Recycling waste walnut shells as a sustainable internal curing material for ultra-high performance concrete matrix

Ultra-high performance concrete (UHPC) is highly prone to cracking due to its relatively high early autogenous shrinkage (AS). Internal curing technology is an effective means to mitigate the AS of UHPC. However, the current mainstream internal curing agents (such as superabsorbent polymers) generally have the problems of high cost, reliance on non-renewable resources, and a large carbon footprint. Meanwhile, a large amount of waste walnut shells (WS) is generated in agricultural production. How to achieve high-value and resourceful utilization of WS has become an urgent issue to be solved. Under this background, this study utilized WS as a novel internal curing agent to mitigate the AS of UHPC. The effect of WS on the AS, internal relative humidity, mechanical strength, hydration heat, and microstructure of UHPC matrix was investigated. The results indicated that the AS of the UHPC matrix at 7 d was reduced by 47.0 %–74.5 % when incorporating 3 %–9 % WS. However, the compressive and flexural strengths of the UHPC matrix at 28 d decreased by 7.0 %–15.5 % and 8.9 %–17.1 %, respectively. After adding 3 %, 6 %, and 9 % of WS, the average chain length of C-S-H increased by 35.5 %, 50.4 %, and 73.8 % respectively, compared with the plain samples. Finally, pretreatment of WS to reduce its negative impact on the mechanical properties of the UHPC matrix is a future research direction.