SE453077B - PROCEDURE FOR IMPROVING CEMENT USE AND CONCRETE PROPERTIES - Google Patents

Abstract

The method is for the improvement of properties of cement and concrete, and involves the addition of waste silicon acid, simultaneously reducing the cement added ingredient. The waste material is obtained from the prodn. of aluminium fluoride from aluminium hydroxide and hexafluorosilicon acid. Up to 15 per cent by wt. of the prescribed cement added ingredient can comprise the silicon acid waste, and the process is enacted at air temperatures of 30 deg C and above. The waste silicon acid contains an acid part which has a particle size of 1-100 micro-metres, and a specific surface of 600 sq. m/kg. It has a residual aluminium content of 2-3 per cent by wt., and one of fluoride of 3-5 per cent by wt.

Description

453 077 DE-OS 22 19 503 and SE-A-8502243-2 ( relate to processes for purifying silicic acid before, by or after treatment with an alkali and producing alkali silicates, in particular sodium silicate. 'Incomplete' purification of silicic acid from aluminium and especially fluorine results in unstable water glass, in particular when diluted with water.

SE-A-8205825-6 describes a process for high-quality purification of specified silicic acid which yields a very pure silicic acid, which can be used in a number of technical fields.

However, there remains a desire to find a direct use for the residual product obtained from the production of aluminum fluoride, i.e. silicic acid contaminated with aluminum and fluorine.

It is known that extremely fine-grained silica fume, a residual product from the production of metallic silicon or the alloying element silicon iron, has been used as a so-called pozzolan in the production of concrete (Sellevold, E. et al: Condensed silica fume in concrete. Institutt for Byggningsmateriallaere, NTH, Trondheim 1982 and Bache, D: Densified cement/ultrafine particle-based materials. Proc. 2nd Int. Conf. on Superplasticizers in concrete. Ottawa June 10-12, 1981). This silica fume consists of spherical particles with a grain size between 0.01 - 1/um (about 70% <0.1/um), i.e. about 100 times smaller than cement particles, and with a specific surface area of 20 uno m2/kg.

A pozzolan reacts chemically with lime hydrate to form calcium silicate hydrates, similar to those formed during cement hydration, which produce a denser cement gel, i.e. the strength-forming component in concrete.

The pozzolanic effect of silica dust is attributed partly to the fineness of the grain, partly to the spherical shape of the particles and partly to the extremely large specific surface area (Sellevold, E.: Silica in concrete.

Wood pulp as pozzolan and filler. BALLASTDAGAR, Luleå 13~15 September 1982l Waste silicic acid from aluminium fluoride production has a grain size and specific surface area similar to those of Portland cement. The grain size is 100 times larger than that of silica fume, it lies between 1 and 100 /um, and the specific surface area according to BLAINE is 600 m2/kg compared to silica fume's 20,000 m2/kg. The particles are not spherical. The relative grain size between cement and silica particles can be illustrated by a calculation where silica and cement are mixed in a weight ratio of 1:10, which is recommended when using silica in concrete. For silica fume, taking into account the compact density, approximately 50,000 silica particles per cement particle are obtained, for silica fume from the aluminium fluoride process 20 - 50 silica particles per cement particle.

Description of the present invention The present invention relates to the use of silicic acid from the aluminium fluoride process as a pozzolan in concrete, particularly in hot climates. If 5 - 10% but even up to 1596 of the intended cement quantity in concrete or cement mortar is replaced or supplemented with a corresponding quantity of silicic acid, the following effect is obtained in addition to lower costs for cement: 1. The strength of the cement mortar or concrete increases and is after 28 days - 35% higher than without silicic acid. 2. The density of the concrete increases. 3. The setting is delayed. The delay is not considered to cause any major practical problems at normal temperature (20°C), but is in hot climates: (ao - ss°chi11 advantage. ' 4. Water separation is unchanged at low dosage and decreases at higher dosage.

. The air content in the concrete does not change. 6. Variations in the silicic acid's content of aluminum and fluorine impurities have no significant effect on the silicic acid's effect.

Despite its completely different chemical and physical form, silicic acid thus has the same properties for improving concrete as silica fume, although with a significantly longer delay in the hardening of the concrete mass.

A fresh concrete mixture without the addition of said waste silicic acid remains workable at a temperature of 20°C for 3 - 4 hours (corresponding to a penetration resistance of up to 8 kp/cm2), at a temperature of 35°C for only about 2 hours. By adding this silicic acid or by replacing 5 - 1096 of the cement quantity with a corresponding quantity of silicic acid, the workability time is extended by 50 - 10096. This is often of great value when casting concrete in areas with a warm climate.

Example 1 Cement mortar corresponding to durability class K 30 was manufactured using Swedish standard cement (Slite Std) and aggregate with a maximum grain size of 8 mm as reference.

In parallel, cement mortar was manufactured with the deviation that 2.5, 5 and 1096 of the cement quantity were replaced with silicic acid waste containing 2.396 aluminum and 3.996 fluorine (designation SN 2.5, SN 5, SN 10). The tests were carried out according to Swedish standards at 20°C and with the following results.

The air content (SS 137111) in all samples ranged between 4 and 696 of the concrete volume.

Water separation (SS 137112) is shown in Figure 1. When 596 is added, water separation decreases insignificantly, when 10% silicic acid is added, water separation can be reduced by half.

Stiffening (SS 137114) is the stage when the concrete gradually changes from a workable to a non-workable mass after casting. Figure 2 shows that the addition of silicic acid causes a retardation, i.e. a delay in the stiffening of between 50 and 10096 of the time course of the reference sample.

The compressive strength according to SS 137210 was slightly worse for all samples containing silicic acid than for the reference sample for the first 2 - 3 days, but then increased sharply and after 28 days was 1596 higher with an addition of 596 silicic acid than the reference sample. An addition of 1096 silicic acid gave a concrete with 2896 higher compressive strength. The reported sample with the designation S0 5 contained 596 silicic acid with an extremely high aluminum content (2.796) and fluorine content (5.396). As can be seen from the figure, the results deviate very little from normal silicic acid values.

Example 2 Cement mortar with and without silicic acid waste was mixed and tested in the same way as in Example 1, but at a temperature of 35°C instead of 20°C. These tests show that the hardening generally decreases by about 1 hour for all samples and is delayed by about 50% with the addition of silicic acid in relation to cement mortar without the addition (Figure 4). The compressive strength of concrete with the addition of silicic acid improves after only 2 days and is 35% higher after 28 days than concrete without the addition of silicic acid (Figure 5).

Example 3 Concrete with a composition corresponding to that prescribed in Sweden for type approval testing of additives and containing stone with a grain size of up to 32 mm was produced, whereby in half of the samples 5% of the cement quantity was replaced with silicic acid waste. The same results regarding water separation, stiffening and compressive strength were obtained as with cement mortar, however, the improvement in compressive strength for the samples containing silicic acid increased by 35% when tested after 28 days compared to the samples without silicic acid addition (Figure 6).

Claims (5)

10 15 20 25 453 077 PATENT REQUIREMENTS Process for improving the properties of cement mortar and concrete, possibly while reducing cement addition, characterized in that silica waste from the production of aluminum fluoride from aluminum hydroxide and hexafluorosilicic acid is added to cement mortar or concrete, whereby up to 15% by weight of the prescribed cement additive can be added. consists of said silicic acid waste, preferably between 5 and 1096 of the prescribed cement weight. Process according to Claim 1, characterized in that the addition takes place at air temperatures of 30 ° C and above. 3. A method according to claim 1, characterized in that up to 1596, preferably 5 - 1096, of the prescribed cement additive is replaced with said silicic acid waste. 4. A method according to claims 1-3, characterized in that the silicic acid waste has a silicic acid moiety, which has a particle size of 1 - 100 .mu.m, and a specific surface area of 600 m 2 / kg. 5. Process according to claims 1-3, characterized in that the silicic acid waste has a residual aluminum content of 2-3% by weight96, and a residual content of 3-5% by weight.