| China possesses tremendous calcium-magnesian carbonate rocks(nearly 70 billion tons). However, due to the mineral co-existence of MgCO3 and CaCO3, the utilization of calcium resources is commonly influenced by magnesium. For example, the reactivity of MgO is low, leading to a slow formation of Mg(OH)2, which always reults in expansion of solid phases at later stage. At present, setting limitations for MgO content in cementitious materias restricts the exploitation of about 40 billion tons mineral resources which are rich in magenisium. In order to utilize these resourses, this thesis deeply investigated the reaction process of MgO-SiO2-H2 O cementitious system. According to the thermodynamic calculation and the composition and structure of reaction products of MgO/silica fume(SF) pastes, the mechanism of dissolution equilibrium in the reaction medium and the reaction kinetics were demonstrated. The influence of MgO reactivity, water to solid ratio, curing temperature, pH of solution, addition of magenisium silicate hydrate(M-S-H) gel and sodium hexametaphosphate(SHMP) on the reaction process of MgO-SiO2-H2 O cementitious system were investigated, the reaction mechanisms and the competition relationship between the formation of M-S-H gel and Mg(OH)2 were illustrated, thus the rapid reaction of MgO and formation of M-S-H gel were achieved. Additionally, the strengths of MgO/SF pastes were significantly improved, and a noval calcium-magnesium composite insulation board was succesfully prepared. These research works are presented in detail as follows.In order to obtain MgO with high reactivity, the preparation procedure of MgO was investigated and the reactivity of MgO was characterized. The reactivity of MgO was mainly depended on its grain size and lattice distortion, which were largely determined by the calcination procedures. When MgCO3 was fully decomposited, a lower calcination temperature resulted in a greater lattice distortion and higher reactivity of MgO. Besides, the reactivity of MgO was also influenced by the fineness, calcination time, rising rate of temperature and cooling mode. In this study, magnesite powder(D50=11.89 μm) was heated at 10°C/min to 850°C and calcinated for 2h. The product was then cooled at natural condition. The prepared MgO presented as fineness(D50) of 6.25 μm, BET specific area of 51.60 m2/g, grain size of 46.2 nm, lattice distortion of 0.208 % and reactivity of 35.83 s.Reaction thermodynamics of MgO-SiO2-H2 O cementitious system was calculated, providing the theory basis to regulate the reaction direction.. The ?fGmθ values of M-S-H gels chemical reaction generating M-S-H gel takes place more easily than the one generating Mg(OH)2, and Mg(OH)2 is able to react with amorphous SiO2 to generate M-S-H gel. The chemical equilibrium constant(K) for the formation of M-S-H gels were higher than Mg(OH)2, indicating that the chemical reaction generating M-S-H gel proceeds more completely than the one producing Mg(OH)2.The reaction products, reaction process and mechanisms of MgO-SiO2-H2 O cementitious system were investigated. The reaction products were Mg(OH)2 and M-S-H gel. Mg(OH)2 was tri-prism crystals(1-4 μm) and M-S-H gel was long flocculent gel(about 100 nm) with a Mg/Si ratio in the range of 0.66±0.03-0.98±0.02. At early ages(7d), Mg(OH)2 rapidly generated while the formation of M-S-H gel was slow. After 14 d, the content of M-S-H gel continually increased and Mg(OH)2 was gradualy consumed by reacting with SF, further producing M-S-H gel. The reaction process was characterized in four parts: dissolution of MgO, dissociation of SF, generation of Mg(OH)2 and formation of M-S-H gel. Mg2+ and OHgenerated with the dissolution of MgO. Mg(OH)2 generated when the concentrations of these ions reached the constant of saturated solubility. The dissolution of MgO promoted the dissociation of SF, forming silicate ions. The silicate ions were then combined with Mg2+ and generated M-S-H gel on the surface of SF particles.According to the reaction mechanisms of MgO-SiO2-H2 O cementitious system and growth process of reaction products, the reaction kinetics models were proposed. At early ages, the reactions were dominated by the ion-solution reaction, which were first order chemical reactions. At later ages, the reactions turned to solid phase transformation and growth process, which were referred as heterogeneous crystal nucleation and one-dimensional growth process. The kinetics formula is = 1- -?, and the reaction rate formula can be expressed as ? = ? -?. When the reaction rate of MgO was lower than the critical value(0.250×10-6s-1), only M-S-H gel generated; under the opposite condition, M-S-H gel and Mg(OH)2 simultaneously generated. As the reaction proceeded, the reaction rate of MgO decreased. After it reached the critical value, only M-S-H gel generated, resulting in gradual decrease of Mg(OH)2 content.Factors influencing the reaction process of MgO-SiO2-H2 O cementitious system were clarified, and the M-S-H gel formation process can be regulated. The main factors were MgO/SF ratio, water to solid ratio, curing temperature, reactivity of MgO, pore solution pH value and additives, respectively. When the addition of MgO was within 30%-40%, MgO completely reacted with SF and only generated M-S-H gel. High W/C ratio was beneficial for the rapid generation of Mg(OH)2, while low W/C ratio was beneficial for the formation of M-S-H gel. Higher curing temperature also accelerated the reaction of MgO together with the formation of M-S-H gel and Mg(OH)2 at early stages. MgO with low reactivity(calcination temperature>1050°C) restrained the generation of Mg(OH)2. The only reaction product was M-S-H gel when using dead-burned MgO(calcinated at 1450°C). High pH value of solution restricted both the dissolution of MgO and generation of Mg(OH)2 while it promoted both the dissociation of SF and generation of M-S-H gel. The addition of SHMP improved the flowability of MgO/SF pastes and significantly lowered the W/C ratio, and the reaction rate of MgO was promoted, resulting in a rapid growth of Mg(OH)2 and M-S-H gel at early age. By adding synthetic M-S-H gel, the reaction of MgO and formation of Mg(OH)2 were halted, but the nucleation sites of M-S-H gel were increased.Finally, a novel M-S-H cementitious material, with a compressive strength of 19.0MPa, 65.0MPa and 75.0MPa at 3d, 28 d and 90 d respectively, was successfully prepared. Futhermore, a novel calcium-magnesium composite insulation board was manufactured, with an apparent density of 230 kg/m3, compressive strength of 0.51 MPa and thermal conductivity of 0.0595W/(m·K) respectively.This study provides theoretical basis for the development of composite silicate-calcium-magenisium materials. Taking advantage of this, the range of available raw materials(rich in magenisium and calcium) are significantly broadened. Huge amount of wasted magnesium-rich carbonate minerals can be recovered and reused in construction and building industry. All of these would considerablely benefit for the environment, economy and society. |
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