Lognormal Simulation of Pore Evolution During Cement and Mortar Hardening
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LOGNORMAL SIMULATION OF PORE EVOLUTION DURING CEMENT AND MORTAR HARDENING
Dexiang Shi, Weiping Ma and Paul W. Brown Materials Research Laboratory The Pennsylvania State University University Park, PA 16802 ABSTRACT A model to describe the pore sizes in cement paste and mortar, as determined by high pressure mercury intrusion porosimetry, has been developed. The model describes porosity using a compound lognormal distribution. For given material under a given set of curing conditions, the weighing factors and shape parameters of two sub-distributions in the lognormal model may be considered as constants, while the location parameters may be related to curing time and the relationship can be quantified. Therefore, it is possible to predict both the pore size distribution in cement and mortar at any age as well as the evolution in pore size during curing. INTRODUCTION Cementitious materials are being used in the immobilization of radioactive waste. Conventional concrete structures are being used as engineered barriers. Low level liquid wastes are being immobilized in cement-based grouts. While these applications differ from those involving normal concrete structures, the porosities of cementitious materials are important in determining the ability of a structure to meet its functional requirements, virtually regardless of the specific applications. It is the distribution of porosity throughout cement matrix and at cement-aggregate interfacial zones that, in concert with environmental variables, determines the macroscopic rate of transport of species. The development of an adequate description of the pore size distributions in cements has recently been accomplished [1]. Our work has shown that the pore size distributions in cement pastes over the size range determined by the high pressure mercury intrusion porosimetry (MIP) may be described by a mixture of two lognormal distributions [1]. The present paper extends the model to cementitious systems containing aggregate. Many physical and chemical processes may be approximated by the lognormal model [2-5]. It is a common physical occurrence that a system can often described by a mixture of two lognormal distributions, which govern two different phenomena occurring in that system [6]. Cement hydration may be regarded as a process of subdivision of void space, or interstices between anhydrous particles [1,51. In cement paste and mortar, the origins and the mechanisms controlling the sizes of larger and smaller capillary pores may be different, suggesting the lognormal model of pore size distribution in cement paste and mortar to have a physical basis. Previous investigations of pore evolution in cement paste have shown that the distribution shifts to the smaller pore size with increasing age, and that the median diameter and threshold diameter of pores decreases with increase age [7]. This paper will show that, for a given set of curing conditions, the weighing factors, f and (1-f), and shape parameters, o1 and 02, of sub-distributions may be considered constant, while th
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