Study On Performance And Compound Method Of Concrete Added With Fly Ash And/or Slag

With the increase of emphasis on environmental protection and sustainable development,"green concrete" has gradually become the focus of people’s attention and has been a important development and research direction in the research field of concrete material. The concrete added with admixture belongs to "green concrete". Nowadays as for the concrete added with fly ash or slag, many research results and some common understandings have been achieved. But as for the concrete double mixed with fly ash and slag(abbreviated as FSC concrete later), few common understandings have been obtained. In the research field of FSC concrete there are numerous work waiting for us to complete. Today FSC concrete is widespreadly used in concrete structures and engineering. Compared with the widespread use of FSC concrete in practical construction, the research on FSC concrete markedly lags behind. So widespread application of FSC concrete in practical construction is influenced, and engineering quality can not be guaranteed very well.A series of experiments on cement concrete and concrete added with fly ash and/or slag were carried out, including these investigations on strength, bleeding resistance, carbonation resistance, frost resistance and microstructure of FSC concrete. Meanwhile a new method of mix proportion design of ordinary concrete-data analysis method is proposed based on a great deal of mix proportion data. Major results in the paper are listed as follows:(1)If the cement in cement concrete (or cement mortar) is partially replaced by slag, the strength of concrete (or mortar) will firstly increase, and then decrease with the increase of the content of slag in binder; When the substituted amount of cement is30%-40%, the strength of concrete(or mortar) reaches to the peak value; When the substituted amount of cement is more than60%, the strength of concrete(or mortar) is less than the strength of control cement concrete(or cement mortar). Substituting a part of cement with fly ash in cement concrete (or mortar), the strength of concrete (or mortar) will always decrease as the increase of the content of fly ash. Partially replacing the cement with slag in fly ash concrete (or mortar), the strength of concrete (or mortar) firstly increases, and then decrease with the increase of the content of slag; When the substituted amount of cement is30%-40%, the strength of concrete (or mortar) reach to peak value; When the substituted amount of cement is more than60%, the strength of concrete (or mortar) is less than the strength of control fly ash concrete (or fly ash mortar).So a conclusion is drawn that if the cement is partially substituted by slag, the law that the strength of concrete changes with increase of the content of slag in cement concrete is similar to that in fly ash concrete. (2)The strength of concrete does not always decrease with increase of the total porosity of pore size≥10nm, it mainly relates to the porosity of pore size≥50nm and the porosity of pore size=1nm-3nm.(3)The strength of binder blended with admixture can be predicted rather accurately by means of simplex-centroid experimental design. By simplex-centroid experimental design and Bolomy formula, concrete mix proportion meeting design requirements can be easily and conveniently gotten when designing mix proportion based on the strength and durability of concrete, because the strength of concrete can rather accurately be gained according to the type and blending content of admixture.(4)Whether in cement concrete or in concrete added with fly ash and/or slag, the paste should be divided into three zones according to its microhardness, being called interface weakness zone, interface strengthening zone and matrix zone respectively. Interface weakness zone is next to aggregate surface, its microhardness is lower than that of other zones. Matrix zone lies in the inside of paste, its microhardness is higher than that of interface weakness zone and lower than that of interface strengthening zone. Interface strengthening zone lies between interface weakness zone and matrix zone, its microhardness is higher than that of other two zones. So interface transition zone between paste and aggregate consists of interface weakness zone and interface strengthening zone.(5)The bleeding resistance of concrete gradually decreases with the increase of porosity of pore size>100nm. Among all concrete (including cement concrete and concrete with fly ash and/or slag) with the same water-binder ratio and binder’s content, there exists a kind of concrete with the best bleeding resistance; so the fly ash-slag-cement ratio of the concrete is known as the optimum ratio. In the paper, the optimum ratio is cement:slag:fly ash=60:20:20.(6)If the cement in cement concrete is partially replaced by slag and/or fly ash, the carbonation resistance of concrete will lower. Among all FSC concrete with similar content of admixture, there exists a kind of concrete with the best carbonation resistance, whose slag-fly ash ratio is optimum ratio.(7)After carbonation, chloride penetration resistance of cement concrete will become weaker slightly, but that of concrete with fly ash and/or slag will be weaker prominently. After immersion into NaCl solution for a long period of time, the carbonation resistance of cement concrete will improve, but that of concrete with fly ash and/or slag will lower. So the concrete subject to double effects of carbonation and chloride salt should be cement concrete, not be the concrete added with fly ash and/or slag. If it is necessary to add admixture into cement concrete, the blending content of fly ash and/or slag must be strictly controlled under a given level.(8)Partially substituting cement with fly ash and/or slag in cement concrete will give rise to the decrease of freeze-thaw resistance of concrete. Freeze-thaw resistance of slag concrete is superior to that of fly ash concrete with the similar water-binder ratio and blending content of admixture. If the slag is partially replaced by fly ash in slag concrete, the freeze-thaw resistance of concrete gradually is decreased with increase of the content of fly ash. Freeze-thaw resistance of concrete will lower with increase of the porosity of pore size≥10nm in paste.(9)Freeze-thaw resistance of cement concrete is stronger than that of concrete with fly ash and/or slag in NaCl solution. Compared with concrete in water, freeze-thaw resistance of both cement concrete and concrete with fly ash and/or slag in chloride salt solution will lower, but the rate of descent of freeze-thaw resistance of the former is lower than that of the latter.(10)After entry of5%air void into concrete, freeze-thaw resistance of both cement concrete and concrete with fly ash and/or slag in NaCl solution greatly improves, but that of the former is more excellent than that of the latter. Therefore in the environments with the interaction of chloride salt and freeze-thaw, cement concrete should be the first option. If concrete with admixture must be used, the content of admixture should be as low as possible.(11) If the unit water content of ordinary concrete without reducing-water agent keeps unchanged, among all concrete with good workability there is a law that the volume ratios of sands to binder linearly increase with the increase of the volume ratios of water to binder. As for the concrete added with reducing-water agents, if reducing-water agents have the same reducing water ratios, there exists the same law among all concrete with good workability. According the law, a new method of mix proportion design of ordinary concrete-data analysis method is proposed.