低热硅酸盐水泥(简称低热水泥)复掺粉煤灰配制的混凝土具有放热慢、放热量低、后期强度较高的特性,为揭示其放热特性与水化机制,开展了胶凝材料的长期水化放热试验,获得了水化状态,借助吉布斯自由能最小化软件(GEMS)进行热力学计算,建立了水泥水化模型。结果表明: ①粉煤灰会显著降低低热水泥的水化热,水化稳定时水泥水化程度为86.0%、粉煤灰反应程度为53.3%;②建立的水化模型可以很好表征低热水泥水化过程,水泥以硅酸二钙(C₂S)为主导矿物,相较于普硅水泥与中热水泥,可生成更多水化硅酸钙(C-S-H)凝胶、更少Ca(OH)₂,这是其后期强度较高的内在原因;③不同类型C-S-H的饱和指数不同,其与Ca(OH)₂饱和指数、OH^-与Si⁴⁺浓度呈线性正相关,与Ca²⁺浓度呈负相关;④掺入粉煤灰后Ca(OH)₂被首先消耗殆尽,形成了钙硅摩尔比较高的C-S-H凝胶,钙矾石溶解形成单硫型水化硫铝酸钙,随掺量增加,C-S-H含量与钙硅摩尔比、pH值均显著降低,水化产物稳定状态在pH值上的拐点是12.86。研究成果可为研制掺粉煤灰的低热水泥混凝土提供理论依据。

[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.