Mechanical performance and microstructural features of various types of alkaline earth- activated slag cements were investigated with microstructural characterization techniques as possible alternatives to OPC to reduce carbon dioxide emissions in this study.
Calcium hydroxide-activated slag cements have been investigated as alternative cements.
However, in general, the compressive strength of calcium hydroxide-activated slag cements has not been competitive with that of OPC. In this study, four different auxiliary activators, sodium hydroxide, sodium carbonate, sodium sulfate, and gypsum, were used with calcium hydroxide which is a major activator for GGBFS to increase the compressive strength of the cements. All auxiliary activators increased early age strength at 3 days but the strength at 28 days was comparable to or even below the sample which had no auxiliary activators. The reduction of water-to-slag ratio appeared to be more effective at improving the strength than the use of the four auxiliary activators.
Microstructures of calcium hydroxide-activated slag cements were highly affected by auxiliary activators where the cements produced different types of hydration products, revealed different reaction degrees of slag, and formed different characteristics of C-S-H depending on the additional activators. Nevertheless, the level of compressive strength did not have big differences, which indicated that the compressive strength of calcium hydroxide-activated slag cements could be determined by physical reasons such as porosity, not by chemical factors (e.g., type of hydration products, or dissolution degree of slag). Thus, to improve the mechanical properties of calcium hydroxide-activated slag cements, a significant reduction in water-to-slag ratio might be required with water reducing agents.
Based on the results of 29Si MAS-NMR, the substantial proportions of monomeric and dimeric silica (i.e., Q1 and Q2 silica) might not belong to the C-S-H, where the mean chain length of C-S-H did not have the relationship with the strength, which indicated that substantial portions of monomers and dimers of silica might be attributed to amorphous phases, not to C-S-H. To verify the presence of amorphous phases in calcium hydroxide-activated slag cements, further research should be conducted.
Barium hydroxide was also used for slag activation as a main activator and the properties of barium hydroxide-activated slag cements were compared to those of calcium hydroxide-activated slag cements. The strength of barium hydroxide-activated slag cements was significantly higher than that of calcium hydroxide-activated slag cement except at 3 days. The lower strength of barium hydroxide- activated slag cement at 3 days might be attributed to the lack of ettringite formation because barite (BaSO4) was more favorable phase than ettringite which could play a role in producing early age strength. Nevertheless, the strength of barium hydroxide-activated slag cements was always higher
than that of calcium hydroxide-activated slag cement because the higher pH value was expected due to the higher solubility of barium hydroxide than calcium hydroxide and more hydration products were produced in barium hydroxide-activated slag cement than calcium hydroxide-activated slag cements.
The hydration products formed in barium hydroxide-activated slag cements were barite, witherite, strätlingite, hydrotalcite-like phases, portlandite and C-S-H while those of calcium hydroxide-activated slag cements were ettringite, portlandite, and C-S-H. Although the C-S-H was formed with both barium hydroxide and calcium hydroxide, the characteristics of C-S-H were clearly different. The hardened matrix in calcium hydroxide-activated slag cements had Ca/Si ratios around ~ 1.4 while that of barium hydroxide-activated slag cements had a Ca/Si ratio below ~ 1.1, which was similar to the typical Ca/Si ratio of C-S-H(I).
The highest dissolution degree of slag was achieved in barium hydroxide-activated slag cement due to higher solubility of barium hydroxide, resulting in producing the highest amount of hydration products. Thus, the highest compressive strength and the smallest porosity were obtained in barium hydroxide-activated slag cements.
Barium hydroxide-activated slag cement could be expected to be used for radiation shielding structures which are generally produced with heavy weight aggregates. However, there is no literature about developing heavy weight binder. Barium hydroxide-activated slag cements could be a good candidate of heavy weight structural binders because the weight of barium element is significantly heavier than the major elements of OPC. Thus, the properties of barium hydroxide-activated slag cements for radiation protection should be investigated in future studies.
In addition, the possibility of calcium substitution by barium in C-S-H was not investigated in this study. The subject remains for future research works.
In previous literature, calcium oxide-activated slag cements had superior mechanical performance to calcium hydroxide-activated slag cements. However, only a few earlier studies investigated the relationship between the material characteristics of slag and the strength development of calcium oxide-activated slag cements. In this study, calcium oxide-activated slag cements from four different slag sources were investigated to verify the relationship between the intrinsic properties of slag and the properties of calcium oxide-activated slag cements.
Despite the same mixture proportion and curing conditions, the strength development of calcium oxide-activated slag cements was significantly different, which indicated that the material characteristics of slag could highly influence the properties of calcium oxide-activated slag cements.
However, strength development was not governed by any single dominant material parameter of the raw slag, but rather by the combination of various favorable factors. Favorable characteristics of slag for high strength were (a) higher calcium sulfate content, (b) finer particle sizes, and (c) higher
basicity and chemical modulus, while the influence of glass content was not notable.
The main hydration products of calcium oxide-activated slag cements were C-S-H and portlandite as well as ettringite in cases containing calcium sulfate and various types of AFm such as monosulfate, monocarboaluminate, and hydroxy-AFm type solid-solution. The slag which contained both calcium sulfate and calcium carbonate, produced the highest compressive strength with CaO activation because of the high amount of ettringite formation. The slag collected from Dubai had the lowest strength due to the presence of bulk-sized porous calcium carbonate, indicating that, although finely ground calcium carbonate could contribute to the strength development of calcium oxide- activated slag cements, the presence of large sized calcium carbonate could be harmful to strength development because it could be a fragile point for crack initiations under loading.
The results of chapter 5 could be a good reference not only for calcium oxide-activated slag cement, but also for slag-blended OPC system because the reaction between CaO and GGBFS is similar to pozzolanic reaction in slag-blended OPC system.
Finally, the influence of calcium carbonate powder on CaO-CaSO4-GGBFS composite cements was investigated. In this study, carbon sequestrated calcium carbonate powder was used as a raw material. The compressive strength of CaO-CaSO4-GGBFS composite cements increased with increasing calcium carbonate substitution up to 20 wt.% but decreased with further substitution.
However, the total porosity of CaO-CaSO4-GGBFS composite cements measured by MIP gradually decreased with increasing calcium carbonate powder.
All samples produced significant amounts of C-S-H and ettringite as main hydration products and formed a small amount of portlandite. The samples which contained 10 wt.% and 20 wt.%
of calcium carbonate powder produced hemicarboaluminate but the samples which contained 50 wt.%
of calcium carbonate powder did not due to the lack of aluminum supplies from GGBFS. While the amount of GGBFS was decreased with increasing the amount of calcium carbonate powder, the amounts of hydration products did not show a large difference and were even increased, slightly, with increasing calcium carbonate powder, indicating that the amounts of hydration products might be determined by the amount of activators or that calcium carbonate powder could increase the dissolution degree of GGBFS due to the filler effects of calcium carbonate powders.
EDS spot analysis clearly showed that the Ca/Si ratio in C-S-H increased with increasing calcium carbonate powder, which was the reason why the compressive strength of the sample, which contained over 30 wt.% of calcium carbonate powder, decreased while the porosity gradually decreased. The mechanical performance of C-S-H decreased with the Ca/Si ratio. Thus, the strength reduction in the samples which contained over 30 wt.% of calcium carbonate powder could be attributed to the poor mechanical performance of C-S-H with a high Ca/Si ratio. However, the relationship between Ca/Si ratio and mechanical properties of C-S-H has been verified only for pure
C-S-H, not for actual cementitious pastes. Thus, the influence of Ca/Si ratio on the properties of actual cementitious materials should be investigated.
Throughout this work, various alkaline earth-activated slag cements were investigated by multiple characterization techniques to verify the microstructural characteristics and its influence on mechanical properties. However, mechanical properties of cementitious materials could not be determined by a single factor. Thus, the influence of each material parameter on mechanical properties of cementitious materials should be more clearly investigated by a comprehensive methodology such as statistical analyses, which remains for future studies.
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