Thermal Characteristic Of Hydrated Cement Paste And Its Effect On Sintering Process Of Recycled Cement

The methods of producing recycled cement from all component waste concrete is benefit to reusing waste concrete with high ratio, which can reduce the environmental load, and to effectively relieve the difficulty about the shortage of cement raw materials. At present, the study on technical road and mechanism of recycled cement is still at the experimental stage. It’s so important about the research on sintering process and influential elements. Hydrated cement paste as only active component exist in waste concrete and the exist of unhydrated cement particles will make important influence on sintering process of recycled cement. Considering to the special character of hydrated cement paste, the paper is major on the thermal characteristic of hydrated cement paste and its effect on recycled cement sintering process. The specific content and relevant conclusions are as follows.Based on the previous work, the high temperature thermal properties of hydrated cement paste are studied firstly in the paper. The effect of cement paste on the burning process of regenerated cement was analysized.The burning regeneration of cement clinker burning into the effect of cement on the stone the influence of burning system, and probes into the cement clinker firing on mechanism of action, and the actual investigation on the stone cement concrete cement clinker regeneration abandoned components from the burned influence; Further, from the waste in ordinary cement concrete difference on the features of the raw material, the material of silicon SiO2form and trace elements of recycled cement burning into the effect of process; In view of the hardening cement exist among part of unhydrated particles, this paper also studies the unhydrated particles of recycled cement clinker.Study on the thermal characteristic of hydrated cement paste:There are two parts about its characteristic, namely low temperature and high temperature. The conclustions about thermal characteristic at low temperature are clear. However, in order to ensuring the integrity of paper, confirmatory study on low temperature thermal characteristic have been taken place with the help of XRD&DTA methods. The experimental results shows that hydrated minerals will completely decomposition when the heating temperature is up to950°C and there are new minerals, such as C2S、C12A7, have been regenerated, which is the same as other researchers’ conclusion; Because the sintering temperature of recycled cement is far higher than100°C, so the high temperature thermal characteristic of hydrated cement paste is very important. Studying on the high temperature thermal characteristic of hydrated cement paste by using the methods of DSC、TG、f-CaO, XRD and petrography analysis’s. The experimental results shows that when the heating temperature is higher than1000°C, solid-phase reaction happened, the liquid phase was produced and the main minerals of cement, such as C3S、C2S、 C4AF and C3A all be regenerated. When the temperature is upon to135O°C,the crystalline degree of various minerals is best, and there are no other kind minerals, which bot belong to ordinal cement clinker, are been produced.Influence of hydrated cement paste on sintering process of recycled cement:Preheating and sintering are the key process of the production of recycled cement. The influence of hydrated cement paste on preheating&sintering process of recycled cement been researched in this section. By testing the f-CaO content in hydrated cement paste under different conditions (the preheating temperature change between850°C and1000°C), the results shows that under the condition, namely the hydrated cement paste was heated under950°C for30min with a very high initial warming speed, the decomposition of hydrated cement paste reach the best state. The heating process of recycled cement was identified as preheating at950°C for30min with a very high initial warming speed, which indirectly confirmed the detrimental element to recycled cement preheating process is hydrated cement paste; Preheating is befit to improve the burnability of recycled cement raw materials and the crystalline degree of clinker minerals. However, the strength of recycled cement after preheating is lower than unprheating, which due to the difference of cement particles distribution. By using the comparison testing methods, the result shows that hydrated cement paste make an positive effect on sintering temperature of recycled cement, which due to three characteristic of hydrated cement paste, the high activity and the special thermal characteristic of hydrated cement paste, namely the minerals, such as C3S^C2S、C4AF and C3A can be regenerated at relative low temperature and the exist of unhydrated cement particles.The existed of quartz sand, the SiO2crystal as the detrimental element make an negative effect on sintering temperature of recycled cement clinker. The experimental results shows that the negative effect of quartz sand will disappear when the finesse of sand particle smaller than45um. On the other hand, the anionic ions or anionic group as important element have an effect on sintering temperature of recycled cement.The source of waste concrete is very wide, thus the content of unhydrated cement particles is so different. This part is mainly research on the influence of unhydrated cement particles on burnability of recycled cement raw materials. The experimental results shows that with the increase of unhydrated cement particles content in harden cement paste, the burnability of raw materials went bad. In order to keep the good burnability of raw materials, the content of unhydrated cement particles should not be more than5%. Study on the improvement measure to high content unhydrated cement particle raw materials, the results shows that CaF2have a positive effect on burnability of this kind raw materials and the suitable mixing content of CaF2is0.5%.