The Influence of Pore System Characteristics on Absorption and Freeze-Thaw Resistance of Carbonated, Low-Lime Calcium Silicate Cement (CSC) Based Materials
Abstract
The new cementitious binder, calcium silicate-based cement (CSC, a.k.a. Solidia cementTM), was developed in order to combat the issue of CO2 emissions during the cement manufacturing process. Currently, the production of 1 tonne of cement results in the emission of approximately 0.81 tonne of CO2, mainly as a result of the decomposition of large quantities of limestone and high clinkering temperatures. CSC dramatically reduces the emission of during the manufacturing process CO2 to levels of approximately 0.565 tonne per tonne of cement (30% reduction). However, unlike ordinary portland cement (OPC), CSC contains low levels of lowlime calcium silicate phases (mostly wollastonite, pseudowollastonite, and rankinite) and is therefore non-hydraulic. Thus, in order to harden, CSC must undergo the carbonation process, which is achieved by exposure to external source of CO2. In ordinary portland cement concrete, the inherent pore structure (i.e., gel pores, capillary pores, and air voids) of the material plays a significant role in its resistance to harsh environments (e.g., exposure to de-icing chemicals during cold weather and frequent freeze-thaw cycles). As an example, a larger number of saturated capillary pores reduces the freeze-thaw resistance of concrete, unless the concrete has been sufficiently air-entrained. Therefore, to facilitate the successful use of CSC in construction projects, in-depth evaluations of the properties of various types of concrete that can be produced using this new material are needed, including evaluation of mechanical performance, durability, and serviceability characteristics. Additionally, evaluating the pore system of CSC is also essential to achieve a comprehensive understanding of its functional behavior. Therefore, an in-depth investigation of the pore system of the carbonated CSC system was conducted, including assessment of the effects of the pore system on water absorption, which affects the freeze-thaw resistance of concrete. Specifically, the primary purpose of this research was to identify mixture design parameters that resulted in improved freeze-thaw resistance of CSC concrete. To achieve this goal, pore characteristics of the carbonated CSC based systems (pastes and mortars that contained various amounts of aggregate) were evaluated using such techniques as mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and image analysis. The types of the carbonated CSC systems investigated (i.e. paste and mortars) were shown to contain two types of pores: large capillary pores (greater than 30 μm) in the bulk matrix and small capillary pores (5 nm ~ 100 nm) between crystals of calcium carbonate. Reduction of paste content in the CSC mortars resulted in percolation (i.e. establishment of spatial connectivity of the pores) and an in generation of larger pores (i.e., pore with diameters between 0.1 and 4 μm) at the interfacial transition zone (ITZ). In contrast, the ITZ in the OPC mortars was relatively less porous. In addition, the CSC system contained a higher volume of capillary pores as compared to the OPC system. The porosity characteristics of CSC affected its ability to absorb water. The CSC specimens had the highest initial rate of water absorption and absorbed the most water among all tested specimens, including the OPC series with various water-to-cement ratio (w/c) values. These tendencies of the CSC system were somewhat alleviated when the CSC was air-entrained (AE).
Degree
Ph.D.
Advisors
Olek, Purdue University.
Subject Area
Analytical chemistry|Atmospheric sciences|Chemistry|Civil engineering|Hydraulic engineering|Industrial engineering|Materials science|Mechanics
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