| Amorphous Calcium Silicate Hydrate(C-S-H)powder owns contact-hardening property,that is,under the molding pressure,the C-S-H powder particles can be in close contact and then immediately harden into a lightweight artificial stone with a high strength and good water resistance.This technique has many advantages,including abundant sources of raw materials,no necessity for further curing after molding,short production cycle and recyclability.It is expected to be used in the development of new building materials with widespread application prospects.This technique is composed of two processes,i.e.,hydrothermal synthesis of C-S-H powders and compression molding.However,the design of the synthesis and compression of C-S-H powders relies largely on the experimental results,and the mathematical model indicating the relationship among the contact-hardening property,the parameters of hydrothermal synthesis and compression pressures has not been established.Therefore,the key factors affecting contact-hardening property are difficult to be determined,and the contact-hardening properties are also out of regulation.Besides,the contact-hardening specimen are relatively brittle,which limits its industrial application to a certain extent.In this study,ground quartz sand and quick lime are used as siliceous and calcareous material,respectively,to prepare the C-S-H powders using a dynamic hydrothermal synthesis method.These powders were prepared into contact-hardening specimen under different compression pressures.The influence of synthesis stage of C-S-H powders and compression stage on the contact-hardening property was investigated to determine the key factors and mechanism.Then a mathematical model was proposed to show the relationship among the contact-hardening property of C-S-H powder,its synthesis parameters and compression pressures.Furthermore,the technological parameters of compression molding were optimized by numerical simulation analysis,and the contacthardening mechanism was revealed from the perspective of energy change in the contacthardening process.Finally,the brittleness and toughness of the contact-hardening specimen was improved through adding fiber and polymer.The research results obtained in this study are as follows.①In the synthesis stage of C-S-H powders,the key factors influencing the contacthardening property of C-S-H powders were the particle size of quartz sand powder and isothermal temperature of hydrothermal synthesis.The reduction in the particle size of quartz sand powder was an effective way to increase the reaction rate and to improve the contact-hardening property of C-S-H powders.When the isothermal period was less than 5h,the contact-hardening property of C-S-H powder could be significantly enhanced through raising the isothermal temperature.The C-S-H powder synthesized at 180℃ for 5h exhibited excellent contact-hardening property.② The amorphous index,wamor,was introduced to represent the content of the amorphous substance.Two key factors influencing the contact-hardening property are amorphous index and compression pressure.The mathematic relationship among the compressive strength F,the amorphous index wamor and compression pressure P of the contact-hardening materials was established as the following equation:F=wamor(a+bP),where a and b are two positive constants related to synthetic C-S-H powders and varied with wamor.③ During the contact-hardening process of C-S-H powder,the density and stress distribution of specimens were analyzed by using AB AQUS finite element software.The optimized parameters for the compressing and compacting process were as follows:the friction coefficient of powder was 0.15,and the height-diameter ratio of the feed was less than or equal to 1,and a bidirectional compressing and compacting method was chosen.The density test results were consistent with the numerical simulation values,thereby validating the reliability of numerical simulation analysis.④ Based on the first law of thermodynamics(ΔU=δW-δQ),the energy evolution of C-S-H powders during the contact-hardening process was investigated.The mathematic relationship between compressive strength of contact-hardening specimen(F)and the variation of internal energy per unit mass(Δu=ΔU/m)could be expressed as F=28714.1-28710.2/(1/e((|Δu|-80.8)/7.0)),where Δu is related to the amorphous index(wamor)and compression pressure(P).The expressions are wamor=41.425.9/(1+e((|Δu|-7.4)/0.9))and |Δu|=11.2P0.26.Improvement of the amorphous index of C-S-H powder and the compression pressure can enhance the contact-hardening properties of specimens,and the essence of this change was the increase of internal energy,i.e.,|Δu|.This theoretically demonstrates that the contact-hardening properties can be controlled by regulating the amorphous index of C-S-H powder and compression pressure.⑤The toughness of the material could be enhanced through the addition of fiber and polymer,and then the machinability would be improved.The fiber dispersion(fiber distribution and orientation)was quantitatively evaluated by digital image processing and electrical resistivity testing.The relationship between the fiber distribution coefficient(αf)and the electrical resistivity(ρc)can be described as αf=0.9236-448.0652/ρc.When the fiber content was 2%,the value of αf reached 0.743,which was considered as that the fibers were distributed most evenly.From the respective of fiber dispersion,the maximum fiber content in the contact-hardening materials can be 3%.When the content of polypropylene fiber and acrylic emulsion was 2%and 2%,respectively,the toughness of contact-hardening specimen reached its maximum value,and its initial flexural-toughness ratio was 0.430 which is 38.4%higher than that of comparison group.The addition of fiber promoted wood-like properties of the contact-hardening specimen which could be nailed and sawn.In conclusion,the results obtained in this study determined the key factors influencing contact-hardening properties of C-S-H powder,revealing the controlling mechanism of the contact-hardening properties.This could provide a new approach for the preparation of this light weight and high toughness artificial stone,and it also establishes a theoretical basis for the development and application of this new material. |
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