Binder Distribution in Macro‐Defect‐Free Cements: Relation between Percolative Properties and Moisture Absorption Kinetics

Abstract

Macro-defect-free (MDF) cement is fabricated from a calcium aluminate cement and a poly(viny1 alcohol-acetate) (PVA) copolymer. For the composites studied, it was determined that the interphase regions comprised 63 vol% of the total binder content, while the bulk PVA regions comprised 37 vol% of this phase. Mercury intrusion porosimetry showed that a bimodal pore size distribution developed as binder was removed in increasing amounts from heattreated samples. Larger pores with a characteristic diameter above 30 nm resulted from the removal of bulk PVA, whereas smaller pores approximately 5 nm in size resulted from the removal of water and PVA from the interphase regions. Simulation results obtained from a hard-corehoftshell continuum percolation model of the MDF microstructure indicate that both the bulk PVA and interphase regions form percolative pathways through the system. Dramatic changes in both moisture absorption kinetics and flexural strength were observed only when a percolative network of larger pores was present in these composites. Hence, the bulk polymer regions are the dominant transport pathway for moisture in MDF cement. Based on this knowledge, processing guidelines have been developed to improve the moisture resistance of these materials.

Contribution of materials chemistry to the knowledge of macro-defect-free (MDF) materials

“…The thermogravimetric determination of the relative contents of hydrated and cross-linked phases in initial and moisture-affected MDF specimens [30][31][32] demonstrates that moisture uptake is accompanied only by increasing contents of cementitious calcium aluminosilicate hydrates and CaCO 3 . The data of MAS-NMR spectroscopy and thermal analysis are in mutual agreement also with earlier reports [7][8][9]13,17] regarding both the persistence of cross-links in the moist environment and the availability of the residual cementitious hydrates and cement particles toward secondary reactions with H 2 O (g) and CO 2 (g) from the moist environment (cf. Fig.…”

“…It is generally accepted that moisture enters the MDF material through the system of pores in a polymer-containing interface and approaches the residual unhydrated cement particles [7][8][9]11,13,17]. In this diffusion process, the only consequence is that the secondary hydrates destroy part of the interphase region, thereby degrading this part of the original interface and the binding matrix.…”

“…This procedure, in addition to eliminating large pores, also helps to induce chemical interactions of polymer and cement [6]. It was also recognized that much of the mechanical strength of these materials is due to the mechanically induced chemical changes [7,8]. Details about the dispute over the correctness of the term "macro-defect-free (MDF)" can be found elsewhere [2,7].…”

“…Nonhydrated particles of the cement become constituents of the developed structure, held together by a polymeric-inorganic matrix [19]. The bonding forces formed in this way are quite strong, and thus the failure of MDF material tends to occur within the polymer phase or clinker grains rather than at the interface [7][8][9]13,17]. Both the introduction of a crosslinked atomic-level structure and the modified particle interfaces conform to the model of functional…”

Toughening the Macro Defect Free (MDF) Cements

“…the flexural strength or order of 200 MPa) (1,2,3). the flexural strength or order of 200 MPa) (1,2,3).…”