Experimental Studies on High Performance Alkali Activated Slag Concrete Mixes

Abstract

In the present study, an attempt has been made to develop high-performance alkaliactivated slag concrete (HPAASC) mixes. A total of fifteen self-compacting concrete mixes were developed based on absolute volume method, with Ground Granulated Blast Furnace Slag (GGBFS) as the principal binder with their compressive strength values targeted to be greater than 70 MPa. A Control, reference OPC-based concrete mix was also developed with similar flow ability and compressive strength values. Further the initial setting times of these mixes were also investigated as per relevant IS: 8142-1976. These concrete mixes incorporated slag sand and Electric arc furnace (EAF) slag, as fine and coarse aggregates respectively. In the background of application of Taguchi’s DOE method, the flow and strength properties of nine of these mixes were evaluated in an initial calibration phase and the performances of the remaining six mixes were analyzed in the validation phase. After ascertaining the self-compacting nature of the mixes, their mechanical strength properties such as compressive, split tensile and flexural strengths, water absorption and moduli of elasticity were evaluated. Regression equations were developed for the different strength characteristics and predictive capabilities of those regression equations were found to be good. Detailed microstructural studies were carried out on all the fifteen mixes using SEM, EDX and XRD. In the second phase of the present investigation, five (of the fifteen above) best performing candidate mixes, in terms of their better flow ability and higher mechanical strengths were subjected to various durability tests in order to assess their resistances on exposure to aggressive chemical environments. Tests were conducted in acid and sulphate-rich environments upto to an extended period of 365 days. Further these five mixes were evaluated for their chloride ion penetration by conducting steady and nonsteady state tests. The same five mixes were further evaluated for their strength characteristics on exposure to sustained elevated temperatures upto 800ºC. Microstructural studies were also carried out on the test specimens of different mixes after them being exposed to aggressive acid and sulphate-rich environments and so also to elevated temperatures using SEM and EDX.In order to enhance the toughness characteristics of these mixes, appropriate percentages of steel fiber were added and the flow ability and strength characteristics of such fiber-reinforced mixes were evaluated. Such mixes were evaluated for their enhanced toughness characteristics based on methods proposed by Barr and Hasso and ACI. The high performance self-compacting alkali-activated slag concrete mixes (HPAASC) mixes were further evaluated by testing the flexural behaviour of beams made of such mixes and reinforced with steel-rebar reinforcement, in order to check their applicability in structural concrete elements. A detailed analysis with respect to the sustainability in terms of ecological performance, of all these concrete mixes is also carried out. A new class of high strength, self-compacting, alkali-activated slag concrete mixes were successfully developed herein, incorporating slag sand and Electric arc furnace slag, both by-products from the iron and steel industry, as fine and coarse aggregates respectively. Addition of small amounts of sodium phosphate, as an additive, has led to a construction-friendly increase in the initial setting times of all the HSAASC mixes tested herein, from an initial-low of 15 minutes to a healthy 60 minutes or so. These mixes developed herein have shown higher mechanical strength properties in terms of compressive, split tensile sand flexural strengths and so also greater modulus of elasticity, and lower water absorption characteristics, possibly due to the formation of more amounts of C-A-S-H gels, as reflected from the morphologies obtained during the microstructural analysis. HSAASC mixes developed herein have also shown better durability performances when subjected to acid, sulphates and chloride environments as compared to the control OPCC reference mix. Thus the present class of alkali activated slag concrete mixes are having the required levels of enhanced flow ability, higher mechanical strengths and better durability properties, and hence can be referred to as HIGH PERFORMANCE ALKALI ACTIVATED SLAG CONCRETE MIXES. Higher residual strength characteristics were also observed in all the HPAASC mixes developed herein, when subjected to sustained elevated temperatures as compared to the control OPCC mix. Addition of steel fibers to the high performancealkali activated slag concrete mixes has been shown to marginally increase the compressive strengths. The ultimate flexural strengths of all the candidate mixes, however, were observed to increase substantially. Increase in the percentage of steel fibers has shown to increase the toughness indices of all the candidate mixes tested herein. The flexural performance of all the reinforced HPAASC beams was found to be, in general, similar to that of the reference HSS-OPCC control beam. High performance alkali activated slag concrete mixes developed herein have shown lesser embodied energy as well as lesser emissions of embodied CO2 as compared to the control OPCC-based mix.