Production of Innovative Angular Fly Ash Aggregate and Its Performance in Light Weight Concrete

Abstract

The present study describes the manufacturing process of a new product, the angular fly ash aggregates (AFAA), that does not involve pelletization process. In previous studies, round fly ash aggregates are produced by using pelletization procedure. Although pelletized fly ash aggregates are commercially used, it has some drawbacks like less interlocking effect due to the round shape, requires more equipment that consumes more energy, need high supervision and lacks reproducibility. To overcome these limitations, an innovative idea for producing angular fly ash aggregates is suggested. AFAA was manufactured by crushing sintered briquettes (obtained from the mixture of fly ash, binder, and water) into angular aggregates. This newly manufactured aggregate possesses a rough surface and greater surface area as compared to round pellets thus result in the better interlocking of aggregates with paste. Different types of AFAA were manufactured in the laboratory by using two different types of fly ash and four different types of binders. Cement, bentonite, glass powder, and lime powder were used as the binder that provides compaction to briquettes and reduces the water absorption of AFAA. The proportions of binders, fly ash and the quantity of water to be used for manufacturing of aggregates were decided after various trials. The aggregates manufactured varying the proportions of binder and fly ash were then tested and concluded that the optimum quantity of cement and lime is 20%, whereas for bentonite and glass powder, it is 15% and the selected water to fly ash ratio is 0.25 for preparation of angular fly ash aggregates. Also, the influence of characteristics of fly ash and binder on the behavior of manufactured aggregates was observed. Performance of AFAA showed that chemical composition and specific surface area of fly ash influence the sintering behavior of ash and as a result, internal microstructure of aggregates also gets affected. It is observed that the performance of bentonite is superior as a binder because of its cohesive nature and swelling property. This study also explains the strength characteristics of structural lightweight concrete produced using manufactured AFAA. Mix design of lightweight concrete was done as per guidelines provided in ACI 211.2-98. Concrete produced using AFAA was found to be lighter in weight compared to the concrete of natural aggregate. It was concluded that the strength of concrete depends on the properties of manufactured AFAA and the nature of the bond between cement and aggregates at the interfacial transition zone (ITZ). 28 day’s compressive strength varies from 22.23 to 35.12 MPa. Split tensile strength varies 2.42 to 3.93 MPa and flexural strength varies from 3.51 to 4.5 MPa. Concrete produced using lightweight AFAA meets the strength requirements of LWC for structural applications as per ASTM C330. The microstructural analysis has been carried out through scanning electron microscopy (SEM) to observe the distribution of pores within aggregates and to understand the interfacial bond between cement and aggregate in produced concrete. It can be concluded from X-ray diffractometer (XRD) analysis that even after the sintering process, there is no change in the chemical composition of AFAA. Regression analysis was done to study the relationship between compressive and split tensile strength and also between compressive and flexural strength of produced fly ash aggregate concrete and equations were suggested. Also, the result obtained was compared with equations proposed by other researchers. Some of the durability tests have been conducted on concrete produced using these aggregates to check its suitability for structural applications. The obtained water absorption of all AFAA are within range of those of good one i.e. below 10%. Sorptivity coefficients are in the range of 0.0138 to 0.0213 mm/√s that shows acceptable performance of concrete. Loss in weight and strength was observed on acid attack that increases with increasing period of acid immersion. Due to penetration of binder within porous aggregate, bond strength is better in lightweight fly ash aggregate concrete and hence surface area is more susceptible than aggregate on acid attack. It was observed that concrete produced using bentonite as binder, showed maximum pulse velocities of 4.896 km/sand 4.983 km/s after 28 and 56 days of curing respectively. Overall, obtained UPV Values of all fly ash aggregate concrete shows good and excellent quality. The present study reveals that fly ash aggregate is adequate to use for structural purpose due to its lightweight nature and acceptable strength. The utilization of fly as in production of lightweight AFAA not only saves our depleting natural resources but also reduce environmental pollution and hence promote sustainable construction practices.