Study On Preparation And Properties Of Humidity Controlling Materials Of CaO-SiO₂-H₂O System

Being the common products in CaO-SiO2-H2O system,calcium silicate hydrate(CSH)is the most fundamental composition of such building materials based on Portland cement or its concrete,in which CSH generally plays a critical role in the mechanical strength and long-term durability of the corresponding products.In most recent years,some kinds of functional materials with the involvement of CSH have been developed,indicating the enormous utilization potential of CSH in the fields of building materials,environmental protection and other fields.Due to the abundant content of nanoscaled pores,CSH matt is so promising to be employed as humidity controlling materials,but CSH products obtained with most of ordinary methods generally possesses less amount of mesoscaled and macroscaled pores,which in turn results in a poor properties of the as-prepared building pieces in water permeability/diffusion and subsequently the capacities of moisture adsorption and desorption.With a purpose to obtain CSH powders with appreciated capacity of moisture adsorption/desorption,silica fume or diatomite was selected as the SiO2 source to prepare CSH products with the incorporation of calcium hydroxide,while the reaction was carried out under the atmosphere of hydrothermal treatment to control the CSH crystals in desired structure.The influence of mixing ratio,holding time and temperature of hydrothermal reaction on the structure and humidity controlling performances of the CSH products were investigated in details,which must be helpful to understand the relationship between the structure and humidity property of CSH matt and to maximize the humidity controlling performances by adjusting the hydrothermal conditions.The microstructures of the as-prepared samples were characterized by scanning electron microscope(SEM),X-ray diffraction(XRD)and nitrogen adsorption method to determine the pore width distribution,while the moisture adsorption/desorption capacity was measured by a static cryo-sorption method.From the hydrothermal reaction between silica fume and calcium hydroxide,one of the targeted products--CSH(B)was generated and showed a morphology of tiny tubular being composed of very thin layers to form a network structure,while another targeted hydrated products—tobermorite(C5S6H5)showed a cluster structure of lamellar crystals.XRD analysis showed that for the preparation of CSH(B),the hydrothermally synthesized specimen behaved not so detectable changes to the intensity of CSH(B)-denoted peaks under XRD analysis with the prolonging of holding time.On the other hand,while tobermorite was selected as the targeted product,the crystal transformation from CSH(B)to tobermorite in the hydratedspecimen was revealed with the prolonging of holding time of the hydrothermal reaction operated at 200? for 6h.From the viewpoints of humidity controlling properties,the hydrated product with a targeted crystal of CSH(B)showed a slight decrease in the moisture absorption and desorption rates after the holding time longer than 4h,while the moisture absorption and desorption rates of the samples with the targeted product of tobermorite showed a trend of first rising and then falling with the prolonging of holding time,which can be attributed to the crystal transformation from CSH(B)to tobermorite.The maximum rates of moisture absorption and desorption were realized from the specimen obtained at 200?,4h.In the hydrothermal reaction between diatomite and calcium hydroxide,the moisture absorption and desorption rates of the as-prepared samples showed a trend of rising first and then decrease with the increasing temperature or prolonging holding time,which gave rise of a maximum capacity of moisture absorption and desorption at the hydrothermal conditions of 200?,4h,SEM observations showed that the crystallinity degree in hydrated specimen increased gradually with the reaction progressed,in which the morphology of diatomite skeleton could be partially preserved.XRD analysis demonstrated the crystal transformation of CSH(B)towards tobermorite,which must be responsible for the lowering of moisture absorption and desorption rates of the hydrated samples.All the experimental results obtained in this work suggests that CSH(B)in high content is of dominant importance for the humidity controlling properties of CSH-based building materials,but the crystal transformation form CSH(B)to tobermorite makes a negative contribution to the same properties of hydrated products.Comparatively,the pores in the range from 3nm to 50nm play a dominant role in the humidity controlling capacity.While diatomite is used as the SiO2 source and mineral template to prepare CSH matt,the moisture adsorption/desorption capacity of such products can be improved to a great extent due to the residual of intrinsic ordered pore structure.