The low absorbency and instability of superabsorbent polymer (SAP) in alkaline solution, as well as its high cost, limit its development and application in concrete. In this work, an eco-friendly SAP with high alkali-absorption for cement-based materials was synthesized through graft copolymerization of organic monomers onto xanthan gum and metakaolin. Meanwhile, the hydration-retarding mechanism of SAP in cement-based materials was revealed by the isothermal microcalorimetric testing technique. The results show that the current study successfully synthesized a new SAP (SAPN). SAPN not only presents better absorption capacity in alkaline solutions, but also extends the time required to reach swelling equilibrium. The main effect for SAP after being swelled by cement filtrate is the decrease of their water absorption capacity due to the internal blockage of the structure of SAP. The addition of SAPN significantly prolongs the initial and final setting times of the cement-based material system. SAPN improved the 28-day compressive strength of mortar by 7.51%. SAP has a certain “negative effect” on the crystallization nucleation and crystal growth period of cement-based materials.