DEVELOPMENT AND PERFORMANCE OF AGRICULTURAL WASTE-BASED ACCELERATOR FOR CONCRETE HARDENING

Abstract

n recent times, construction industries have been shifting toward the use of precast structural elements to minimize cost and construction time. To meet the demand, the precast industries are constantly looking for various alternative ways to enhance the production rate so that the overall construction process can be accelerated. The production cycle of the precast element is closely linked with the formwork removal time, which in turn depends on the early strength development of fresh concrete. Production of cement, which is the primary binding material in concrete, releases huge quantities of CO2 into the atmosphere causing environmental concern. In view of this, the development of sustainable concrete by reducing the cement content and utilizing industrial/agricultural waste materials, such as fly ash, etc., has gained considerable popularity in the construction industry. Replacement of cement with fly ash significantly reduces the initial hydration rate resulting delay in the process of early strength gain, which affects the pace of the overall construction process. In order to overcome the limitations of the use of waste materials, costeffective hardening accelerators are used to enhance early strength development. The use of conventional methods such as high-temperature curing and chloride-based hardening accelerators are generally costly, generate greenhouse gas and environmental-related issues. Moreover, such methods alter the kinetics of the hydration process and chemical structure of hydration products resulting in durability-related problems in concrete. On the contrary, calcium silicate hydrate (C-S-H) based hardening accelerators have been found to be more suitable for early strength gaining as they act as seeds which are similar to that of naturally formed gels during the hydration process. Existing synthesis methods of C-S-H seeds, namely, pozzolanic, solgel, hydro-thermal and precipitation methods etc., pose various limitations like slow kinetics, formation of semi-crystalline phases, high energy consumption, and the presence of harmful elements (like chloride, nitrate, etc.). Moreover, these products are generally cost-prohibitive and not readily available in the Indian market. Therefore, there is a need to develop C-S-H–based hardening accelerator for concrete utilizing cost-effective, locally available raw materials. The present thesis aims to develop an impurity-free preformed C-S-H–based hardening accelerator from the agricultural waste product using a novel ion-exchange–based synthesis process. Such an ion exchange-based novel synthesis process overcomes the limitations associated with the conventional methods of early strength gain and existing C-S-H seed synthesis methods. The synthesized C-S-H seeds, when added to concrete, provide additional nucleation sites, thereby reducing the free energy of nucleation and accelerating the early hydration process. Maintaining the effectiveness of synthesized C-S-H seeds towards early strength development in concrete remains a major challenge. The synthesized C-S-H seeds have the tendency to agglomerate during their shelf life, thereby reducing their expected performance. In order to keep the seeds dispersed, a suitable, cost-effective dispersing agent without having any adverse effect on concrete was selected in the present work. Further, the performance of the synthesized C-S-H-based hardening accelerator in concrete was investigated in terms of setting time, compressive strength and durability performances. Different characterization studies such as X-ray diffraction (XRD), thermal analysis (TGA), Field- emission Scanning Electron Microscopy (FE-SEM), the heat of hydration etc., were performed to determine the structural, morphological, and chemical properties of the synthesized C-S-H seed. The heat of hydration and compressive strength studies proved that the material is effective in achieving early strength in concrete. The presence of synthesized C-S-H seeds in concrete effectively accelerated the initial strength gain development by approximately 30% during the early period of the setting. Therefore, the developed hardening accelerator has the potential to accelerate the production cycle of precast elements. Moreover, the presence of C-S-H seeds in concrete accelerates the precipitation of hydration products in the pore spaces between the cement particles, resulting in a compact microstructure. The long-term performance of the developed hardening accelerator when incorporated in concrete was investigated through various durability studies such as chloride penetration, sulphate attack, acid attack and accelerated corrosion tests. The compact microstructure and tortuous pores space observed in C-S-H seeded concrete provides hindrance towards migration of harmful chemicals responsible for the degradation of concrete.