水化-碳化耦合反应条件下水化硅酸钙凝胶体系的形成机制研究

The CO2 originated from the flue gas has been used as a curing agent for concrete products with lots of merits in achieving high early strength, low shrinkage and low carbon footprint. Compared to the conventional water curing method, CO2 curing technique have been wildly reported to render the calcium silicate hydrates (C-S-H) very distinctive in the physico-chemical features, while the underlying cause of which is still uncertain. The proposed project aims to explore the formation mechanism of C-S-H in the CO2-cured Portland cement paste by comparing with the formation of C-S-H in the normal hydrated cement paste. Therefore, the dissolution/ hydrolysis of alite/belite in the CO2-H2O solutions will be performed to reveal the reaction kinetics of cement in a mild acid medium. By determining the polymerization degree and crystal structural parameters of silicate tetrahedra in the C-S-H formed under different curing conditions, the impact of carbonation reactions on the growth and polymerization of C-S-H will be uncovered, as well as the formation mechanism of C-S-H under the coupling effects of hydration and carbonation. Based on these results, the interactions between the pozzolan and cement will also be investigated under the CO2 curing regime, in order to reveal the effects of carbonation at the early age on the pozzolanic reactions in the binary cement blends, and also to establish linkages between the material characteristics of pozzolan and the physico-chemical properties of the C-S-H in the binary cement blends. The results are expected to perfect the basic theory of the CO2 curing technique for manufacturing the concrete products, and are also of great importance for the low-carbon development of building materials industry.

利用工业废弃CO2养护混凝土制品有诸多优点：高早强，低收缩，低碳足迹。与常规养护手段相比，CO2养护所形成的水化硅酸钙凝胶(C-S-H)体系在物化特征上有明显的区别，而背后机理却尚未明确。本项目旨在通过与水泥的常规水化进程作对比，探讨硅酸盐水泥在CO2养护条件下C-S-H凝胶体系的形成机制。为此，拟对水泥熟料成分在CO2水溶液中溶解-水解机制展开研究，探明水泥在弱酸性溶液中反应动力学行为；通过测试C-S-H中硅氧四面体的聚合形态和晶体结构参数，掌握碳化反应对C-S-H凝胶的生长，聚合的作用机理，揭示在水化-碳化耦合作用下C-S-H凝胶体系的形成机制。在此基础上，进一步研究在CO2养护条件下火山灰材料与水泥的相互作用机制，探讨在二元复合胶材体系中早期碳化对火山灰反应的影响，建立火山灰材料特性与复合凝胶体系特性之间的联系。本研究有助于完善混凝土制品的碳化养护理论，对建材工业的低碳化发展意义重大

CO2养护条件下，硅酸盐水泥基材料暴露的反应媒介发生明显改变，水泥熟料组分的水化-碳化耦合反应进程尚未明确，反应产物形成机理不清，火山灰材料的加入更使硅酸盐水泥复合材料的性能发展变得扑朔迷离。本项目通过与水泥的常规水化进程作对比，研究水泥在弱酸性溶液中反应动力学行为；揭示在水化-碳化耦合作用下C-S-H凝胶体系的形成机制；阐明在CO2养护条件下粉煤灰材料与水泥的相互作用机制。研究结果表明：在CO2水溶液中，水泥熟料物相中Ca，Mg元素迅速溶出并在数分钟内即可达到峰值，随后因转化成碳酸盐沉淀致使溶液浓度急剧下降；而Si元素的溶出影响并不明显。碳化反应生成的碳酸钙晶体，其表面可与C-S-H凝胶以离子共价键形成强键粘结，从而在反应早期充当C-S-H凝胶生长的晶核，加速水泥熟料成分的水解和C-S-H凝胶的生长，实现了24小时碳化养护硅酸盐水泥的反应程度和28天水养护相当。加速碳化处理会将水泥水化产物转化为碳酸钙和活性硅胶，二者均可与新拌水泥浆发生反应，从而发挥“火山灰效应”；水泥砂浆中的微细石英砂颗粒可发挥“成核效应”，为水泥水化提供额外的成核位点，有效促进水泥早期水化。粉煤灰的加入会降低复合胶凝材料对CO2的吸收能力，碳酸钙的沉积也会阻滞其后期的火山灰反应。该研究内容属于水泥基复合材料基础研究，成果丰富了水泥制品的碳化养护理论，对建材工业的低碳化发展意义重大。