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SUMMARYSubjected to the wet surrounding, hydrated cement paste (HCP) exposed to high temperature may exhibit rehydration behavior. This paper presents the influence of the dehydration temperature and the initial water/cement ratio on the rehydration activity of dehydrated cement paste (DCP). Original HCPs were prepared with two water/cement ratios of 0.3 and 0.5, respectively, and cured under the fog-spraying standard condition for 30 days. The DCP powders used were obtained by grinding dry HCP less than 75 m and then subjecting to different temperatures, up to 900 • C. The rehydration properties of DCP were evaluated by the required water for standard consistency, the setting time, the rehydrated compressive strength and the microstructure evolution. X-ray diffraction (XRD) was employed to identify the crystalline phases before and after rehydration. Experimental results showed that the coupled rehydration effect from the dehydrated hydration products and the initially unhydrated cement determined the rehydration behavior of DCP. The rehydration of DCP strongly depended on the dehydration temperature and the water/cement ratio of the original HCP.

A newly developed version of a three-dimensional computer model for simulating the hydration and microstructure development of slag cement pastes is presented in this study. It is based on a 3-D computer model for Portland cement hydration (CEMHYD3D) which was originally developed at NIST, taken over in the authors' group and further developed. Features like the digitized 3-D microstructure, the cellular automata (CA) algorithm for simulating the random walking, phase transformation for simulating the chemical reactions, are retained. But, the 3-D microstructure was reconstructed allowing for slag particles as binder in the system. Algorithms and rules are developed to account for the interaction between Portland cement hydration and slag reaction in the paste, of which the mechanisms were revealed in the studies by Chen and Brouwers [(2007) J Mater Sci 42(2):428; (2007) J Mater Sci 42(2):444] Methods for considering the various factors on the reactivity of slag in hydrating slag cement pastes are proposed, mainly for the oxide composition of slag and the alkalinity in the pore solution composition. A comparison between the model predictions and the experimental results in literature shows that the presented computer model can successfully predict the hydration process and the microstructure development of hydrating slag cement paste.

Due to environmental reasons and the shortage of natural resources, it is greatly valuable to recycle construction and demolition waste (CDW) as much as possible. One of effective ways to reuse more CDW is to produce a cemented road base material. The recycled CDW however is a mix of recycled masonry and concrete with a wide variation in composition. This implies that the mechanical properties of cement treated demolition waste are not only determined by cement content and degree of compaction, but also by the ratio of crushed masonry content to crushed concrete content. In order to optimize its mixture proportioning, this paper explores the response surface and contour plot of the combined effect of mixture variables on the mechanical properties including the unconfined compressive strength (UCS), the elastic modulus (E) and their ratio. It has been recognized that optimizing the mixture proportioning of cement treated demolition waste should not only consider its material properties, but also needs to take into account its structural behavior as a pavement layer. Analytical results indicate that increasing the degree of compaction is an economic technique to obtain the required strength, but it is not an efficient method to enhance the admissible elastic strain (the ratio of UCS to E) and to improve the flexural rigidity of the road base layer. Obtaining a desired low flexural rigidity certainly needs adjusting of the masonry content and the cement content.

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