Activated hybrid cementitious systems, including high volume fly ash with high loss on ignition (LOI) content and sodium sulfate as activator, are studied to explore more sustainable alternatives to Portland cement (PC) for reducing CO2 emissions in the concrete industry. Most of the background of this project is on mortars with low LOI fly ashes. Performance and deterioration initiation periods have never been studied before for concretes with these materials. None of the following factors have been considered previously in one study: fly ash replacement level, nature of fly ashes obtained from Colombian sources (of high LOI), and the type and amount of activator. In addition, no specific study had encompassed all the parameters necessary for the development of systems viable for the Colombian concrete industry, on performance, environmental and economic grounds. Therefore, this study aims to address this.
This research covers characterization of raw materials before and after treatment, mortar evaluation, fresh and hardened state concrete evaluation of both laboratory samples and large size concrete elements cured outdoors, durability characterization, prediction of corrosion initiation period, CO2 emissions and cost calculations. The characterization includes the evaluation of four different fly ashes (Termopaipa, Fabricato, Termoguajira, Tampa) before and after sieving and the evaluation of different activators; sodium sulfate, lime and quicklime at different dosages. The mortar and concrete studies were carried out for a period of up to one year. The concrete study evaluates the performance of a 50/50 Termopaipa fly ash/PC system with 1% sodium sulfate by weight of cementitious material. Beside compressive strength and maturity, the performance evaluation includes water permeability, sorptivity, chloride penetration, chloride diffusion, carbonation, sulfate attack and alkali silica reaction. Prediction models for corrosion initiation time are developed by correlating results from laboratory cured samples to those cured outdoors. Efficiency curves were developed to correlate CO2 emissions and costs to compressive strength for the different cementitious systems.
Modifying the particle size distribution of the fly ash, through sieving, affected the compressive strength due to changes in the amorphous content. The benefits of sodium sulfate in terms of compressive strength are highlighted, with 1% found to be the optimum dosage for use in concrete. The higher ettringite formation, portlandite consumption and early compressive strengths are some of the characteristics of mixes incorporating sodium sulfate. In terms of concrete performance, it is found that the chloride diffusion coefficient is reduced significantly with time for the activated system compared to control samples (100% PC and 80% PC - 20% fly ash) of the same water to cementitious material ratio (W/CM). This behaviour is exhibited by samples cured under controlled laboratory conditions (100% RH and 23°C). On the other hand, outdoor curing increases concrete permeability for all concretes. Long term carbonation is also explored, and samples under outdoor curing have a significant carbonation depth. Alkali silica reaction and sulfate attack problems are mitigated with this activated hybrid system. The prediction equations developed take into account chloride and carbonation diffusion and the influence of other parameters such as the W/CM, fly ash replacement level and compressive strength. From knowledge of the 28-day compressive strength of concrete, the time for critical levels of chloride or carbonation to reach the steel can be predicted considering the cover depth and the level of cement replacement with or without activator. Reduction of CO2 emissions and costs and the observed technical characteristics, demonstrate the viability of this green alternative in the short term.
