碱激发矿渣固化淤泥质渣土冷结颗粒的强度及其碳化耐久性OA

Insufficient thermal and water resistance of cold-bonded sludge soil granules,coupled with the under-recognized long-term degradation induced by atmospheric carbon dioxide,remains a critical obstacle to their wide-scale adoption as secondary aggregates in geotechnical engineering.To close this knowledge gap,a comprehensive laboratory programme was designed to(i)benchmark three alkali-activation strategies—sodium silicate(Na2SiO3)alone,calcium hydroxide(Ca(OH)2)alone,and their combined use—for early-age strength gain and durability,and(ii)elucidate how the calcium-to-silicon molar ratio(Ca/Si)governs the balance between short-term mechanical excellence and long-term carbonation resistance.Granules were manufactured by pelletising dredged sludge soil from a viaduct project in Shaoxing(China)with 80 wt％ground-granulated blast-furnace slag(GGBS)and 20 wt％activator powders.A 0.5 M NaOH solution was used as the mixing fluid(liquid/solid=0.20)to ensure a uniform alkaline milieu.After 24 h air curing,all pellets(d≈12 mm,ρ ≈1.9 g cm-3)were conditioned at 20℃and 95％RH until testing ages of 7 d and 28 d.Accelerated carbonation was performed in a thermostatic chamber at 60℃,90％CO2 for 2 h,4 h and 6 h to simulate decades of natural exposure in a humid,CO2-rich atmosphere.Mechanical retention was quantified through single-particle crushing strength(σd)and water-softening coefficient(K=σw/σd),while microstructure evolution was tracked by X-ray diffraction(XRD),thermogravimetry(TG-DTG)and field-emission SEM.The combined activator system delivered a clear synergistic benefit.At Ca/Si=1.5,the 28-day σd peaked at 9.08 MPa,K reached 0.847,and the residual strength after 24 h at 105℃still exceeded 5.34 MPa—performance metrics that surpass those of conventional cement-stabilized spoil pellets reported in the literature.However,this optimized formulation exhibited high sensitivity to CO2 attack:after only 2 h of accelerated carbonation,σd plummeted by 61％to 3.53 MPa,and the once-dense matrix became visibly porous under SEM.XRD patterns revealed a pronounced attenuation of the C-S-H hump at 29-31° 2θ and a concomitant rise in calcite reflections at 23.0° and 36.9° 2θ,confirming decalcification of the calcium-silicate-hydrate skeleton and reprecipitation of CaCO3.TG data corroborated these findings:the high-temperature decarbonation step(600-710℃)increased from 1.23％for the uncarbonated reference to 2.81％after 6 h of CO2 exposure,while the low-temperature mass-loss associated with bound water decreased from 4.81％to 3.21％,signaling a net consumption of hydrated phases.Raising the Ca/Si ratio to 2.0 mitigated the carbonation penalty.Although the 28-day σd decreased slightly to 3.57 MPa,the strength loss after carbonation was only 10％,resulting in a residual σd of 3.21 MPa,which was close to that of the Ca/Si=1.5 granules under the same carbonation condition(3.53 MPa).SEM inspection showed that the higher portlandite inventory in Ca/Si=2.0 pellets acted as a sacrificial reservoir,preferentially carbonating to form an intergrown calcite network that partially sealed surface pores and slowed further CO2 ingress.At Ca/Si=2.5,the initial σd dropped to 2.23 MPa because excess Ca(OH)2 remained unreacted,disrupting the continuity of the C-S-H gel.Yet,after 6 h of carbonation,σd rebounded slightly to 2.31 MPa,implying that calcite infilling could provide a modest strengthening effect when the original binder matrix was weak.TG profiles for Ca/Si=2.5 exhibited the largest carbonate loss(4.11％),confirming that higher Ca availability intensifies carbonation uptake.The water-softening behavior followed a similar trend.K for Ca/Si=1.5 remained above 0.84 even after 6 h carbonation,whereas Ca/Si=2.5 suffered a drastic fall from 1.08 to 0.27,indicating severe micro-cracking and loss of gel cohesion.Taken together,the results demonstrate that short-term strength alone is an unreliable predictor of long-term service performance.A Ca/Si window of 1.8-2.0 appears to offer the best compromise between high initial strength,reasonable thermal stability,and acceptable carbonation resilience for spoil-soil granules destined for road base or embankment applications in temperate,CO2-rich climates.Future work will explore nano-silica and alumina-rich additives to refine the pore-size distribution and further enhance carbonation resistance without sacrificing early strength.