Field of the Invention

The present invention relates to a hardening accelerator containing no chloride and a method to accelerate the hardening of hydraulic binders, in particular Portland cement and fly ash with high levels of Portland cement replaced by fly ash and mixtures containing hydraulic binders, in particular of mortar and concrete. The hardening accelerator is applied for manufacturing of prefabricated elements and accelerating the manufacture of concrete on the construction site.

Background of the Invention

Portland cement is a hydraulic binder that is most commonly used in construction. Portland cement mainly includes a mixture of calcium silicate and calcium aluminate minerals that can react with water forming a dense, solid paste. However, Portland cement has some drawbacks when used as a binder for concretes. Materials using Portland cement can be degraded by sulphate attack from seawater or drainage water. Steel used in reinforced Portland concrete is corrosive when pH in the system is under a certain level. Some aggregates can cause alkali-silica reactions in the materials using high alkali Portland cement resulting in cracks and degradation. In addition, Portland cement has higher heat of hydration than other cement types, and therefore it is not suitable for use in some environments. For example, high heat of hydration can cause cracking when Portland cement is used in hot climates.

Fly ash is a by-product obtained from coal-burning thermal power plant. Fly ash contains mainly glassy, amorphous components including alumina and silica. ASTM C 618-00 has classified fly ash into two classes: Class C and Class F, depending on the total amount of calcium oxide, silica, alumina and ferric oxide content present. Class F contains more than 70% of the above mentioned oxides , but less than 20% lime (CaO). Class C contains less than 70%, but more than 50% of the abovementioned compounds. Class C fly ash generally contains more than 20% lime (CaO). Class F fly ash has pozzolanic properties, which means that it can react with calcium hydroxide in the presence of water to make calcium aluminate and calcium silicate hydrates. Class C fly ash, in addition to having pozzolanic properties, also has hydraulic properties.

Replacing one part of Portland cement by fly ash in cement and concrete productions brings some advantages to environment, technology and economy. Using fly ash reduces exploitation of natural recourses for Portland cement production such as limestone, clay and consumes industrial by-product. That helps to decrease the amount of CO2 emitting and fuel burnt during cement production. Those contribute to the protection of the environment and lower the price of the cementitious binder used for concrete production. Concrete comprising fly ash has improved workability, lower risks of thermal cracking and decreased damages caused by alkali-silica reactions in the system using high alkali Portland cement. However, fly ash has one main drawback originating from its slow pozzolanic reaction in early ages. This slow reaction, particularly at low temperature, results in low early compressive strength that causes the extension of operating time at the jobsite and delays the applications of concrete using this kind of binder into service. It is generally accepted that the amount of Portland cement replaced by fly ash should not exceed about 20% to avoid a significant reduction in early compressive strength. That means the amount of fly ash used in concrete industry is still limited.

The early compressive strength of hydraulic binders containing Portland cement and fly ash, particularly at high level of Portland replacement can be increased by applying some alkali compounds such as alkali metal hydroxides, alkali sulphate salts, etc to increase the alkalinity of the system. The increase in alkalinity helps to activate the pozzolanic reaction of fly ash due to the increased solubility of fly ash in high pH environment (pH > 13). However, the amount of solid alkali substances required to be added into the system to cause a significant accelerating effect is quite high and this will expense the final strength and long term durability of material using this kind of binder.

The prior art also focuses on using chemicals to shorten the setting time to reduce the working time on the construction site and increase the circulation of the forms and moulds. KR 200801 1 1645 A describes an accelerator for increasing the fly ash mixing ratio in concrete, comprising polycarboxylic acid salt, triethanol- amine, sodium thiocyanate, glycine, nitrate and water. KR 20060018594 A describes non-alkaline accelerators for concrete admixtures. US 2004/0244655 A1 relates to an accelerator used for setting and hardening of hydraulic binders. The main components of the accelerator are nitrates, aminoalcohols, hydroxyl- carboxylic acids and polyalcohols. US 5605571 A discloses an accelerator used for setting and hardening of hydraulic binders containing at least a nitrate- or sulfite component, at least a thiocyanate component, at least an alkanolamine component and at least a carboxylic acid component.

There is a demand to develop a new hardening accelerator for hydraulic binders composing of Portland cement and fly ash with high level of Portland cement replacement at both low and normal temperatures. The new accelerator should contain no chloride and sharply diminish the drawbacks caused by adding high dosages of chemical into binder systems.

Summary of the invention

The composition of the new accelerator admixture of the present invention includes: at least one inorganic thiocyanate, at least one organic alkanolamine, at least one organic polyol, and optionally water. All these compounds are known individually producing an accelerating effect on hydration of hydraulic binders, but it has been surprisingly found a synergistic effect when they are combined in the proportion of the present invention.

The accelerator according to the invention contains no chloride. The accelerator according to the invention is suitable for accelerating the hardening of hydraulic binders, particularly for binders comprising of Portland cement and fly ash with high proportions of fly ash. The accelerator is suitable for manufacturing prefabricated concretes or for accelerating the hardening of concrete produced on the construction site.

The components of the accelerator according to the invention can be combined and then added together to other ingredients (binders, aggregates, fillers) to make concrete or they can be added separately. The accelerator can also be added to mixing water before being added to the mixture of binders and other ingredients. Figures

Figure 1 : Diagram showing rate of heat of hydration in OPC samples with combinations of the components and components individually and sample with no admixture.

Figure 2: Diagram showing rate of heat of hydration in OPC samples using 2- and 3-component admixture and with no admixture.

Figure 3: Diagram showing rate of heat of hydration in OPC (70%) and FA (30%) samples to combinations of the components and components individually and sample with no admixture.

Detailed Description of the Invention

The accelerator according to the invention comprises at least one inorganic thiocyanate, at least one organic alkanolamine, at least one organic polyol and optionally water.

A preferred thiocyanate is an alkaline thiocyanate, alkaline earth

thiocyanate or mixtures thereof. A preferred example is sodium thiocyanate.

A preferred alkanolamine is mono-, di-, triethanolamine, triisopropanol- amine, or mixtures thereof. Diethanolamine is especially preferred.

A preferred polyol is a lower alcohol with carbon chain from C2 to C6, or mixtures thereof. 1 ,2,3-propanetriol (glycerol) is especially preferred.

The accelerator according to the invention contains from 5 to 50 % by weight of the at least one inorganic thiocyanate, from 1 to 20% by weight of the at least one organic alkanolamine, from 1 to 20 % by weight of the at least one polyol, and any remainder being water. Preferably, the accelerator according to the invention comprises from 10 to 40 % by weight of the at least one inorganic thiocyanate, from 5 to 15 % by weight of the at least one organic alkanolamine, from 5 to 20 % by weight of the at least one polyol , and any remainder being water.

The accelerator according to the invention is suitable for accelerating the hardening of hydraulic binders, particularly for binders comprising of Portland cement and fly ash with high proportions of fly ash (>20 % by weight). The accelerator is appropriate for manufacturing prefabricated concretes or for accelerating the hardening of concrete produced on the construction site. The proportion of fly ash in hydraulic binders can be more than 20 % by weight relative to the total weight of the hydraulic binders.

The accelerator can be added to hydraulic binders, particularly binders comprising Portland cement and fly ash, in the dosages of from 0.5 to 5 % by weight based on the total weight of the hydraulic binders, more preferably in the dosages of from 1 to 3 % by weight.

The components of the accelerator according to the invention can be combined in dispersed form in water and then added together to other ingredients (binders, aggregates, fillers) to make concrete or they can be added separately.

Examples

The following examples are given to demonstrate the effects of the accelerator according to the invention at low temperature (5°C) and normal temperatures (20°C).

The accelerator according to the invention that was used in the following test mixtures has a composition as disclosed in Table 1 .

Table 1

EXAMPLE 1 :

The effects of the accelerator on the hardening of mortar were tested. The compressive strength was tested with samples of mortar 4x4x16 cm size. The compositions of mortars are made up as disclosed in Table 2. Table 2

Table 3 shows the effect of the accelerator on the hardening of mortars at 5°C after ageing 2 days and 28 days.

Table 3

Table 4 shows the effect of the accelerator on the hardening of mortars at 20°C after ageing 1 day and 28 days.

Table 4

EXAMPLE 2

The effects of the accelerator on the hardening of concrete were tested. The concrete samples with size of 10x10x10 cm were made for compressive strength test with compositions as disclosed in Table 5: Table 5

Table 6 shows the effect of the accelerator on the hardening of concretes at 5°C after ageing 2 days and 28 days.

Table 6

Table 7 shows the effect of the accelerator on the hardening of concretes at 20°C after ageing 18 hours, 2 days and 28 days.

Table 7

EXAMPLE 3

The effects of the accelerator on the hardening of concrete with CEM ll/A-V (cement containing minimum 20% FA) were also tested. The concrete samples with size of 10x10x10 cm were made for compressive strength test with

compositions as disclosed in Table 8:

Table 8

Table 9 shows the effect of the accelerator on the hardening of concretes at 5°C after ageing 2 days and 28 days.

Table 9

Table 10 shows the effect of the accelerator on the hardening of concretes at 20°C after ageing 18 hours, 2 days and 28 days.

Table 10

Compressive strength (MPa)

Mixture

18 hours 1 day 28 days

E (without 9.0

19.3 53.0

accelerator)

F (with accelerator) 17.1 21 .7 54.0 EXAMPLE 4

The effects of the accelerator on the hardening of concrete with CEM I (containing no fly ash) were also tested. The concrete samples with size of 10x10x10 cm were made for compressive strength test with compositions as disclosed in Table 1 1 :

Table 1 1

Table 12 shows the effect of the accelerator on the hardening of concretes at 5°C after ageing 2 days and 28 days.

Table 12

Table 13 shows the effect of the accelerator on the hardening of concretes at 20°C after ageing 18 hours, 2 days and 28 days.

Table 13

Compressive strength (MPa)

Mixture

18 hours 1 day 28 days

G (without

24.8 32.5 60.0

accelerator)

H (with accelerator) 30.5 35.1 63.3 To conclude the accelerator effect in mixtures A-H, the relative increased compressive strength is shown in Table 14.

Table 14

The accelerator has strongest effect at early age or at low temperature. EXAMPLE 5

The heat development cement added the accelerator components combined / not combined are presented in the Tables 15 and 16 below and in Figures 1 to 3.

The binder pastes contained Portland cement CEM I (OPC) and fly ash io class F (FA) with added water water/binder = 0.4. The paste parallels had no admixture (control) and with added NaSCN, DEA, and Glycerol to the water individually with dosages of 0.2%, 0.1 % respectively as well as with the 3- component admixture composed of (0.2% NaSCN + 0.1 % DEA + 0.05% Gly). i s Table 15: The cumulative heat and maximum rate of heat evolution effect of admixture in OPC.

* A _¾2 ^C>H Table 16: The cumulative heat and maximum rate of heat evolution effect of admixture in OPC (70%) and FA (30%).

Tables 15 and 16 show that the heat evolutions from the combination of the components are higher than the heat evolutions from the individual components. This synergistic effect is found in Portland cement CEM I and also when 30% of OPC is replaced with fly ash.

Figures 1 and 3 show how the combination of components affects the heat as function of time. This synergistic effect is clearly shown for the combination of the 3 components admixture in Figure 2.