Incorporation of wollastonite whiskers into alkali-activated composite adhesive for FRP-based structural rehabilitation : microstructural modification and its effect on bonding properties

This paper employs wollastonite whiskers (WS) to modify bonding properties of alkali-activated metakaolin-blast furnace slag (MK-BFS) adhesive. A systematic investigation is conducted on the effect of WS content and curing age on failure mode, bonding property, compressive strength and microstructural evolution. The results show that as WS content or curing age increases, the failure mode of double-shear specimens gradually transitions from failure at the end of anchorage zone to delamination failure of carbon fiber reinforced plastic (CFRP) fabric. The bonding strength exhibits a pattern of initially increasing and then decreasing as WS content increases. The bonding strength of 7d specimen containing 5% WS is increased by 27.12%, 8.66% and 6.01% respectively compared with blank specimen and specimens with 3% and 7% WS. The compressive strength also exhibits a similar trend of increasing and then decreasing with rising WS content. Moderate WS incorporation optimizes chemical bonding state of products, promoting the formation of high-polymerization-degree C-(A)-S-H and N-A-S-H gels, thereby enhancing microstructure densification. However, excessive WS induces adverse effects, leading to agglomeration and consequently causing pore defects and non-uniform particle distribution. The bonding property is synergistically optimized through multiple pathways, including mechanical interlocking, promoting gel formation, inducing adsorption nucleation and crack bridging. Furthermore, the bonding strength of MK-BFS adhesive is assessed utilizing existing models, with an analysis conducted to identify the causes of predicted deviations. Considering the effects of WS characteristic and content, a modified model for predicting interfacial bonding strength is established and the model prediction results match well with test data.