WO2020199907A1

WO2020199907A1 - Low-shrinkage alkali-activated dry mix repair mortar

Info

Publication number: WO2020199907A1
Application number: PCT/CN2020/079516
Authority: WO
Inventors: Chung Kong Chau; Xiwen Guan; Shiyin LIU; Man Lung Sham; Wai Pong CHAN
Original assignee: Optimix Eco Building Material Limited
Priority date: 2019-04-03
Filing date: 2020-03-16
Publication date: 2020-10-08
Also published as: CN112930329A

Abstract

A dry-mix, OPC-free,alkali-activated repair mortar includes a base material selected from ground granulated blast furnace slag, fly ash, or a mixture thereof in an amount from approximately 32 weight percent to approximately 37 weight percent. Metakaolin is provided in an amount from approximately 3 weight percent to approximately 8 weight percent. As an alkali activator, modulated potassium silicate is used, the potassium silicate having been modified (by KOH) such that a molar ratio of K ₂O to SiO ₂ ranges from approximately 1.8 to 2.2. A filler is used in an amount from approximately 50 weight percent to approximately 54 weight percent. The drying shrinkage of the repair mortar is less than or equal to approximately 0.06% and the bond strength of the repair mortar is greater than or equal to approximately 0.5 MPa, the acid resistance measured by a weight loss after 28 days of acid exposure is less than or equal to approximately 3.4 %, and the setting time of the repair mortar is greater than or equal to approximately 60 minutes.

Description

LOW-SHRINKAGE ALKALI-ACTIVATED DRY MIX REPAIR MORTAR

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from a Hong Kong patent application number 19121825.4 filed on April 3 ^rd, 2019, and the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an alkali-activated dry mix repair mortar and, more particularly, to an alkali-activated dry mix repair mortar that exhibits very low shrinkage and excellent chemical resistance to the working environment.

BACKGROUND

Repair of concrete structures is a large percentage of all construction work performed. The maintenance and renovation of aging infrastructure places high demands on repair materials which need to adhere well to concrete substrates yet have sufficient strength to restore the structures to near-original load-bearing capacity. Often concrete repairs occur in structure regions that are subject to harsh chemical or mechanical environments and must be highly durable. Originally, repair mortars were based on ordinary Portland cement. However, many of these repair mortars lacked volume stability, chemical resistance, strength and adhesion to the original concrete structures. Newer repair mortars have included various polymers such as epoxy, acrylic, or polyurethane; these may demonstrate increased strength and adhesion. However, volatile organic compounds that emanate from the repair material may cause concern for workers applying the material. Moreover, organic based materials are not compatible with the parent cement-based substrates.

Because ordinary Portland cement manufacture generates a high volume of carbon dioxide, alternatives to the use of cement in repair have been investigated as environmentally-friendly substitutes. Consequently, different recycle or waste materials are being considered for use in repair mortars. Recycle and waste materials have been added to concrete compositions in the past. However, as noted above, repair mortars have special mechanical and chemical resistance requirements. In particular, requirements such as volume stability with very low shrinkage, high chemical resistance, and strong bonding to the concrete substrate to be repaired, of the repair mortar, are highly desirable for repair mortars. Thus, there is a need in the art for environmentally-friendly, OPC-free repair mortars demonstrating good bond strength and low drying shrinkage. Further, there is a need in the art for repair mortars that have high resistance to chemical attacks.

Therefore, a novel, low-shrinkage, ordinary Portland cement-free, alkali-activated dry mix repair mortar is developed to tackle the above-mentioned issues.

SUMMARY OF THE INVENTION

The present invention provides a low drying shrinkage, high chemical resistance, OPC-free repair mortar including a substantial proportion of waste materials. A dry-mix, alkali-activated repair mortar includes a base material selected from ground granulated blast furnace slag, fly ash, or a mixture thereof in an amount from approximately 32 weight percent to approximately 37 weight percent. Metakaolin is provided in an amount from approximately 3 weight percent to approximately 8 weight percent. As an alkali activator, modulated potassium silicate is used, with the potassium silicate having been modified by KOH such that a molar ratio of K ₂O to SiO ₂ ranges from approximately 1.8 to 2.2. Different kinds of fillers are used in an amount from approximately 50 weight percent to approximately 54 weight percent. The drying shrinkage of the repair mortar is less than or equal to approximately 0.06%and the bond strength of the repair mortar is greater than 0.5 MPa, the chemical resistance measured by a weight loss after 28 days of acid exposure is less than or equal to approximately 3.4 %, and the setting time of the repair mortar is greater than or equal to approximately 60 minutes.

DETAILED DESCRIPTION

The environmentally-friendly repair mortars of the present invention include a base aluminosilicate recycled or waste material. This recycle or waste material may be selected from granulated ground blast furnace slag (or a related slag) or fly ash. Slag is the material left over when a metal has been separated (e.g., smelted) from its respective metal ore. Granulated ground blast furnace slag is produced by quenching of molten iron slag (a by-product of iron and steel-making) from a blast furnace followed by grinding. The main components of granulated ground blast furnace slag are CaO (30-50%) , SiO ₂ (28-38%) , Al ₂O ₃ (8-24%) , and MgO (1-18%) .

Fly ash is the leftover product of coal combustion. The range of components depend on the composition of the coal that was burned and tends to be regionally-specific. The main components of fly ash are silicon dioxide (SiO ₂) in an amount from 45-60 weight percent, aluminium oxide (Al ₂O ₃) in an amount from 18-32 weight percent and calcium oxide (CaO) in an amount from 2 to 10 weight percent. Note that the base materials necessarily have variable compositions because they are recycled or waste materials and, as such, come from a wide variety of sources. Therefore, the above compositions are merely exemplary of base material compositions. The GGBS and/or fly ash is present in the repair mortar in an amount from approximately 22 to approximately 33 weight percent.

Metakaolin is also used in the repair mortars of the present invention. Metakaolin is formed from calcined kaolin, a clay mineral. It has a low calcium content, being primarily composed of silicon dioxide (SiO ₂) , about 48-58 weight percent, and aluminum oxide (Al ₂O ₃) , about 40-50 weight percent, with small amounts of iron oxide, calcium oxide, and titanium dioxide. In particular, metakaolin that is produced by calcining at temperatures in the range of approximately 750 to 800℃ is selected for use in the repair mortars. The metakaolin is present in the repair mortar composition in an amount from approximately 3 to approximately 8 weight percent.

Metakaolin has been used as a replacement material in conventional concrete mixes, particularly in environments that may be exposed to chemical attack, as, for example, chemical attack by chloride ions. However, in the present invention, it was determined that the use of metakaolin in the repair mortars contributed to a very low dry shrinkage percentage. A low dry shrinkage percentage allows a repair mortar to be applied to a concrete structure, bond to the region in need of repair, and dry and cure without cracking. It was determined that metakaolin, combined with either GGBS or fly ash, produces a sufficiently strong binder phase, good bonding to the structure to be repaired, and high durability.

Metakaolin is an aluminate-rich material. As the alkali activation produces SiO ₄ and AlO ₄ tetrahedral frameworks linked by shared oxygens as poly (sialates) or poly (sialate–siloxo) or poly (sialate–disiloxo) , the addition of metakaolin yields an improved structure with a dense amorphous phase including a semi-crystalline 3-D alumino-silicate microstructure. The dense 3-D structure is volumetrically stable and has very low drying shrinkage.

Alkali activation of the base materials produces a variety of reaction products that bind together. In alkali activation of low calcium-content materials, a reaction occurs involving the dissolution of an aluminosilicate material by the alkaline material. That is, the minerals are “de-polymerized” into their constituent “monomers” by condensation of the alkaline material. The dissolution results in formation of reactive units of covalently bonded Si-O-Si and Al-O-Si units. Aluminum atoms enter the Si-O-Si units, forming aluminosilicate gels. Following the gel formation, a condensed structure is generated with crystallization into a polymeric network. Due to the formation of a polymeric network, some alkali activated materials are referred to as “geopolymers. ”

For alkali activation of higher-calcium raw materials, calcium silicate hydrates-like phases as reaction products are formed. Similar to the alkali activation of the low calcium-content materials, an aluminosilicate gel is formed and a hardened polymeric network creates a binder phase for the repair mortar.

In the present invention, modulated potassium silicate (K ₂SiO ₃) may be used as the alkali activator. The potassium silicate is modified by potassium hydroxide (KOH) such that a molar ratio of SiO ₂ to K ₂O ranges from approximately 1.8 to 2.2. The modulated activator is present in a dry powder form, avoiding the many safety hazards associated with the use of liquid alkali activators (transport issues, chemical burn issues, etc. ) . Alternatively, other modulated alkali activators may be used such as sodium-based activators or lithium-based activators. The modulated alkali activator may be present in the repair mortar in an amount from approximately 5 percent to approximately 6 percent.

Other materials may be added to the repair mortar in order to improve fresh mechanical properties such as workability and setting time. For example, a retarder such as NaHCO ₃ may be added in an amount from approximately 0.01 to approximately 0.2 weight percent. The retarder NaHCO ₃ achieves a controllable setting time through buffering action in the alkali-activated mortar. In the fresh mixed matrix, NaHCO ₃ is a buffering agent to balance the alkalinity for the matrix. It can help to counteract and limit the increase of alkalinity in the matrix and slow down the chemical reaction of the alkali-activation in the early stage. Consequently, the setting and hardening process of the alkali-activated material can be well-controlled. In particular, the setting time of the repair mortar can be controlled to be greater than 60 minutes and, in some cases, greater than 90 minutes. A long setting time is important in the repair environment as applied mortar may need to be shaped into complex configurations that require time to be formed.

A thickener such as starch ether may be added in an amount from approximately 0 to approximately 0.1 weight percent. A rheology modifier such as hydroxy propyl methyl cellulose (HPMC) may be added in an amount from approximately 0 to approximately 0.01 weight percent. The rheology may be selected for different applications. For example, the horizontal applications may use a repair mortar that is less viscous than repair mortars for vertical surfaces or overhead applications.

Fibers may be added to the repair mortar in an amount from approximately 0 weight percent to approximately 0.2 weight percent. Glass fibers may be used as the fiber additives. Because mortars typically have weak tensile properties, fiber reinforcement may enhance tensile strength. Fibers may help reduce shrinkage and cracking during drying of the repaired structure. Fibers can act as crack arresters as the fibers may prevent cracks from propagating by adhering parts of the repair mortar to each other.

Fillers provide compressive strength and bulk to the repair mortar and may be chosen based on the desired durability, strength, and workability of the repair mortar. The repair mortar of the present disclosure may include sand and/or lightweight aggregate in varying amounts. A total amount of aggregate may range from approximately 50 weight %to approximately 54 weight %. Of this amount approximately 50 to 52 weight %may be sand and approximately 0-2 weight %may be lightweight aggregate. In particular, different grades of sand may be used. For example, graded silica sand having a diameter on the order of 0.5 mm or less may be used in an amount of approximately 20 weight percent to approximately 21 weight percent and graded silica sand having a diameter on the order of approximately 0.5 mm to approximately 1.2 mm may be used in an amount of approximately 30 weight percent to approximately 31 weight percent.

During mixing, water is added in an amount from approximately 15-18 weight percent, more particularly, 16-17 weight percent. Because the alkali activator is already present in the dry-mix repair mortar, no other liquids are required to prepare the repair mortar for application. By adding the alkali activator as a dry ingredient to a dry-mix repair mortar, the precise activation amount is assured in an even distribution throughout the mix because the mix is prepared in a controlled environment rather than at the worksite. Further, the proper amount is guaranteed from batch to batch using a dry-mix formula. The repair mortars of the present invention exhibit excellent chemical resistance, and may be applied to repairs that require corrosion resistance (e.g., resistance to chlorine ions penetration to reduce corrosion of reinforcing steel bars- “rebar” ) . The repair mortars also demonstrate superior acid resistance. Particular applications that require chemical resistance include concrete structures in contact with contaminated acidic chemical sources. The repair mortar further exhibits extremely low drying shrinkage, on the order of approximately lower than 600 microstrain (that is, less than or equal to approximately 0.06%or 0.6 mm/m) . The setting time is controllable and may be controlled to set in approximately 60 minutes or more and, in some embodiments, 90 minutes or more. The inventive repair mortars also demonstrate a high compressive strength, both at 7 days and at 28 day which is 30 MPa and 35MPa, respectively. The strength may be selectively modified depending upon the repair application. In order to ensure excellent attachment to existing concrete structures, the repair mortar of the present invention possesses a good bond strength, on the order of at least approximately 0.5 MPa, more preferably, at least approximately 2.0 MPa. In part, the repair mortars of the present invention may penetrate and chemically react with the concrete substrates being repaired, providing the good bond strength.

Examples

The alkali-activated dry mix repair mortar specimens of the Examples were prepared and handled with the following procedures:

a) All dry powder components in certain amounts were mixed evenly within an M-tec MS 1.1N dry powder mixer for 120 s. Then the alkali-activated dry mix repair mortar is obtained and packaged.

b) The alkali-activated dry mix repair mortar in powder form has water added. The mixing water was mixed with dry mortar in a Hobart mixer at a speed of 140 r/min for 120 s. Subsequently, the mixing speed was changed to a higher speed of 285 r/min for another 120 s. Upon completion of the above mixing procedures, the mixture was prepared and ready for casting or testing operations.

c) The alkali-activated repair mortar mixture was cast in moulds to form different specimens for testing.

d) Cast specimens in the moulds were wrapped with polyethene sheets for moisture curing at room temperature of 23±2 ℃ and relative humidity of 60±5 %for 24 h;

e) After 24 h curing, specimens were demoulded and maintained wrapped by polyethene sheets at the previous temperature and humidity conditions for further curing until the designated testing age.

The fresh performances, mechanical and durability properties of the alkali-activated dry mix repair mortar mixtures were measured. Mortar setting times is measure by vicat needle, flowability is measured with an ASTM standard flow table, compressive strength results at different curing ages of 1, 7 and 28 days and elastic moduli are acquired by a universal testing machine and bonding strength is measured with a pull-off tester. Besides the durability properties, properties such as drying shrinkages are measured according to the linear length changing method and acid resistance is tested by weight loss with cylindrical specimens which were immersed in 5%sulphuric acid solution. Chloride ion penetrability is measured by the rapid electric flux method.

Example 1 (Comparative Example Without Metakaolin)

Constituents of AAM	Proportion
Slag	0.28
Fly ash	0.12
Metakaolin	0
Potassium silicate powder	0.0453
Potassium hydroxide powder	0.0046
Sodium bicarbonate	0.0020
Graded silica sand (0-0.5 mm)	0.2
Graded silica sand (0.5-1.2 mm)	0.3
Mixing proportion	5 kg mortar to 615 kg water

Example 2 (Comparative Example With 2%Metakaolin)

Constituents of AAM	Proportion
Slag	0.28
Fly ash	0.10
Metakaolin	0.02
Potassium silicate powder	0.0453
Potassium hydroxide powder	0.0046
Sodium bicarbonate	0.0020
Graded silica sand (0-0.5 mm)	0.2
Graded silica sand (0.5-1.2 mm)	0.3
Mixing proportion	5 kg mortar to 615 kg water

Example 3 (Inventive Composition With 3.2%Metakaolin)

Constituents of AAM	Proportion
Slag	0.28
Fly ash	0.088
Metakaolin	0.032
Potassium silicate powder	0.0453
Potassium hydroxide powder	0.0046
Sodium bicarbonate	0.0020
Graded silica sand (0-0.5 mm)	0.2
Graded silica sand (0.5-1.2 mm)	0.3
Mixing proportion	5 kg mortar to 615 kg water

Example 4 (Inventive Composition With 6%Metakaolin)

Constituents of AAM	Proportion
Slag	0.28
Fly ash	0.06
Metakaolin	0.06
Potassium silicate powder	0.0453
Potassium hydroxide powder	0.0046
Sodium bicarbonate	0.0020
Graded silica sand (0-0.5 mm)	0.2
Graded silica sand (0.5-1.2 mm)	0.3
Mixing proportion	5 kg mortar to 615 kg water

Example 5 (Inventive Composition With 4.8%Metakaolin)

Constituents of repair mortar	Proportion
GGBS	0.28
Fly ash	0.072
Metakaolin	0.048
Potassium silicate powder	0.0465
Potassium hydroxide powder	0.0047
Sodium bicarbonate	0.0020
Starch ether	0.0008
HPMC	0.00008
Glass fiber	0.002
Light weight filler (0-0.2 mm)	0.02
Graded silica sand (0-0.5 mm)	0.205
Graded silica sand (0.5-1.2 mm)	0.307
Mixing proportion	5 kg dry mortar to 675 kg water

Performance of Example 1

Example 1 is a comparative example formulation without metakaolin. The setting time is around 90 min and the compressive strength at 7d and 28d is 29.3 and 30 MPa, respectively. However, the drying shrinkage at 28 days is as large as 0.333%, which is too large for a repair mortar. The weight loss in acid solution after 28 days is 3.5%, chloride ion penetrability is low and the bonding strength is higher than 0.5 MPa. It is found that without metakaolin in this comparative example repair mortar formulation, drying shrinkage is a serious problem and it is unsuitable for use as a repair mortar.

Performance of Example 2

Example 2 is a comparative example formulation with only 2%metakaolin. The setting time is 47 min and compressive strength at 7d and 28d is 43.4 and 46.5 MPa, respectively. However, the drying shrinkage at 28 days is 0.113%, which, while smaller than in example 1, is still unacceptable for a repair mortar. The weight loss in acid solution after 28 days is 3.6%, chloride ion penetrability is low and bonding strength is higher than 0.5 MPa. Although the metakaolin has contributed to reducing the drying shrinkage, the level of drying shrinkage is still unacceptable for a repair mortar.

Performance of Example 3

Example 3 (inventive composition) is a formulation with 3.2%metakaolin. The setting time is 46 min and compressive strength at 7d and 28d is 42.3 and 43.6 MPa, respectively. The drying shrinkage at 28 days is 0.052%, which is acceptable for a repair mortar. Typically, a drying shrinkage of less than or equal to 0.06%is needed for a low-shrinkage repair mortar. The weight loss in acid solution after 28d is 3.6%, chloride ion penetrability is low and bonding strength is higher than 0.5 MPa. The higher amount of metakaolin of 3.2%in this low-shrinkage alkali-activated dry mix repair mortar formulation has contributed to further control the drying shrinkage to an acceptable level.

Performance of Example 4

Example 4 is a formulation within the range of the present invention with 6%metakaolin. The setting time is 60 minutes and compressive strength at 7d and 28d is 41.4 and 46.8 MPa, respectively. The drying shrinkage at 28 days is only 0.048%. The weight loss in acid solution after 28d is 3.8%, chloride ion penetrability is low and bonding strength is higher than 0.5 MPa. With 6%of metakaolin in this low-shrinkage alkali-activated dry mix repair mortar formulation, the drying shrinkage is minimized to 0.048%. The workability of the repair mortar, however, may be further enhanced by utilizing other functional admixtures.

Performance of Example 5

Example 5 is a formulation of the inventive low-shrinkage alkali-activated dry mix repair mortar. Thickener, rheology modifier and fibers are added in this formulation to adjust the workability and durability properties. The setting time is longer than 90 minutes and compressive strength at 7d and 28d is 34.6 and 36.8 MPa, respectively, which are sufficient for a repair mortar. The 28-day drying shrinkage is only 0.048%, weight loss in acid solution after 28 days is 3.4%, chloride ion penetrability is low and bonding strength is higher than 0.5 MPa.

Those skilled in the art will appreciate from the foregoing description that the broad techniques of the embodiments can be implemented in a variety of forms. Therefore, while the embodiments have been described in connection with particular examples thereof, the true scope of the embodiments should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the specification, and following claims.

Claims

A low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar, comprising:

a base material selected from ground granulated blast furnace slag, fly ash, or a mixture thereof in an amount from approximately 32 weight percent to approximately 37 weight percent;

metakaolin in an amount from approximately 3 weight percent to approximately 8 weight percent;

modulated potassium silicate as an alkali activator, the modified potassium silicate having been modified by potassium hydroxide such that a molar ratio of K ₂O to SiO ₂ ranges from approximately 1.8 to 2.2;

filler in an amount from approximately 50 weight percent to approximately 54 weight percent;

wherein drying shrinkage of the repair mortar is less than or equal to approximately 0.06%, the bond strength of the repair mortar is greater than or equal to approximately 0.5 MPa, the acid resistance measured by a weight loss after 28 days of acid exposure is less than or equal to approximately 3.4 %, and the setting time of the repair mortar is greater than or equal to approximately 60 minutes.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein the metakaolin is produced by calcining at temperatures in the range of approximately 750 to 800℃.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, further comprising fiber additives in an amount less than or equal to approximately 0.02 weight percent.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein a bond strength is at least approximately 2 MPa.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein the repair mortar has a low chloride ion penetrability.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, further comprising hydroxy propyl methyl cellulose in an amount less than or equal to approximately 0.008 weight percent.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein the filler is graded silica sand.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 7, wherein a portion of the silica sand is less than 0.5mm.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 7, wherein a portion of the silica sand is 0.5mm to 1.2mm.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein a cured repair mortar has a 7-day compressive strength of at least approximately 30 MPa.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein a cured repair mortar has a 28-day compressive strength of at least approximately 35 MPa.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein drying shrinkage of the repair mortar is less than or equal to approximately 0.048%.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein a cured repair mortar has a 28-day compressive strength of at least approximately 46 MPa.
The low shrinkage, ordinary Portland cement-free dry-mix, alkali-activated repair mortar as recited in claim 1, wherein the setting time of the repair mortar is greater than or equal to approximately 90 minutes.