Resumen de Activated industrial sugarcane bagasse ash as an ecoefficient mineral addition to enhance the durability of cement-based materials
This thesis investigates the feasibility of using low-reactive industrial sugarcane bagasse ash (SCBA) as mineral addition in the production of more sustainable cementitious materials with a focus on the durability prospective. Besides being the most produced crop cultivated in the world, gaps still exist in literature on the understanding SCBA in cementitious materials. Research on i) the impact of treatments on low-reactive SCBA when used in the substitution of aggregates, ii) the comparison between cement and fine aggregates substitution, iii) the environmental impact of treatments , iv) the durability properties of mortars and concretes or v) the chloride binding capacity promoted by SCBA. The work seeks to provide the most eco-efficient activation method (able to counteract the inherent drawbacks) and usage for low-reactive SCBA while providing a thorough understanding on the mechanisms responsible for the positive improvements observed. The research finds a solution for a low-quality SCBA not suitable for cement substitution, resulting in mortars and concretes with improved performance. By exploring the influence of different activation methods (recalcination, grinding and physical separation) and substitution scenarios (cement and fine aggregates substitution), on the ultimate performance of mortars and evaluating environmental impact, grinding was found to be the optimal activation method. The use of 20% of ground SCBA replacing fine aggregates, increases the compressive strength on mortars by 62% after 28 days, decreases the porosity by 35% and reduced the migration of chlorides by 10 times. The environmental assessment showed that the eco-strength efficiency increased by 46.75%, enabling the reduction of structural elements and, therefore, the material consumption. This, resulted in the reduction of the specific embodied carbon (term provided in this research) of 49.47% and the reduction of the water consumption by 17%. The combination of performance-based methods (compressive and flexural strength, open porosity, apparent density, water capillary absorption, surface electrical resistivity, v rapid chloride migration coefficient) with a set of analytical techniques (XRD, FT-IR, SEM, TGA-DTA, MIP and physisorption) enabled the identification of the mechanisms behind the improvements. It was found that, at early ages, activated SCBA performs as an inert filler, increasing the packing density and potentially providing more nucleation sites. The pozzolanic activity is highly boosted at 28-56 days refining the pore structure and narrowing the interfacial transition zone by the formation of additional secondary C-SH gel. Besides the densification of the matrix, the research revealed that SCBA provide a higher capacity to chemically bind external chlorides, slowing down the diffusion through the matrix. Additionally, it was found that the reason behind the durability improvements provided (also by untreated SCBA regardless the higher porosity) was down to a lower threshold pore diameter. The use of the activated SCBA in other systems resulted in being more efficient in i) mixes with lower amounts of cement, providing results comparable to more cement-rich mixes ii) non-well graded sands. Finally, to close the loop, it was found that the activation of the low-reactive SCBA counteract the drawbacks initially observed reducing the water demand, accelerate the increase in the compressive strength gains providing at 7 days the same CS of control samples at 28 days, improving the chloride penetration resistance by 58.8% at 28 days, increasing the eco-strength activity up to 19.2% and delaying the corrosion initiation period up 3.5 times for curing periods of 90 days.
