The Research On Cement Paste-aggregate Interface Based On Functional Design Of Aggregate

Concrete is a multi-phase and multi-scale composite material and its structure and performance is predominantly influenced by the compatibility between each phase and the presence of the weak interfacial transition zone (ITZ). The interface between phases, especially between cement paste and aggregate is of paramount importance to the mechanical properties and durability of concrete. Previous research on the improvement of ITZ was focused on the amelioration of hydration products, the enhancing effect of supplementary cementitious materials and some treatment on aggregates, which failed to make a breakthrough in the overall optimization of composition and structure of concrete. In this thesis, the concept of function aggregate is put forward to improve the ITZ between cement paste and aggregate so that the performance, especially the durability, of concrete can be substantially improved and hopefully it will provide a theoretical basis for the design, development and application of high technology concrete.The main results obtained in this study are as follows:This study examines the characteristics of ITZ and its the formation mechanism, develops the concept of function aggregate. In concrete, function aggregate can enhance and optimize ITZ, and also has the ability to adjust and control the formation of cement paste. The ideal structure of functional aggregate is envisioned as composed of by a porous matrix of high strength and a reactive layer of hydration activity. In concrete proportioned with functional aggregate, the adhesion between cement paste and aggregate is enhanced by the highly reactive surface of function aggregates and its the porous structure can be a moisture reservoir for internal curing, both of which can essentially improve ITZ and provide a new direction on the research and development of high technology concrete.The study proposes the design and processing method of functional aggregate, explores the influence of the material and mineralogical composition of the matrix and processing parameters on the microstructure and properties of concrete. Results indicate that the incorporation of cordierite with excellent thermal stability can reduce the micro-cracks formed during the rapid cooling of aggregates, so as to increase its strength and decrease the water absorption. The effect of additives on the material composition of the aggregate surface is analyzed by using different surface active layer materials. A function aggregate is obtained with an outer layer ofβ-C2S and mullite and cordierite inside; its crushing strength is up to 13.5 MPa and vacuum absorption is less than 8%.By employing the test method of SEM, EDXA and microhardness, the hydration characteristics of the outlayer active minerals is studies. Sinceβ-C2S has a lower hydration activity at early age and a glassy phase is likely to be formed on the outerlayer when produced under high temperature, little hydration activity is exhibited until 28d. Ca2+ on the surface of aggregate immigrates gradually to cement paste, leading to a decreased Ca/Si; the hydration products can heal the cracks generated at early age and compact the porous ITZ. By studying the effect of w/c, supplementary cementitious materials and curing conditions on the property and performance of ITZ, the internal curing effect of functional aggregate is researched and the formation law and property adjustment mechanism are elucidated.This study examines the mechanical properties, volume stability and impermeability of concrete proportioned with functional aggregates and establish the linear relation between internal relative humidity and autogenous shrinkage: AS=κRH+b. The autogenous shrinkage is increased with a decreased internal relative humidity, which can be improved by the internal effect of pre-wetted functional aggregate. The interfacial enhancement and internal curing of functional aggregates can improve the structure and performance of ITZ, which is the main reason for the better mechanical property and impermeability of concrete.Based on the design principle of material science, the fine design method and idea of concrete is proposed. The cementitious system is optimized based on the compatibility of strength and elastic modulus between cement paste and aggregate. By fine-tuning the grading of aggregate, the compactness of aggregate is maximized and the porosity is minimized; the thickness of cement layer on the aggregate surface is optimized based on the specific performance of concrete.