Carbonation-Induced Hardening Mechanism Of Calcium Silicate Minerals And The Performance-Enhancing Strategies Of The Carbonated Materials

With the proposal of“maritime power”strategy and“Belt and Road”initiative,the infrastructure construction and major engineering projects extend to the tropical open sea and polar regions,where the severe exposure conditions demand higher durability of the building materials.The use of traditional cement and concrete materials in these extremely harsh environments is limited,on account of the disadvantages of Portland cement hydration products on the resistance to ultra-high or low temperature and erosion.Limestone is the raw material for Portland cement manufacturing,with excellent mechanical property and resistance to ultra-high and low temperature and erosion.If a building material with its composition and structure similar to limestone could be prepared,the infrastructure construction and major engineering projects could be applied in wider regions.Calcium silicate minerals can react with CO₂ and produce a binding effect.The main binder is calcium carbonate which is also the main component of limestone.This principle is helpful for the design and preparation of a new kind of high-performance engineering material（Engineered Limestone,ELS for short）.However,this new engineering material is still in the stage of exploration and development with particularly a lack of fundamental research,which is difficult to effectively guide the design and preparation of ELS materials.This dissertation proposes a systematic design and preparation technology and methodology of high-performance ELS materials based on high-activity calcium silicate mineral（γ-dicalcium silicate,γ-C₂S for short）.The main work and innovative achievements in this dissertation are as follows:（1）The carbonation hardening mechanism of calcium silicate mineral and the microstructure formation mechanism of hardened matrix were revealed,and the kinetic equation of carbonation reaction based on a modified shrinking-core model was established,which provided a theoretical basis for the preparation of high-performance ELS material.Based on the dissolution and precipitation characteristics of calcium silicate mineral carbonation,by monitoring the changes of conductivity,p H value and ion concentration in the carbonation process of a highly-diluted suspension,the acceleration of calcium ion dissolution and the preferential formation of calcium carbonate in carbonate-rich environment were investigated.The carbonation of calcium silicate finally forms a three-dimensional reticular matrix with calcium carbonate as a continuous phase embedded with amorphous silica gel.The essence of this formation mechanism is that the Ca²⁺dissolved from calcium silicate has opposite driving forces to Si-O groups.The calculation of the solid phase volume evolution in the carbonation process shows a 75%volume expansion by the complete reaction ofγ-C₂S,which contributes to the connection of adjacent calcium carbonates to form the structural skeleton of the hardened matrix.The kinetic equation of carbonation reaction based on the modified shrinking-core model reveals that the obstruction of the diffusion process of Ca²⁺and CO₃^2-by carbonation products is the main controlling factor of carbonation reaction.（2）An ideal structural design method for ELS materials based on the inter-particle spacing factor of calcium silicate particles was proposed.Based on the microstructure formation model of carbonated matrix,the optimal range of spacing factor of adjacent calcium silicate particles and the bulk porosity of the packing entity is determined,from which the reasonable molding pressure and water-to-solid ratio of ELS material are determined to be 30 MPa and 0.15,respectively.Based on the kinetics of carbonation reaction,reasonable curing conditions including temperature and humidity and partial pressure of CO2 for ELS materials were determined.ELS material with a carbonation degree of 50%,compressive strength of65 MPa and porosity of 14%at 24 h is prepared.Carbonation of ELS materials is a process in which CO₂ gradually diffuses from outside to inside,resulting in excessive densification of the surface and a lower degree of internal carbonation.The formation model of carbonation structure of ELS materials was proposed,and pointed out that controlling the homogeneity of CO₂ diffusion and reaction process is the key to prepare ELS materials with high performance.（3）The mechanism of performance-enhancing strategies of ELS materials based on the modification of the structure and morphology of calcium silicate mineral,the dissolution rate of calcium ions and the crystallinity of calcium carbonate was proposed.The physical chemical property of calcium silicate minerals determine the basic performance of ELS materials.Barium doping increases the ion leaching and self-pulverization property ofγ-C₂S,and the cracks could absorb water which benefits the moisture holding and Ca ions supplement.The prolonged carbonation reaction of barium dopedγ-C₂S results in the 50%enhancement of 24 h compressive strength.The main controlling factor of carbonation reaction is the barrier effect of the already formed product layer on the ion’s dissolution and migration.High salinity solutions,such as KNO₃ solution,can destruct the silica gel layer and promote the dissolution of calcium silicate minerals.The boiling point of KNO₃ solution is also elevated,leading to the reduction of moisture evaporation.The carbonation reaction can be carried out fast and persistently,strengthening the calcium carbonate framework of hardened matrix.At 24 h,the reaction degree is increased by 20%,the porosity is decreased by37%,and the compressive strength is increased from 65 MPa to 90 MPa.The strength of ELS materials comes from the continuous calcium carbonate,so increasing the strength of calcium carbonate skeleton is a key to improve the overall mechanical properties of ELS material.500℃heat treatment can improve the crystallinity of calcium carbonate skeleton and enhance the ELS material’s mechanical property by60%.Through the combined strengthening effect of high salinity solution and heat treatment,the 24 h compressive strength of ELS material is increased by about 100%to 133 MPa,and the porosity is reduced by 30%.