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A cementitious slurry comprising fly ash having a predominant particle size of up to about 10 microns, and/or aluminous material having a predominant particle size of up to about 150 microns. The additive acts as a water reduction agent and can replace either wholly or partially a conventional plas

A cementitious slurry comprising fly ash having a predominant particle size of up to about 10 microns, and/or aluminous material having a predominant particle size of up to about 150 microns. The additive acts as a water reduction agent and can replace either wholly or partially a conventional plasticiser.

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What is claimed is: 1. A cementitious slurry comprising: cement; water; a plasticiser; a dewatering additive including a component selected from the group consisting of: (i) a fly ash having particles with a predominant size up to about 10 microns forming about 30 wt % to 100 wt % based on the weig

What is claimed is: 1. A cementitious slurry comprising: cement; water; a plasticiser; a dewatering additive including a component selected from the group consisting of: (i) a fly ash having particles with a predominant size up to about 10 microns forming about 30 wt % to 100 wt % based on the weight of said cement in the slurry; (ii) aluminous material having particles with a predominant size less than about 150 microns forming about 5-30 wt % based on the weight of said cement in the slurry, wherein the aluminous material is selected from the group consisting of hydrated alumina, partially hydrated alumina and unhydrated alumina; and (iii) both the fly ash of (i) and the aluminous material of (ii); wherein the components (i), (ii), or (iii) of said dewatering additive are added to provide water reduction, and the amount of said fly ash having said particle sizes is greater than the amount of said cement by weight percentage. 2. The cementitious slurry of claim 1, wherein said fly ash comprises about 25-60% silica, about 10-30% Al2O3, about 5-25% Fe2O3, up to about 20% CaO and up to about 5% MgO. 3. The cementitious slurry of claim 1, wherein the ratio of hydrated alumina to said fly ash is between about 1:1 and 1:10. 4. The cementitious slurry of claim 1, wherein said cement is selected from the group consisting of common Portland cements, fast setting or extra fast setting cement, sulphate resisting cements, modified cements, alumina cements, high alumina cements, calcium aluminate cements and cements which contain secondary components. 5. The cementitious stuffy of claim 4, wherein said secondary components are selected from the group consisting of fly ash and slag. 6. The cementitious stuffy of claim 1, wherein said cement is between about 10 and 50 wt % of total dry ingredients. 7. The cementitious slurry of claim 1, wherein the amount of said plasticiser is between about 0.3 to 3 wt % based on weight of the dry cement. 8. The cementitious slurry of claim 1, wherein said cementitious slurry contains about 5 to 30 wt % of fillers. 9. A cementitious slurry comprising: a hydraulic binder for cement; water; a water reducer selected from the group consisting of: (i) fly ash having particle sizes up to about 10 microns; (ii) an aluminous material selected from the group consisting of hydrated alumina, partially hydrated alumina, unhydrated alumina and mixtures thereof; wherein said water reducer reduces by about 20%-40% the amount of water required in the slurry to achieve substantially the same slurry viscosity as a cementitious slurry of an equivalent formulation substantially identical in composition to the formulation of the cementitious slurry but without said water reducer. 10. The cementitious slurry of claim 9, wherein said aluminous material comprises a plurality of particles having size up to about 150 microns. 11. The cementitious slurry of claim 9, further comprising a plasticiser.