MDF cement

As MDF cements generally have very high flexural strength, it is expected that ideal composite materials can be made by reinforcing them with fibers. An example of the development of high flexural strength by the fiber reinforcement of MDF cements is demonstrated in Table 8.1.  

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Figure 9.12. Bend strengths of ordinary and MDF cements (after Birchall el al. 1982),...

In terms of reinforcement, hydrocalumite/polymer nanocomposites may also be considered as promising in the field of cement-related materials. The so-called macro-defect-free (MDF) cements are based on the filHng by polymer of the macroscopic voids responsible for the breakdown of cement. The hydrocalumite phase is formed during the hydration of cement and is known to readily incorporate polymers between its layers [71-75]. 

The tensile strength of Portland cements is limited by the presence of relatively large pores (0.1-1.0 mm). Modern research has produced cements that are substantially free of large pores (macro-defect-free or MDF cements) and have very promising mechanical properties for example, MDF cements can be formed into cement springs.70 d... 

Kendall el al. (K64) considered that MDF cements owed their high strengths primarily to the absence of large flaws, which they held to be the reason for the relative weakness of normal cement pastes. This view has been challenged (B115,K65,E4). Beaudoin and Feldman (B115) discussed ways in which stress concentrations were reduced in very strong cement pastes, and both they and Eden and Bailey (E4) considered that in MDF pastes the polymer plays a major role. 

Recently, it has been found that the use of alumina cement and autoclave cure after moist and heat cure are most effective for improving the water resistance of MDF cements.P Figure 8.6 shows the flexural strength of heat plus autoclave cured MDF cements using the alumina cement before and after 48-hour water immersion. 

Figure 8.6 Moist curing period vs. flexural strength of heat plus autoclave cured MDF cements using alumina cement before and after 48-hour water immersion.

Table 8,1 Comparison of Phj ical Properties of Fiber Reinforced MDF Cement and Aluminum, f 199L, Materials Research Society, reprinted with permission.)... 

Development of MDF cement using water-soluble resins [33-34]... 

Development of highly durable (or water-resistant) MDF cement [74]... 

Y. Ohama, T. Kobayashi and K. Demura Improvement in water resistance of macrodefect-free (MDF) cements by application of hot press molding process (in Japanese). Journal of the Society of Materials Science, Japan Vol. 50, No.8 (2001), pp.873-876. 

Different cement and polymer types [2, 7-9] can be used for the production of MDF cement. However, highest flexural strengths are always obtained when calcium alumina cements (CAC) as a... 

Table 1 Mix proportions of MDF cements produced with diglycidyl ethers of bisphenol A and F...

Production Procedures. Birchall et al. [2] proposed the MDF cement production method by using a two-roll mill and the method was optimized by Russell [3]. However, obtaining the product in a sheet form is not that easy and procedure can vary with respect to the properties of the two-roll mill and the used materials. Therefore, an optimized process was developed before starting the study [15]. At least four different 100x100x1.7 mm sheets were trimmed with a knife after production. Then, the sheets were pressed at 80°C and 5 MPa for 10 min between 2 sheets of nylon... 

Testing. In order to characterize the obtained series, the MDF cement specimens were subjected to mechanical tests, contact angle determinations and atomic force microscopy (AFM). 

The surface topography of the obtained MDF cement specimens was studied by AFM (NT-MDT model NTGRA PREVIA EC), operated at ambient atmosphere. AFM data were collected in semi contact mode using a silicone cantilever (CSG 10, force constant 0.15 N/m, tip radius 10 nm). The average roughness was calculated with the AFM manufacturer s provided software on images of 5 x 5 pm scan size. 

Biaxial Flexural Strength of MDF Cements with Diglycidyl Ethers of Bisphenol A and F. 

Figure 1 Biaxial flexural strengths of MDF cements produced with diglycidyl ether of bisphenol A...

It seems that the addition of epoxy resin improves wet strength for some MDF cements. It was reported that epoxy resin can harden in the presence of hydroxide ions (OIT) from the cement [19] and as consequence, an epoxy network may form, which may decrease degradation. 

Dry MDF cements means the ones stored in dry conditions (in desiccators), while the water-stored MDF cements are the samples stored in water for 28 d. 

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