Dissolution, Early Hydration And Chloride Binding Behaviour Of Aluminum Compounds In Cementitious System

Hydration of cementitious materials is absolutely central for concrete to transform from a fluid suspension into a rigid solid.Dissolution and hydration of cementitious materials greatly affect the macro properties and durability of concrete.Especially,the dissolution of aluminum compounds directly determines the development of early-age properties of concrete.Also,aluminum compounds and their products are able to bind chloride ion by the formation of chlorine-containing compounds,which can improve concrete resistance against chloride attack and prevent steel bar corrosion.Mineral dissolution,the first step of hydration process,and the nucleation and growth of products constitute cementitious materials’hydration.Considering that mineral dissolution and product nucleation and growth proceed in pore solution environments,understanding the changes in nature of the solid-liquid interface during dissolution and hydration processes in solution environments contributes much to identifying the hydration and chloride ion binding mechanisms of aluminum compounds,providing theoretical bases for the design of chloride ion binding process and the application seawater concrete,and saving freshwater.Bases on the background,methods and scientific issues,this research can be divided into five aspects.The first aspect is about the dissolution and hydration of tricalcium aluminate（C₃A）,which mainly focused on the measurements of C₃A’s key kinetic and thermodynamic parameters,like dissolution rate and solubility product,and on the investigation of the effects of product shell and sulfate salts on C₃A’s hydration.The second one is about the dissolution and hydration of tetracalcium aluminate ferrite（C₄AF）and its interaction with C₃A,to study the dissolution behaviour of C₄AF in ultrapure water and sulfate salts environments and to characterize the interaction between C₃A and C₄AF during the dissolution and early hydration processes.The third one is about the dissolution and hydration of tricalcium silicate（C₃S）and its interactions with C₃A or C₄AF.After measuring the dissolution rate of C₃S,the interaction between C₃S and C₃A and the interaction between C₃S and C₄AF during dissolution and hydration processes were characterized and identified.The fourth one is about the dissolution of C₃A in the presence of admixtures,focusing on the modifications of C₃A’s dissolution behaviour and exothermic behaviour caused by sugars and superplasticizers to unravel the interaction mechanism of admixtures.The last aspect is the influence of aluminum compounds on cement hydration and chloride ion binding behaviour of paste.The chloride ion binding mechanisms of aluminum compounds were explored and the key role of aluminum compounds’dissolution in chloride ion binding process was determined.On the basis of the five-part experimental research,the main results are:1)The dissolution and precipitation of C₃A were decoupled by the in-situ,real-time monitoring of surface changes of C₃A during hydration.The dissolution rate of C₃A in ultrapure water was measured as 0.5～2 mmol·m^-2·s^-1 and the solubility products of C₃A at10°C,20°C,30°C and 40°C were determined.The effect of product shell on dissolution mechanism transition was also investigated.Sulfate salts are able to significantly inhibit C₃A’s dissolution and there exists a cation-specific effect.By excluding the inhibiting effects of calcium ion,sulfate ion adsorption and ionic strength,the adsorption of sulfate ion pairs was proposed to result in the strong inhabitation of C₃A’s dissolution.Conversely,chloride salts can promote the dissolution of C₃A and affect the phase compositions of hydration products.2)The dissolution rates of C₄AF in ultrapure water and sulfate salts environments were measured,and the cation-specific inhibition effect of sulfate salts was also investigated.Combining the change in nature of the solid-liquid interface,the conductivity evolution of C₄AF suspension and the exothermic behaviour of C₄AF paste,the mechanisms of sulfate salts and chloride salts on C₄AF’s hydration were determined.When C₄AF and C₃A coexisted,C₄AF and C₃A can interact with each other by the common ion effect and the competitive adsorption effect under an environment with a large water-solid ratio,but there was no obvious interaction between C₄AF and C₃A in paste with a water-solid ratio of 0.5.3)By comparing the dissolution rate of C₃S under an environment far from equilibrium(44.1±0.495μmol·m^-2·s^-1)and the dissolution rate of C₃S in suspensions(4.79～12.4μmol·m^-2·s^-1),the change of driving force of dissolution was found to be responsible for the mismatching of dissolution rates.The dissolution and early hydration interaction between C₃S and C₃A and the interaction between C₃S and C₄AF were investigated by a combination of multiple methods,and various interaction mechanisms such as common ion effect,ion poisoning effect,and adsorption effect were characterized.4)The presence of sucrose can inhibit both the dissolution of C₃A in ultrapure water and the formation of products on C₃A’s surface.Sugars can weaken the inhibition of C₃A’s dissolution caused by gypsum through inhibiting the adsorption of calcium sulfate ion pair on C₃A’s surface and through inhibiting the dissolution of gypsum.Similarly,PCE can also affect the dissolution and early hydration of C₃A in ultrapure water and sulfate salts environments,and the influence was closely related to the molecular structure of superplasticizers.5)The chloride ion binding mechanisms of four aluminum compounds with different aluminum forms were identified.Na Al O₂ and Al（OH）₃ are with huge capacities for binding chloride ion and are able to react with chloride ion to form chlorine-containing compounds.Hydrotalcite possesses a weaker chloride ion binding capacity,and the chloride ion is bound in the way of competitive ion intercalation.Though alum can react to form aluminum-containing products,the transition to form chlorine-containing compounds is inhibited by the presence of sulfate ion,resulting in the reduction of paste’s chloride ion binding capacity.Chloride salts can affect paste’s chloride ion binding capacity by adjusting cement hydration process and changing the p H value of pore solution.Through linking the dissolution,early hydration and chloride ion binding behaviours of aluminum compounds,the key role of aluminum compounds’dissolution in affecting the chloride ion binding rate were identified.