WO1987002354A1

WO1987002354A1 - Method of preparing building materials

Info

Publication number: WO1987002354A1
Application number: PCT/SE1986/000473
Authority: WO
Inventors: Fredrik W. A. Kurz
Original assignee: Kurz Fredrik W A
Priority date: 1985-10-14
Filing date: 1986-10-14
Publication date: 1987-04-23
Also published as: AU590020B2; FI881714A0; DK300187A; FI881714A; EP0272267B1; JPS63502501A; AU6476286A; EP0272267A1; US5073198A; SE8504754L; SE8504754D0; BR8607194A; FI84594C; DK300187D0; FI84594B

Abstract

A method of making a building material by activation of latently hydraulic finely ground granulated basic blast-furnace slag to form a direct acting hydraulic binder. The slag is mixed with water, sand and ballast material as well as with a combination of acidic and basic components. The acidic components consist of phosphates, optionally in combination with strongly acting sulfates, and the basic components consist of oxides or other compounds of earth metals, optionally in combination with zinc. Thereby, a concrete having great mechanical strength and high chemical resistance is obtained.

Description

METHOD OF PREPARING BUILDING MATERIALS.

Portland cement is generally considered as the best hydraulic binder, which only by addition of water hardens to form a rock-like material (concrete) within a few hours to obtain finally its ultimate strength within about one month. The effect is due mainly to chemical reactions between basic lime and silicic acid.

Analysis of Portland cement shows about 64 % CaO,

20 % Si0₂, 2.5 % MgO, 6 % Al-O- 3.5 % e₂0_, + FeO,

2 % K-0 + Na-O, 1.5 % S0_. A disadvantage of cement is that not all lime is bound in the concrete and that a consistently appearing excess of unstable hydrate of lime which is formed towards the end of the hardening process, is relatively easily leached out by the action of water and carbon dioxide in the air, involving a danger of detrimental carbonization. Furthermore, the chemical resistance to acidic and basic attack is very limited. Also, the chemical resistance to acidic and basic attack is very limited. Example: Destruction of concrete surfaces on roads by road salts or of concrete bridges by sea-water.

Risk of rust attack on steel reinforcement and great difficulties with glass fibre reinforce¬ ment. To avoid the sometimes disturbing weaknessess of Portland cement, materia-s having a similar composition but with an enhancement of the components improving both the physical and the chemical resistance have long been sought. It was natural to experiment with finely ground granulated basic blast-furnace slag as it has just a high percentage of highly resistant substances. The analysis is as follows, varying with the source: about 30 to 40 % CaO, 35 to 40 % SiO-, 7 to 10 % MgO, 10 to 20 % A1-0-, 0.5 to 2 % Fe-0₃ + FeO, 1 to 2 % K-0+Na₂0, 0.5 to 3 % SO-, . Compared to Portland cement, the lime content is only about _^• , but Si0₂ and Al-0_ is about double, and MgO almost 4 times higher. These substances, however, impart to silicates the highest mechanical and chemical resistance, i.e. increased compressive and tensile strength and resistance to chemical action. Blast furnace slag is obtained largely as a useless residual product in the manufacture of iron and steel and is present internationally in hundreds of millions of tons. "Granulated" generally means "subdivided", but in connection with slag it is commonly meant that the slag in a still red-hot state has been subjected to rapid cooling with water or with a combination of cold water and cold- air, whereby the slag becomes vitreous and amorphous. In spite of the favorable chemical composition the finely ground granular blast-furnace slag is only "latently" hydraulic, i.e. it does not bind directly after admixture with water. The reason is that there is formed a dense gel rich -in silicic acid, said gel enclosing the slag grains and preventing hydration. A condition for accomplishing activation is that this gel is broken. Thus, activators have a double task, they must first break down the gel and then react with the slag itself. However, the gel formation also has a positive effect, since the gel pores are uniformly distributed, whereby a better resistance to frost is obtained than with capillary pores in Portland cement concrete.

Already toward the end of the nineteenth century attempts to activate blast-furnace slag were made. The oldest patent goes back to 1892 (Passow) , wherein a mixture of slag with Portland cement is recommended and wherein the free lime in the form Ca (OH) „ formed in the final stage of hydration functions as an activator. Thereby, the reaction with slag occurs late and gives rise to a slow development of strength. Also the risk of shrinkage in cooling is rather great. This is the reason why the so-called slag cement is hardly used at present. In addition to lime the activators known for a long time (see H. Kϋhl, Zement-Chemie, Berlin 1951) are alkalies and sulfates. Hitherto, it- was thought that activation with alkalies results in the highest strength values, but it results in a number of disadvantages. The long-term strength is not satisf ctory, and there are great risks of shrinkage, salt deposition and carbonatization. Setting occurs tooquickly, in between 20 and 30 minutes, wherefore casting in a building site is not possible. Use is limited to the manufacture of prefabricated concrete elements. Activation by alkalies also has the disadvantage that strong caustic NaOH is formed. Activation with lime or sulfates has the disadvantage of an inferior short-term strength and a risk of swelling. For all these known methods it is also difficult to control the rate of setting which is either too rapid or too slow.

With the-prior art activating powders it was found that their admixture with slag powder during extended storage but also during transport will often result in some binding action. It is important to avoid this disadvantage in a novel activating system.

A much more reliable activation technique is obtained by the cooperation of acidic and basic components, the acidic substance consisting preferably of phosphates, optionally in combination with strongly acting sulfates, and the basic component of oxides or compounds of earth metals or optionally also zinc, and addition of water resulting in a hydraulic reaction. Earth metals, apart from magnesium, are calcium, strontium, barium, aluminium, beryllium, gallium, indium, thallium, titanium and zirconium and the so-called rare earth metals. Most effective is magnesium oxide, which has the best improving effect on silicates, as it en- hances the compression and tensile strengths and the elasticity, reduces shrinkage and results in a non hygroscopic product. Normally MgO can be incorporated in silicates only by melting at a high temperature. Together with phosphates, optionally in combination with sulfates, a hydraulically acting reaction is achieved with finely ground granulated basic blast¬ furnace slag. The best action is obtained with calcined magnesia (fired at about 1750 C, whereby all water and carbon dioxide have been driven off). Less suitable are MgO containing minerals, e.g. dolomite, which act more as fillers. The earth metal compounds are suitably employed in an amount of 0.3 to 3 % by weight, based on the dry concrete (i.e. slag + sand + aggregate) or 2 to 20 % by weight based on the slag.

The acidic components are suitably included in an amount of 0.3 to 6 % by weight, based on the dry concrete, or 2 to 40 % by weight based on the slag. Furthermore, it was found that the reaction will be much more active if also a detergent or nitrate is added which reduces surface tension, disperses and ' prevents lump formation. The same effect will be achieved by taking a phosphate having detergent action, e.g. Na tripolyphosphate. MgO and phosphate per se do not react with slag and water, only in combination.

Sometimes it is advantageous with a cooperation of MgO with other earth metal compounds. Al_>0- has positive effects similar to those of MgO, improves the reactivity of the slag and its resistance to chlorides. Titanium oxide imparts resistance to acidic actions, e.g. in contaminated air (sulfur deposition) , and forms resistant crystals with silica gels._ Zr0₂ gives a reliable security against alkaline attack.

An example of a strongly acting sulfate is sodium bisulfate, NaHSO., which on account of its strongly acidic reaction is often used industrially instead of sulfuric acid. The hitherto known activators mostly bind too rapidly (in the case of Portland cement too slowly) , and no suitable control could be achieved. This is possible with the new method, either by addition.of surface-action reducing agents or fluxing (plasticizing) agents, e.g. lignosulfonate, melamine, naphthalene- formaldehyde, sodium gluconate or the like, or by plaster of Paris or anhydrite (about 3 %) or by mixing different phosphates having different times of reaction. Thus, it will be possible to obtain a binding agent which will harden within half an hour for prefabricated concrete elements, whereby more castings can be made per day, or it will be possible to increase the binding time to about 2.5 hours which will be necessary for casting or a building site.

By the addition of amorphous silicic acid, e.g. in the form of the filtered residual product from el^'ectrometallurgical processes (such as silicon ferrosilicon or silicon chrome manufacture) , having an SiO- content between 75 and close to 100 % and usually a specific area of at least 20 m 2/g, so called silica fume or silica, the compressive strength and density can be further improved, preferably in combination with plasticizing agents. The amorphous silicic acid is suitably used in an amount of 0.6 to 2 % weight, based on the dry concrete or 4 to 15 % by weight, based on the slag.

The novel material is denser than concrete from Portland cement, is brighter in colour and lighter in weight. The new concrete can also be used as a plaster or porous or light-weight concrete, if a pore-forming agent or light-weight ballast of the type of perlite or vermiculite is added. Of course, it is possible to admix concrete aggregate, or steel, glass, mineral or plastic fibres or fly ash. A combination with bitumen (asphalt) is possible. The advantage of the improved slag concrete according to the present invention as compared to common concrete from Portland cement is above all a higher compressive and tensile strength, as seen from the table below. This includes both a higher short-term strength which enables removal of moulds in building sites to be carried out after about 10 hours for wall mouldings and af er about 16 hours for vault mouldings, which results in great savings,. and also an increasing strength for several months, while conventional concrete reaches maximum values after about 28 days.

The resistance to salt was tested at Chalmers Institute of Technology in Gothenburg for 4 months in a 30 % calcium chloride solution. No deterioration or cracking could be observed, as occurs in common concrete after a few weeks in highly concentrated calcium chloride solution.

Protection against attack by rust is achieved in common concrete by the free lime in the form of

Ca(OH)₂ formed in the final stages of hydration being deposited on the steel surfaces and protecting by its high pH the steel -from oxidation by penetration Of water, oxygen or CO_ from the air. However, calcium hydroxide is an unstable substance which is dissolved by water and converted by C0₂ (carbonatization) . In the new concrete, MgO which has a higher pH than lime forms the rust protection. Calcined MgO is resistant to water, oxygen and C0₂ and therefore more reliable than lime. In addition, the new concrete is much denser (less, porous) and therefore makes more resistance to penetrating water, oxygen or C0₂, which also results in an improved adherence to the steel reinforcement. The stability of the high pH value in the new concrete was also checked at Chalmers Institute of Technology by means of a bath of phenolphthalein which is a pH indicator. Permanent high pH is seen from unchanged red colour which is not the case with Portland cement concrete.

The combination of MgO and phosphate is hitherto known mostly from the manufacture of refractory ceramics but will also result in an improved fire-resistance of the activated blast¬ furnace slag. Usual concrete does not withstand temperatures higher than about 500 C. The reason for the sensitivity to high temperatures of Portland cement is substantially the presence-of chemically bound water. The physically bound water (capillary water) is removed at about 105 C without any deleterious action. The chemically bound water is released later, but with cracking which will then result in decomposition. The unstable free lime Ca (OH) ₂ will be converted into CaO and H₂0. At the same time the liberated water will also attack the tri- and dicalcium silicates formed durinσ hydration which will be converted into unstable calcium silicate hydrate. In addition, the alpha phase of quartz (Si0₂) present in the concrete will be converted into a different crystal form with increase in volume, which will also contribute to cracking (see R.K. Her "Chemistry of Silicates") . In the combination of blast-furnace slag, phosphate and MgO there is no free lime and the Si0₂ of the granulated slag is amorphous, wherefore these risks are not present. In uses where tenneratures above 1000 C can occur, it may be suitable to replace the stone material of aggregates which may expand in too high heat, by refractory ceramic materials which however is needed only exceptionally.

The new concrete may also be combined with bitumen (asphalt) in road pavings. As examples of the action of the novel combina¬ tion of activators with regard to compressive and tensile strengths, reference may be made to the follow¬ ing test results obtained with a mixture of 100 units of slag, 10 units of Na tripolyphosphate, 7.5 units of MgO, 353 units of sand and 40 units of water.

Age Compressive strength Ten;sible strength MPa MPa

10 1 day 6.2 1.2

3 days 26.0 3.7

7 days 40.9 6.3

28 days 81.3 10.4

* - These values are more advantageous than the corresponding values of Portland cement (after 28 days 49.0 and 7.9 MPa, respectively) . By the additions mentioned above, the tabled values can be further improved.

As compared to Portland cement concrete the

20 novel concrete achieves the following advantages.

1. Higher mechanical resistance, i.e. higher compressive and tensile strengths.

2. Higher chemical resistance.

3. No carbonatization, i.e. precipitation of 5 unbound lime, which may result in deterioration of the concrete.

4. No salt attack. A road paving will not be damaged by road salt. A higher life for concrete bridges. A possibility of making resistant concrete boats. 0 5. Not alkaline in spite of pH 12. No unbound lime, wherefore reinforcement glass fibres is possible. (If desired, a special type with ZrO- may be manufactured) .

6. Lighter than Portland cement concrete, 5 structures may be made thinner.

7. The possibility of making thinner layers or thickness makes the construction cheaper, apart from the fact that slag is cheaper than Portland cement. 8. Much denser.

9. Thereby better adherence to steel reinforce¬ ment and protection against -attack from rust on the steel reinforcement. 10. Higher refractoriness (fire resistance) .

11. Resistance to frost.

12. Facilitates casting in cold weather.

13. The characteristics also enables it to be used in sealing compounds. 14. Better material than cement mortar for plastering.

15. Lower requirements for moist hardening of freshly cast concrete.

16. Similar to usual concrete, the new material may be rendered porous to obtain a light-weight concrete which has great advantages as compared to traditional porous concrete a light-weight concrete, since the cell structure is mechanically stronger and the new material is not hygroscopic. 17. Lighter colour.

Claims

1. .A method of making a building material by activation of latently hydraulic finely ground granulated basic blast-furnace slag to form a direct acting hydraulic binder, characterized by mixing the slag, in addition to water, sand and ballast material, with a combination of acidic and basic components, the acidic components consisting of phosphates, optionally in combination with strongly acting sulfates, and the basic components consisting of oxides or other compounds of earth metals, optionally in combination with zinc, whereby even without heating a concrete of great mechanical strength and high chemical resistance is formed.

2. A method according to claim 1, characterized by adding surface-tension reducing agents.,

3. -A method-according to claim 1 or 2, characterized in that the earth metal compounds consist of MgO 1₂0-, TiO , ZrO-, BaO and/or ZnO in an amount of 0.3 to 3 % by weight based on the dry concrete or 2 to 20 % by weight based on the slag.

4. A method according to any of claims 1 to 3, characterized in that the acidic components consist of sodium tripolyphosphate, optionally in admixture with other phosphates or with a strongly acting sulfate, such as NaHSO. in an amount of 0.3 to 6 % by weight based on the dry concrete or 2 to 40 % by weight based on the slag.

5. A method according to any of claims 1 to 4, characterized by adding agents controlling the binding period, such as plaster of Paris or anhydrite, and/or plasticizing agents.

6. A method as in any of claims 1 to 5, characterized by adding amorphous silicic acid, e.g. as the filtered amorphous residual product from electrometallurgical processes, such as from the manufacture of silicon, ferrosilicon or chromium suicide, having a SiO~ content of from 75 % and up to almost 100 % and a specific

2 area of at least 20 m /g, so-called silica fume or silica, in an amount of 0.6 to 2 % by weight of the dry concrete or 4 to 15 % by weight of the slag.

7. A method according to any of claims 1 to 6, characterized by adding concrete ballast, or steel, glass, mineral or plastic fiber reinforcement or pore-forming agents or light-weight ballast.