Effect of Sand Type and Content on the Fiber-Bridging and Matrix Properties of K- and K/Na-Activated Metakaolin-Based Engineered Geopolymer Composites

This study explored the effects of sand type [i.e., natural river sand (RS) and manufactured microsilica sand (MS)], sand content (i.e., 0%, 30%, 45%, and 60% by volume), and fiber content (i.e., 1% and 1.5% by volume) on the fiber-bridging and matrix properties of ambient-cured K-activated and K/Na-activated metakaolin (MK)-based engineered geopolymer composites (EGCs) reinforced with ultrahigh molecular weight polyethylene (UHMWPE) fiber. Experimental tests showed that incrementing sand content (i.e., from 30% to 60% by volume) generally enhanced the compressive and tensile strengths and reduced the tensile strain capacity of the composites. Additionally, the composites incorporating MS generally exhibited higher compressive and tensile strengths and lower tensile strain capacity compared to the RS composites with 1% by volume UHMWPE fiber. Based on the strength, ductility, and workability of EGCs, the optimum sand content was determined to be 45% by volume. All composites exhibited pseudostrain hardening (PSH) behavior with PSH strength and energy indices exceeding 1.3 and 2.7, respectively. Relative to the benchmark engineered cementitious composites (ECC) (M-45), the best-performing EGC developed (i.e., K/Na331 with 45% by volume MS and 1.5% by volume UHMWPE) presented greater tensile strain capacity and a slightly lower tensile strength.