Investigation On The Utilization Of Dehydrated Aluminosilicate Phases

Utilizing construction wastes sufficiently is one of the effective methods to resolve the problems from energy, resources and environment. However, in the waste concrete, excepting that part of the the coarse aggregates have been separated and reused, a large mount of hydrated cement paste are regarded as the waste and abandoned. The hydrated cement paste is the one that consume the largest mount of energy, resource and money. Hence, effective and reasonable utilizing of the hydrated cement paste from construction wastes is the key point to solve the problem above. Some researches show that after heat treatment, the harden cement paste can change into another substance called dehydrated cement paste, which contain the ability of rehydration and strength recovery. Based on such theory, this paper focus on the utilization of dehydrated cement paste in several ways and bring theoretical support for the application of harden cement paste from construction wastes.The harden cement paste is used to simulate the hydrated cement paste from construction wastes in this paper. After a series of treatments, the dehydrated aluminosilicate phases (dehydrated cement paste/DCP) is produced from harden cement paste. In the second chapter, the basic characteristics of DCP are investigated, which mainly including the rehydration characteristics of hydrated cement pastes that subjected to high temperature, influence of the content of dehydrated phases on rehydration characteristics of hydrated cement pastes after heat treatment, and the modified investigation of DCP. The experimental results show that the DCP is a new kind of cementitious material. However, due to the limitation of microstructure (e.g.huge suface area), some of the properties of DCP are negative compared to pure cement, such as excessive water requirement, short setting times, relatively low compressive strength et al. All in all, the setting and hardening characteristics of DCP are hard to control. Consequently, the investigation of regarding the DCP as the main part of cementitious material is hard to get more progress. The DCP can only be utilized reasonablely when the advantages of its structural characteristics are considered. In chapter 3, considering the fact that the DCP contain high activity and high pH value, the DCP is utilized as a kind of activator of fly ash to produde a new kind cementitious material, which contains a large mount of fly ash. In this chapter, a new concept-designing the activated fly ash/DCP system by using the theoretical ratio of calcium to silicon (T-Ca/Si) is proposed. When the T-Ca/Si value is 0.953, the compressive strength of the new cementitious material is maximum, even higher than that of the neat cement. By adjusting the value of T-Ca/Si, the compressive strength of the new cementitious material may be controlled according to practical requirements. Results of the XRD analysis and SEM imaging indicate that when the T-Ca/Si is reasonable or optimum, the secondary reactions of fly ash are relatively complete and the microstructure becomes dense. As a result, the activated fly ash/DCP system reaches a high compressive strength. And also, when the raw materials are chosen, adjusting the value of T-Ca/Si to control the compressive strength of new cementitious system is reasonable.In the fourth chapter, the effect of DCP addition on the cementitious characteristics of cement at the early age has been investigated. With the increase of the content of DCP, the DCP/cement systems need more water to keep the same standard consistency, and shorter times to reach the initial and final setting time. A concept called strength contribution percent in theory (TSC) was put forward to help to evaluate the contribution of DCP to the compressive strength of the whole system in early age. When the content of DCP is about 10%, the value of TSC can reach the maximum. Results of XRD analysis, TG-DSC analysis and SEM imaging indicate that when the content of DCP reaches about 10%, the hydration speed of the DCP/cement systems can be accelerated in the largest extent, which results in more ettringite to be produced and the microstructure of the whole system can become denser after ld.In order to develop the function of modification of DCP to cement and other materials furtherly, the effect of nano-particles in DCP addition to the cementitious characteristics of cement is investigated in chapter 5. The experimental results show that the separation method that designed in this part can separate the DCP particles effectively, and some nano-particles can be observed. Such nano-particles can enhance the mechanical properties of cement obviously. When 4% of nano-particles are added in to cement, the compressive strength of such system in 3d,7d and 28d can be enhanced by 69.9%,62.0%and 60.4% respectively. Results of XRD analysis and SEM imaging indicate that the addition of DCP nano-particles haven't resuled in the appearance of new hydration products, but a large mount of crystal nucleus, which bring many tiny reaction groups in the early ages and help to make the the system to become dense with low porosity and high compressive strength.According to this study, it can be concluded that the investigation of regarding the DCP as the main part of cementitious material is hard to get more progress, excepting that when the DCP is considered as the activator of pozzolanic materials, early strength agent and performance regulator that containing nano-particles of cement or concrete. The investigation in this paper provides some theoretical foundation and technical support to the utilization of aluminosilicate dehydrated phases, which can not only release the pressure of resources and environment, but also bring great economic benefits.