Modified cements

The ZOE cements are susceptible to modification. Modification by the addition of accelerators has already been discussed in Sections 9.2.4 and 9.2.5. 

One of the oldest additives is rosin (abietic acid) which improves working, hardening rate and strength (Wallace Hansen, 1939 Molnar Skinner, 1942). El-Tahawi Craig (1971) report that hydrogenated rosin inhibits the formation of crystallites. 

Some materials intended for temporary cementation and cavity lining are formulated as two pastes. One paste is formed by blending the zinc oxide powder with a mineral or vegetable oil and the other by mixing an inert filler into the liquid. These cements are much weaker, with 

Sometimes antimicrobial agents such as thymol or 8-hydroxyquinoline may be present (Wilson, 1975b). The latter is also capable of forming a cementitious chelate with zinc. 

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Fegulated-set cement, called jet cement in Japan, is formulated to yield a controlled short setting time, <1 h, and very eady strength (73). It is a modified cement that can be manufactured in a conventional Pordand cement kiln. It incorporates set control and eady strength development components. 

One approach for ameliorating the highly brittle nature of these cements has involved the use of tougher, more ductile fillers (62,63). Another approach for improving the overall properties of traditional glass—ionomer cements involves the development of hybrid cement-composites and resin-modified cements (64—68). 

Portland cement is susceptible to corrosion by CO2 and H2S. The chemical attack by CO2 is called carbonation. A microsample technique has been developed to study the CO2 corrosion in cements, because the corrosion is difficult to monitor with common test procedures [264]. This technique is also advantageous as an accelerated testing method. A polymer-modified cement has been tested in field studies [694]. The addition of silica also improves chemical resistance [146], in particular brine corrosion. 

The presence of heavy metals is often observed to significantly interfere with proper setting of Portland cement. The incorporation of fly ash into Portland cement helps to partially compensate for this set-retardation, and a subsequent supercritical C02 treatment of such modified cement should perform even better. 

The incorporation of fly ash lowers the initial heat evolution during setting, thereby reducing the incidence of cracking and spalling. It is desirable to maintain the modified cement in monolithic form for optimum leach resistance. 

Fly ash, as it is a large-volume industrial waste, is both cheap and abundant, so that there is an economic incentive to use fly-ash-modified cements. In addition, C02 is also produced as a waste by-product of industrial processes (power generation, cement manufacture, etc.), and its permanent sequestration into cement is an added environmental benefit. A fully carbonated Portland cement permanently sequesters about 130 L of C02 per kilogram of cement. Figure 15.8 shows the structural and chemical modifications produced in cemented fly ash microspheres as a result of the supercritical C02 treatment. As is the case with fly ash, kiln dusts are primarily siliceous, so that the same benefits can be derived from their use as modifiers in immobilization and S/S matrices. 

Shellac (described in Vol 6, G187-L under Gum Lac) became a strategic material during WWII, and a mixt of rosin and ethyl cellulose was substituted for it. The modified cement possesses drying time and waterproofing props similar to those of the original PC (Refs 2 3)... 

Figure 5.8, which is the result of extensive research by Rohm Haas (1991), illustrates the comparative durability of polymer-modified cements. A comparative study carried out by Rohm Haas (1989) of different cement modifiers concluded with the following results ... 

Figure 5,8 The comparative durability of polymer-modified cements. - - -. - = unmodified cement -------= acrylic-modified cement ------= styrene-butadiene-rubber-modified...

In polymer-modified cement systems, there occurs air entrapment resulting from the generation of foam while mixing. Air has an adverse effect on the mechanical strength properties of cement mortars as expressed by Feret s relationship ... 

It is important that acrylic-modified cement mortars be marketed based on their end-use for example mortars for concrete repair and for use as ceramic tile adhesives are sold as separate products. Therefore, one should not become confused that a single mortar is used for all of the above-mentioned functions. 

Acrylic-modified cement coatings protect atmospherically exposed reinforced concrete structures from attack by chloride ions, oxygen and water. 

Up till now there is only few safe knowledge about stress-induced deformations (E-modulus, creep) and stress-independent deformations (shrinkage, swelling, temperature-expansion). But due to long experiences with other mortars containing polymers (e.g. lightweight mortars with expanded polystyrene as aggregate, polymer modified cement mortars PCC), basic problems are not to be expected. 

O Ordinary plain cement mortar A Ordinary cement mortar with methyl cellulose SBR polymer modified cement mortar + SBR and superplastidzer modified cement mortar — Curve drawn with hypothesis that there is correlation between two characteristics... 

Polymer modified cement mortar 4 Honeycomb shaped woven net 5 Bonding mortar 6 Joint-filling mortar 7 Ceramic tile... 

EMU Powder 120 FD. [BASF AG] Styrene copolymer powder binder for paints, paper coatings, modifying cement-containing diy mortar. 

M. Lackhoff, X. Prieto, N. Nestle, F. Dehn, R. Niessner, Photocatal3dic activity of semiconductor-modified cement-influence of semiconductor type and cement ageing . Applied Catalysis B Environmental, 43, 205-216, (2003). 

Setting time of an SBR modified cement composition is delayed. However, in actual use, such mortar toppings may often be walked on in 12 to 18 hours, and may take light rubber tire traffic in 48 hours. These times will vary, depending upon the temperature, humidity and the percentage of latex in the mix. Table 2 shows the setting time for SBR latex/cement compositions. ... 

Anti-foam agents are almost always used in SBR latex modified cement compositions because they increase mix density by minimizing air-entrainment, thereby upgrading physical properties. Anti-foam agents may be added to the SBR latex by the manufacturer of the latex and additional amounts may be added by the formulator of the final cementitious mortar composition. The amount of anti-foam added may range from 0.1% anti-foam solids on latex solids up to 3.5%. 

Mixing of all latex modified cement compositions should be held to a minimum, in any circumstance. Antifoam agents can lose their efficiency if they are over-mixed. They tend to become emulsified after continuous shear action and they no longer give adequate performance. ... 

The acrylic resins may be so polymerized that a resultant product may be obtained which can be dispersed in water to form a true polymer emulsion. Such a product is now available as a cement mortar modifier. By dry blending this resin with appropriate cementitious materials, it is possible to formulate a one-package acrylic modified cement mortar. Addition of water to the dry blend will produce a mortar with superior adhesion and physical strength properties. 

In the modification with water-soluble polymers such as cellulose derivatives and polyvinyl alcohol, small amounts of the polymers are added as powders or aqueous solutions to cement mortar and concrete during mixing. Such a modification mainly improves their workability because of the surface activity of the water-soluble polymers, and prevents the dryout phenomena (explained in Ch. 4, Sec. 3.1, Water Retention). The prevention of the dry-out is interpreted in terms of an increase in the viscosity of the water phase in the modified cement mortar and concrete and a sealing effect due to the formation of very thin and water-impervious film in them. In general, the water-soluble polymers hardly contribute to an improvement in the strength of the modified systems. 

Latex Modified Cement Mortars, Dow Latex 460 and Dow Latex 464, p. 42, Midland, Michigan, The Dow Chemical Co. (1965)... 

International Organization for Standardization, ISO 9917-2 Dentistry - Water-Based Cements, Part 2 Resin-Modified Cements, ISO, Geneva, Switzerland, 2010. 

Architectural Institute of Japan (AIJ) Recommendation and Guideline (Japan) PMC, PC and PIC Recommendation for practice of concrete-polymer composites Recommendation for practice of survey, diagnosis and repair for deterioration of reinforce concrete structures Guideline for polymer-modified cement waterproofing membrane work... 

Architectural Institute of Japan (AU) Guideline for Polymer-Modified Cement Waterproofing Membrane Work (in Japanese), AU, Tokyo (2006). 

Abstract. The presence of water-soluble polymers affects the microstructure of polymer-modified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks. 

The paper describes the backscattered electron image analysis procedure, as well as the effects on thicloiess and composition of the ITZ and the cohesion and cracking of the bulk cement paste of water-soluble polymer-modified cement mortar. 

Figure 6 BSE image of 1 % PVAA modified cement mortar. Black lines indicate the 20pm distance from the sand particle surface. Lines are drawn manually.

Shrinkage properties of poiymer-modified cement mortars (PCM)... 

Keywords shrinkage, PCC, PCM, polymer modified, cement stone, adhesive bond, longitudinal change behaviour, shrinkage stress, porosity... 

Determination of shrinkage on polymer-modified cement stones... 

After the investigations on PCCs in modified shrinkage channels had not led to the expected results, the aim was to verify the gained findings by means of experiments on polymer-modified cement stones (Fig. 3). CEM I - cement stone recipes were used. The water requirement of the stone granulation as to its w/c ratio was taken into account. 

Figure 4 Shrinkage of cement stone, with polymer 1 modified cement stone (ZSl) and with polymer 2 modified cement stone (ZS2)...

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