Experimental investigation on dynamic mechanical properties of sandy clay treated with alkali-activated metakaolin cement and discrete polypropylene fibers

The dynamic mechanical properties and dynamic energy absorption capacity of marine sandy clay,which was stabilized by cement with partial substitution of alkali-activated metakaolin(AAMK)and discrete polypropylene fibers,were experimentally investigated at strain rates of 80-280 s^(-1).The AAMK,as partial replacement of cement,is eco-friendly and economical,and polypropylene fibers with corrosion resistance can withstand severe environmental conditions.Dynamic mechanical properties of 16 different mix ratios were experimentally examined via split Hopkinson compression pressure bar(SHPB)tests.Typical macroscopic post-impact fragment patterns Ⅰ and Ⅱ were observed in dynamic stress-strain curves and macroscopic fragmentations.The results confirmed an obvious enhancement in the dynamic compressive strength and energy absorption density due to the use of cement with partial substitution of AAMK and the addition of polypropylene fibers and sand.Based on scanning electron microscopy(SEM)tests and nuclear magnetic resonance(NMR)tests,cemented sandy clay specimens treated with 0.2%fiber contents or higher exhibited a denser network of soil particles with hydration products.The connection mechanism and typical interface between fiber-sand-hydrate-sandy clay particles were observed via SEM tests.Furthermore,an optimal mix ratio was proposed to satisfy the demands of high dynamic mechanical properties,energy absorption capacity,and economic and environmental constraints.The optimal mix ratio corresponded to 0.2% fiber content or higher and sand content of up to 16%.Additionally,it was observed that the dynamic compressive strength of samples with 0.1% fibers or less deteriorated.Based on absorption energy density and failure modes analysis,the fiber content should be higher than or equal to 0.2% to effectively reduce the degree of fragmentation and increase the size of fragments.