The Strength and Fracture Characteristics of One-Part Strain-Hardening Green Alkali-Activated Engineered Composites

Abstract

Alkali-activated engineered composites (AAECs) are cement-free composites developed using alkali activation technology, which exhibit strain hardening and multiple micro-cracking like conventional engineered cementitious composites (ECCs). Such AAECs are developed in this study by incorporating 2% v/v polyvinyl alcohol (PVA) fibers into alkali-activated mortars (AAMs) produced using binary/ternary combinations of fly ash class C (FA-C), fly ash class F (FA-F), and ground-granulated blast furnace slag (GGBFS) with powder-form alkaline reagents and silica sand through a one-part mixing method under ambient curing conditions. The mechanical and microstructural characteristics of eight AAECs are investigated to characterize their strain-hardening performance based on existing (stress and energy indices) and newly developed tensile/flexural ductility indices. The binary (FA-C + GGBFS) AAECs obtained higher compressive strengths (between 48 MPa and 52 MPa) and ultrasonic pulse velocities (between 3358 m/s and 3947 m/s) than their ternary (FA-C + FA-F + GGBFS) counterparts. The ternary AAECs obtained a higher fracture energy than their binary counterparts. The AAECs incorporating reagent 2 (Ca(OH)2: Na2SO4 = 2.5:1) obtained a greater fracture energy and compressive strengths than their counterparts with reagent 1 (Ca(OH)2: Na2SiO3.5H2O = 1:2.5), due to additional C-S-H gel formation, which increased their energy absorption for crack propagation through superior multiple-cracking behavior. A lower fracture and crack-tip toughness facilitated the development of enhanced flexural strength characteristics with higher flexural strengths (ranging from 5.3 MPa to 11.3 MPa) and a higher energy ductility of the binary AAMs compared to their ternary counterparts. The tensile stress relaxation process was relatively gradual in the binary AAECs, owing to the formation of a more uniform combination of reaction products (C-S-H/C-A-S-H) rather than a blend of amorphous (N-C-A-S-H/N-A-S-H) and crystalline (C-A-S-H/C-S-H) binding phases in the case of the ternary AAECs. All the AAECs demonstrated tensile strain-hardening characteristics at 28 days, with significant improvements from 28% to 100% in the maximum bridging stresses for mixes incorporating 40% to 45% GGBFS at 365 days. This study confirmed the viability of producing green cement-free strain-hardening alkali-activated composites with powder-form reagents, with satisfactory mechanical characteristics under ambient conditions.

Effect of PVA Fiber on the Mechanical Properties of Seawater Coral Sand Engineered Cementitious Composites

“…Fracture energy refers to the energy consumed per unit area of the SCECC material during crack expansion. The calculation for fracture energy is as follows [ 38 ]: where G F is the fracture energy (J/m 2 ), W is the work carried out by the SCECC in the three-point bending test (J), m is the mass of the specimen between the supports (kg), g is the acceleration of gravity (9.8 m/s 2 ), δ is the final displacement at failure (m), A is the actual area of the fracture surface of the specimen (m 2 ).…”

“…Fracture energy refers to the energy consumed per unit area of the SCECC material during crack expansion. The calculation for fracture energy is as follows [38]:…”

Shrinkage, Permeation and Freeze–Thaw Characteristics of Ambient Cured High Calcium-Based Alkali-Activated Engineered Composites

“…SEM/EDS analysis of the chip specimens taken out from the cube samples was conducted to characterize the reaction products formed in the matrix. Detailed procedures of sample preparation and SEM/EDS analyses can be found in [21].…”

“…High calcium fly ash class C (FA-C), low calcium fly ash class F (FA-F), GGBFS, two powder-based reagents (reagent 1 and 2) and constant dosage (0.3 by mass of binder content) of silica sand (maximum particle size 600 µm) were used to develop AAMs as per authors' research works [20]. AAEC mixes were made by using 2% v/v PVA fibers to the AAMs [21]. Detailed physical and chemical properties of all the materials can be found in [20].…”

“…PVA fiber addition to AAECs and ECC slightly increased their compressive strength compared to their AAMs (ranging between 34 and 42 MPa) and cementitious mortar (FPcM) counterparts without fiber (Table 3). Detailed compressive and flexural strengths of all these mixes can be found in [21]. Negative indicates shrinkage and mass loss, and positive values denotes expansion and mass gain; AAMs: alkali activated mortars, AAECs: alkali activated engineered composites, ECC: engineered cementitious composite.…”