Research On Properties Of Alkali Slag Concrete Subjected To Elevated Temperatures

The producing procedures of Alkali-activated Slag cement （AAS） is notcomplicate and has Low energy consumption, besides, its mixture cement ischaracterized of desirable mechanical property and durability. However, researchershold different opinions towards this cement’s high temperature’s property. On one hand,its boasts of desirable high temperature’s property because its hydro products nearlycontain no elements as Aft and Ca(OH)2that can be easily decomposed under hightemperature; On the other hand, since AAS has greater contraction under hightemperature, the thermal deformation difference between its mortar and aggregatesresults in undesirable high temperature’s property. Therefore, researches with numericalmodels on Alkali Slag concret（eASC）’s high temperature’s property to discover rules ofmechanical behavior and microstructure under high temperature are significant toenhance related theoretical studies, providing guidance for ASC structural design andapplication.This paper explores heating system, incubation time, strength grade, aggregate, andfiber’s influences on ASC sample’s appearance, quality and strength; TG-DSC, XRD,FTIR, SEM and the nitrogen adsorption tests are used to analyze AAS’s products and itsmicrostructure under high temperature; Multiple physical fields transmission model,Numerical method for solving the coupling and the analyses of numerical example havealso been used during the research. Altogether, the principal rules and research resultscan be listed as below.ASC’s strength and structural changes correlates with the heating temperature.When temperature ranges from room temperature to200℃, hydration reaction getenhanced due to such high temperature, so cement’s structure get more compacted andcompress strength gains comparably; when temperature is over200℃, its cement andaggregate interface bond strength reduces and shows a decreased tendency; during600℃to700℃, its non-evaporable water gets less and its strength reduces dramatically;Solid state reaction occurs and formulates gehlenite after800℃, what’s more, theaggregate is partially decomposing and cement’s structure becomes severe worse withdrastically increasing mass loss rate.ASC’s high temperature property is related to the concrete strength grade. Thelower strength grade, the less influence high temperature will bring to ASC; the higher strength grade and much more compact structure the ASC has, more structural damagewill be seen due to steam pressure and thermal stress under high temperature. Comparedwith common cement, AAS has greater thermal contraction. Therefore, the crack ofmortar and aggregate produced in the course of degradation will not only lessen cementstrength obviously, but also steam pressure brings less damage to the cement structure,additionally, the spalling can be avoided. Compared with ordinary concrete in the samestrength class, ASC’s temperature degradation increases100℃to200℃.ASC’s high temperature property is relevant to heating system. When temperatureis over400℃, ASC gets more damage as the heat increases; when heating sustains for1h～6h even longer, ASC’s strength decreases more quickly. Furthermore, air coolingand water cooling make no difference for ASC’s high temperature property. If theheating temperature is below400℃, its strength gains under standard high temperaturemaintenance; If the heating temperature is over600℃, its hydration process andstrength gains turns to be invalid.ASC’s high temperature property subjects to aggregate’s thermophysical properties.The basalt has less volumetric deformation and its mortar structure is stable under hightemperature, so steam pressure brings larger damage to the cement because steam ishard to evaporate. The limestone decomposes around800℃and the ASC structurechanges obviously. The shale has big porosity, so steam pressure’s damage on shale’scement structure becomes smaller under high temperature, thus its cement strengthsuffers less loss. Comparably, shale AAC boasts better high temperature property.Fiber plays a small role for ASC’s high temperature property. When thetemperature ranges from room temperature to600℃, steel fiber exerts bigger influenceto ASC; its bonding strength between fiber and paste drops due to severe oxidation ofcopper coated steel fiber coating when temperature is800℃. Cracks produced duringASC’s structural degradation may reduce pore pressure effectively. However,polypropylene fiber is not an important factor for ASC’s high temperature property.ASC’s structural change conforms with the theory of heat and mass transfer inporous media. Based on porous media theory, the ASC’s model can be transformed asporous media composed of solid skeleton and pore, thus heat transfer mechanism forASC under high temperature can be established; Given that elastic deformation’sinfluence on heat transfer mechanism under the full coupled calculation betweenvolumetric strain rate of solid skeleton and pore pressure,“heat-pore-elastoplasticcoupling mathematical model” can be established In terms of Bazant model. The strength gained in the course of transmission medium can be calculated withthe radial regression method and Newton-Raphson’s backward euler method.Considering the “heat-pore-elastoplastic coupling mathematical model”,Consistent tangent modulus of stiffness matrix is mentioned to ensure the quadraticconvergence with the formulation of transient finite element format. Combing theresearch on ASC’s thermophysical properties, the analysis of numerical example withABAQUS software has been explored. The analysis indicates that the calculated resultsare similar to the measured results, thus this method is efficient to detect the ASC’sstructural changes under high temperature.