Investigation of rice husk ash as a sustainable source material for blended low calcium fly ash based alkali activated binders

Alkali activated binder has been found to be mechanically efficient and an eco-friendly concrete with the utilization of waste by-products such as fly ash, blast furnace slag, rice husk ash (RHA) and metakaolin. Furthermore, the utilization of waste for concrete production leads to the conservation of landfills and storage lagoons and converts a waste product into a useful by-product. Low calcium class F fly ash is a widely used aluminosilicate-rich by-product for alkali activated concrete. RHA is an agricultural waste product, rich in silica, which initiates numerous environmental and health issues. Both low calcium fly ash and RHA are considered as waste industrial and agricultural by-products, respectively, that can be utilized in alkali activated concrete as a precursor material. It is vital to understand the chemistry behind the blended fly ash-RHA alkali activated binders in different construction applications (i.e., concrete and bricks). This research study aims at developing a fundamental understanding of RHA as a sustainable source material in blended fly ash-RHA based alkali activated binders. A comprehensive literature review was initially conducted to identify the factors affecting the properties of RHA based alkali activated binders. A comprehensive experimental methodology was designed and executed to understand the influence of the properties (physical, chemical and mineralogical) of precursor materials (low calcium fly ash and RHA) which affected the compressive strength of alkali activated binders. A series of experiments were conducted using state of the art techniques to characterize the precursor RHA and fly ash and the alkali activated binders. The addition of RHA lead to a reduction in strength of the blended fly ash-RHA alkali activated mortar. This is attributed to a higher rate of dissolution of RHA compared to the fly ash which negatively affects the formation of the gel matrix due to increased silica concentration which results in rapid precipitation of the gel matrix. This study shows that the addition of RHA replaces aluminium with silica in the system and leads to an increase in the Si/Al ratio (3.70 - 3.89), thus negatively affecting compressive strength. The second phase of the study investigated the mechanical and durability performance of blended fly ash-RHA alkali activated concrete up to a one-year period. The experimental results are explained based on comprehensive chemical, micro structural and pore structure analysis. The addition of 10% RHA leads to a reduction in all mechanical properties, i.e., compressive strength, flexural strength splitting tensile strength and elastic modulus for all ages up to 365 days. Lower workability and lower density are observed with the addition of 10% RHA. This is due to the higher unburnt carbon content and specific surface area of RHA. Furthermore, the long-term mechanical properties decreased beyond 28 days up to 365 days. Crack propagation negatively influences the microstructure and undermined the mechanical properties post 28 days for alkali activated concrete with or without RHA. Alkali-activated binder degradation through long-term crack propagation (initiated during heat curing) due to the formation of efflorescence products is identified as one of the major factors contributing to the lower strength properties over the long-term period (180 and 365 days). The concretes displayed a decrease in air and water permeability from 28 to 180 days followed by an increase from 180 to 365 days for both 100NFA an 10RHA concretes. The air and water permeability of 100NFA concrete is significantly lower than 10RHA concrete at all ages. The combined effect of the degree of crack propagation and the homogeneity within the structure are the main factors that governs the long-term mechanical and durability properties of blended fly ash- RHA alkali-activated concrete. The third phase of this study presents the test results abased on the engineering properties of the blended fly ash-RHA alkali activated concrete bricks and compared with PC brick having equivalent binder content. RHA was used as a replacement (20%) for low calcium fly ash to evaluate the mechanical and durability performance of blended low calcium fly ash - RHA bricks over a period of 90 days. The blended fly ash-RHA alkali activated concrete bricks showed a compressive strength growth rate of 4% between 7 and 28 days as compared to 13.8% for PC bricks, with both achieving similar 28 day compressive strength (approximately 17 MPa).  The strength data indicates that blended fly ash-RHA alkali activated concrete bricks can be used for structural masonry purposes as load bearing bricks. A denser interfacial transition zone (ITZ) between aggregates and paste in blended fly ash-RHA brick is identified as the reason for the increase of tensile strength as compared to PC brick. All durability properties (i.e., water absorption, initial rate of absorption, coefficient of expansion and contraction, salt attack) showed improved performances for blended fly ash-RHA alkali activated brick when compared with PC bricks except efflorescence analysis. Finally, a life cycle assessment (LCA) was conducted based on environmental and economic factors of alkali activated concrete and bricks prepared using low calcium fly ash and RHA. The results are compared with an equivalent strength PC concrete and brick. The cost analysis was conducted to assess the economic costs of the use of fly ash and RHA in low calcium fly ash alkali activated concrete and bricks. The results indicated that the utilization of waste fly ash and RHA provides significant benefits in terms of fresh and marine water ecotoxicity by avoiding the disposal of waste at dumpsites, rivers and storage lagoons for blended fly ash-RHA alkali activated concrete. Overall, the low calcium fly ash RHA blended alkali activated concrete displayed a deterioration in mechanical and permeability properties over the long term, up to 365 days. Therefore,  future research is required to understand the dominant factors that caused to the deterioration of strength and durability over the long term. However, the blended fly ash-RHA alkali activated brick satisfies the requirement of the 28 day brick strength mix. Hence, this can be used for structural masonry purposes utilizing as load bearing bricks.