Study on the Strength Properties of Sink Beads-Fly Ash-Based Geopolymers Under Carbonic Acid Water Environment

The conventional silicate cements exhibit poor resistance to carbonation corrosion, failing to meet the long-term sealing needs of carbon sequestration wells. Geopolymers, widely studied in civil engineering, demonstrate good carbonation resistance under ambient curing. However, systematic research on how precursor components affect their strength in well applications is lacking. Sink beads, high-density microspheres in fly ash, are promising precursor additives for enhancing geopolymer mechanical properties. Therefore, this study investigates the effects of sink bead content and curing time on the compressive strength of fly ash-based geopolymers under 70 °C water bath and carbonic acid water environment. The results showed that during the early stage of water bath curing (2d), the highest compressive strength of 23.41 MPa was achieved with a 60% sink bead content. Over the 28-day water bath curing period, with increasing curing time, samples with higher sink bead content exhibited the first signs of strength degradation (2–28d), while samples with lower sink bead content also showed a significant strength decline later (7–28d). Samples of fly ash-based geopolymers without sink beads did not exhibit strength degradation. Notably, when cured in a carbonic acid water environment, all geopolymers showed no strength degradation and, instead, exhibited an overall strength increase of approximately 5.08%–58.82% as compared to those cured in water bath environment. ‌ ‌IR analysis revealed that the carbonic acid water environment promoted amorphous gel polymerization, increasing the Q3 + Q4 proportion by 17.02%. Additionally, XRD indicated that the accompanying crystallization transformation of the gel contributed to the strength decline.