WO2017065703A1

WO2017065703A1 - Production of building materials from high free lime content fly ash

Info

Publication number: WO2017065703A1
Application number: PCT/TH2016/000083
Authority: WO
Inventors: Narissara Bussayajarn; Pornlert PIYA-AREETHAM
Original assignee: Siam Cement Public Co Ltd
Current assignee: Siam Cement Public Co Ltd
Priority date: 2015-10-15
Filing date: 2016-10-12
Publication date: 2017-04-20
Anticipated expiration: 2018-04-15

Abstract

An embodiment of the invention is a building material comprising an aggregate proportion, a cement proportion and a fly ash composition. The fly ash composition includes a first proportion of high-free lime fly ash and a second proportion of low-free lime fly ash to maintain an acceptable range of free lime. A mixture made from this fly ash composition can be pressed at low pressures. Cement can be added to the mixture. A steam-curing process can be used to obtain building materials from fly ash.

Description

PRODUCTION OF BUILDING MATERIALS FROM HIGH FREE LIME

CONTENT FLY ASH

Technical Field The present disclosure relates to building materials. More particularly, the present disclosure relates to the production of building materials from high free lime content fly ash.

Background Art

Fly ash particles are formed by the combustion of coal and biomass or other solid fuels such as wood waste, rice husks, bagasse, palm residue, corn, cassava and palm leaf. Fly ash particle size is about 0.5 to 300 microns. Fly ash is not acutely toxic or harmful but it can in the long term affect the respiratory system. Landfill disposal is one way to manage fly ash, however: 1) the space for landfill is limited; 2) fly ash dust is formed during the landfill disposal; and 3) groundwater can be contaminated by fly ash leachate. Fly ash can be used in concrete mix, in cement or building materials. However, the most suitable fly ash for those applications is fly ash with small size and spherical physical shape.

Fly ash can be created by commercial combustion processes such as Pulverized Coal Combustion (PCC) at high temperature (see 1-1 of FIG. 1) and Circulating Fluidized Bed combustion (CFBC) at lower temperatures (see 2-1 of FIG. 2). CFBC is a clean-energy technology that can use low quality of coal, biomass and solid fuel. With CFBC, the sulfur dioxide (S0₂) and nitrogen oxides (NO_x) emission are lower than other combustion processes due to the low temperature operation (800-900 ° C). The low amount of S0₂ in flue gas can be scrubbed by adding limestone into the combustion process. This addition can cause the presence of gypsum (CaS0₄), free calcium oxide (free lime or CaO) and sulfur trioxide (S0₃) in fly ash. Therefore, the CFBC fly ash is sensitive to pozzolanic reaction, can react with water on its own (self-cementitious), and may reduce the strength of products. If fly ash is used as a building material, the high sulfur trioxide (S0₃) in fly ash can cause the expansion of the building materials over time. Thus, the fly ash containing high free lime and sulfur trioxide is not typically suitable for the production of building materials. The more suitable free lime and S0₃ quantity of fly ash for cement industry, concrete ready mix, and building materials should be less than 4% and 5%, respectively. Research has previously disclosed use of fly ash with high free lime (more than 4%) and high sulfur trioxide (more than 5%), using a high pressing pressure. For example, Chinese patent No. CN 100425561 C has developed a formulation to produce building materials from CFBC fly ash, with high free calcium oxide and sulfur trioxide, and steel slag with pressing pressure about 100 MPa. Chinese Patent No. CN101805162A has developed a formula to produce building materials from CFBC fly ash mixed with gypsum and lime by applying pressing pressure between 20-25 MPa. These techniques require high pressing pressure and therefore high energy usage in the production of building materials from fly ash.

Summary A primary embodiment of the invention is a process for the production of building materials using at least a proportion of high free lime fly ash in the fly ash composition. The high free lime fly ash is mixed with low free lime fly ash to maintain a specific range of free lime and sulfur trioxide in the fly ash composition.

In a second embodiment, low pressing pressure is used to conserve energy consumption in the production process, extend machine lifetime and reduce maintenance costs.

In a third embodiment of the invention, cement is also included in the building material, in the range of 0-20% by dry weight of the mixture.

Brief Description of the Drawings

Embodiments of the present disclosure are described herein with reference to the drawings in which:

FIG. 1 pictures PCC fly ash under a microscope.

FIG. 2 pictures CFCB fly ash under a microscope.

FIG. 3 is a flowchart representing an embodiment of invention. Detailed Description

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments can be utilized, and other changes can be made, without departing from the spirit or scope of the subject matter presented herein. Unless specified otherwise, the terms "comprising," "comprise," "including" and "include" used herein, and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, un-recited elements. Steps may be conducting in the order recited in independent or dependent claims or a different order.

A primary embodiment of the invention is a process for the production of building materials using at least a proportion of high free lime fly ash in the fly ash composition. The high free lime fly ash is mixed with low free lime fly ash to maintain a specific range of free lime and sulfur trioxide in the fly ash composition.

In a second embodiment, low pressure pressing is used to conserve energy consumption in the production process, extend machine lifetime and reduce maintenance costs.

In a third embodiment of the invention, cement is also included in the building material, in the range of 0-20% by dry weight of the building materials. FIG. 3 represents a flowchart of an embodiment of the invention's process 3-1. The first step 3A is to create a mixture containing an aggregate, a cement, water and a fly ash composition. The second step 3B is to incubate the mixture in the temperature range of 25-45 ° C. The third step 3C is to press the mixture at pressing pressure in the range of 5-15 MPa to create green brick. The fourth step 3D is to cure the green brick in autoclave with temperature range of 150- 250 ° C.

In first embodiment of the invention, for the fly ash using in the process of slurry mixing (fly ash + water), fly ash can be prepared by mixing two types of fly ash including 1) high-free lime fly ash, 2) low-free lime fly ash. The fly ash come from Circulating fluidized bed combustion (CFBC) or Pulverized Coal Combustion (PCC) processes which coal and/or biomass is used as fuel. The amount of free lime and sulfur trioxide of "high-free lime fly ash" are 6-20% and 7- 12%, respectively, while that of "low-free lime fly ash" are 0-6% and 0-7%, respectively. Both types of fly ash is blended together, controlling the amount of free lime and sulfur trioxide of the blended fly ash between 0-12% and 0-1 1%, respectively. The high free lime fly ash can be in the range of 25-75% by dry weight of the total fly ash composition. Then, the blended fly ash is mixed with water to make a slurry, where appropriate ratio between fly ash and water is 1 to 10 or 10% by dry weight of dry fly ash. For mixing aggregate with the slurry, according to this embodiment of the invention, slurry is mixed with coarse sand, bottom ash or slag from steel. The compositions of mixture consist of 35-75% of fly ash and 20-60% of aggregate. The particle sizes of aggregate is between 0.16 to 5 mm. In addition, this embodiment of the invention can use bottom ash, steel slag to replace some or all of coarse sand. The ratio for steel slag or bottom ash or steel slag is 30-55% by dry weight. Cement can be selected from Portland cement, or mixed cement, or white cement, or white lime, or lime cement etc. Cement can reduce water absorption, increase strength, and reduce the shrinkage of the building materials. The ratio of cement can be 0 to 20% by dry weight, or preferably between 10-15% by dry weight. The suitable composition for building material using fly ash are: fly ash = 45-50%, sand (or another aggregate) = 40-50%, cement = 5-10%. The mixing process can be performed using forge-type mixer or vertical cone mixer or twin shaft mixer. The water is added to the mixture with the ratio of 5-15% by weight, or the optimal ratio is 10-12% by weight.

After mixing, the mixture is pre-cured at a temperature of 25-45 °C in a closed system to retard pozzolanic reaction, and prevent serious reactions from free calcium (free lime, CaO) during autoclave process, which lead to be cracked inside construction specimens and result in reduction of the strength later. The pre-curing period of 1 -5 hours is appropriate, or the optimum pre-curing period is 1.0-2.0 hours. After pre-curing process, the mixture is grinded again using a grinder roller (Wheel mill) with the moisture in the mixture around 8-10%. The grinding process can separate hard particle from the mixture. And the metal in the mixture can be eliminated through magnetic metal separation unit.

The pre-cured mixture, after grinding and separation, is formed to become building material by the hydraulic press with pressing pressure of 5-15 MPa, or the optimum pressing pressure is 8- 10 MPa. Normally, for the building material production using fly ash, pressing pressure less than 10 MPa would cause the building material to have low density which affect the strength of the product, while pressing pressure more than 40 MPa would make the material density too high, and such material would be difficult to use for conveying to a high level, cutting, and drilling. However, the process in this embodiment of the invention can use pressing pressure lower than 10 MPa, and the building material still has high strength, durable, and suitable for use. This is a result of a mixture of fly ash with cement addition for increasing strength and durability, and decreasing water absorption of the building material. The reduction of the pressing pressure for the hydraulic press can reduce the energy consumption and machine lifetime in the production of building materials from fly ash.

After pressing, the specimens is incubated at a temperature of 25-45 °C to reduce undesired reactions from free calcium (free lime, CaO) during a steam curing step using autoclave process with the temperature of 150-250 °C, which lead to be cracked on construction specimens and result in reduction of the strength and durability.

In this application, a steam curing step and autoclave process can be used interchangeably.

Production steps for an embodiment can include:

a. Fly ash is mixed with water to make a slurry (Slurry), where fly ash containing 0-1 1% free calcium oxide (free lime, CaO) and 0-12% sulfur trioxide (S03). The amount of fly ash is 40-65% by dry weight.

b. Sand or bottom ash or slag from steel factory and slurry is mixed in ratio of 30-55 % by of dry weight.

c. Cement and water is filled to the mixture with a ratio of 0 to 20% and 5-15% by dry weight, respectively.

d. Pre-curing temperature of 25-45 ° C.

e. Pre-cured mixture is formed by the pressing unit with pressing pressure of 5-15 MPa, called green brick.

f. After pressing, the green brick is incubated at a temperature of 25-45 ° C.

g. Green brick is steam-cured in the autoclave process with the temperature of 150-250 ° C to obtain building materials from fly ash. Table 1 lists example brick quality compressive strength and water adsorption for various embodiments of the invention with different raw material composition, pressing pressure, autoclave temperature.

Table 1

While various aspects and embodiments have been disclosed herein, it will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit of the invention being indicated by the appended claims.

Claims

1. A process for production of building materials from fly ash comprising the steps of:

(a) mixing a mixture comprising:

(1) an aggregate in the range of 20-60% by dry weight of the mixture;

(2) a cement in the range of 0-20% by dry weight of the mixture;

(3) a water component in the range of 5-15% by weight of the mixture.

(4) a fly ash composition in the range of 35-75% by dry weight of the mixture, wherein in the fly ash composition includes:

calcium oxide in the range of 0- 1 1 % by weight of the fly ash

composition; and

sulfur trioxide in the range of 0-12% by weight of the fly ash

composition; and

(b) press the mixture in the pressure range of 5-15 MPa.

2. The process of claim 1 , further comprising an incubation step after the mixing of the mixture, wherein the step of incubating the mixture is conducted in the temperature range of 25- 45 °C.

3. The process of one of claims 1-2, wherein: the aggregate is in the range of 30-55% by dry weight of the mixture;

the fly ash composition is in the range of 40-65% dry weight of the mixture.

4. The process of one of claims 1-3, wherein the step of pressing the mixture is conducted in the pressure range of 8- 10 MPa.

5. The process of one of claims 1 -4, further comprising the step of steam-curing the mixture in the temperature range of 150-250 °C.

6. The process of claim 5, wherein the step of steam-curing is conducted in the temperature range of 190-210 °C.

7. The process of one of claims 1-6, wherein the aggregate is selected from the group consisting of sand, bottom ash, blast-furnace slag, and any combination thereof.

8. The process of one of claims 1-7, wherein the fly ash composition includes a first proportion of high-free lime fly ash and a second proportion of low-free lime fly ash.

9. The process of claim 8,

wherein the first proportion has calcium oxide in the range of 6-20% by dry weight of the first proportion; and

wherein the first proportion has sulfur trioxide in the range of 7-12% by dry weight of the first proportion.

10. The process of one of claims 8-9,

wherein the first proportion is in the range of 25 to 75% by dry weight of the fly ash composition.

1 1. The process of one of claims 8-10,

wherein the first proportion is obtained from an incinerator using fuel selected from the group consisting of coal, biomass and any combination thereof.

12. The process of one of claims 8-11,

wherein the second proportion has calcium oxide in the range of 0-5% by dry weight of the second proportion; and

wherein the second proportion has sulfur trioxide in the range of 0-8% by dry weight of the second proportion.

13. The process of one of claims 1-12, wherein the the cement is selected from the group consisting of Portland cement, mixed cement, white cement, lime cement, calcium oxide, and any combination thereof.

14. The process of one of claims 1-13, wherein the cement is in the range of 10-15%» by dry weight of the slurry.

15. A building material comprising:

(a) an aggregate proportion;

(b) a cement proportion; and

(c) a fly ash composition including a first proportion of high-free lime fly ash and a second proportion of low-free lime fly ash: wherein the first proportion has calcium oxide in the range of 6-20% by dry weight of the first proportion; and

16. The building material of claim 15, wherein the first proportion is in the range of 25 to 75% by dry weight of the fly ash composition.

17. The building material of one of claims 15-16,

18. The building material of one of claims 15-17, wherein the the cement is selected from the group consisting of Portland cement, mixed cement, white cement, lime cement, calcium, oxide and any combination thereof.

19. The building material of one of claims 15-18, wherein the cement is in the range of 10- 15% by dry weight of the building material.

20. The building material obtainable from the process of one of claims 1-14.