Lightweight Gypsum Products Having Enhanced Water Resistance

Abstract

The product is produced from a settable aqueous calcium sulphate dispersion which has a water to solids ratio of less than 0.4 to 1, and has distributed therethrough lightweight hollow bodies having water-impervious surfaces (such as expanded polystyrene beads). The dispersion contains a hydratable cement (such as calcium sulpho aluminate) which is capable of hydration in the presence of the calcium sulphate dispersion. The hydratable cement is such that it reacts with excess water in the dispersion thereby enhancing the water resistance of the resulting product.

Claims

1. A method of producing a water-resistant gypsum product, in which the product is produced from a settable aqueous calcium sulphate dispersion having a water to solids ratio of less than 0.4 to 1, the dispersion having distributed therethrough lightweight hollow bodies having water-impervious surfaces; and a hydratable cement which is capable of hydration in the presence of the calcium sulphate dispersion, the hydratable cement being reactive with excess water in the dispersion, wherein the hydratable cement is present in an amount of 5 to 15%, based on the weight of hydraulic powder.

2. A method according to claim 1, wherein the hydratable cement comprises calcium sulpho aluminate.

3. A method according to claim 1, wherein the hollow bodies are of expanded polystyrene.

4. A method according to claim 1, wherein the hollow bodies are present in an amount of 0.5 to 2% by weight, based on the weight of hydraulic powder.

5. A method according to claim 1, wherein the dispersion contains an amphiphilic compound.

6. A method according to claim 5, wherein the amphiphilic compound is a soap.

7. A method according to claim 1, wherein the dispersion contains a water repellent agent that cures to form a hydrophobic silicone resin in an alkaline environment.

8. A method according to the invention, wherein the dispersion further contains glass fibre reinforcement.

9. A method according to claim 1, wherein the gypsum product has surface reinforcement.

10. A method according to claim 1, wherein the gypsum product is produced without externally applied heating.

11. A water-resistant gypsum product made by the process of claim 1.

12. A slurry comprising a settable aqueous calcium sulphate dispersion having a water to solids ratio of less than 0.4 to 1, the dispersion having distributed therethrough lightweight hollow bodies having water-impervious surfaces; and a hydratable cement which is capable of hydration in the presence of the calcium sulphate dispersion.

Description

EXAMPLE 1

[0024] Laboratory samples were produced using a Kenwood KM300 Chef Mixer, with planetary mixing action. The hydraulic material was pure plaster (Saint-Gobain Formula, Crystacal Base) or a 9:1 blend of plaster: Belitex CSA cement. The density was reduced with 1 mm diameter Expanded Polystyrene Beads at 1.2% w:w of hydraulic material dry blended in a plastic bag. The calcium stearate powder, namely SM-Microfine grade available from FACI (UK), was also blended at this stage in subsets 1.3 and 1.4. Tap water was pre-heated to 40 C. to replicate a typical plant slurry temperature when mixed with the powder. The water was weighed at 0.35:1, water:solid powder and added to the mixing bowl first, followed by the liquid hydrophobic additive BS1260 In the subsets 1.5 and 1.6.

[0025] The hydraulic material with EPS beads was poured on the liquid over 30 seconds, left to stand for 30 seconds and mixed over one minute starting at setting 1 and ramping up the speed incrementally each 10 seconds of mixing until finishing on setting 6. The slurry was deposited into a silicone rubber mould to cast 6cuboids measuring 20 mm20 mm100 mm. After hydration of the plaster was complete (this was typically 1 hour and was determined by temperature measurement) the samples were de-moulded and then sealed for 48 hours in a plastic bag to allow the CSA to hydrate further. The samples were then dried at 40 C. for 12 hours or more. Preparation included cutting 5 mm off the edges using a bandsaw to expose the core, then conditioning in a chamber at 23 C./50% RH for 12 hours or more. The weight and dimensions were taken at this point to give the initial weight and density.

[0026] The results obtained are shown in the following Table 1.

TABLE-US-00001 TABLE 1 Average dry Hydraulic Water repellent density Water uptake (%) Ref powder additive (kg/m.sup.3) 2 hrs 24 hrs 72 hrs 1.1 plaster None 733 16.5 16.3 16.1 1.2 None 783 12.4 12.2 12.2 1.3 plaster + 1% calcium stearate 800 9.1 10.3 10.0 1.4 CSA (9:1) 5% calcium stearate 757 5.1 8.7 8.7 1.5 0.1% Wacker BS1260 717 5.8 8.2 10.0 1.6 0.5% Wacker BS1260 749 1.5 2.6 3.1

EXAMPLE 2 (COMPARATIVE)

[0027] For comparative purposes, a commercially available piece of Glasroc was obtained from Saint-Gobain Gyproc. Polymethylhydrogensiloxane was added to the glass reinforced product at the mixer stage and coated with a polymer for further protection against surface water ingress. It was cut into 125 mm125 mm pieces for water absorption testing. Results are shown in comparison to those from Example 3 in Table 2.

EXAMPLE 3

[0028] The method used was the same as detailed in Example 2, apart from the following differences.

[0029] The hydraulic powder was a 9:1 blend of plaster: CSA cement. Gauging water at 40 C. was slightly higher than in Example 1, at water:solids ratio of 0.4. An excess of slurry was deposited into two moulds with a glass tissue facing material from Johns Manville beneath. The mould made with brass square section measuring 152 mm152 mm12.5 mm (internal dimensions). A top piece of glass tissue was then placed on top of the slurry, which was forced through by sliding a metal bar across, thus impregnating the tissue. The mould was weighted down between two Perspex sheets.

[0030] After hydration, the samples were de-moulded and dried at 60 C., 20% RH in a Vtsch climatic chamber for 12 hours. Preparation included cutting off the edges using a bandsaw to expose the core and produce samples 114-122 mm in length and width.

[0031] The results of water absorption tests are shown in Table 2. The results of wet strength tests are shown in Table 3.

TABLE-US-00002 TABLE 2 Average Water dry Hydraulic repellent density Water uptake (%) Ref powder additive (kg/m.sup.3) 2 hrs 24 hrs 72 hrs 240 hrs 2 plaster Poly- 709 2.7 40.3 51.8 64.4 methyl hydro- gensil- oxane 3.1 None 846 15.2 15.9 16.2 17.1 3.2 plaster + 0.25% 823 6.1 9.8 11.9 15.9 CSA Wacker (9:1) BS1260 3.3 0.5% 793 4.7 5.8 6.8 9.7 Wacker BS1260

TABLE-US-00003 TABLE 3 Wet strength error Hydraulic Water repellent Wet strength (1 standard Ref powder additive (Average; N) deviation; N) 2 plaster Polmethyl 18.7 1.3 hydrogensiloxane 3.1 plaster + None 120.6 11.6 3.2 CSA (9:1) 0.25% Wacker 95.7 6.4 BS1260 3.3 0.5% Wacker 99.4 10.0 BS1260

Water Absorption Test

[0032] The water absorption test was done on by immersing the samples in tap water at 23 C. such that a head of water of 30 mm between the top of the sample and water line was maintained. After given time periods the samples were removed and the excess water blotted before re-weighing. The method used was the same as that given in EN520: 2004 section 5.9.2, but the sample size was smaller and longer immersion times than 2 hours were used to demonstrate the improved water resistance of the invention. Example 1 results are averages from triplicate measurements, whereas Examples 2 and 3 are from duplicate measurements.

Wet Strength Test

[0033] The wet strength of the samples immersed for 240 hours (that is, saturated samples) was determined using a method similar to ASTM-C 473 section 12 for core and edge hardness measurements of gypsum panels. The maximum force required to drive a 2 mm diameter steel punch (ASTM-C 473 specifies 2.5 mm) through 13 mm of the sample was measured with a Mecmesin force gauge at a crosshead speed of 30 mm/min. The test was repeated around the sample edge 16 times to determine the average.