Thursday, May 3, 2007

FEATI UNIVERSITY-CIVIL ENGINEERING DEPARTMENT THESIS AND RESEARCH 2007

ABSTRACT

Title : “SPRAYABLE CEMENT-BASED COMPOSITES FOR SEWER REPAIR”


Researchers : BRIAN BAILEY K. ARQUERO, VALENTINO AQUINO, MELBORNE M.
BOSTON, FELIX B. CERDA and RICHARD S. USI

Adviser : Prof. Gerard V. Paguibitan

School : FEATI University

Date : March 2007

Degree : Bachelor of Science in Civil Engineering

A sprayed-on phosphate cement coating formed from the combination and reaction of a phosphoric acid solution and a base metal solution. The acid solution and base solution may be intermixed prior to spraying, during spraying, or on a substrate. The curing reaction rate of the phosphate cement coating and its final physical properties may be controlled by adding various retardants, accelerants, reducers, wetting agents, superplasticizers, buffers, water reducers, adhesive agents, hardening agents, and/or sequestrates to the precursor solutions. The curing rate and properties of the cement coating may be further controlled by adjusting the temperature of the precursor solutions and/or the target substrate


SUMMARY OF FINDINGS
The present invention relates to a sprayable phosphate cement material with a controlled curing reaction time and viscosity. The cement composition includes a phosphoric acid component, a metallic alkali or base component, and water. The phosphoric acid component and the metallic base component are mixed with water separately to form component slurries (i.e., an acid slurry and a base slurry), and each slurry is maintained separately until the application step. The acid and base slurries may each be thought of a first and/or second precursor constituent of the phosphate cement composition. Depending upon the order of usage either could be the first or second constituent. The application step preferentially involves first coating a desired surface with the phosphoric acid mixture and then with the metallic base slurry. Alternately, the application step may involve first coating the desired surface with the base slurry and then the phosphoric acid solution, or simultaneously spraying the desired surface with the both the phosphoric acid solution and the base slurry from separate sources, wherein the acid and base components mix in transit or in situ on the desired surface.
Preferentially water, metallic base and one or more retardants, emulsifiers, deflocculates, sequestrates and/or dispersants are added to cold water and mixed in with the silica source(s) to form a slurry. Next, the liquid phosphoric acid or phosphate salts are quickly mixed into the slurry, and the slurry is then preferably immediately sprayed onto a desired target, although the use of cold precursors and strong retarders can extend the shelf-life of the mixed phosphate cement slurry such that immediate spraying is not a requirement. Alternately, with the use of strong retardant additives, dry powder phosphate salts, silica sources, and metallic oxide alkali powders can be mixed together to form a slurry having a long enough shelf-life to make spraying possible.
After the phosphoric acid and the metallic base components are mixed, the phosphate cement slurry is preferably used promptly. The individual cement components may be mixed in spray cans or any clean containers and mixed right on the job, preferably in a cool environment. Preferably, the water used in the mixture is added cold in order to retard the progression of the exothermic acid-base phosphate cement-forming reaction.
Alternately, the phosphoric acid and/or the base coat may be brushed on, with the other coat also either sprayed or brushed on. One coat of the slurry with acid and base and silica sources is usually enough to provide good coverage, although subsequent coats are easy to apply and may be applied immediately after the first coat is applied. In the preferred embodiment, the phosphoric acid coating is applied first. More preferably, the phosphoric acid coat contains a silica source admixed therein. Alternatively, the base coating is applied, preferably by spraying, such that it penetrates the existing phosphoric acid layer and allows the cementitious reaction to begin. The reaction progresses rapidly since the reactants are spread as a thin coating over a large surface area. Also, the heat generated by the reaction is dissipated quickly, again because the reaction occurs over a large area and is generated in a thinly spread film having a very high surface area to volume ratio. In an alternate embodiment, the base coat is applied first, followed by the phosphoric acid coat, thereby catalyzing the in-place base slurry.Some preferred phosphoric acid components include potassium phosphate, magnesium phosphate, sodium phosphate, aluminum phosphate, ammonium phosphate, iron phosphate, zinc phosphate and combinations thereof. By using controlled combinations of different phosphate salts, each one spiking in temperature at a different time, the overall temperature profile of the composition is controlled so as to substantially minimize the maximum temperature reached. Therefore, the controlled combination of the above-listed phosphate salts has the same effect as the addition of a temperature retarder. In addition the resultant mix of different shaped and size crystals can yield denser packing and gives a "granite effect" to a composition formed has improved fracture strength as cracks cannot as easily propagate through a composition with no
common cleavage lines. The phosphoric acid component may be either a solid (preferably a powder) or a liquid. Some preferred metallic base components include magnesium oxide, dolomite, zinc oxide, aluminum oxide, potash, calcium oxide, lithium carbonate, barium carbonate, molybdenum oxide, calcium hydroxide, aluminum hydroxide, tin oxide, nickel oxide, magnesium hydroxide, iron oxide, titanium oxide, dolomite, manganese oxide, and zirconium oxide, zirconium hydroxide and wood ash. One means of controlling the reaction rate of the cement is by controlling the temperatures of the cement components. The colder one or both of the components are kept, the slower the reaction progresses. One way of controlling the temperature of the phosphoric acid component and the metallic base component is by cooling the water used in the admixture of each. Another means of temperature control is cooling one or both of the components' containers and/or the spraying apparatus; such is in an ice bath or by refrigeration. Another means of controlling the reaction rate is to keep the surface to be sprayed cold, such as with ice or cold water or dry ice. Various combinations of these cooling techniques may be employed to obtain maximum temperature control of the reaction.
Another means of controlling the reaction rate is the use of the retarders (surfactants, retarders, dispersants, water reducers, super plasticizers, and sequestrates) in the cement-forming components. Preferably, the retarders are added to the water before it is added to the powdered phosphoric acid solution and/or the metallic base precursors (minerals, metal oxides, and the like) to form the base slurry. This approach provides that no water contacts the component materials (usually powders) without a dispersant/retarders present. Since cement-forming powders are reactive, the retarders slow the setting time by keeping them apart, eliminating or reducing rapid agglomeration and aiding to control the reaction of the cement.
Another embodiment of this technology contemplates pre-mixing the phosphoric acid solution and the metallic base slurry before spraying. In this embodiment, it becomes necessary to reduce the reaction rate of the cement sufficiently to keep the mixed cement slurry from becoming too viscous to remain sprayable as a thin coating. This is achieved through cooling the mixed solution, by using chilled water and/or refrigeration of the container and sprayer and/or through the use of retarders. Retarders are used to keep the component particles dispersed in order to slow the chemical reaction and prevent agglomerations from forming inside the sprayer. Another method of controlling the speed of the phosphate cement-forming reaction is through the use of pH buffers to regulate the pH of the solution and thereby its reaction rate. Yet another means of regulating the reaction rate is by controlling the concentration of the acid and base components or, conversely, the water component. Increasing the water concentration will slow the reaction rate of the cement. Traditionally phosphate cement manufacturers want a low water/cement ratio as they believe that like Portland cement, the lower the w/c ratio the greater the compressive strength. Through the addition of more mix water, the crystals continue to grow/form as long as there is unreacted acid

CONCLUSION
It is preferred that the phosphate cement be mixed thoroughly. If even stronger and less porous cement is desired, it is more preferred that a plastic resin and/or catalyst/initiator be admixed therein to yield strong phosphate cement that’s less porous and more water resistant. The additions of MMA (methyl methacrylate), EMA (ethyl methacrylate), BMA (butyl methyl methacrylate) and other epoxies, urethanes and plastics can also yield harder or tougher cements. Moreover, the addition of an emulsifier helps to better disperse the above additives in the cementitious mixture. Phosphate cements cure exothermically, generating substantial amounts of heat quickly. The heat generated by the curing phosphate cement likewise speeds the curing of endothermic plastics and plastic coatings, such as 2 part epoxies. Additionally, the heat generated by the curing phosphate cements is often sufficient to raise the energy of a system containing an exothermically curing component enough to initiate the reaction (in other words, if the system includes a component that requires an predetermined energy influx in order to begin reacting, the heat spike produced by the curing phosphate cement usually exceeds the predetermined energy influx requirement). phate cements are compatible with catalysts such as BPO.

RECOMMENDATION
It is preferred that the sprayed surface first be cleaned in order to optimize the bonding of the reactive phosphate cement. It is not necessary to abrade or acid etch a surface in preparation for cement spraying, although a wash with phosphoric acid (or other acids) or NaOH or KOH solutions does tend to enhance bonding. Other cementitious or plastic based products for overlaying concrete require that the concrete surface first be cleaned and then either etched or abraded. A smooth surface finish may be produced by limiting the size and/or amount of the aggregate component of the cement. Also, the additions of diatomaceous earth or potassium silicates, fumed silica, colloidal silica, silica flour or the like may improve the surface finish without substantially diminishing the cement's strength or chemical stability. The aforementioned clays and diatomaceous earth and combinations thereof retard the initial setting or curing of the phosphate cement, as well as enhancing the flow ability and workability of the cement (i.e., producing cement that is self-leveling and self-consolidating). Diatomaceous earth and/or bentonite additions (preferably at levels of about 0.1 to 4%) may be thoroughly mixed into the cement precursor to achieve the result of reducing the number and size of surface pores. Likewise, the surface finish may be controlled by the additions of dispersants and/or sequestrates that control the distribution of the aggregates
in the mix.

1 comment:

Unknown said...

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