continued to be hydrated during the thawing period. This was particularly notable before the 130th cycle. It was the continuing hydration of the cement that led to the increase in the resistance of the cement paste, partly counteracting the decrease in the resistance caused by micro-fractures. This resulted in the fact that the decrease in resistance between freeze-thaw cycle 100 and 150 was not pronounced. However, during the same time, the strength of PC paste dramatically decreased, suggesting substantial dispersion of hydrated gel in the PC paste.
3.2. Under the Effect of Chemical Corrosion
Figure 4 shows the resistances of net paste samples during standard curing, after corrosion by seawater and by MgSO4 solution at the testing frequency, 837.8 Hz. The resistance of cement paste under the effects of chemical corrosion is thought to be affected by three main factors.