Investigation On Composition, Structure And Performance Of Calcium Magnesium Silicate Aerated Concrete

Aerated concrete is a widely used building energy-saving wall material by virtue of light weight,heat insulation,fire prevention,environmental protection,wide source of raw materials,high production efficiency.Quicklime is a kind of the important raw materials,while the associated magnesium in lime is considered to affect the volume stability of the slurry,thus a large amount of high-quality limestone is consumed every year,at the same time,a great deal of limestone containing magnesium is discarded.Based on the early study of the cementitious properties of MgO-SiO₂-H₂O system by our research group,calcium magnesium silicate aerated concrete?MCS?was investigated in this paper,which provided theoretical guidance and technical support for the utilization of low-grade limestone and the development of aerated concrete.This paper focused on MCS,the formation process,microstructure,mechanical and thermal properties of hydration products of the calcium magnesium silicate cementitious material under the autoclaving condition were systematically studied from the point of the products of CaO-MgO-SiO₂-H₂O system under high temperature;lightweight and high-strength MCS was prepared based on the match of expansion and thickening of the slurry,and the theory of bubble stability,then the compositions,structure,and the relations among them with the thermal properties of MCS were also investigated.At the same time,computer technology and image analysis were used to reconstruct the pore structure,and a heat transfer model for MCS was builded to predict the thermal performance.Finally,evaluation of the thermal insulation of MCS was proposed through tracking and testing the temperature changes of concrete box in actual environment,and the thermal stability of the MCS structure was also estimated.These research works are presented in detail as follows:?1?According to the cementitious characteristics of MCS matrix,hydrothermal synthesis method was used to ascertain the influence mechanism of Mg²⁺on the formation and microstructure of the hydrothermal products of CaO-MgO-SiO₂-H₂O system.On this basis,the hydration products and basic properties,such as strength,specific heat,thermal conductivity,of the autoclaved calcium magnesium silicate slurry were studied using the same temperature and time regime,which laid the theoretical foundation for development of calcium magnesium silicate cementitious materials and provided basic data for building the structural model of MCS.The types and formation process of hydrothermal products of CaO-MgO-SiO₂-H₂O system with different compositions and reaction temperature and time were studied by XRD,TG-DSC and SEM analysis.The results show that Mg²⁺replaced part of Ca²⁺,then solidified in the structure of calcium silicate hydrate,which had a stabilizing effect on tobermorite.Combined with the crystal structure of calcium silicate hydrate,the influence mechanism of Mg²⁺was analyzed.In addition,MgO promoted the formation of calcium silicate hydrate in the autoclaved calcium magnesium silicate slurry,increased the crystallinity of tobermorite,inhibited the formation of hydrogarnet,and generated a small amount of Mg?OH?₂.At the same time,the crystal size of tobermorite was reduced,which tended to slender structure and provided more contact points and denser microstructure.The compressive strength and specific heat of the autoclaved slurry were increased by proper content of MgO,where MgO/?MgO+CaO?of 12-18%resulted in the highest compressive strength.?2?Starting from the initial rheological properties of MCS slurry and the formation and development of bubbles,the key preparation technology of MCS was mastered based on the matching principle of expansion and thickening rates of the slurry.The density,mechanical,thermal,and hygric properties of MCS and the relations among them are studied.The ultimate yield stress and plastic viscosity of MCS slurry with different compositions were tested using a rheometer.The influence of MgO on the cohesion of the slurry was primarily evaluated.The relationship between rheological and expansion properties of MCS slurry with different compositions was investigated using the expansion rate,the loss of fluidity over time,and the internal temperature as evaluation indexes The optimum mix proportion of the slurry was determined based on the thickening and expansion curves.The results show that the yield stress and plastic viscosity increased with the increase of MgO content,so the expansion of the slurry delayed and its thickening accelerated.Based on the initial rheological properties of the slurry and its matching principle of expansion and thickening,the optimum MgO/?MgO+CaO?was 6-20%,the optimum content of quicklime was 17%and the optimum content of gypsum was 3%or 5%.The basic performance tests of MCS shows that the thermal conductivity of MCS was increased by introducing MgO,and the specific heat and thermal effusivity also increased.?3?The microscopic and macroscopic pores were characterized by nitrogen adsorption and image analysis,respectively,and three major structural parameters?porosity,average pore size,and pore size distribution?of macropores were systematically analyzed.The influence law of main factors,such as the water to solid ratio,the amount and fineness of the gas generating agent,and the curing temperature,on the pore structure parameters and thermal properties of MCS were investigated.The correlation among the influencing factors,pore structure and thermal properties was established by the grey correlation analysis.Finally,based on the pore structure parameters and the thermal parameters of the autoclaved slurry,the pore structure of MCS was reconstructed using the“random generation”method through the computer program design,the steady-state heat transfer equation of the reconstructed structure was solved by the“Finite Volume Method”.The main conclusions include:the microscopic pore characteristics were almost not affected by gas generating agent;the macro porosity and average pore size of MCS increased with decreasing density,and the pore size distribution met the lognormal distribution.Aluminum powder content and water to solid ratio were the main influence factors on the pore structure and thermal conductivity,while the pores with size of 500-1000?m had the greatest correlation with thermal conductivity,and those with size<2000?m had considerable correlation with thermal conductivity.The influence mechanism of the overlap and merger of pores on the thermal conductivity was revealed through the reconstruction of pore structure and numerical simulation of thermal conductivity,and a structural model that could accurately predict the thermal conductivity of MCS was builded.?4?The dynamic temperature field characteristics of different MCS structures were tested using the indoor box temperature control method and the actual outdoor environment in order to comprehensively characterize the thermal insulation properties.The results show that the temperature decrement factor increased with the increase of density,and the amplitude of variation of decrement factor with density of the sandwich panel was less than that of aerated concrete panel.The time lags of aerated concrete panels with different densities could not be distinguished,but the time lags of sandwich panels were greater than those of aerated concrete panels.In addition,the time lags increased with the thickness of the aerated concrete panels.Through calculation of the temperature field and thermal stability analysis,the comprehensive thermal inertia of aerated concrete with density range of 700-900 kg/m³ was better.Outdoor test results show that aerated concrete had a strong peak weakening effect on hot weather.The sandwich structure could more effectively reduce the internal temperature and improve the thermal insulation of the overall structure.