What kind of acid dissolves concrete

_Concrete

The effect of chemical attack on concrete is in DIN 1045-2 Structures made of concrete, reinforced concrete and prestressed concrete - Part 2: Concrete - Definition, properties, production and conformity regulated. According to this, three degrees of attack are differentiated and assigned to corresponding exposure classes:

  • XA1: weak attack
  • XA2: moderate attack '
  • XA3: strong attack
Concrete is not acid-resistant due to its chemical composition alone, but it can be made acid-resistant through appropriate additives and the choice of raw materials. The concrete technology for achieving increased resistance to acid attack is based on two essential principles, the water-binding agent ratio (w / c value) and the impermeability of the cement stone. The lowest possible w / c value in the range of 0.35-0.40 with cement contents of 340 + 20 kg / m³ leads to concrete compressive strengths of approx. 60 N / mm² and the associated low capillary porosity results in a high level of concrete tightness .

For individual high-performance concretes with increased acid resistance requirements, further measures are required. To increase the acid resistance, a limitation or conversion of the calcium hydroxide content Ca (OH)2as well as a fine distribution of Ca (OH)2- Particles are essential in the cementitious structure. These two requirements are met by the main constituents of the cement or by reactive additives such as fly ash and / or silica dust.
 
The mechanism of action of the substances that is responsible for the conversion of calcium hydroxide into the CSH phases (calcium silicate hydrates and calcium aluminate hydrates) is the pozzolanic or the latent hydraulic reaction, in which the additives, which are characterized by a high glass content, are dissolved by the high pH value of the pore solution (approx. 13.4 + 0.2). Their components (dissolved silicon and aluminum) react with the calcium hydroxide to form CSH phases. In addition, on the basis of the Fuller parabola (the ideal grading line which Fuller and Thompson viewed as a favorable grain distribution), the highest possible packing density of the largely acid-resistant aggregates should be aimed for in order to increase the tightness of the concrete. As a result of the high packing density, there is a simultaneous minimization of the cement content and thus a reduction in the available calcium hydroxide.

In some cases, special concretes with increased acid resistance, such as concretes used in power plant construction, fall below the standard specifications with regard to the minimum cement content. These special concretes require a high level of competence from all those involved in the construction with regard to production, installation, processing and monitoring.

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