EP0830325A1

EP0830325A1 - A method of slowing the setting rate of magnesium phosphate cements

Info

Publication number: EP0830325A1
Application number: EP96911867A
Authority: EP
Inventors: Peter Allan Williams; Gary Ralph Dennis; Nilmini Sureka Joshua; Brian Patrick Sloane
Original assignee: James Hardie Research Pty Ltd
Current assignee: James Hardie Research Pty Ltd
Priority date: 1995-05-10
Filing date: 1996-05-09
Publication date: 1998-03-25
Also published as: JPH11505204A; EP0830325A4; AUPN291195A0; KR19990014664A; WO1996035647A1

Abstract

A method for slowing the setting rate of a magnesium phosphate cement comprising adding a suitable base to an uncured or partially cured aqueous mix of the magnesium phosphate cement in an amount sufficient to raise the pH and minimise MgO/Mg(OH)2 dissolution. Suitable magnesium phosphate cements include ammonium magnesium phosphate cements and potassium magnesium phosphate cements.

Description

A METHOD OF SLOWING THE SETTING RATE OF MAGNESIUM PHOSPHATE CEMENTS

FIELD OF THE INVENTION

This invention relates to a method of slowing the setting rate of hydrated

cements.

BACKGROUND OF THE INVENTION

The most commonly used hydrated cement is portland cement formed by

burning a mixture of clay and limestone. In some applications portland cement has

properties that render it less than ideal. These include situations where the cement is

required to be rapid setting or in which the product concerned is likely to be exposed

to high temperatures. Additionally, when cured concrete made from ordinary portland

cement is subjected to carbonation due to atmospheric carbon dioxide exposure the

alkaline cement reaction products carbonate to calcium carbonate reducing its

alkalinity and eventually causing micro-cracking of the concrete. This leads to

corrosion of reinforcing steel where there is inadequate concrete cover over the steel. The use of magnesium ammonium phosphate (NH_4-vlgPO₄.6H₂O), commonly known as struvite, as a hydrated cement is well documented. This

compound occurs as a natural mineral or may be synthesised. Known magnesium

ammonium phosphate cements have an initial setting time of about 1 to 5 minutes.

This time is too short in many specialised applications such as fabrication and it would

be desirable to slow the setting time down. Previous methods to slow down the

setting time have involved the addition of borax or halides (such as sodium chloride or

barium salts) as setting rate retardants. Borax has disadvantages including in that it is

very soluble in water and in order to slow the setting time, a substantial amount of

borax must be added with consequent loss of strength of the final product. Halides are

also not desired setting retardants. Chlorides in general are not desirable additives

since materials containing them are liable to give off hydrogen chloride gas at high

temperatures and their retarding effects occurring within a couple of minutes is not

very significant. Barium salts are not desirable in view of cost and toxicity problems.

Magnesium ammonium phosphate cements have the additional disadvantage

of inadequate mechanical properties and moisture resistance compared to portland

cement for general purpose use. Despite the problems struvite cements have been

used in applications requiring rapid set times such as the repair of cracks in concrete

roads and have also been used in the manufacture of furnace lining boards.

OBJECT OF THE INVENTION

It is an object of this invention to substantially ameliorate at least some of the

disadvantages of the prior art. DISCLOSURE OF THE INVENTION

In a first aspect, this invention consists in a method for slowing the setting

rate of a magnesium phosphate cement comprising adding a suitable base to an

uncured or partially cured aqueous mix of the magnesium phosphate cement in an

amount sufficient to raise the pH and minimise MgO/Mg(OH)₂ dissolution.

In another aspect, the invention consists in a cementitious article formed

using the method of the first aspect.

PREFERRED EMBODIMENTS OF THE INVENTION Suitable magnesium phosphate cements include ammonium magnesium phosphate cements and potassium magnesium phosphate cements (MgKPO₄ nH₂O).

The cement is chosen depending on its end application.

When the magnesium phosphate cement is ammonium magnesium

phosphate, a suitable base is ammonia. Preferably, in order to prepare the cement, the

ammonia is added to mono ammonium phosphate to which magnesium oxide is

subsequently added. Preferably the ratio of ammonia to phosphate is 0.5-2.0: 1 , most

preferably 1 : 1 and the ratio of magnesium to phosphate is 7-1 : 1 , most preferably 3:1.

Ratios of ammonia to phosphate above 2:1 tend to produce crumbly cements.

When the magnesium phosphate cement is a potassium magnesium

phosphate cement, a suitable base is potassium hydroxide or potassium carbonate.

Similarly in order to prepare the cement the base is preferably added to mono

ammonium phosphate prior to the addition of magnesium oxide. Preferably the ratio

of potassium to phosphate is 0.5-2.0:1, most preferably 1:1 and the ratio of

magnesium to phosphate is 7- 1 : 1 , most preferably 3:1. Ratios of magnesium to phosphate greater than 7: 1 tend to result in cements having reduced setting times and

increased viscosity and are generally unworkable.

In a preferred aspect, the invention consists in a method of slowing the setting

rate of an ammonium magnesium phosphate cement comprising the steps of reacting

ammonia with an arnmonium phosphate compound in the presence of water and

subsequently reacting the mixture with a magnesium compound to form a compound

having the formula NH MgPO₄.nH₂O, wherein the amount of ammonia used is

sufficient to raise the pH and minimise MgO/Mg(OH)₂ dissolution.

In another preferred aspect, the invention consists in a method of slowing the

setting rate of potassium magnesium phosphate cement comprising the steps of

reacting an ammonium phosphate compound with a potassium compound in the

presence of water and subsequently reacting the mixture with a magnesium compound

to form a compound having the formula:

MgKPO₄.nH₂O

wherein the amount of potassium compound used is sufficient to raise the pH and

minimise MgO/Mg(OH)₂ dissolution.

It is found by adding the potassium compound to the ammonium phosphate

compound prior to addition of magnesium oxide that the rate of set is slowed to such

an extent that the cement is suitable for fabrication.

The ammonium phosphate compound is preferably mono ammonium

phosphate, the potassium compound is preferably potassium carbonate and the

magnesium compound is preferably magnesium oxide. It will be apparent, however, that other materials may be used as the source of phosphate, potassium and

magnesium.

The setting time of the hydrated cement according to this invention can be

controlled by adjusting the pH of the cement at the time of setting. Preferably the pH

is raised to a value between 4 to 10, more preferably to a value between 8 and 9. For

potassium magnesium phosphate cements, the pH is preferably controlled by using

potassium carbonate or potassium hydroxide as a source of potassium in the

preparation of the compound. Preferably potassium carbonate is used as it has some

buffering effect. It has also been found that the setting time of the hydrated cement

may be further delayed by the gradual addition of excess water to the final mixture or by mechanical agitation and working of the mix.

Although there is no limit on the amount of water used in order to maximise

strength, it is desirable to use only a slight excess from that stoichiometrically required

to form the desired hydrated cement so as to form a smooth paste before setting. Once the cement has started to set any amount of water can be added to slow the setting time with either none or minimal loss of strength.

The hydrated cement prepared by the method of the invention can

additionally contain density modifiers such as flyash, expanded polystyrene beads,

cenospheres, vermiculite, perlite or predigested calcium silicate hydrate. Other

additives can include fibre reinforcements such as cellulose, glass, polymer or

wollastonite fibres.

The hydrated cement can be prepared as a foam cement. The magnesium potassium phosphate cement prepared by the method of this

invention has been found to have a high strength at ambient temperature, to retain

shape without distortion at high temperatures, exhibit low shrinkage at high

temperatures, to have low water permeability and is crystalline in appearance.

The hydrated cement prepared according to this invention may be readily

formed from commercially available raw materials and a high degree of purity is not

required for reliable manufacture of the product.

The invention will now be described, by way of example only, with reference

to specific examples.

The hydrated cement prepared according to this invention may be formed, for

example, in a conventional concrete mixer using the following method:-

(1) Dry mono ammonium phosphate NH₄H₂PO₄ is mixed with potassium

carbonate K₂CO₃ in approximately stoichiometric quantities and a controlled

amount of water added.

(2) Magnesium oxide is then added to these materials and a further controlled

amount of water added. The cement will now react and set.

(3) The setting time may be further delayed by the addition of yet more water to

the mixture which displays thixotropic properties during the setting process.

The method can form cements with various levels of hydration in terms of

water molecules bound to the compound. If no excess water is added then when the

compound sets, there is no surplus water to cause drying shrinkage, the cement has a

very low porosity, and there is no need to dry excess water from articles formed from

the cement. For a rapid or slightly delayed set, minimal quantities of water are added,

forming cements with the minimal quantities of bound water. For a further delayed

set larger quantities of water are added, forming compounds up to the maximum level

of bound water. For magnesium potassium phosphate cements the chemical reactions which

occur can be represented as follows:-

NH₄H₂PO₄ + K₂CO₃ → K₂HPO₄ + CO₂ + NH₃ + H₂O

MgO + H₂O → Mg(OH)₂

Mg(OH)₂ + 2H⁺ → Mg²⁺ + 2H₂O

Mg²⁺ + K⁺ + PO^ + nH₂O → MgKPO₄.nH₂O

Although the above reactions are depicted around mono ammonium

phosphate as the source of phosphate, potassium carbonate as the source potassium

and the method of controlling the pH and consequently setting time, and magnesium

oxide as the source of magnesium, other materials may be used or the materials may

be used in other combinations.

A typical formulation for preparing a magnesium potassium cement

according to the method of the invention is:-

NH₄H₂PO₄ 42 kilograms

K₂CO₃ 25 kilograms

MgO 45 kilograms

H₂O 20 litres (minimum) or an

amount adjusted to suit The mole ratio of K₂C0₃:NH₄H₂PO₄:MgO:H₂O is suitably 1:2:6:6. Using

such a mole ratio the pH is raised from a pH of 4 to 5 (corresponding to the pH of a

saturated solution of NH₄H₂PO₄) to a pH of 8 to 9.

The purity of the raw materials is not crucial to the formation of the cement

and the normal commercial purity of bulk commodity raw materials is of sufficient

quality for reliable manufacture of this invention. The batching accuracy of the raw

materials is not critical to the formation of the cement and normal site mortar mixing

accuracy of around 10% is adequate. The dry raw materials can be pre-packaged in

approximately stoichiometric quantities. For potassium magnesium phosphate

cements a two component hydrated cement mix can be provided comprising a first

component containing a mixture of mono ammonium phosphate and potassium

carbonate and a second component containing magnesium oxide. Accurate addition of

water can be achieved by use of a meter or fixed volume containers.

Using commercially available grades of the raw materials cured magnesium

potassium phosphate cement made in accordance with the above typical formulation,

has been found to exhibit strengths of about 50% of that obtained from cured

specimens of ordinary portland cement and to exhibit a pore structure which is

relatively impermeable to water. The linear shrinkage of cured magnesium potassium

phosphate cement specimens subjected by heating to temperatures of about 1000°C

then cooling in a dissicator has been measured to be about 2% of the original length.

No distortion is evident in such specimens.

The pH of the cured magnesium potassium phosphate cement is suitably

alkaline such that reinforcing steel bound by concrete formed from the cement will be passivated and therefore not subject to corrosion, and the permeability of the cement is

such that the ingress of water to corrode the steel is inhibited. Additionally, cured

magnesium potassium phosphate cement is not subject to any known reaction with

atmospheric carbon dioxide in the same way as ordinary portland cement. Thus

concrete formed using the cement of this invention is expected to be dimensionally

stable with age and to maintain its non corrosive features to reinforcing steel

indefinitely.

The applications of the cement according to this invention include:-

a raw material in the manufacture of refractory materials;

a binding cement in the installation of refractory materials;

a binding cement in structural concrete;

a binding cement in extruded and pressed mortar articles;

a binding cement for fibre reinforced articles such as those containing fibres

including cellulose, glass, wollastonite and/or polymers; and

a rapid setting concrete for use in marine environments.

Example 1 - Retarding an ammonium magnesium phosphate cement containing flvash.

The formulation used in this Example was as follows:

concentrated aqueous ammonia (25%) 23mls

mono ammonium phosphate 36g

flyash 25g

magnesium oxide 38g

and water 14mls This formulation corresponds to a mole ratio of NH₃:NH₄H₂PO₄:MgO:H₂O

of 1.0:1.0:3.0:5.3.

The mono ammonium phosphate, ammonia and water were first mixed

together thoroughly, the flyash was then blended into the mixture and magnesium

oxide was then added resulting in a hexahydrated ammonium phosphate cement.

Example 2 - Retarding a potassium magnesium phosphate cement optionally

containing flvash.

The formulation used in this example was as follows:

potassium carbonate 22g

mono ammonium phosphate 36g

(flyash) 25g

magnesium oxide 38g

and water 17ml

This formulation corresponds to a mole ratio of K₂CO₃:NH₄H₂PO₄:MgO:H₂O

of 1:2:6:6.

The mono ammonium phosphate was mixed with water and the potassium

carbonate gradually added, with agitation to allow the CO₂ evolved in the reaction to

excape. The mono ammonium phosphate and potassium carbonate particle size was

<500μm. (Slight warming can be used at this stage to aid the further removal of CO₂

but intense heating is detrimental as it results in loss of ammonia). The mixing

resulted in the formation of a white slurry (weighing ~70g). The magnesium oxide

was then added to the slurry and stirred to form a smooth paste which could be cast

after lmin, but also could be kept workable up to 15mins by continual agitation. If longer set times were required, water was added gradually, as stiffening occurred. A

gradual addition of a further 10ml of water extended working time to 60mins. When

the cement was made with flyash (added to the slurry prior to the addition of the

magnesium oxide) then a further addition of 31ml of water extended workability to

60mins.

The foregoing describes only some aspects of the present invention and

modifications made thereto without departing from the scope of the invention.

Claims

1. A method for slowing the setting rate of a magnesium phosphate cement

comprising adding to a suitable base to an uncured or partially cured aqueous mix of

the magnesium phosphate cement in an amount sufficient to raise the pH and

minimise MgO/Mg(OH)₂ dissolution.

2. A method according to claim 1 wherein the magnesium phosphate cement is

an ammonium magnesium phosphate cement and the base is ammonia.

3. A method according to claim 1 wherein the magnesium phosphate cement is

a potassium magnesium phosphate cement and the base is potassium hydroxide or

potassium carbonate.

4. A method of slowing the setting rate of an ammonium magnesium phosphate

cement comprising the steps of reacting ammonia with an ammonium phosphate

compound in the presence of water and subsequently reacting the mixture with a

magnesium compound to form a compound having the formula NH₄MgPO₄.nH₂O,

wherein the amount of ammonia used is sufficient to raise the pH and minimise

MgO/Mg(OH)₂ dissolution.

5. A method according to claim 4 wherein the ammonium compound is mono

ammonium phosphate, and the magnesium compound is magnesium oxide.

6. A method of slowing the setting rate of potassium magnesium phosphate

cement comprising the steps of reacting an ammonium phosphate compound with a

potassium compound in the presence of water and subsequently reacting the mixture

with a magnesium compound to form a compound having the formula:

MgKPO₄.nH₂O - 13 - wherein the amount of potassium compound used is sufficient to raise the pH and

minimise MgO/Mg(OH)₂ dissolution.

7. A method according to claim 6 wherein the ammonium phosphate compound

is mono ammonium phosphate, the potassium compound is potassium carbonate and

the magnesium compound is magnesium oxide.

8. A method according to any one of the preceding claims further comprising

delaying the setting time by adding excess water and/or mechanically agitating the

mix.

9. A method according to any one of the preceding claims wherein the cement

additionally contains fibres and/or fillers.