Department of Urban and Environmental Engineering (Urban Infrastructure Engineering)Modified sulfur polymer composite has been developed to replace hydraulic cement concrete in specific applications. Because it has superior properties including the rapid development of high compressive strength, resistance to water environments, and resistance to strong acid and saline attack compared with the hydraulic cement concrete. Most of all, the fabrication of modified sulfur composites excludes water due to the thermoplastic properties of modified sulfur. Thus, all the raw materials such as aggregate and micro-fillers can be easily mixed with plastic modified sulfur to produce the modified sulfur composite. Early in the development of sulfur composites, however, elemental sulfur mostly from the distillation of crude oil in petroleum refineries was used as a binder. The composites made of elemental sulfur presented not only severe curing contraction but also inferior durability in weathering and chemical environments. Considering the aforesaid problems, many researchers developed modified sulfur polymers by reacting the elemental sulfur with a variety of chemical additives at a certain reaction temperature and time. The modified sulfur polymers had alternating compositions according to the reaction conditions such as the types of chemical additive, reaction temperature, and reaction time. Through a modification process, the composition of modified sulfur mixture was composed of abundant polysulfide products along with a reduced free elemental sulfur such as orthorhombic, monoclinic, and amorphous sulfur chains. Such a converted composition considerably enhanced the mechanical and durability properties of sulfur composites. The term, modified sulfur polymer, indicates the final reaction product from a mixture of elemental sulfur and chemical additive. If a certain micro-filler accounts for a part of the modified sulfur polymer, the mixture is usually called as sulfur polymer cement.
Among various modified sulfur polymers, this study employed a dicyclopentadiene (DCPD)-modified sulfur polymer as a binder, which is one of the commercially available products. Based on this modified sulfur polymer, total five research topics were carried out step by step in this study.
First, different proportions of modified sulfur composites were developed by replacing a portion of the modified sulfur polymer by fly ash and rubber powder. Both the fly ash and the rubber powder were employed to be a substitute for fine aggregate and to enhance the mechanical properties of modified sulfur composites by reducing thermal curing shrinkage. An increasing portion of the fly ash of up to 50 vol.% resulted in a continuous rise of compressive strengths with a given portion of rubber powder. Moreover, the rubber powder also significantly reduced the unit weight of modified sulfur composites without sacrificing the compressive strength. Finally, a series of microstructural analysis suggested the rationales for the enhanced mechanical properties in terms of crystalline phase transition, morphological transition, and porosity.
Second, in a similar way as the first research topic, modified sulfur composites included rubber powder, and a blend of Portland cement and fly ash as the binary cement that rendered the different particle characteristics (i.e., particle shape, particle size distribution) of micro-fillers as compared with sole fly ash or Portland cement. An increasing portion of the binary cement generally induced the higher compressive strengths of the modified sulfur composites than those with only fly ash. This was likely due to the larger indentation modulus of Portland cement than fly ash. In addition, the use of rubber powder contributed to a reduced unit weight of sulfur composites without a severe strength reduction. Most of all, because all the crystalline phases from the binary cement remained intact in hardened sulfur composites, the feasibility of using the binary cement as a self-healing material for cracked sulfur composites was confirmed empirically.
Third, considering the brittleness of modified sulfur composites except those containing rubber powder identified in the first and second research topics, a series of fiber-reinforced sulfur composites were developed and examined to convert the brittleness of modified sulfur composites into a more ductile manner, and to acquire multiple micro-cracks especially under flexures. Two micro fibers including steel and electrical chemical resistant (ECR) glass fiber were incorporated in the mixtures. By varying the total fraction of micro fibers and adjusting the relative volumetric ratios between steel and glass fiber, the flexural stress-strain responses of modified sulfur composites were greatly modified, which was supported by the change of porosity and the uniform strain distribution revealed by a digital image correlation (DIC).
Fourth, the combined effect of particle characteristics of micro-fillers and temperature on the rheological properties of fresh sulfur composites was examined through a rheometer test. Because both the portion of micro-filler and mixing temperature have been critical in determining the workability of modified sulfur composites, the author adjusted the surface areas of a given portion of binary cement and set the temperature at 120??? or 140???. Overall, both the yield stress and plastic viscosity of the modified sulfur composites were higher in 140??? than 120???. Especially, the result of mini slump flow of fresh modified sulfur composites was compared with the rheology test results at 140???. Thus, the results from the rheology and the mini slump test were deemed to suggest an optimal temperature range favorable for placement.
Finally, considering the feasibility of using cementitious materials as self-healing materials in the sulfur composites, total eight mix proportions of self-healing modified sulfur composites were developed and examined. The modified sulfur composites were comprised of a binary cement of calcium sulfoaluminate (CSA) expansive agent and Portland cement, superabsorbent polymer (SAP) powder, and fine aggregate. After through crack was made on each sample, all the samples were exposed to two wet environments: one was water curing, and the other was water permeability test. Each of them was dedicated to building different self-healing conditions. While the water curing gave a stable self-healing condition to the cracked samples, the water permeability test without any water curing was analogous to a real water intrusion through the cracks of a certain structure. For each sample, the extent of self-healing was monitored and evaluated by the optical microscope images of surface crack and elastic wave tests, respectively at specified ages. Moreover, computed tomography was used to confirm the recovery of inner crack width after 7 days of water curing. Through a series of tests, it was revealed that a higher ratio of CSA expansive agent than Portland cement in a binary cement promoted the self-healing of through cracks further with the swelling of SAP particles on crack surfaces.clos
