Open Access

ARTICLE

Combined Effects of Carbonation and Fly Ash on the Sulfate Resistance of Recycled Aggregate Concrete: Compressive Strength Evolution and CNN-LSTM Prediction

Zhixi Chen¹, Jie Zhong¹, Qingsong Li¹, Sen Yang¹, Yi Sun¹, Changming Bu¹, Mingtao Zhang¹, Jiehong Li^1,*, Yang Yu^1,2,*

1 School of Civil and Hydraulic Engineering, Chongqing University of Science and Technology, Chongqing, China
2 School of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia

* Corresponding Authors: Jiehong Li. Email: ; Yang Yu. Email:

Computer Modeling in Engineering & Sciences 2026, 148(3), 11 https://doi.org/10.32604/cmes.2026.089217

Received 16 July 2026; Accepted 08 September 2026; Issue published 28 September 2026

Abstract

Recycled aggregate concrete (RAC) enables construction waste reuse, but surface micro-cracks on recycled aggregates (RA) and multiple ITZs from old adhered mortar create extra sulfate transport paths, undermining RAC’s sulfate resistance. Current studies largely examine single modification methods’ effects on durability, yet lack sufficient predictive power for how such modifications influence final material properties. This study evaluated nine concrete mixtures comprising 297 cube specimens and employed sulfate wet-dry cycling experiments combined with XRD and SEM microscopic characterization to investigate the coupled influence of carbonation-treated RA and fly ash addition on the compressive strength development and resistance of RAC to sulfate-induced deterioration. Subsequently, a convolutional neural network–long short-term memory (CNN-LSTM) hybrid model was utilized for performance prediction. SHAP (SHapley Additive exPlanations) analysis was applied to examine the contribution of each factor to the prediction outcomes, thereby enhancing the model’s interpretability. The results showed that, under standard curing, carbonation at 0.5 MPa provided the greatest strength improvement among the carbonation pressures investigated. After 120 d of continued curing, concrete incorporating RA carbonated at 0.5 MPa (CRAC-0.5) reached a compressive strength of 47.5 MPa, equivalent to 95.6% of that of natural aggregate concrete (NAC) and 13.9% higher than that of untreated RAC. Among the combined modification levels tested, the mixture incorporating RA carbonated at 0.5 MPa and 20% fly ash (FCRAC-20%) exhibited the best overall performance. After 120 sulfate wet-dry cycles, it retained a compressive strength of 25.6 MPa, corresponding to 82.8% of its baseline strength and comparable to the strength retention of NAC. By contrast, untreated RAC retained only 1.0 MPa, or approximately 3.0% of its baseline strength. Carbonation generated CaCO₃ that filled old mortar cracks, while fly ash improved the new paste via filling and pozzolanic reaction; together, they mitigated sulfate ion ingress, thus reducing gypsum and ettringite formation. X-ray diffraction (XRD) phase analysis further showed weaker gypsum and ettringite reflections in FCRAC-20%, consistent with reduced sulfate reaction product accumulation. Moreover, the CNN-LSTM model (R² values of 0.94 and 0.93 on the training and test sets) accurately predicted strength under various modifications and erosion stages. SHAP ranked wet-dry cycles as the top influencing factor, followed by carbonation pressure, highlighting carbonation’s crucial role in strength and sulfate resistance. These findings identify FCRAC-20% as the combination with the best performance among those tested, aid durability assessment and mix design for carbonated RAC in sulfate-rich environments, and provide a comparative laboratory basis for future validation at full scale.

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