Admixture in concrete.

ABSTRACT

Some major projects have been proposed immediately, not only does it have a huge demand for concrete, but it also has very high demands on the quality of concrete. Concrete admixtures are chemical substances added to concrete. Reasonable use of admixtures can effectively improve the properties of concrete. In recent years, with the continuous improvement of the quality requirements of concrete in the construction industry and advances in chemical technologies, many new chemical materials have been used in concrete admixtures, and it has become necessary to study the effect of admixtures in concrete. The types of admixtures are various. In this paper, the effects of antifreeze, hardener and compound admixtures in concrete are explored, which provides technical guidance for the application of admixtures in concrete.

Introduction

Concrete is still a very important building material during the construction of a building project. With the continuous development of science and technology, the application of concrete is becoming more and more widespread. In this process of change, the use of admixtures is extremely important. New infrastructure construction has maintained a rapid growth, and the demand for concrete admixtures has continued to flourish in railways, highways, airports, coal mines, municipal engineering, nuclear power plants, and dams .
High-performance concrete (HPC) is a recent development in concrete technology. High strength concrete having more than 60 MPa compressive strength with improved properties when designed to fulfill specific performance requirements is normally termed as HPC. In many field applications concrete is required to meet certain specific performance requirements besides high strength. For prestressed concrete bridges, offshore structures, highway and airport pavements and in machine foundations, concrete should possess high fatigue strength. For nuclear containers exposed to very high temperatures, the concrete must have high resistance to thermal cracking. All these needs have made the researchers to think seriously to find out an appropriate technology through research and HPC was the outcome.

Antifreeze
The daily average temperature is lower than 5°C for 5 consecutive days, and the concrete works will enter the winter construction. At this time, the concrete project needs to consider the low temperature from the design of the mix ratio, the initial temperature of the raw materials, to the pouring, forming, curing, and removal of the mold. The impact. Low temperature mainly affects the hydration rate of cement and volume expansion after water freezing, leading to prolonged coagulation hardening, internal structural damage and slow growth of strength. If the curing temperature of the concrete is reduced to 10°C, the setting time will be doubled. If the concrete curing temperature is lowered to -5°C, the fresh concrete will be subjected to freezing damage, and the compressive strength at the later stage will be lost by more than 50%. Therefore, a series of winter construction techniques in cold regions is to protect early-stage concrete from freezing damage. Under negative temperature, incorporation of antifreeze in concrete and proper insulation measures are common methods in winter construction of concrete in cold regions.
Mechanism of antifreeze
Antifreeze refers to a chemical substance that makes a concrete mixture free from freezing damage in a negative temperature environment. Many inorganic salts and some organic substances have antifreeze function. The mode of action can be divided into two categories: one is a very low eutectic temperature with water, which has the ability to reduce the freezing point of water, and allows the concrete to perform hydration at negative temperatures, such as sodium nitrite and sodium chloride. However, if the amount is insufficient or the temperature is too low, it will still cause freezing damage. The other is to make it possible to reduce the freezing point of water as well as to seriously deform the lattice structure of ice containing this type of material, thus failing to form frost heaving stress and destroying the hydration mineral structure to impair the strength of the concrete, such as urea and methanol. When the dosage is insufficient, the intensity stops growing at negative temperature but the positive temperature has no effect on the final strength; The second type is that although the aqueous solution has a low eutectic temperature, it cannot significantly reduce the freezing point of the water in the concrete. Its role is to react with the cement directly to accelerate hydration of the concrete and accelerate the setting and hardening of concrete, which is beneficial to the development of concrete strength, such as calcium chloride and potassium carbonate.

Hardener
Concrete hardener refers to an admixture that can improve the earlynstrength of concrete and has no significant effect on the later strength. Its main role is to accelerate the speed of cement hydration and promote the development of early strength of concrete. The admixtures with early strong functions and certain water-reducing enhancement functions are called water-reducing hardener.

Early strength agent is a special admixture that specifically solves the problem of obtaining the strength of cement concrete as soon as possible or as soon as possible. 

It is mainly used in highway cement concrete projects in the following situations:
1. Fast access to the cement concrete pavement or bridge deck pavement, especially the level crossings of the first, second and third grade highways.
2. Construction of cement concrete structures in low temperature environments where the lowest temperature is not lower than -5°C requires the use of early strength agents to accelerate the setting and hardening of cement concrete to prevent freezing damage of cement concrete at lower temperatures.
3. Pre-stressed reinforced concrete structures require the use of early-strength agents to speed up prestressing and increase the speed of component fabrication.
4. Rapid restoration of cement concrete pavement and bridges.

Mechanism of hardener
Concrete hardener is one of the earliest additive species used in the history of admixture development. So far, people have successively developed various harder admixtures other than chlorine salts and sulfates, such as nitrites, chromates, etc. As well as organic hardener, such as triethanolamine, calcium formate, urea, etc., and on the basis of hardener, production and application of a variety of composite admixtures, such as water reducer hardener, antifreeze hardener and pump delivery hardener. These types of hardener admixtures have been used in practical projects and have played an important role in improving concrete performance, increasing construction efficiency, and saving investment costs.

Compound admixtures

Compound admixture refers to the mixing of several admixtures with different properties by means of mechanical mixing. In the highway project, in order to meet the standards required by the “Test Procedure for Cement and Cement Concrete for Highway Engineering”, the use of additives should first enable the strength of the cement to reach a certain standard, so that the concrete can maintain certain workability. Therefore, the determination of a suitable compound admixture is an unavoidable important link in the project. Now three different compound admixtures are used in the experiment. According to the results of measured cement paste fluidity and mortar strength, the results are optimized and compared. The best solution.

EXPERIMENTAL STUDY

1. Cement:

The cement used in this experimental investigation is ordinary Portland Cement of 53 grade confirming to IS:12269: 1987

2. Fine aggregate: 

In the present investigations fine aggregate is natural sand obtained from local marked is used

3. Coarse aggregate: 

The coarse Aggregate used in this experimental investigation is crushed granite of 12.5 mm maximum size, which was obtained from the local crushing plant.

4. Water: 

Potable tap water available in the laboratory with pH value of 7.0±1 and confirming to the requirements of IS: 456 - 2000 was used for mixing concrete.

5. Fly Ash:

The fly ash used 

6. Silicafume:

Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys. The Silica Fume used in this investigation was obtained from Pragathinagar, Hyderabad.

7. Metakaolin: 

Metakaolin is a dehydroxylated form of the clay mineral kaolinite.

8. Super plasticizer:

Conplast SP43 complies with IS:9103:1999 and BS:5075 Part 3 and ASTM-C-494 Type 'F' as a high range water reducing admixture and Type G at high dosage

Mix proportioning:

Concrete mixes were designed for M60 to study the compressive strength at different w/b ratios. The w/b ratios of 0.3, 0.35 and 0.4 were adopted. At each w/b ratio, silica fume, metakaolin and fly ash content were varied as 0%, 5%, 10% and 15% by weight of cement. The cementitious material was taken as 450 kg/m3 and Sand content was650kg/m3. The quantity of coarse aggregate was calculated by allowing 2% airentrainment. Concrete with different w/b ratios with different content of silica fume, metakaolin and fly ash was studied at different ages, namely 3, 7, 28, 56 and 90 days. A concrete mixer machine was used for mixing the dry as well as wet concrete for sufficient time till a uniform mix was achieved.

Application of Mineral Admixture in High Performance Concrete

High performance concrete and high strength and performance concrete

High performance is a new requirement for concrete at an international conference in 1990. High performance concrete is also a basic direction of concrete technology development in the future.

There are different definitions of high performance concrete at home and abroad, but they can be summarized as the following five aspects;

• High durability 

• High construction performance 

• Higher strength

• High volume stability 

• It can meet the requirements of environmental protection and sustainable development

Main technical approach of preparing high performance concrete.

• Using high-quality mineral admixtures with large amount is the technical core of preparing high performance concrete.

• Low water-binder ratio is adopted. The high durability formed by high content of mineral admixtures can only be shown when concrete adopts low water-binder ratio, otherwise it may be counterproductive.

• To achieve high content of mineral admixtures and low water-binder ratio,  water reducing agent with high quality and high efficiency is certainly required. Polycarboxylic water reducing agent developed and popularized in recent years is an ideal material, 

• In addition to the above special requirements, the selection of raw materials, mix design and production control of concrete should be strictly carried out in accordance with the requirements of the standards.

Effect of mineral admixture in high performance concrete.

I. Enhancement effect

When mineral admixtures are added, the composition of cement paste’s gelatinous substance  can be improved; especially the free lime (Ca (OH)2) can be reduced and removed. For SiO2 in the active mineral admixture, Ca(OH)2 and tobermorite with high alkaline can react pozzolanic reaction, which can produce tobermorite with low alkaline, higher strength and better stability.

II. Filling effect

The average particle size of cement is 20-30 microns, while the average particle size of fly ash is 3-6 microns, and the silica fume is smaller than both of them, which is between 0.1-0.26 microns. It can fully fill the gap between the cement particles, so that the compressive strength and permeability performance are significantly improved. Close concrete prevents moisture from entering the interior of concrete. Freezing water in concrete is very scarce. Therefore, under the condition of freeze-thaw alternation, the frost resistance of concrete is greatly improved.

III. Reduction of hydration temperature peak effect.

After adding mineral admixtures, the amount of cement in concrete is reduced, so the calorific value of cement hydration in concrete is reduced. Although these active mineral admixtures will produce pozzolanic reaction and release hydration heat in concrete, this reaction lags behind the hydration reaction of the main body of cement and lasts a long time. This can restrain the early strength of concrete, but the later strength will not decrease.

IV. Improvement effect of concrete durability

1. Improve impermeability: The structure of cement paste and the interface between cement paste and aggregate are more compact, blocking the possible

permeability pathway. 

2. Reduce the harmfulness of alkali aggregate reaction: Due to the incorporation of mineral admixture, a large amount of calcium silicate gel with low alkalinity is formed in concrete hydrates. They can absorb and maintain large amounts of Na+ and K+ ions, thus greatly reducing the effective alkali content in solution of concrete pore. Therefore, the harmfulness of alkali aggregate reaction is greatly reduced. 

3. Improve frost resistance: When water can’t enter the concrete, the frozen water in concrete is very scarce. Therefore, under the condition of freeze-thaw alternation, the frost resistance of concrete is greatly improved.

V. Relation of high fly ash content and reduction of alkalinity

The possible negative effect of adding active mineral admixtures makes the  alkalinity of concrete, the carbonization resistance of concrete, and the ability of protecting steel bar decrease. But the decline rate of concrete alkalinity is not very fast. 

Green high performance concrete

I. More clinker cement is saved and environmental pollution is reduced. Because a  large number of industrial residues are used for high performance concrete as active mineral admixture to replace a large number of cement, and these fine waterquenched slag and high-quality fly ash, silica fume or their composite materials become the main components of binding materials. Compared with the production of clinker cement, the emission of CO2 is greatly reduced, and resources and energy are also saved.

II. Adding more active mineral admixtures (mainly industrial waste) is of advantages of improving the environment, saving land and limestone resources and energy, reducing the hydration temperature rise of concrete, and enhancing the volume stability and wear resistance.

III. Give full play to the advantages of high performance and reduce the amount of cement and concrete. By reducing the environmental burden fundamentally, concrete can become a sustainable building material as the largest artificial material in the contemporary era. It is the direction of concrete development and the future perspective of concrete.

DISCUSSIONS ON RESULTS

1. From the test results it was observed that the maximum compressive strength is obtained for mixes with 10% silica fume at all ages and for all water binder ratios

2. When comparing5% replacement levels at of 0.4, the metakaolin gave the better result than silica fume at all ages.

3. In all w/b ratios, upto 28 days, fly ash attained least strength even less than normal concrete, but at 56 and 90 days the strength of fly ash concrete was increased significantly.

4. Due to the slow pozzolanic reaction, the initial strength of fly ash concrete was lower than that of concrete without fly ash.

5. Due to continued pozzolanic reactivity fly ash might have developed greater strength at later age, which was more than that of the concrete without fly ash.

6. For all w/b ratios, at all ages, the optimum replacement level of silica fume and metakaolin was found to be 10%.

CONCLUSIONS

1. Concrete with 10% of silica fume gives better strength at all water binder ratios and at all ages.

2. Metakaolin also performs well and the strength improvement is almost close to the strength development in silica fume concrete.

3. The main aim of silica fume and metakaolin replacement is to increase strength whereas the aim of addition of fly ash was for economy and for improving the strength of hardened concrete.

4. The early age strength can be achieved by adding silica fume and metakaolin

5. At all ages of 7, 28, 56 and 90 days, the compressive strength of HPC with silica fume and metakaolin is more than that of normal concrete.

6. It may be noted that addition of silica fume and metakaolin causes an increase in strength at all ages. But fly ash gains strength at 56 and 90 days.

7. From the results, it was found that the optimum replacement of silica fume, metakaolin and fly ash are 10%, 10% and 15%, respectively.