GB2075963A - Setting-time controlling agents for mouldable inorganic materials - Google Patents

Abstract

The setting times of mouldable inorganic materials such as gypsum based plasters are regulated by adding dried milk solids as a controlling agent. Tartaric acid may be present in the mouldable inorganic materials.

Description

SPECIFICATION Setting-time controlling agents for mouldable inor ganic materials The invention relates to setting-time controlling agents for mouldable inorganic materials such as gypsum-based plasters.

Gypsum plasters and the like materials are well known and are used widely as building materials for stucco and plaster work. In addition, they are used in artistic and medical modelling work, and for prefabricated components.

This versatility is only possible if the natural setting behaviour of these materials can be modified in such a way asto allow adequate time forthe completion of the desired processing times.

In the processing of, for example, lime-gypsum plaster for interior finishing work, the so-called stiffening of the plaster is required to begin only some 40 to 80 minutes after gauging with water with an equally long, or longer, time interval following, in which the plaster can still be worked and smoothed on the wall. In certain other processes, however, it is desirable that the stiffening of a gypsum-based plaster be delayed for a very long period, but for the final setting to begin shortly afterwards.

The setting characteristics of gypsum plaster can be controlled to a certain extent by modifying the manufacturing process to regulate the crystal structture and size. In most cases, however, it is preferable to add, setting-time controlling agents, usually known as setting-retarders.

These agents react in the setting process in various, generally complex, ways. For example, alcohols and certain lower fatty acids have a retarding effect by reducing the solubility of the an hydrous or hemi-hydrate phases of the gypsum plaster. Other substances, such as inorganic phosphates produce filmy deposits on the soluble grains which greatly lower the rate of dissolution of the plaster in the mixing water. The effectiveness of many commercially obtainable retarders is based on the fact that they prevent precipitation of calcium sulphate dihydrate by hindering nucleation for crystal growth. The complexing of calcium ions also influences the mechanism of setting; therefore, typical chelationcomplex forming compounds, such as dicarboxylic acids and polyhydroxy carboxylic acids are also useful setting-retarders.Many of these compounds are also known to be effective retarders for Portland cement, but despite the large number of systems which have been investigated, it is still almost impossible to determine from the structure of a compound whether it will be effective and suitable for use as a setting-retarder for either gypsum plaster or Portland cement.

Problems have also been encountered when calcium hydroxide in the form of dry hydrated lime is present in a gypsum plaster formulation. Although the presence of lime is known to improve many properties of the hardened plaster and indeed, to impart improved handling characteristics to the wet plaster, it has also been found that the functioning of many ofthe previously known setting retarders is impaired by the presence of lime. This is especially marked in the case of the common protein retarders such as keratin.

It is known that L-tartaric acid produces a substan tial delay in the setting of gypsum plasters, espe cially when these contain a proportion of hydrated lime. D, L-tartaric acid (a racemic mixture) is also known to exhibit this favourable property when used as an additive to gypsum, and a combination of meso-tartaric acid with a D, L-mixture has been recommended for use with lime-gypsum plasters. In this last case, the pH value of a 5% by weight sus pension of the plaster in water should be at least 12.0.

The disadvantages of any of the above tarta ric acid setting-retarders are their high price and, more important, the fact that a reliably constant supply is not available.

An object of the present invention is to provide a setting-retarder for materials such as gypsum plaster in which some, or all, of the tartaric acid normally used is replaced with a cheaper and more readily obtainable ingredient which is compatible with either lime-gypsum plasters, or lime4ree gypsum plasters.

The present invention provides a method of regulating the setting times of mouldable inorganic materials such as gypsum-based plasters including the step of adding to the materials in their dry, particulate condition a setting-time controlling agent comprising dried milk solids.

If preferred, the quantity of the dried milk solids may be reduced and an amount of L-, D-, DL- and/or meso-tartaric acid added singly or in selected mixtures, said amount of tartaric acid being less than would be required for adequate setting retardation in the absence of the milk solids.

The dried milk solids may be a commercially available grade of skimmed milk solids and may contain added vegetable fats, sugars, emulsifiers, preservatives (a nti-oxidants), vitamins or sodium aluminium silicates. In practice, most commercial dried skimmed milks, contain some or all of these additives, at least some of which may have plasticising, dispersing or retarding effects, none of which is incompatible with gypsum plaster.

The setting retarders used according to the invention preferably contain dried skimmed milk solids, as commercially obtained, in a proportion of from 60 to 100%, and more particularly in a proportion of from 85 to 95% by weight. The setting-retarders may be used in quantities of from 0.05 to 5% by weight and preferably in quantities of from 0.15 to 1.5% by weight, as a percentage in addition to the total weight of plaster.

Accordingly, gypsum plasters which, on account of their origin, their manufacture ortheirfabrication, e.g. with lime-gypsum plaster, formerly could be retarded efficiently only with D-, DL-, L-, or mesotartaric acid, can be retarded efficiently and more economically to give technically acceptable settingtimes with the retarders used according to the invention.

The invention therefore also provides for use, when admixed with water, as a mould able material, a dry particulate inorganic material such as gypsum-plaster containing dried milk solids to act as a setting-time controlling agent Preferably the material may also contain L-, D-, DL-, or meso-tartaric acid, singly or in selected mixtures, this amount being less than would be necessary for adequate setting retardation in the absence of the milk solids. For example a material may include an addition of between 0.6 and 1.0% by weight of dried milk solids and give comparable setting times to those of a material including an addition of between 0.2 and 0.4% of a 9:1 mixture or dried milk solids and tartaric acid.

It will be found that by varying the amounts and reiative quantities of the setting retardant agents, it is possible to control not only the overall setting time, but,withinthattime period, also the point at which so-called stiffening, i.e. initial setting, commences.

It will be appreciated that the constituents of the retardant agent, hitherto tartaric acid, and according to the present invention, dried milk solids are effective when used alone. For example a gypsum/lime mixture which will set after 2 or3 hours using either agent by itself will, where a mixture of the two is used, set finally at about 4 hours, although the amounts of each are considerably reduced compared with the amount used when present on its own.

The efficiency of the four stereo-isomers of tartaric acid is known to vary, the most efficient being L-tartaric acid. However, supplies of the somewhat less efficient D-tartaric acid are normally more readily available than any of the other three, but, when admixed with milk solids according to a preferred example of the invention, the D-isomer gives an improved efficiency,making its use entirely acceptable.

The invention will now be further described with referece to the following examples. It will be understood that the description is given by way of example only and not by way of limitation.

In the examples, the terms used are defined as follows.

Gypsum plaster - hemihydrate gypsum plaster to B.S.1191 Part 1: 1973: CLASS A.

Hydrated lime - Hydrated high calcium lime to BS 890:1972 Limestone filler - ground limestone filler at least 96% Cacao3, 99% by weight passing a 425 mic ron B.S. 410 test sieve and 100% passing a 1.18 mm BS 410 sieve.

Exfoliated vermiculite - material belonging to the class of aluminium-iron-magnesium silicates having a laminar structure, exfoliated by sud den exposure to temperatures of 700-1 000 C at which moisture between the laminae forces them apart, 100% ofthe material passing a 1.18 mm BS 410 sieve.

D-tartaric acid - Commercial grade D (+) tartaric acid, usually less than 500 micron grain size.

Keratin-based retarder - commercially obtainable retarder based on the protein keratin.

Example I Taking a gypsum plaster of the following composi- tion: Calcium(%CaO) 31.5 Sulphate (%SO3) 40.2 Insoluble residue (%) 15.7 five identical batches (A-E) of dry plaster mix were made up having the following constituents: % by weight of dry mix Gypsum plaster (as above) 52.50 Hydrated lime 7.50 Limestone filler 38.95 Hydroxypropylmethyl cellulose 0.05 Exfoliated vermiculite 1.00 Setting agents were added to four of the batches according to the following table, with no setting agent added to batch E which served as a control.

The characteristics of the five batches are given below.

Table I

Batch A B C D E Retarder Dried 90 parts D-tartaric Keratin Nil added milk dried milk aric acid based % on dry solids solids/10 retarder weight parts D-tartarics acia 0.8 0.25 0.15 0.25 - Water addition 38.9 42.5 52.8 48.7 53.0 (% on dry weight) Initial setting 94 85 80 53 8.5 time (mins) Final setting 142 160 128 76 12.5 time (mins) Consistence retentivity 66 62 49 44 Water retentivity 95 94 90 89 Modulus of rupture 2.1 2.1 1.8 1.7 (Nlmm2) It will be observed that, compared with conventional keratin-based retarders, there is a marked lengthening of both initial and final setting times.

The longest final setting time is that of batch B, which included both dried milk solids and D-tartaric acid, the quantities used being far less than that of each of these two constituents by itself required to achieve comparable results.

Comparing the batches from the point of view of manufacturing expense, the cost of the dried milk solids used with batch A was approximately half that of the cost oftartaricused to obtain similar setting times with batch C. A further cost saving accrued for batch B, the milk solids/tartaric acid mixture costing only 75% of the cost of the milk solids alone in batch A.

Moreover, besides the cost advantage of using a retarding agent including milk solids, the resulting plaster work made up from batch B, when dry, was of a higher quality, harder, stronger and with less surface cracking, than the plaster work done with batch C, which used tartaric acid alone. This may possibly be due to the plasticising action of the vegetable matter in the skimmed milk.

Example lI Taking a gypsum plaster of a differentcomposi- tion, viz: Calcium (% CaO) 37.8 Sulphate (% S03) 51.6 Insoluble residue (%) 1.6 five further identical batches (F-J) were made up having the following constituents: % by weight of dry mix Gypsum plaster (as above) 52.50 Hydrated lime 7.50 Limestone filler 38.95 Hydroxypropylmethyl cellulose 0.05 Exfoliated vermiculite 1.00 Setting agents as detailed in Table II below were added to batches F-l, and J was the control.

Table 11

Batch F G H I J Retarder Dried 90 parts D-Tartaric Keratin Nil added milk dried milk acid based % on dry solids solids/l0 retarder weight parts D-tartaric acid 0.8 0.25 O.15 0.;;E5 ~ Water addition 37.2 41.5 48.9 45.3 50.1 (% on dry weight) Initial setting 90 82 81 41 4.5 time (mins) Final setting 135 154 120 63 6.0 time (mins) Consistence retentivity 68 64 52 56 Water retentivity 98 95 91 92 Modulus of rupture 2.2 2.3 2.3 1.9 Example 111 Five further samples (K, L, M, N, P) were made up which incorporated the gypsum plaster of Example These batches contained the following:: %by weight of dry mix Gypsum plaster (as Example I) 33.3 Hydrated lime 17.9 Limestone filler 48.6 Hydroxypropylmethyl cellulose 0.2 Exfoliated vermiculite nil Setting agents were then added to batches K-N according to Table Ill with batch P acting as the control. The characteristics of the batches are given below.

Table 111

Batch K L M N P Retarder Dry 90 parts D-tartaric Keratin added milk dried milk acid based nil %on dry solids solids/10 retarder weight parts D-tartaric acid 0.8 0.25 0.15 0.25 - natter addition 48.0 49.5 54.7 53.4 56.0 on on dry weight) Initial setting 93 98 79 32 9 time (mins) Final setting 134 158 340 45 time (mins) Consistence retentivity 74 70 60 63 % water retentivity 99 99 94 95 Modulus of rupture 1.5 1.4 1.2 1.0 (Ntmm2) It will be observed that the performance of batch M showed a particularly long final setting time, using relatively expensive D-tartaric acid. Batches K and L however, produced entirely adequate setting times however using retarder materials according to the invention at a fraction of the cost of the retarder addition to batch M.

Example IV Five samples (Q-U) were made up in a limefree mix which incorprated the gypsum plaster of Exam ple I. These batches contained the following: % by weight of dry mix Gypsum plaster (as Example I) 60.00 Hydrated lime nil Limestone filler 38.95 Exfoliated vermiculite 1.00 Hydroxylpropylmethyl cellulose 0.05 Batch U was the control.

Table IV

Batch O R S T U Retarder Dried 90 parts D-tartaric Keratin nil added milk dried acid based % on dry solids milk retarder weight solids!1 0 parts D-tartaric acid 0.8 0.25 01.5 (IR5 Water addition 36.4 40.2 51.0 4i9 51.5 (%on dry weight) Initial setting 78 77 36 6 8.0 time (mins) Final setting 150 134 42 102 12.5 time (mins) Consistence retentivity 150 134 42 102 12.5 time (mins) Consistence retentivity 48 45 41 44 Water retentivity 83 80 78 79 Modulus of rupture 2.2 2.3 2.4 2.0 (Nlmm2) It will be observed that although the performance of D-tartaric acid alone is considerably impaired when used in batch S, the retarders according to the present invention used in batches 0 and R give satisfactory results.

Claims (15)

1. A method of regulating the setting times of mouldable inorganic materials such as gypsum based plasters including the step of adding to the materials in their dry particulate condition a settingtime controlling agent comprising dried milk solids.

2. A method as claimed in claim 1, including the addition of tartaric acid to the materials.

3. A method as claimed in claim 2, wherein the addition of tartaric acid to the material is accom panied by an addition of the dried milk solids in a quantity less per unit weight of inorganic materials then when no tartaric acid is present, to achieve the same level of control of the setting times.

4. A method as claimed in either one of claims 2 or3, wherein the tartaric acid is selected from the group comprising L-, D-, DL- and meso-tartaric acid, added alone or in mixtures thereof.

5. A method as claimed in any one of claims 2 to 4, wherein the proportion of dried milk solids to tar taricacid is irr the region of 60% to 90%.

6. A method asclaimed in claim 5 wherein the proportion is from 850/oto 95%.

7. A method as claimed in any one of thepreceding claims,. wherein the mouldable inorganic materials do not contain free lime.

8. A method as claimed in either one of claims 1 and 2, wherein the addition of controlling agent is between 0.05% and 5% by weight of inorganic materials.

9. A method as claimed in claim 8 wherein the addition is between 0.15% to 1.5% by weight.

10. Adry particulate inorganic material for use, when admixed with water, as a mouldable material, said material containing dried milk solids to act as a setting-time controlling agent.

11. A material as claimed in claim 10, containing tartaric acid.

12. A material as claimed in claim 10, containing between 0.6 and 1.0% by weight of dried milk solids.

13. A material as claimed in claim 11, containing between 0.2 and 0.4% by weight of a mixture of dried milk solids and tartaric acid.

14. A method of regulating the setting time of mouldable inorganic materials such as gypsumbased plasters, substantially as hereinbefore described with reference to the Examples.

15. A dry particulate inorganic material for use, when admixed with water, as a mouldable material, produced substantially as hereinbefore described with reference to the Examples.