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Advantages and disadvantages of polycarboxylate water reducer in application

Polycarboxylate Superplasticizer is a high performance water reducer and a cement dispersant for cement concrete applications. Widely used in highways, bridges, dams, tunnels, high-rise buildings and other projects. The product is environmentally friendly, non-flammable, non-explosive, and can be safely transported by train and car. In many concrete projects, traditional high-efficiency concrete such as naphthalene is less and less able to meet engineering needs due to the limitations of technical performance. A new generation of water reducing agent and polycarboxylate high performance water reducing agent that has attracted much attention at home and abroad. Because it has designed an effective molecular structure based on the mechanism of dispersing cement, it has a super-dispersion type and can prevent concrete slump loss. It does not cause obvious retardation, exerts high plasticizing effect under low dosage, has good fluidity retention, has large freedom of cement to adapt to wide molecular structure, has a large amount of synthetic technology, and has high room for high performance. The effect is remarkable, which can reduce the shrinkage of concrete and the technical properties of extremely low harmful substances, which gives the concrete excellent workability, good strength development, excellent durability, and good performance of polycarboxylate high-performance water reducer. Comprehensive technical performance advantages and environmental protection features meet the needs of modern concrete engineering. Therefore, polycarboxylate superplasticizers are becoming the preferred admixture for the preparation of high performance concrete. According to reports, the use of polycarboxylate admixtures in Japan has accounted for more than 80% of all high-performance admixture products, and North America and Europe account for more than 50%. In China, polycarboxylate water reducers have been successfully applied to large-scale water conservancy, bridges, nuclear power and railway projects in the Three Gorges Dam, Sutong Bridge, Tianwan Nuclear Power Plant, Beijing-Shanghai High-speed Railway, etc., and have achieved remarkable results. At the same time, polycarboxylate water reducing agent also has some problems: 1. Insufficient heat retention in high temperature environment; 2. Strong temperature sensitivity, the same kind of polycarboxylic acid water reducing agent is constructed in different seasons, and the concrete protection property is far apart; 3. Less functional products, it is difficult to meet the requirements of ultra-high, ultra-long-distance concrete pumping, negative temperature construction, preparation of super early strength concrete and high durability of concrete; 4. High viscosity, high blending material, low In the preparation of water-cement ratio concrete, the viscosity of concrete is high, which is not conducive to construction; 5. It is sensitive to the mud content of sand aggregate. 6. The adaptation to the mechanism sand is also poor, and the dosage is sensitive to the construction.

Chapter 8

Dental cements

DEFINITION

A luting cement is a material that bonds, seals or cements objects together (Figure 8.1a–c). It should have low solubility, low viscosity and high fracture resistance. Luting cements are used in dentistry in order to cement crowns, bridges and appliances (e.g. orthodontic appliances) either temporarily or permanently. Depending on the needs of the treatment, the operator will choose the cement based on strength, antibacterial properties, ability to create a good marginal seal, ability to be used with an adhesive, solubility, tensile strength, resistance to wear, ease of manipulation, translucency and operator’s preference. There is not one luting agent that will meet the desired needs in all situations, hence the reason for a large number of products available for this use.

ZINC PHOSPHATE

Although zinc phosphate has a function as a base (see Chapter 7), it may also be used as a luting cement. It will be discussed in this context in this chapter.

Material constituents/composition

Powder Liquid Zinc oxide Phosphoric acid Magnesium oxide Water   Aluminium and zinc ions

Properties

Once the powder and liquid are mixed together, heat is produced, i.e. an exothermic reaction takes place. This reaction speeds up the setting of the material. To control the setting of zinc phosphate, it should always be mixed on a cool, dry glass slab, and the whole surface area of the slab should be used during the mix to minimise heat production. The manipulation technique is very important, as a warm slab, mixing too fast, or contamination by water may speed up the setting time of the material. Incorporating the powder increments too fast or too slow will also affect the setting of zinc phosphate. Zinc phosphate is fast setting and has a moderate to high solubility and low acidity (once set). The pH is 1–2 but the acidity decreases over time (about 24 hours).

Figure 8.1 (a) Teeth prepared for crowns. (b) Crowns cemented in place with a luting cement. (c) Operator holding the crowns in place until the initial set of the desired luting cement. (Photos courtesy of Dr. Shuichitsubura).

• Acidic
• Gives off an exothermic reaction (gives out heat when mixed)
• Strong material (reaches two-thirds of strength in less than one hour)
• May be used as a base (thicker mix) or a luting cement (thinner mix)
• Mixing times may be extended by mixing the material over a large surface area (dissipates the heat given off as a result of the exothermic reaction)
• Mixing on a cool glass slab can also extend the working time and allow for greater powder incorporation (gives the material a higher strength and reduces solubility)
• Care must be taken to not reduce the ratio of powder to liquid as this results in a more soluble, more irritant and weaker material
• 50% of the strength of the material is reached after 10 minutes, reaching its final strength after 24 hours
• Moderate solubility when used as a base and high solubility when used as a luting cement

Advantages

• Long shelf-life
• Low thermo-conductivity
• Rapid setting time
• Low cost
• Long clinical history

Disadvantages

• Does not release fluoride
• Freshly mixed material has a high acidity (reduces with setting), but has the potential to cause pulpal irritation
• Moisture sensitive
• Slight shrinkage during setting
• No adhesive properties
• Brittle

Indications and contraindications for use

Indications

• Permanent cementation of crowns, bridges, inlays, onlays, orthodontic appliances and orthodontic bands

Contraindications

• Zinc phosphate is acidic at the time of placement, and care should be taken to protect the pulp

Trade names

Figure 8.2 (a) DeTray® – Dentsply. (b) Flecks® – Keystone group. (c) Zinc Cement – SS White. (d) Zinc Phosphate Cement – Bosworth.

This is the same manipulation technique as mixing zinc phosphate as a base, except with altered powder/liquid ratios, mixing times and final consistency.

Wearing personal protective equipment:

• Tooth is appropriately prepared and isolated
• Ensure you have a clean, disinfected, cool, dry glass mixing slab
• If the chosen method is to cool the glass slab under cold water, ensure that all moisture is removed prior to dispensing materials

Step 1

• Fluff the zinc phosphate powder in the bottle (shake the bottle, ensuring the lid is securely in place)

Step 2

• Dispense the powder on the glass slab according to the manufacturer’s instructions, replacing the cap immediately after dispensing
• Dispense liquid as per the manufacturer’s instructions, according to the appropriate amount of powder dispensed (more liquid is added for a luting consistency than a base)
• To ensure uniform drops of the liquid, hold the bottle perpendicular to the glass slab during dispensing and immediately replace the cap
• Care must be taken to replace the cap on the liquid once it has been dispensed. The zinc phosphate material is water based, and the water will evaporate if the cap is left off, which will prolong the setting time

Step 3

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• Section powder into increments according to the manufacturer’s instructions (two techniques are shown)

Step 4

• Mix in powder increments at 15-second intervals
• Using a Weston spatula (thin, flexible spatula) incorporate the first mound of powder into the liquid, spatulating in a figure-of-eight motion over the entire surface area of the slab for 15 seconds

Step 5

• Ensure that all the powder has been incorporated into the liquid and add the second powder increment, following the same mixing technique for 15 seconds

Step 6

• Repeat steps 5 and 6 until you have achieved the desired creamy consistency (the mixed material should form a ‘string’ from the spatula to the glass slab when the spatula is lifted approximately an inch (2 cm) above the glass slab)

Step 7

• Using the spatula, gather the material into one area and use a flat plastic instrument to load the restoration or prosthesis to be cemented under the direction of the operator, alternatively pass the material and the flat plastic instrument to the operator for loading

Step 8

• Have some gauze at hand to wipe away excess cement from the flat plastic instrument

Step 9

• Immediately clean the spatula and glass slab; if there is not enough time to clean immediately, immerse the slab and spatula in water until cleaning can take place

Step 10

• 60–90 seconds

Mixing time

• 3–6 minutes

Working time

• 5–14 minutes

Setting time

Figure 8.3 (a) Zinc phosphate set-up. (b) Step 4 – Zinc phosphate dispensed – powder divided into six equal portions. (c) Step 4 – Zinc phosphate dispensed – powder divided into fractional portions (8.3b and 8.3c are two different techniques for dispensing zinc phosphate powder). (d) Step 5 – figure-of-eight mixing motion. (e) Step 7 – luting consistency of zinc phosphate (2cm ‘string’ from glass slab to spatula).

Instruments and materials used in set-up

• Zinc phosphate powder and liquid
• Weston spatula
• Cool, dry glass slab
• Gauze
• Flat plastic instrument

POLYCARBOXYLATE CEMENTS (ZINC POLYCARBOXYLATE CEMENTS)

Polycarboxylate cement is a derivative of zinc phosphate cement in which the zinc phosphate has been replaced with a polyacrylic acid. It has the ability to bond to both enamel and dentine.

Material constituents/composition

Supplied in powder and liquid forms:

Powder Liquid Zinc oxide Polyacrylic acid Magnesium oxide Water

Properties

• Acceptable to mix polycarboxylate cement on either a waxed paper pad or a glass slab
• Mixing polycarboxylate cement on a cooled glass slab enables extension of the working time
• Reaches 80% of its final setting in one hour
• Do not store the liquid in the fridge as this will cause it to gel

Advantages

• Does not release fluoride, although some materials have added properties which allow fluoride release
• Bonds to enamel, dentine and alloys
• Low irritation
• Easy manipulation

Disadvantages

• Short mixing/working times
• Sensitive to manipulation techniques
• Lower compressive strength when compared to zinc phosphate

Indications and contraindications for use

Indications

• Cementation of metal crowns and bridges (also suitable for porcelain fused to metal crowns and bridges)
• Orthodontic bands and appliances

Contraindications

• Does not bond well to untreated gold restorations

Trade names

Trade name Manufacturer Poly-F® Plus (Figure 8.4) (this material has Dentsply added properties to release fluoride)  

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