NEW ROOFING TILE WITH ENHANCED SURFACE DURABILITY AND PROCESSES FOR MANUFACTURING THE SAME

FIELD OF THE INVENTION

The invention relates to a novel roofing tile with enhanced surface durability, especially smoothness and acid-erosion resistance. The invention also provides processes for making the same. BACKGROUND OF THE INVENTION

In the following, the cement chemistry notation will be used: C = CaO; A = Al ₂ O ₃ ; S = SiO ₂ ; s = SO ₃ , H = H ₂ O.

WO-A-0172658 discloses a hydraulic binder for forming a non-efflorescing cementitious body, comprising a source of calcium aluminate, a source of calcium silicate, a source of sulfate and a source of reactive silica, especially comprising 10-49% by weight of a source of active silica (based on the weight of the dry hydraulic binder) , the balance comprising 40-90% by weight of the source of calcium aluminate selected from calcium sulfoaluminate cement or clinker, said source of calcium aluminate having at least 25% alumina or a C/A ratio of less than 3, 5-55% by weight of Portland cement or clinker as the source of calcium silicate, and 3-50% by weight of a source of sulphate, at least 25% of which is SO ₃ . The said ingredients are in relative proportions to form, upon hydration, the products monosulfate (C ₃ A. s.12H), hydrated alumina (AH ₃ ), as well as ettringite (C3A.3Cs .32H) and stratlingite (C ₂ ASHs) . This document aims at avoiding efflorescence, the efficacy of which is based on the presence of sulfate and reactive silica to form said hydration products and to minimize the formation of portlandite (CH) . The presence of a sulfate source and a reactive silica source is thus mandatory in this document.

EP-A-0356086 discloses a hydraulic cement forming compositions which comprise a high alumina cement blended with a latently hydraulic or pozzolanic material, especially blast furnace slag and silica fume, such that upon hydration,

stratlingite is formed. This hydrate effectively blocks the conversion of calcium aluminate hydrates such as CAHio and C ₂ AH ₈ to the denser C ₃ AH ₆ phase, thus counteracting the normal strength loss of CAC binders during aging (a phenomenon in the art known as "conversion") . Here the technical problem is to maintain a high mechanical strength, which is not a critical property for the application intended in the current invention.

There is however still a need to provide a cementitious mixture that will provide a hard, durable surface, which has excellent acid erosion resistance and an ability to maintain a smooth surface during natural weathering. An additional objective is to provide a means of applying said cementitious mixture as a thin coating (0.5-2.0 mm) on various substrates, particularly roofing tiles. SUMMARY OF THE INVENTION

The invention provides a roofing tile comprising: (a) a substrate; and (b) a coating disposed on said substrate, said coating resulting from hydration and hardening of a mixture comprising a hydraulic binder, said hydraulic binder comprising at least 60% by weight of a source of calcium aluminate and no more than 1% by weight of sulfate.

In one embodiment, the hydraulic binder comprises less than 0.5% by weight of sulfate, and preferably is substantially free of sulfate.

In another embodiment, the hydraulic binder comprises at least 35% by weight of alumina.

In another embodiment, the source of calcium aluminate comprises a calcium aluminate cement having an alumina content higher than 35%, preferably higher than 45%.

In another embodiment, the hydraulic binder further comprises a pozzolanic material, preferably having an alumina content of at least 20% by weight, preferably at least 30%.

In another embodiment, said hydraulic binder comprising at least 80% by weight of a source of calcium aluminate.

In another embodiment, the hydraulic binder further comprises a fine limestone filler, preferably in an amount of 0-10 parts for 100 parts of calcium aluminate cement.

In another embodiment, the coating comprises additives such as retarders, accelerators, superplasticizers, rheological modifiers, defoamers, and thickeners, and preferably a combination of a superplasticizer and retarder.

In another embodiment, said hydraulic binder comprises 20- 50 parts of pozzolanic material for 100 parts of calcium aluminate cement.

In another embodiment, the hydration is obtained with a water to calcium aluminate source ratio of less than 0.4, preferably between 0.25 and 0.40.

In another embodiment, the tile material further comprises sand with a maximum diameter of less than 1 mm.

In another embodiment, tile material comprises at least 32.5% by weight of binder, preferably about 50%.

In another embodiment, the coating has a density of from 2.0 to 2.3. In another embodiment, the substrate binder is based on Portland cement.

In another embodiment, the tile further comprises a coating on it.

The invention also provides a process of manufacturing the roofing tile of the invention, comprising the steps of coating an exterior surface of a freshly made substrate with a paste and curing the article and the coating together.

The invention further provides a process for manufacturing a roofing tile, comprising the steps of: (i) providing a substrate; (ii) mixing water and a cementitious mixture comprising a hydraulic binder, said hydraulic binder comprising at least 60% by weight of a source of calcium aluminate and no more than 1.0% by weight of sulfate; (iii) and applying on said substrate a coating slurry of the mixture of step (ii) ; and (iv) causing hydration of said coating on the substrate of step (iii) into the roofing tile. This

process is useful for the manufacture of the tiles of the invention.

In an embodiment, the coating slurry has a slump value between 135-175 mm, preferably 140-160 mm, for a period of at least 20 minutes, preferably at least 30 minutes.

In an embodiment, the air content of the slurry is less than 10%, and preferably closer to 5%.

In one embodiment, steps (ii) and (iii) are carried out as a co-extrusion. In one embodiment, step (iii) is carried out by a brush application.

In one embodiment, step (iii) is carried out by the bell process .

In another embodiment, the process further comprises a curing step at a relative humidity of from 70% to 100% and at a temperature of from 0 ⁰ C to 60 ⁰ C and for a period of from 30 minutes to 24 hours.

In another embodiment, the process further comprises a post-treatment acid wash. The invention is based on the use of a high alumina content hydraulic binder, with very limited amount of sulfate and preferably no sulfate. The invention may thus provide a binder with a maximum alumina content, which would not have been possible in prior art formulations due to the diluting effect of added sulfate. A high alumina content in the binder is desirable for acid resistance due to the low solubility above pH 4 of hydrated alumina, the ultimate decomposition product of calcium aluminate hydrates in acidic or leaching environments. Moreover, the minimization of sulfate in the invention reduces the water demand in the system, thus allowing for higher densities and improved durability in the invention. Prior art compositions have correspondingly lower densities due to the high water demand of sulfoaluminate compounds such as monosulfate and ettringite. These compounds represent the principle phases formed in calcium aluminate systems containing high additions of sulfate. Monosulfate and ettringite are substantially non-existent in the invention

(i.e., they may, at most, represent a few percent, e.g. less than 5%) . BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a process of one embodiment of the invention.

Figure 2 is a schematic representation of the process of another embodiment of the invention.

Figure 3 is a Scanning Electron Microscopy (SEM) micrograph of a cross section of a tile, according to example 2, exposed for ten years to natural weathering. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Cementitious material

One characteristic of the invention is the very low amount of source of sulfate and preferably its substantial absence. The term "sulfate" or "source of sulfate" is intended to designate the reactive sulfate species, designated SO ₄ .

Components comprising low amounts of sulfate may thus be present, as long as their content is such that the upper limit for the sulfate content in the final composition is not exceeded.

Calcium aluminates are combinations of aluminum oxide, AI ₂ O ₃ , and calcium oxide, CaO. Calcium aluminate may be generated in situ by the use of separate sources of calcium oxide and alumina as ingredients, or, alternatively, alumina can be added afterwards to obtain the required alumina concentration. The source of calcium aluminate cement is preferably a hydraulic binder such as Fondu (37% AI ₂ O ₃ ) , Secar 51 (50% Al ₂ O ₃ ), and Secar 71(68% Al ₂ O ₃ ). The calcium aluminates may be crystallized in several different anhydrous phases such as (using cement notation) CA, C ₃ A and C ₁₂ A ₇ . The source of pozzolanic material can be ground blast furnace slag or fly ash.

In the current invention the pozzolanic material, if present, represents less than 50% by weight of cement, preferably between 10 and 30% by weight. The pozzolanic material should preferably have a high Al ₂ O ₃ content — at least above 20%, and preferably above 30%. The particle size

distribution of fly ash can vary widely depending on the process. The D50 (mean particle size) of the fly ash can range can be between 2 - 60 μm as measured in the wet process on a Malvern laser granulometer . The amount of fly ash added may be limited by its granulometry . The preferred D50 granulometery of the fly ash is 10 - 20 μm.

A fine limestone filler may also be added to the slurry for improved compaction of the slurry or for reduced strength loss. The latter effect may arise from the formation of a carboaluminate phase, C4ACH ₁₁ . This phase tends to form at the expense of C2AH ₈ and C ₃ AH ₆ , thus hindering (but not necessarily permanently preventing) conversion (see H. F.W. Taylor, Cement Chemistry, 2 ^nd ed. , Thomas Telford, London, 1997). The limestone filler, which may serve as a replacement or partial replacement of the pozzolanic material, can represent up to about 10 parts per 100 parts cement in the slurry. The fine limestone can have a D50 mean particle size of 1 -10 μm, and preferably between 1 - 5 μm. It should be noted, however, that the intrinsic acid resistance of the slurry may decrease with increasing limestone content, due to the higher solubility of limestone relative to AH ₃ . The specific dosage of limestone filler will ultimately depend on the priorities placed on the rheological, strength, and durability properties of the slurry/coating. Sand of a fine grading, from 0.0-0.6 mm, is used to ensure a smooth surface. This will also enhance the acid and freeze- thaw performance of the body. Durability can also be enhanced by optimizing the type and quantity of sand.

Additives known in the art to be effective with calcium aluminates based binders including defoamers, retarders typically citric acid, plasticizers, superplasticizers (e.g., polycarboxylate ether), pigments (e.g., inorganic or organic), rheology enhancers (e.g., acrylates emulsion), and thickeners

(cellulose or wellan gum) may be added to achieve the required fresh slurry properties.

A typical composition for a slurry would be the following (in parts by weight) :

^a Cited for a coextrusion process. ^b Expressed in terms of solids content. ^c Expressed in terms of active component.

As illustrated in the preferred embodiment, a water to calcium aluminate source ratio lower than 0.4 can be used. This relatively low amount of water results in a dense final product . Process for manufacturing

A general process for manufacturing the roofing tile comprises the following steps:

(i) preparing a concrete roofing tile substrate; (ii) preparing a calcium aluminate cement rich slurry; (iϋ) applying the slurry to the roofing tile substrate; (iv) curing the coating and substrate together; and (v) optionally post-treating the coating on the substrate, notably for aesthetic and/or durability reasons.

The preparation of the concrete roofing tile substrate of the invention may be carried out using the processes disclosed in US-P-5017230 or US-P-4986744 , incorporated herein by reference.

US-P-5017230 describes a process for manufacturing stratified pieces such as roofing tiles by successive and independent extrusion of concrete, which is deposited on a

limited part of the moulds for the first layer. Thereafter follows a compacting and beveling step before the application of the second layer to completely fill the mould. US-P-4986744 describes the high pressure application of the top finish coating layer of a roofing tile onto the already pre-shaped and pre-compacted bottom layer.

The process of manufacturing the roofing tiles of the invention comprises first producing the concrete tile body or substrate by standard methods of mixing and extrusion of the tile body material using with a standard tile machine. This technology is known in the art and is described in patents US- P-3193903 to White, and US-P-4666648 to Brittain, and which are incorporated by reference.

The calcium aluminate based slurry can be prepared in a batch process or in a continuously mixed process. The preferred method is the batch mixed process where the wet and dry ingredients are placed in a mixer and agitated for 1 - 5 minutes as is well known in the art of preparing cement slurry. The continuous mix method may be advantageous in some industrial settings where the wet and dry ingredients can be pre-blended independently, and then combined in a high shear short mixer for a period of 5 - 20 seconds before the slurry is applied to the substrate. In a third process step the tile is then coated by extrusion with a slurry. The slurry is batched and prepared prior to mixing. The method of applying the slurry may vary, depending on the particular factory process, as well as the desired surface aesthetics (e.g., mono- versus multi-colored). Typical slurry application processes include coextrusion, brush application, and the so-called "bell" (or curtain coating) process . Each process usually possesses distinctive rheological and formulation requirements. After the slurry is applied, the tile and coating are cured together under standard curing conditions (e.g., relative humidity of 70- 100%, temperature 10°C-60°C for a period of 30 min-24 hours) .

One embodiment of the present invention uses a coextrusion process, as illustrated in Figure 1. The body of the tile is first extruded (1) using the tile body material (2). Then the slurry formulation (3), after mixing, is pumped via a pump (4) and extruded directly as a layer (5) onto the underlying fresh body. This inline process thus obviates the need to transport the tile body to a separate slurry application unit. By this process it is possible to produce a flat or profiled roofing tile (6), which, with the action of the smoothing bars (7), possesses a smooth and homogeneous surface. The tile is then cured in standard conditions. The double slipper method for coextrusion (e.g. as disclosed above and in relation with US- P-4986744) may be alternatively simulated by a double roller method, whereby the body and slurry are extruded sequentially, rather than simultaneously.

In another embodiment, the slurry is applied by a brush application, as illustrated in Figure 2. In this process, a relatively fluid slurry flows from its reservoir down a declined surface, and ultimately to the junction between the rotating inter-roller and brush. The bristles of the brush project the slurry as droplets onto the target tile. The size and speed of the droplets depend on the flow rate of the slurry as well as the rotation speeds of the roller and brush. Multi-colored tile designs are possible from this process. The body and slurry coating are then cured under standard conditions .

In yet another embodiment, the slurry is applied by the bell process, where a fluid slurry is passed over a bell- shaped piece, thereby creating a curtain through which the body substrate is passed. After the slurry is deposited on the body, the two are then cured together under standard conditions .

In the above processes, the mortar for the tile body is usually produced by first mixing the solids in a batch mixer (any batch mixer suffices) , then introducing the liquids and mixing the slurry in 2 stages, with a moisture control test conducted in between. The overall moisture content of the tile

body is adjusted to a sufficiently low value (typically 6.5- 8.0% depending on the initial state of the raw materials) so as to give the desired rheology and to prevent undesirable bubbles in the overlying slurry. The tile bodies may be extruded using roller and smoothing bars as known in the art. The tile body weight may range from 3.0 to 6.0 kg for tile dimensions that are typically 4300 mm by 3300 mm.

As already indicated above, the slurry is mixed (e.g. using a high-shear mixer) for 2 to 5 minutes, depending on the particular type of mixer used. The rheology of the fresh mix can be assessed with a slump test. The equipment typically used for such a test is described in DIN 1048-T1 (slump table) and EN 196-3 (test sample receptacle) . The process, adapted from various standards, involves first filling a cylindrical sample receptacle (8 cm diameter, 4 cm height) with the fresh slurry. The receptacle, which is placed in the center of the slump table, is then removed. A coextruded slurry should preferably show nearly zero initial slump under its own weight. After the slump table is lifted 15 times (15 knocks) to a fixed height, the slump of the slurry is measured; an average of 2 perpendicular diameters is taken. Optimal coextrusion performance is obtained with slump values between 135-175 mm, preferably 140-160 mm. To ensure consistent large- scale production this slump should be maintained for a period of at least 20 minutes, preferably at least 30 minutes, depending on the particular manufacturing process. Furthermore, a good quality slurry has a smooth and creamy consistency and a lack of excessively large air bubbles. While there is no rigorous limit on the air content of the fresh slurry, it should be less than 10%, and preferably closer to 5%.

To reach the target fresh slurry properties mentioned above for the coextruded slurry, one will use the proper combination and type of additives, as appreciated by the skilled worker in the art. A combination of a superplasticizer based on polycarboxylate polyox molecules and an effective calcium aluminate retarder (e.g., citric acid) can provide excellent

fluidity and workability for 1 hour or longer. Plasticizers based on naphthalene-type and especially melamine-type resins are not preferred since the workability may not be sufficient for practical use. Rheological modifiers are added to facilitate the pumpability of the slurry. Defoamers are used to reduce the overall air content as well as the large, obtrusive bubbles that have deleterious effects on the aesthetics and the durability of the surface.

After curing, the tile product may be further treated by acid washing for cleaning purposes, or by coating with paints, clear-coats, and other protective coatings. Roofing tile

The resulting roofing tile of the invention comprises a relatively thin coating, or slurry, based on a calcium aluminate cement binder, typically with a thickness of 0.5-2 mm, and especially about 1 mm.

The high aluminate content of the slurry of the roofing tile of the invention encourages the formation of a dense layer of hydrated alumina (AH ₃ ) gel. Depending on the particular curing conditions, this hydrate may form during the initial curing process. The formation of AH ₃ can also be significantly accelerated upon exposure of the slurry to acidic solution. This acid exposure can be an intentional post-treatment, such as a brief acid wash of the slurry at pH values above 4, or preferably, the acid can be introduced naturally in the form of acid rain during natural weathering. Both routes will produce a dense AH ₃ layer due to the infilling of surface pores by the AH ₃ gel. The layer, whose thickness can be tens to hundreds of microns, thus serves as a protective barrier layer that is dense, hard, and subsequently resistant to erosion and other forms of degradation. The low solubility of AH ₃ above pH 4 further increases the erosion resistance of the surface. Moreover, the formation of the AH ₃ layer results in a smooth surface, which is useful in reducing the potential for colonization by microorganisms such as algae and lichens since they would not grow in the irregularities of the surface.

The substrate, generally 6-20 mm in thickness, possesses coarser aggregates than the slurry, using standard sand 0-4 mm in diameter. Small amounts of inorganic standard pigments and of plasticizers may be added. The binder in the substrate is generally comprised of OPC (Ordinary Portland Cement) or OPC and pozzolanic material (with the ratio of OPC to pozzolanic material ranging from 100:0 to 70:30). A typical composition for the tile body is given below.

Component Kg

Sand, 0-4mm 1150

Ordinary Portland Cement 290

Pigment ³ 8.0

Superplasticizer ^a 0.0-0.5

Water/Cement 0.4

Expressed in terms of solids content.

EXAMPLES

The following examples, which are based on the coextrusion process, are illustrative of the invention and shall not be considered limiting the scope thereof. The amounts are expressed in kg.

Example 1.

Expressed in terms of solids content ³ or active component*

The particular example is produced from the coextusion process depicted in Figure 1.

The slurry is mixed using a high-shear mixer for 2 to 5 minutes .

Table. Fresh slurry properties of Example 1.

The slurry is then pumped using an eccentric screw pump operating at standard conditions and is delivered by a pressure pipe to the dosing hopper. Again, for extrusion standard roller and smoothing bars of the prior art may be used at standard operating conditions.

Example 2.

Expressed in terms of solids content ¹³ or active component

The tiles of example 2 have been produced using a similar process to that depicted in example 1.

Figure 3 is a Scanning Electron Microscopy (SEM) micrograph of a cross section of the tile from example 2, exposed for 10 years in natural weathering in a typical western European climate. SEM analysis was conducted in a back scattered electron (BSE) mode using 12kV, InA (measured with a Faraday cup) and a 12mm working distance.

As seen in Figure 3, a dense, protective layer of AH ₃ has formed spontaneously during natural weathering. This layer

(here about 50 μm in thickness) serves as a protective and durable barrier, especially against erosion and roughening during weathering.