Cement matrix

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

Liquid resin-based systems which, like the chemical surface hardeners, penetrate into the surface of a concrete topping or directly finished slab and protect the acid-susceptible cement matrix from attack and, at the same time, strengthen the surface of the concrete are now being increasingly used. These in-surface seals leave the slip resistance of the concrete floor virtually unchanged but the treated floors are easier to clean and are more durable. 

Unfortunately, the protection provided by concrete can be overcome by contamination of the concrete by chloride. Chloride, when entering the concrete as a contaminant of the mix constituents, is to a large extent (about 90%) complexed within the cement matrix and only a small percentage is free in the pore solutions. The present codes of practice ban the use of chloride-bearing additives and restrict the amount of chloride present in concrete. For normally reinforced concrete made with ordinary Portland cement it should be not more than 0.4% chloride ion with respect to the cement content weight/weight. 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

Base -I- Add = Salt -I-Water (powder) (liquid) (cement matrix)... 

The cement sets as the result of an acid-base reaction between a zinc oxide dental powder and a poly(alkenoic acid). The pH increases and an insoluble amorphous salt is formed which acts as the cement matrix. A general account of the gelation processes is given in Section 5.4. 

X-ray diffraction shows that both the cement matrix and the salt are amorphous (Wilson, 1982 Smith, 1971 Steinke et al., 1988). On the basis of chemical analysis, Wilson (1982) assigned the following empirical formula to the zinc polyacrylate salt ... 

Figure 5.11 The time-dependent variation, in setting and hardening cements, of the concentration of soluble ions Al ", Ca, F and PO (expressed as P Oj). These ions are released from the glass powder into the cement matrix (Crisp Wilson, 1974b).

The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility oil-1 % compared with the near insolubility of silica gel, it would... 

Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix. Journal of Biomedical Materials Research, 16, 549-57. 

Finch Sharp (1989) found the mole ratio of MgO to A1(H2P04)3 to be an important parameter that affected both the reaction rate and the nature of the reaction products. The critical mole ratio was 2 1. When the ratio was less than 2 1 cements were not formed at all, and when it was exactly 2 1 the paste set slowly and always remained tacky. Further increases in the ratio caused cements to set faster with greater evolution of heat. Finch Sharp (1989) also found that this ratio affected the proportion of crystalline phase to amorphous phase in the cement matrix. The proportion of newberyite in the matrix reached a maximum when the MgO/A1(H2P04)3 ratio was 4 1 and decreased to a low level when the ratio was 8 1. 

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner Phillips, 1960). Wilson Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson Mesley, 1968). 

These findings support the view that during the reaction ions are extracted from the surface of the glass particles, migrate to the aqueous phase where they form the matrix, and leave a silica gel relict. This explains why the glass particles appear to be unattacked when examined under the microscope. The presence of both A1 and P in the cementing matrix and the... 

Eugenol can also be extracted from the cement matrix by methanol (Molnar, 1967) this is further evidence of the weakness of the chelate, which is decomposed during the extraction. 

Figure 9.5a Electronmicrograph of a ZOE cement matrix, showing zinc oxide particles covered by zinc eugenolate (Wilson, Clinton Miller, 1973).

Cementite, 4 647 23 272, 274, 275 Cement kiln dust, 5 492, 495 Cement kilns, 13 178 scrap tire fuel in, 21 464 Cement-matrix composites, 26 751 Cement plants, sulfuric acid and, 23 787 Cement rock, raw material for cement,... 

Kim, J.K., Zhou, L.M. and Mai, Y.W, (1993a), Interfacial debonding and pull-out stresses part 111, Interfacial properties of cement matrix composites. J. Mater. Sci. 28, 3923-3930. 

The descriptions presented in the foregoing sections are concerned mainly with composites containing brittle fibers and brittle matrices. If the composite contains ductile fibers or matrix material, the work of plastic deformation of the composite constituents must also be taken into account in the total fracture toughness equation. If a composite contains a brittle matrix reinforced with ductile libers, such as steel wire-cement matrix systems, the fracture toughness of the composite is derived significantly from the work done in plastically shearing the fiber as it is extracted from the cracked matrix. The work done due to the plastic flow of fiber over a distance on either side of the matrix fracture plane, which is of the order of the fiber diameter d, is given by (Tetelman, 1969)... 

The recorded data on lignosulfonate water-reducing agents indicate that, as far as freeze-thaw durability is concerned, because of the low water-cement ratios possible, an enhancement to the durability will invariably be obtained. When the admixtures are used to effect a reduction in the cement content, there are strong indications that a considerable enhancement of durability is obtained, presumably due to a reduction in the cement matrix which is the part of the concrete susceptible to frost damage. The higher aggregate content would therefore allow easier dissipation of stresses. 

The properties exhibited by polyelectrolytes make them nearly-ideal candidates for dental material formulations. Dental polyelectrolytes are generally considered to be nontoxic and are able to adsorb chemically to the hydrophilic surface of tooth material through ionic interactions. Ionic cross-linking of the polyelectrolyte with multivalent cations (Zn2+, Mg2+, Al3+, Ca2+) results in the formation of a rigid and insoluble cement matrix. The stability and strength of the cement is attributed to the fact that, if a bond is broken, it can be reformed as long as the other bonds are maintained. Even today, polyelectrolytes are the only materials which are known with certainty to form a bond, which is stable with time, to tooth material [120]. In addition to long-term stability, many polyelectrolytes are translucent and possess cariostatic properties [121]. 

In general terms, the binding of heavy metals and metalloid ions in the cement matrix is quite well understood. There appear to be three basic types of binding mechanism (e.g., Cocke Mollah 1993 Cougar et al. 1996 Glasser 1997). A metal ion may be ... 

Precipitated in the alkaline cement matrix as an oxide, mixed oxide or as other solid... 

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