Silica Calcium Products Using Waste Concrete By Hydrothermal Synthesis At Low Calcined Temperature

At present,the recycling of waste concrete indomestic and abroad research mainlyincludes backfill materials,foundation mats and preparation of recycled materials.In the reuse of waste concrete,there are shortcomings such as low-added value and unstable performance of recycled products,and it is urgent to solve the problem to maximize utilization of waste concrete resources.The waste concrete is mainly composed of sand and gravel andthe chemical composition is mainly composed of calcium and silicon oxides.It can be used as calcium raw materials or（and）silica raw materials for the production of cement and silica calcium products.This aim of thesis is to study the use of waste concrete as the main raw material and prepare silicon-calcium products.On the one hand,it can realize the recycling of sand and gravel materials in waste concrete.On the other hand,it can reduce the consumption of natural silicon-calcium raw materials in the production of silicon-calcium products.This research has a good economy,benefits and broad development prospects.Nowadays,the waste concrete is mainly used as silica raw materialsin the related research on the preparation of silicon-calcium products from waste concrete,and the limestone（calcium aggregate）only serves as aggregate,and additional Ca（OH）₂ is needed as Calcium raw material.In this paper,low-temperature calcination-hydrothermal synthesis technology is used to prepare siliceous calcium products,which can make full use of limestone resources in waste concrete and use it as calcium raw material in silicon calcium products.There are two key problems to be solved in this thesis: one is to reduce the decomposition temperature of CaCO₃ in waste concrete by using suitable additives and to obtain high reactivity CaO under the premise of reducing energy consumption.The other is to find suitable mixture ratio and steaming synthesis system to prepare silicon-calcium products with good performance.In this thesis,the whole component waste concrete without separation treatment is used as the main raw material,and the types of additives（gypsum,fly ash,calcium fluoride,oxalic acid,magnesium carbonate,copper oxide and barium sulfate）and the amount of additives are changed,and the weight is passed.Analytical,chemical testing,XRD analysis and other methods were used to study the CaO content and CaO activity in calcined products under different calcination systems,and the suitable additive doping scheme and calcination system were determined.Further,the water-solid ratio,the calcium-to-silicon ratio,the pressingtemperature and the pressing time were changed,and the hydrothermal synthesis was studied by the compressive strength test,the pore solution alkalinity test,the water resistance test,the mercury intrusion analysis,the XRD analysis,the SEM analysis,and the like.The performance of the product,determine the appropriate ratio of ingredients and the steaming system.The main research conclusions of the paper are as follow:（1）Thermogravimetric analysis showed that the decomposition temperature of CaCO₃ in waste concrete was 841 ℃;gypsum and fly ash had the best effect on reducing the decomposition temperature of CaCO₃,while oxalic acid did not promote the decomposition temperature of it.The calcination test showed that there was no obvious correspondence between the thermal decomposition of calcium carbonate in waste concrete and the holding time when the content and activity of CaO were used as the evaluation indexes.The optimum decomposition temperature of control group adding 1% gypsum in waste concrete was 900 ℃.The optimum decomposition temperature of waste concrete adding 10% fly ash group and10% CaF₂ were 850 ℃.The waste concrete mixed with 10% CaF₂ was calcined and steamed.However,the specimens could not be formed.Under the experimental conditions of this paper,the suitable additive doping scheme for waste concrete was doped with 10% fly ash.The suitable low temperature calcination system was 850 ℃ and the holding time was30 min.（2）As the water-solid ratio increased,the compressive strengthen of the autoclaved specimen decreased.The optimum water-solid ratio was 0.36 under the experimental conditions.Under different steaming systems,the compressive strengthen of the specimens increased with the increase of the ratio of calcium and silicon.The autoclave at lower pressure should prolong the holding time,while the higher pressure autoclave should shorten it.With the increase of steaming holding time,the softening coefficient of the specimens increased when the pressure was 1MPa,and the softening coefficient of the specimens decreased when the pressure was 2MPa.With the increase of calcium-silicon ratio,the softening coefficient of the specimens decreased before increased.Most of the steamed specimens have a softening coefficient greater than 0.6 and water resistance was acceptable.The pressure of steaming system has the greatest influence on the alkalinity of the solution pores,followed by the steaming holding time,while the ratio of calcium to silicon has little effect on the alkalinity of the solution.the mercury intrusion method showed that the porosity of the specimens was9.22% when the autoclave pressure was 2 MPa,the heat retention was 3 h,and the calcium to silicon ratio was 1.As the pressure of autoclaving increased,the holding time of autoclaving increased,and the ratio of calcium to silicon also increased,and the hydrated calcium silicate formed in the autoclaved specimens gradually changed from the initial amorphous CSH gel to tobe mullite or hard wollastonite.the hard wollastonite was synthesized in an autoclaved product when the ratio of calcium to silicon was 0.9,pressure was 2 MPa,holding time was 6h and the calcium to silicon ratio was 1.0,pressure was 1.5 MPa and holding time was of 6 h.