Mixture of silica sols

Details Mixture of silica sols Mixture of silica sols

2001-06-15

2003-07-22

Marcantoni, Paul (Department: 1755)

Chemistry of inorganic compounds

Silicon or compound thereof

Oxygen containing

C501S012000, C501S133000

Reexamination Certificate

active

06596250

ABSTRACT:

The present invention relates to a silica sol mixture and a method for the preparation and use thereof. The invention further relates to a concrete composition comprising said silica sol mixture and a method for preparation and use thereof.
BACKGROUND OF THE INVENTION
Silica sols have been known for decades and are nowadays used in a great variety of fields and applications, such as in paper production, coatings, catalysts, ink-jet for paper, in the photo industry, investment casting, refractory fibre bonding, anti-skid products, polishing agents, and as additives in concrete.
In the concrete industry, durable concrete and mortar are being used as building material, in which aggregates or ballast of suitable sizes are bound together by a matrix of a hydraulic binder, usually of cement type, e.g. Ordinary Portland Cement. For simplicity, where otherwise not stated, mortar is here included in the term “concrete”.
The durability of concrete is influenced by physical and chemical exposure in the environment. Concrete will thus have a varying durability depending on the environment where it is used. The composition of the concrete will also play an important role how well it resists chemical and physical attacks. Often, several physical and chemical processes will occur simultaneously. One damaging factor often contributes to rendering the concrete more susceptible to attack of another. Corrosion is the most common attack on concrete leading to cracking or spalling thereof. This is due to the corrosion of steel present in concrete constructions. A concrete composition can be protected by providing a stabilising high pH value, resulting in a protecting oxide film on the steel. If the pH is lowered, the steel will destabilise to such an extent that carbonation and leaching occur. Concrete is used in many applications where it is exposed to chloride ions, e.g. constructions immersed in seawater, concrete in de-icing salted roads, and concrete compositions containing calcium chloride serving as e.g. accelerator. In ground water, concrete is often liable to sulphate ions, which render the concrete weaker in strength. The attack of the sulphate ions lead to expansion of the concrete due to the formation of gypsum, i.e. CaSO
4
, e.g. in the presence of MgSO
4
, Na
2
SO
4
or other sulphate sources, which may result in spalling and disintegration of the concrete. The formation of gypsum occurs as a result of reaction with Ca(OH)
2
, present in the concrete.
It is thus a big concern to provide concrete compositions with a good resistance, strength and durability, especially in environments exposed to chemical attacks of e.g. sulphate and chloride ions.
There is a further concern to provide concrete compositions having a high early strength. This is especially important in the precast industry, allowing early deforming thereby decreasing the moulding time, when constructing concrete roads making the roads readily open to traffic, and for shotcrete and concrete for industrial flooring as well as other applications where high early strength is required.
Silica sols influence the strength of concrete or mortar by reacting with calcium hydroxide present in the hydraulic binder thereby preventing the formation of large Ca(OH)
2
crystals. The adhesion between hydraulic binder and aggregates is also increased. Both of these two properties provide a more homogeneous, denser and stronger mortar or concrete. The presence of silica sols also influence the ion permeability in concrete by forming insoluble calcium silicate compounds rendering the mortar or concrete less permeable and less susceptible to chemical attacks.
U.S. Pat. No. 5,932,000 discloses a concrete composition comprising a concrete mixture and a silica sol containing a broad particle size distribution having a specific surface area of 50-170 m
2
/g. The concrete mixture is stated to be resistant to chemical attack.
However, there is still a need to improve the protection against a combined chemical attack of e.g. chloride and sulphate ions. It is also desirable to further improve the early strength.
The present invention solves the problems above.
SUMMARY OF THE INVENTION
The Invention
The present invention relates to a silica sol mixture comprising at least two silica sols. The first silica sol has a broad particle size distribution and a relative standard deviation of the particle size distribution of at least about 30% by numbers, preferably 40%, and most preferably 50%. The second silica sol has a narrow particle size distribution, and a relative standard deviation of the particle size distribution lower than about 15% by numbers, preferably lower than 10% by numbers, and most preferably lower than about 8% by numbers. Ideally, the second silica sol is a substantially monodisperse sol, i.e. a silica sol where all the silica particles have substantially the same size.
The relative standard deviation of the particle size distribution corresponds to the ratio between the standard deviation of the particle size distribution and the average particle size by numbers. “Variation coefficient” and “coefficient of variation” are terms synonymous to “relative standard deviation”. The relative standard deviation of the particle size distribution is measured by use of the dynamic light scattering method. By particle size is meant the particle diameter of the silica particles.
DETAILED DESCRIPTION OF THE INVENTION
The specific-surface area of the first broad silica sol suitably is from about 20 to about 300 m
2
/g, preferably from about 30 to about 200 m
2
/g, and most preferably from about 50 to about 170 m
2
/g. The silica particle size distribution of the broad silica sol suitably ranges from about 10 nm to about 100 nm, preferably from about 5 nm to 200 nm.
The silica particles of the second narrow silica sol suitably have a particle diameter ranging from about 2 nm to about 20 nm, preferably from about 3 nm to about 15 nm. The specific surface area of the second narrow silica sol suitably ranges from about 130 to about 1200 m
2
/g, preferably from about 200 to about 900 m
2
/g.
The dry weight ratio between the first and second silica sols in the mixture suitably is from about 1:4 to about 20:1, preferably from about 1:1 to about 10:1, and most preferably from about 3:1 to about 7:1. The silica particles are suitably dispersed in water or organic solvents e.g. alcohols, preferably in water. The pH of the aqueous dispersion suitably is from about 1 to about 12, preferably from about 2 to about 11. The silica sol mixture suitably has a silica concentration of from about 1 to about 70% by weight, preferably from about 5 to about 50%. The silica particles are preferably anionic and dispersed in presence of stabilising cations such as K
+
, Na
+
, Li
+
, NH
4
+
or the like or mixtures thereof. The silica particles, dispersed in water or solvent, may also occur in aluminate modified form, as described by Dr. Ralph Iler in “The Chemistry of Silica”, 1979, pages 407-409.
Suitably, the silica sol mixture contain a mixture of ready available aqueous silica sols mixed together. The silica particles of the silica sol mixture may also be dispersed in other solvents, e.g. organic solvents such as alcohols, or mixtures of solvents.
The present invention also relates to a method for the preparation of a silica sol mixture comprising a first silica sol having a broad particle size distribution and a relative standard deviation of the particle size distribution of at least about 30% by numbers, preferably 40%, and most preferably 50%, and a second silica sol having a narrow particle size distribution and a relative standard deviation of the particle size distribution suitably lower than about 15%, preferably lower than 10%, and most preferably lower than 8% by numbers. The proportions and concentrations of the mixed silica sols are further described above.
The invention further concerns the use of a silica sol mixture as an additive to concrete or mortar.
The invention further relates to a concrete or mortar composition comprising a hydra

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