Effects of Microstructure on Fracture Behavior of Hardened Cement Paste
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EFFECTS OF MICROSTRUCTURE ON FRACTURE BEHAVIOR OF HARDENED CEMENT PASTE ASIF AHMED AND LESLIE STRUBLE University of Illinois, Department of Civil Engineering, Urbana, IL. ABSTRACT Mechanical properties of any material, including hardened cement paste, are assumed to be controlled by its microstructure. An attempt has been made here to establish a link between bulk fracture parameters of hardened cement paste and its microstructure. Paste microstructure has been varied by changing the initial w/c ratio, curing time and curing temperature, and by addition of chemicals to change the calcium hydroxide morphology. It has been found that, like compressive strength, fracture parameters depend directly on porosity. Contrary to our initial hypothesis, CH morphology was found to have no effect on the fracture parameters. INTRODUCTION Hardened cement paste (HCP) is produced by hydration reactions that start as soon as water is mixed with dry portland cement. Microstructure of hardened paste is controlled by various parameters: particle size distribution and composition of cement, morphology of individual hydration products, and composition and age of the paste. In turn, the microstructure determines bulk mechanical properties like strength and toughness. The size scales that are important in the three levels - hydration reactions, microstructure and bulk mechanical properties - differ by orders of magnitude. This wide range of scale is probably why studies of HCP tend to focus on one of the three levels, but very rarely on all three levels at once. From a materials science point of view identifying the microstructural parameters that affect the various bulk mechanical properties is the essential first step in establishing a microstructure-property relationship. The research reported here is an attempt to identify microstructural parameters that affect the fracture behavior of HCP. MICROSTRUCTURE OF HCP At the microstructural level HCP is an intimate but inhomogeneous mixture of a variety of crystalline and quasi-crystalline phases and pores of different sizes and shapes. Most of the volume is calcium silicate hydrate (C-S-H), the principal hydration product, which is a highly disordered quasi-crystalline material of variable composition. The molecular structure of C-S-H is layered and provides for a very high internal porosity. The other primary hydration product is calcium hydroxide (CH). In contrast to C-S-H, CH is highly crystalline and has a fixed composition. The crystals appear as thin hexagonal platelets, often layered, typically tens of •tm across. With continued hydration they grow massive, lose their hexagonal outline, and encapsulate other regions of the paste. Massive blocks of CH crystals can be easily identified in mature pastes. Unhydrated residues of clinker grains, which are crystalline in nature, are present even in well hydrated systems. Pores are essential components of HCP microstructure. There are two major sources for these pores: (1) C-S-H, which is inherently porous, containing gel pores with characteri
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