Cementitious mixture

Details Cementitious mixture Cementitious mixture

2001-03-07

2003-02-04

Marcantoni, Paul (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Inorganic settable ingredient containing

C106S745000

Reexamination Certificate

active

06514334

ABSTRACT:

The present invention relates to a process for making a cementitious mixture and, in particular, a cementitious mixture which comprises a filter cake waste material. The inclusion of such a cementitious mixture in concrete results in an improvement in cohesion, workability and compressive strength and provides a less porous structure compared with many of the known cements. The present invention also provides a new process for disposing safely of certain filter cake waste materials.
Known materials produced from cementitious mixtures generally fall into two groups: mortars and concretes. Mortars and concretes include a filler and a hydraulic or non-hydraulic cement. Fillers are typically classified as either fine or coarse aggregates. Fine aggregates, such as sand, generally have a particle size of less than about 5 mm size. Coarse aggregates, such as gravel, generally have a particle size of greater than about 5 mm. Whilst concretes and mortars commonly contain fine aggregates, only concretes contain coarse aggregates.
The purpose of the cement is to coat the aggregate particles and to bond the aggregates into a monolithic product. Hydraulic cements harden by the chemical reaction of hydration and common examples thereof include ordinary Portland cement (OPC), limestone, gypsum plaster, lime, ground granulated blast furnace slag (GGBS), pulverised fuel ash (PFA) and pozzolanic materials. The essential binding component formed when the cement hardens upon addition of water is calcium silicate hydrate (CSH) or tobomorite gel. Owing to the very complicated chemistry of cements, it is common practice to use a reduced nomenclature, where CaO=C, Al
2
O
3
=A, SiO
2
=S and H
2
O=H.
It is estimated that thousands of tonnes of waste filter cake material go to landfill sites in Great Britain annually. Consequently, a method of disposing safely with such a material would be beneficial to the environment. The present invention addresses this problem and, furthermore, provides a cementitious mixture which can be used in the construction industry and which possesses properties which substantially match and, in some cases, exceed the properties of conventional cements.
Accordingly, in a first aspect the present invention provides a process for making a hydraulic cementitious mixture, which process comprises the steps of:
(i) providing a first granular composition (A) comprising:
Weight %
SiO
2
55-80
Al
2
O
3
10-20
Na
2
O
 1-10
K
2
O
 1-10
Fe
2
O
3
0.5-2  
SrO
0-7
BaO
0-7
Ce
2
O
3
0-4
CaO
0-2
La
2
O
3
0-2
Nd
2
O
3
0-1
Pr
2
O
3
  0-0.5
P
2
O
5
  0-0.5
ZrO
2
  0-0.5
Sb
2
O
3
  0-0.5
TiO
2
  0-0.5
MgO
  0-0.2
MnO
  0-0.1
wherein the chloride content in composition (A) is from 0.013 to 0.05 weight % and the sulphate ion content is preferably from 0.03 to 0.15 weight %, and
(ii) providing a second granular composition (B) comprising:
Weight %
CaO
80-99
SiO
2
 1-10
Fe
2
O
3
0.1-1.5
MgO
0-8
Al
2
O
3
0-3
BaO
0-2
K
2
O
  0-0.4
Na
2
O
  0-0.2
SrO
  0-0.2
MnO
  0-0.2
Ce
2
O
3
  0-0.1
La
2
O
3
  0-0.1
Pr
2
O
3
  0-0.1
P
2
O
5
  0-0.1
Nd
2
O
3
  0-0.1
TiO
2
  0-0.1
wherein the chloride ion content in composition (B) is from 0.001 to 0.008 wt. % and the sulphate ion content is preferably from 0.01 to 0.04 wt. %, and
(iii) mixing granular compositions (A) and (B) to form a substantially homogeneous cementitious mixture.
By blending compositions (A) and (B) together at a given ratio it is possible to improve the rate of hydration and also reduce porosity, thereby providing a higher earlier strength than GGBS and PFA. Granular compositions (A) and (B) are preferably mixed in relative proportions to form a substantially homogenous cementitious mixture having a total chloride ion content of 0.02 wt. % or less in order to meet industry standards for reinforced concrete.
The oxide composition of the materials may be assessed using conventional techniques including, for example: US Environmental Protection Agency procedure USEPA SW-846 method 3051 (November 1990) for microwave digestion (nitric acid) to dissolve samples for analysis in solution, lithium metaborate fusion to prepare solutions for major element determination, ICP-Atomic Emission Spectrometry for determination of major oxides (except silica) and trace metals, X-ray fluorescence (silica), ICP-Mass Spectrometry for determination of rare earth elements. Soluble chloride and sulphate ion contents may be determined by ion chromatography.
Loss on Ignition (EN 196-2 section 7, EN 197) tests may also be carried out to determine amount of volatile matter by drying the starting materials at approximately 110° C., followed by heating at 925° C. to 975° C. ±25° C. At this temperature carbonates, for example calcium carbonate, decompose to the oxide and CO
2
is given off. This results in a loss of weight. The samples may then be analysed by the conventional techniques above, the chemical composition being expressed in terms of weight percent on the basis of the oxide, excluding the weight of the volatile material. Insoluble residues can be determined EN-196-2 Section 9.
Both compositions (A) and (B) may contain trace amounts of rare earth elements or oxides thereof, including Lutetium, Thulium, Holmium, Ytterbium, Terbium, Erbium, Yttrium, Europium, Dysprosium, Samarium and Gadolinium.
In general, the weight ratio of composition A to composition B will be in the range of from 1:4 to 4:1, more preferably from 1:1 to 7:3. For example, if both compositions (A) and (B) were blended on a 1.1 basis, then the silica content in the resulting mixture would be approximately the same as that of GGBS, whilst the alumina content would be approximately the same as that of OPC, although the calcium oxide content would remain lower than either GGBS or OPC. On a 3:7 basis, the silica content would be approximately the same as that of OPC, and the calcium oxide approximately the same as that of GGBS. Therefore the relative amounts of compositions (A) and (B) to be included in the cementitious mixture can be derived from the required performance of the cement.
If the weight ratio of composition A to composition B is chosen to be in excess of 1:1, then the silica content in the mixture will generally be present in an amount of from 40 to 70 wt. %, more typically from 45 to 60 wt. %, whilst the alumina content will typically be present in an amount of from 5 to 20 wt. %, more typically from 8 to 15 wt. %. Such levels of alumina and silica are higher than those found in OPC and have been found to result in higher compressive strengths in the early stages (1 to 7 days) of hydration, and substantially the same compressive strength at 28 days compared with OPC for the same water ratio. The higher levels of alumina also mean that higher water ratios can be used compared with OPC, whilst still maintaining equivalent mechanical properties. This is because the higher levels of alumina and silica form a gel paste, which improves the performance of the material, and the strength gain compensates for the increase in water demand. This is advantageous because higher water ratios facilitate mixing and processing. In addition, the relatively high silica levels improve the CSH gel at the aggregate-cement paste interface. Also the amount of free lime is reduced in the composition during reaction with the soluble silica in composition (A), thus reducing the risk of efflorescence in the concrete.
The use of the cement according to the present invention in the manufacture of concrete provides a more cohesive mix compared with OPC, and the slump of the concrete is reduced considerably. Additionally, the cementitious mixture is very mobile when vibrated and this improves compaction, being particularly suited to pumped concrete. The initial or plastic set is reduced and bleeding of the concrete is also reduced. Furthermore, the bulk density o

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