Contributions And Mechanisms Of Waste Slags Characteristics And Their Multiple Combination On High Performance Of Cement Based Materials

Based on the characteristics of dam concrete, the effects of Class F fly ash on the properties of concrete used in Three Gorge Dam were fully demonstrated in this dissertation firstly. This not only had no negative effects on the performance of dam concrete, but also effectively increased the cracking-resistance and volume stability and durability of dam concrete. Therefore, the service life of the Three Gorge Dam will be enhanced. The meso-scope and micro-scope mechanisms were also revealed.Whereas, the early strength and the carbonation resistance of fly ash concrete were decreased with the addition of fly ash because of the low activity and low CaO content of Class F fly ash. These negative aspects are no problem for dam concrete because high early strength is not necessary for dam concrete, furthermore, dam concrete has no steel bars. But, the early strength and carbonation resistance are important properties for lots of high performance structural concrete reinforced with steel bars.In order to resolve those issues, the influencing pattern and mechanism of four kinds of industry waste slags with different activity and different CaO content, on water-reducing effect, early strength and carbonation resistance ability were revealed. The waste slags were pulverized slag powder, Class C fly ash, super-fine Class F fly ash and Class F fly ash. The main problems because of the use of Class F fly ash in structural concrete could be effectively resolved by means of reducing water-to-binder ratio and fully exerting the mutual compensation effect possessed by the combination of different waste slags. Further, the influencing pattern of waste slags with different characteristics and thire compounds on the volume stability of cement based materials with low water-to-binder ratio were discussed. The quantitative relationship among chemical shrinkage and drying shrinkage and autogenous shrinkage were discussed. A structural and mathematical model was advanced to exactly evaluate the effective degree of waste slags with different characteristics on autogenous shrinkage of cement based materials with lower water-to-binder ratio. The Simplex-centroid Design was applied to establish the quantitative mathematics equations among concrete mechanical performance and waste slag combining ratio and curing age. The rationality of mix proportion design for multiple combined waste slag concrete was enhanced and the scientific forecast for concrete performance was realized. Finally, a mathematics model that could exactly estimate the contribution of superposition effect due to combined waste slag to concrete structure (paste porosity) and performance (compressive strength of concrete) was put forward. The main research results were as follows:Effect of Class F fly ash content on dam concrete: Because of the pore refining effect and the densifying of interfacial zone resulted from the pozzlanic reaction of Class F fly ash that started at 7d, the strength increasing ratio at later age, flexural-to-compressive strength ratio, frost resistance of fly ash dam concrete were all increased with the increase of fly ash content. The drying shrinkage strain was decreased with the increase of fly ash content due to the water-reducing effect of Class F fly ash. And the relationship among drying shrinkage strain or frost resistance and fly ash content and curing age could be described quantitatively by different regressive equations. It is fully illuminated that high performance hydraulic engineering concretewith high cracking-resistance, volume stability and durability could be prepared by combining high content fly ash and superplasticizer and air-entraining agent. The lack of calcium should not present for fly ash dam concrete. Whereas, earlier strength and carbonation resistance ability of fly ash concrete were reduced with the increase of fly ash content.The water reducing effects of waste slags and their mutual compensation effects: The water reducing effect of waste slag lowered following the sequence of Class C fly ash, pulverized slag powder, super-fine class F fly ash and Class F fly ash at high cement replacement. The water reducing effect of waste slag had close relation to its particle morphology, glass activity, density, specific surface area, bulk density and average prticle size etc. When combining Class C fly ash with Class F fly ash or pulverized slag powder, the excellent water reducing effect of Class C fly ash could compensate the negative effect due to the other waste slag. Therefore, the proportion of other waste slag could be raised. When pulverized slag powder was combined with super-fine fly ash, the former could improve the problem of large stickiness caused by the latter, but the latter could improve the problem of bleeding caused by pulverized slag powder.Early activity of waste slags and their mutual compensation effects: The early activity of waste slag is the main reason that affect the early strength of cement based materials. The early age hydration processes of different waste slag particles were observed by the Environmental Scan Electron Microscope (ESEM). The combined water content of silicate and aluminate, the time when the pozzlanic reaction strated and pozzlanic reaction degree were calculated and analyzed. The early age activity of waste slags declined following the sequence of pulverized slag powder, Class C fly ash, super-fine class F fly ash and Class F fly ash. The increasing ratio of slag powder mortar strength at later age was not as good as that of fly ash mortar. But increasing ratio of slag powder mortar strength at later age was increased with the increase of slag powder content. When combining fly ash with pulverized slag powder, the contribution of slag powder to early age strength and fly ash to later age strength were fully exerted, this not only increased the earlier strength, but also increased the later strength.The early age hydration process of different waste slag particle observed by ESEM demonstrated that the time of early hydration products appeared on the particle surface with the sequence of cement particle, Class C fly ash particle, Class F fly ash particle. This conclusion has great difference to the traditional viewpoint that Class F fly ash grain is a nucleus center of hydration products and the rate of cement hydration is accelerated at earlier ages.Studies on the carbonation resistance of waste slags: The main reason that waste slags affect the carbonation resistance of cement based materials were the basic reserving content(token by CH content of paste), porosity and permeability. At the same replacement level of cement, the basic reserving content of slag paste was much higher than that of fly ash paste. The CH content of Class C fly ash paste was not higher than that of Class F fly ash because the pozzlanic reaction of Class C fly ash need consume much more CH. The porosity and permeability were increased with the sequence of pulverized slag powder, Class C fly ash, super-fine class F fly ash and Class F fly ash., so did the carbonation resistance. The carbonation resistance of cement based materials could not be improved by increasing the CaO content of fly ash. The combination of pulverized slag powder and fly ash could improve the carbonation resistance of fly ash -cement based materials at higher cement replacement level. It was the most economic and effective measure to combine pulverized slag powder and fly ash.Effect of waste slag on volume stability of cement based materials with low water-to-binder ratios: when cement was replaced partially by waste slags, the autogenous shrinkage of the mortar containing active binder was smaller than that of the control mortar. Effects of waste slags on autogenous and drying shiinkage were affected by the activity of waste slags and the amount of cement replacement. Large desiccation shrinkage in cement-based materials with high content of Class F fly ash could decreased by the combined incoiporation of pulverized slag powder and fly ash.Class C fly ash contained certain amounts of highly active f-CaO, hydration of which could produce some expansion compensating for he autogenous and drying shiinkage, especially for the latter. Thus, it is an effective measure that the multiple compound binders which contain Class C fly ash, cement and other active binders may be adopted to decrease the autogenous and drying shrinkage in cement-based materials with low water-to-binder ratios.At the ambient condition, the autogenous shrinkage of cement-based materials was smaller than the desiccation shiinkage. The ratios of the autogenous shrinkage to the desiccation shrinkagedecreased gradually with the increase of curing age indicated by a regression model ￡l = eafb, in￡which a and b reflected the influence of different waste slags and their combination on autogenous and drying shrinkage.The contribution of different waste slags to autogenous shrinkage could be estimated by measuring the non-evaporable water, and calculating the capillary pressure resulted from chemicaly cos#shrinkage by the model: pw(t) = -^-------— pHv it).In order to increase the rationality of mix proportions design for the concrete with the combination of waste slags, the Simplex-cetroid Design was applied to establish the quantitative relationship among mechanical performance, incorporation ratios in waste slag combination and curing age for four series of concrete, i.e. cement-pulverized slag powder-Class C fly ash, cement-pulverized slag powder-super fine Class F fly ash, cement- Class C fly ash-super fine Class F fly ash, and cement- Class C fly ash- Class F fly ash. The high strength concrete produced by high content of combined waste slags could match the requirements for frost resistance even if air-entraining agent was not used.A mathematical model describing the relationship among cement parameters, paste porosity and concrete strength was established based on rational hypothesis and choice of model parameters.This model could be expressed as follows:R(t) = cdl.3la(t)^In R3 it) = -0.0345P3 (t) + 4.932In which, the parameter c was related to the non-evaporable water of the hydration hydration products from combined binders at Id and can be gained from an experience equationW (C) lnc = 0.558—------—0.496 . The parameter d was related to the addition of waste slag incombined binders and curing age. Waste slags with different properties have different d values.