[Objective] This study aims to reveal the exothermic properties and hydration mechanism of low-heat Portland cement (LHC) (hereafter referred to as low-heat cement) mixed with fly ash, the composition of pore solution during hydration, and the evolution mechanism of hydration products. [Methods] Long-term exothermic tests were conducted on the cementitious materials, and a quantitative relationship between heat release and hydration progress was established, from which the hydration state was obtained. Based on the theory of Gibbs free energy minimization using GEMS software, thermodynamic calculations were performed to construct a cement hydration model, whose validity was verified through comparative analysis between simulated and measured Ca(OH)₂ contents in neat paste. [Results] (1) The exotherm from low-heat cement hydration was mainly concentrated within 28 days. Fly ash significantly reduced the hydration heat of low-heat cement: a 20%-50% replacement ratio resulting in a reduction of 14.6%-32.7% in total exotherm after 720 days of hydration. Predictive models for exotherm and degree of hydration of the cementitious system were established. At hydration stabilization, the degree of hydration of low-heat cement reached 86.0%, and the reaction degree of fly ash was 53.3%. (2) The Ca(OH)₂ content calculated by the cement hydration model showed small deviation from measured values, indicating that the developed model adequately characterized the hydration process of low-heat cement. The cement was dominated by dicalcium silicate (C₂S). Compared with ordinary and medium-heat Portland cements, it produced more calcium silicate hydrate (C-S-H) gel and less Ca(OH)₂, which explained its higher later-age strength. (3) Different types of C-S-H were found to exhibit distinct saturation indices, ranked from highest to lowest as C_1.5S_0.67H_2.5, C_0.83S_0.67H_1.83, C_1.33SH_2.17, and C_0.67SH_1.5 (the last being unstable). All types showed linear positive correlations with the saturation index of Ca(OH)_2, as well as with OH^- and silicon ion concentrations, and a negative correlation with calcium ion concentration, with the relevant relationships established. (4) After fly ash incorporation (0-25%), the ettringite (AFt) content in the cementitious system gradually decreased to zero, while the monosulfate (AFm) content continuously increased. (5) At low fly ash dosages (0-20%), Si phases in fly ash first reacted with Ca(OH)₂ to form C-S-H with a higher Ca/Si ratio, increasing the average from 1.61 to 1.63. At higher dosages (20%-50%), C-S-H content decreased by 16.5%, and both Ca/Si ratio and pH declined markedly, primarily due to exhaustion of CH, reduced Ca phase, and increased Si and Al phases. At very high dosages (65%-80%), severe deficiency of Ca phase along with excess Si, Al, and Fe phases and lower pH caused extensive dissolution of C-S-H and a reduction in its Ca/Si ratio. (6) A relationship among pH, solution composition, and product saturation indices was established. At pH=12.86, the saturation indices of the above products were highly similar. When pH≤12.86, significant changes in the stable states of the products occurred. [Conclusion] Incorporation of fly ash further reduces the exotherm of low-heat cement, and heat release is significantly positively correlated with hydration progress. At low dosages, both the content and properties of C-S-H gel are improved, which benefits macroscopic mechanical performance and durability. pH 12.86 appears to be the pH inflection point for the stable state of hydration products, which should be considered when designing low-heat cement concrete with high fly ash dosages.