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Influence of various auxiliary activators on CaO-activated cementless binder

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In the samples with Ca(NO3)2, an additional supply of Ca ion from Ca(NO3)2 resulted in a common ion effect, which appears to have a relatively low pH as measured by lowering the solubility of Ca(OH)2 . However, after 28 days of curing, the samples with NaNO3 clearly showed a reduction of C-S-H as the weight of the salt increased.

Global warming and problems of cement use

As shown in Figure 1.3, the annual production of Portland cement is expected to increase further, and the amount of carbon dioxide generated by the cement industry is expected to increase further. Therefore, reducing the use of Portland cement and developing a material that can replace it is a way to reduce carbon dioxide emissions and prevent global warming.

Figure 1.3 World portland cement production 1990-2050 [1]
Figure 1.3 World portland cement production 1990-2050 [1]

Alternative materials for Portland cement

Low carbon cement

Geopolymer

Alkali-activated GGBFS

Ca-based activation

However, depending on the characteristics of the sample, improper methods of stopping hydration can cause damage to the sample. As the CaCl2 dosage was increased, the intensity of the initial peak also increased. Therefore, the use of CaCl2 caused an acceleration of the reaction and generated a greater amount of reaction products.

Two possible explanations can be suggested for these observations of the unclear trend in the influence of CaCl2. Thus, the use of Ca(NO3)2 seemed to be more beneficial in the CaO-activated GGBFS system to improve the bond strength at all days compared to the addition of NaNO3. In this study, the influence of nitrate salts Ca(NO3)2 and NaNO3 on the CaO-activated GGBFS system was investigated.

However, Ca(NO3)2 still improved the strength of the samples during the early days of curing. On day 3 of cure, the use of nitrate salts was clearly beneficial in producing C-S-H, regardless of the cation type in the salts. 81] studied the addition of CaCl2 as an auxiliary activator to improve the early strength of the CaO-activated GGBFS.

In the study, the addition of CaCl2 significantly increased the early and 28-day potencies of CaO-activated GGBFS. The results of the analysis of the pore structure of the hardened samples are shown in figure 5.4 and table 5.3.

Figure 2.1 Procedure of laser diffraction particel size analyer
Figure 2.1 Procedure of laser diffraction particel size analyer

Outline of Dissetation

EXPERIMENTAL TECHNIQUES AND BACKGROUND

  • Laser diffraction particle size analyzer
  • X-ray fluorescence
  • Hydration stop methods
  • X-ray diffraction
  • Thermalgravimetric analysis
  • Mercury intrusion porosimetry
  • Scanning electron microscopy
  • Isothermal calorimetry
  • Measurement of pH
  • Electrical resistivity of cementitious material

Therefore, the selection of the appropriate hydration stop method is very important for accurate microstructural analysis of cementitious material [21]. To prevent the charging phenomenon, the surface of the sample is pretreated with a metal such as gold, platinum or palladium.

Figure 2.2 XRF mesurement process
Figure 2.2 XRF mesurement process

EFFECTS OF CALCIUM CHLORIDE ON HYDRATION AND PROPERTIES OF CAO-

Introduction

EFFECTS OF CALCIUM CHLORIDE ON THE HYDRATION AND PROPERTIES OF CAO-ACTIVATED BINDER GGBFS/FLY ASH. CaCl2 was reported to increase the early and late compressive strengths of F-grade and GGBFS fly ash-based binders, but C-grade fly ash-based binders showed less significant effects than the other binders. The main reason for the positive effects of CaCl2 on strength was the rapid consumption of Ca(OH)2 in the presence of CaCl2 [55, 56].

The aim of this study was to investigate a possible use of CaCl2 to increase the early and ultimate strengths of CAS 4:4:2 and thus develop a new price-competitive cementless binder for concrete production. Conduction calorimetry was also performed to assess the possibility of excessive reaction heat generation, which could be caused by the addition of CaCl2. Reaction products and microstructure of hardened pastes of CAS 4:4:2 binders were characterized using powder X-ray diffraction (XRD), thermogravimetry (TG), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM).

Experiment

In addition, electrical resistivity measurements (based on Wenner four-probe resistivity measurements) were performed to evaluate the corrosion resistance of the samples. The size distribution of GGBFS was similar to distributions in the literature, but the fly ash had relatively larger particles above 80 μm. In particular, the mixed feedstock had a similar amorphous hump from GGBFS because a relatively small portion of fly ash was included in the mixed feedstock, and the intensity of the amorphous fly ash hump generally appeared much smaller than that of GGBFS in XRD.

Although this may lead to confusion in the reader's understanding, the notation of two separate GGBFSs was necessary to mimic a mixed triplet mixture. Since the presence of CaCl2 in the mixture may produce an excessive heat of reaction. For this purpose, samples of 2 mm thickness along the length of the cylindrical samples were made using a precision saw.

Because, in the presence of CaCl2, steel corrosion can be a serious problem when CAS 4:4:2 binders are applied to reinforced concrete structures, electrical resistance was measured to evaluate the potential for steel corrosion using Wenner measurements of the electrical resistance with four probes [ 60 ].

Figure 3.1 Particle size distributions of raw materials
Figure 3.1 Particle size distributions of raw materials

Results and discussion

  • Compressive strength tests
  • Heat of reaction
  • XRD
  • TG
  • MIP
  • Scanning electron microscopy
  • Electrical resistivity of prismatic mortar samples of CAS 4:4:2
  • Further discussion

This lower cumulative heat production of CAS 4:4:2 binders after 10 h was probably due to the fact that the chemical reactivity of GGBFS was lower than that of clinker [68]. These observations indicate that the presence of CaCl2 greatly accelerated the dissolution of the glass phase of GGBFS and fly ash in the early days, regardless of the CaCl2 dosage, when the CaCl2 content was incorporated above 1.0 wt. Thus, the increased amount of reaction products can more efficiently fill the capillary pores in the matrix, resulting in an increase in the early compressive strength.

In Table 3.4, the Na/Al ratio of the samples increased with the increase in the CaCl2 content, and it experienced a large jump above the addition of 2% CaCl2 (i.e. 2CSF). However, it should be noted that the XRD results at 28 days in Figure 3.7 (b) apparently showed similar amorphous bumps at ~22-37° regardless of the CaCl2 dosage; thus, the XRD results cannot be consistent with the latter suggestion. The primary role of CaCl2 in the CAS 4:4:2 binder is to increase the solubility of raw materials.

The similarity in behavior of the CaO-activated fly ash/slag systems could be attributed to the influence of CaCl2 on strength development.

Figure 3.4 Compressive strength testing results of CAS 4:4:2 binders: (a) w/b = 0.3, (b) w/b = 0.35,  and (c) w/b = 0.4
Figure 3.4 Compressive strength testing results of CAS 4:4:2 binders: (a) w/b = 0.3, (b) w/b = 0.35, and (c) w/b = 0.4

Chapter summary

In general, the addition of CaCl2 can shorten the initial setting times and increase the early strength of PC concretes. The effect of CaCl2 on the CAS 4:4:2 binder is similar to the effect of CaCl2 on PC systems. 52, 67] pointed out that the key role of CaCl2 in GGBFS/fly ash systems is to increase Ca(OH)2 solubility.

According to the XRD and TG results, with the addition of larger amounts of CaCl2, Ca(OH)2 gradually decreases. XRD analysis also illustrated that the presence of CaCl2. greatly accelerated the dissolution of the glass phase of GGBFS and fly ash in the early days. In each mixture, in total weight losses up to 1000 °C in TG, more weight was lost at 28 days compared to that at 3 days, regardless of CaCl2 dose; however, with increasing CaCl2 content, the change in total weight loss between 3 and 28 days decreased significantly.

Thus, the use of CaCl2 not only caused the acceleration of the reaction, but also generated a larger amount of reaction products.

INFLUENCE OF CALCIUM AND SODIUM NITRATE ON THE STRENGTH AND REACTION

  • Introduction
  • Experimental procedure
  • Results and discussion
    • Compressive strength
    • Measurement results of the pH levels
    • XRD analysis
    • TG/DTG
    • MIP
  • Chapter summary

INFLUENCE OF CALCIUM AND SODIUM NITRATE ON THE STRENGTH AND REACTION PRODUCTS OF THE CAO ACTIVATORS. The XRD patterns of hardened samples are provided with the reference patterns for the identified phases in Figure 4.5. These results support that the use of nitrate salts—regardless of the type of cation of the salts—was clearly beneficial in producing C-S-H (up to 1 wt%).

Further investigation of the pore size distribution showed that it depended significantly on the cation type of the nitrate salts. In summary, these results indicate that average pore size was more important than total porosity in determining the strength of the SN group at 28 days of curing. The results showed that on the third day of curing, the strengths almost doubled for all samples, regardless of the type of cation of the nitrate salts.

In this study, all samples mostly lost their pores larger than 0.05 μm as the curing days progressed (from 3 to 28 days), resulting in a reduction of the average pore size and an improvement in their strength.

Figure 4.1 Particle size distribution of GGBFS
Figure 4.1 Particle size distribution of GGBFS

STRENGTH ENHANCEMENT OF CAO-ACTIVATED GGBFS BINDER THROUGH ADDITION

  • Introduction
  • Experimental procedures
  • Results and discussions
    • Compressive strength
    • Mercury intrusion porosimetry (MIP)
    • X-ray diffraction (XRD)
    • Thermogravimetry (TG)
  • Chapter summary

The number on the sample label represents the replacement rate of CF; for example 3CFCS. The total porosity and average pore diameter tended to decrease as the amount of CF increased after 3 days, and thus the use of CF clearly induced a refinement of pore size [4] in all samples. Given that in XRD, as a greater amount of CF was replaced, a greater amount of C2AH8 was produced, the presence of CF was likely the main driver of C2AH8 generation.

Thus, the formation of C2AH8 could contribute to the increased strength after CF addition after 3 days. Therefore, the use of CF above 1 wt% probably did not result in further C-S-H formation in this study. The main reasons for the increase in strength due to the use of CF are summarized as follows:.

However, when the amount of CF was used above a certain limit (e.g., 3 wt%, in this study), the formation of C-S-H began to be disturbed, although more C2AH8 was formed.

Figure 5.1 shows the particle size distributions of GGBFS, and its median particle size was about 15  µm
Figure 5.1 shows the particle size distributions of GGBFS, and its median particle size was about 15 µm

SUMMARY

Schöler, A., et al., Hydration of quaternary Portland cement mixtures containing blast furnace slag, siliceous fly ash and limestone dust. Alahrache, S., et al., Chemical activation of hybrid binders based on silica fly ash and Portland cement. Lee, N.K., et al., Cement-free alkali-activated controlled low-strength materials (CLSM) using industrial by-products.

Park, H., et al., Strength development and self-activation hydration behavior of commercial granular blast furnace slag in soil mixed with purified water. Jeong, Y., et al., Strength development and microstructural characteristics of granular blast furnace slag activated by barium hydroxide. Yang, K.-H., et al., Hydration products and strength development of calcium hydroxide-based alkali-activated slag mortars.

Yum, W.S., et al., Effects of CaCl2 on hydration and properties of lime (CaO)-activated slag/fly ash binder.

수치

Figure 1.2 Globally averaged greenhouse gas concentrations [2]
Figure 1.3 World portland cement production 1990-2050 [1]
Figure 2.4 Schematic of scanning electron microscope
Figure 2.5 Schematic of isothermal conductivity method
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