Questions and Answers on basic theory of ready-mixed mortar

2022/11/18 10:57

1. Why can wet-mixed mortar maintain its mappability for a long time?

A certain retarder is added to the wet mortar in order to delay the hydration and hardening speed of cement, so that the newly mixed mortar can remain plastic in a long time, and can ensure the product transportation and construction.  It should be noted that weather, basic materials, and construction habits all affect erasability.

2. What are the advantages of special retarder for wet mortar?

The slow-setting admixture dedicated to wet mortar should have the property of delaying the initial setting time of cement, so that the mortar in a closed container can maintain a maximum of 24 hours of non-condensation, more than the above time or the water in the mortar is absorbed and evaporated, mortar can still set and harden normally.  Mortar special retarding admixture must also have less influence on the strength of mortar properties.  For example, the quality index of special retarder produced by Beijing Forten Technology Co., Ltd. is shown in Table 2-1.

Table 2-1 Quality indexes of special retarder

Item

Chlorine ion content

Mortar setting time /h

Quality requirements

≤0.40

≥24

3. What role does fly ash play in mortar?

It is recommended to add grade Ⅱ fly ash to mortar in general.  Fly ash has pozzolanic effect, with fine particles and a large number of vitreous microbeads (drift beads). When mixed into mortar, it can exert three effects, namely morphology effect, activity effect and micro-aggregate effect.

(1) Morphological effect  

Fly ash contains a large number of glass beads, which can reduce the internal friction resistance of mortar and improve the workability of mortar when mixed into mortar.

(2) Activity effect  

Active silicon dioxide, aluminum oxide, iron oxide and other active substances content more than 70%, although these active ingredients alone do not have hydraulic, but under the excitation of calcium hydroxide and sulfate, calcium silicate hydrate, ettringite and other substances can be generated, so that the strength increases, especially the late strength of the material increases significantly.

(3) Microaggregate effect  

The particle size of fly ash is mostly less than 0.045mm, and it is finer than cement particles on the whole. It is filled with pores and pores in cement gel, so that the mortar gel is more dense and has higher strength.

4. What is the significance of reasonable selection of fly ash in mortar?  

 Because the quality of fly ash has a great influence on the performance of mortar, it is necessary to choose fly ash reasonably, and determine the most appropriate dosage according to the test.

(1) Performance of mortar mixture  

Fly ash with good quality has the effect of water reduction, so it can reduce the water requirement of mortar.  The morphology effect and micro-aggregate effect of fly ash can improve the compactness, fluidity and plasticity of mortar, and reduce bleeding and segregation.  In addition, the setting time of mortar can be extended.  After the addition of fly ash, the mortar becomes thick and soft, which can prevent bleeding to a certain extent, and has a good hand feeling, which improves the construction performance of the mortar.  (Note: if the combustion loss of fly ash is too large, the admixture will be greatly increased;  If it is denitrification ash, it will make the mortar and water produce odor. )

(2) Intensity  

Generally, with the increase of fly ash content, the decrease of mortar strength increases, especially in the early stage, but in the later stage, the strength increases under the favorable use environment.  The amount of cement replaced by fly ash is related to the excess coefficient. By adjusting the excess coefficient of fly ash, the strength of mortar can be equal to that of reference mortar.

(3) Elastic modulus  

The elastic modulus of fly ash mortar is directly proportional to the compressive strength.  Since the unburned carbon of fly ash absorbs water, the higher the burn loss of fly ash, the greater the shrinkage of fly ash mortar under the same working condition.  The consistency of elastic modulus is an important index of the compatibility of building materials.

(4) Durability  

It is generally believed that because fly ash improves the porous structure of mortar, its impermeability is better than that of ordinary mortar.  With the increase of fly ash content, the impermeability of fly ash mortar will be improved.

5. What is the significance of the tensile bond strength of wet mortar?  

Plastering mortar engineering quality is the final measure of plastering layer does not produce cracking, hollow and burst, which depends on the quality of the material and the level of construction operation.  The main index of mortar hardening is its bond strength with the base.  Generally speaking, in a certain range, the amount of cement is large, the mortar bond strength is also high, but not completely proportional;  The compressive strength of mortar is too high, but the bond strength is reduced, that is, the compressive strength, tensile strength can not characterize the bond strength of mortar and base.  Therefore, plastering mortar should have the provisions of tensile bond strength, the strength level M5 plastering mortar tensile bond strength is not less than 0.15MPa, M10 plastering mortar tensile bond strength is not less than 0.20MPa.

6.  What is remolding of wet mix mortar?  

If there is a small amount of bleeding in wet mixed mortar during storage, it should be mixed well to meet the requirements before use (if the bleeding is serious, it should be re-sampled for quality inspection).  Reshaping means that the consistency of mortar is large within the specified use time, but the consistency cannot meet the construction requirements. Under the premise of ensuring the quality, the mortar can be mixed with appropriate amount of water to regain the original consistency after being confirmed by the on-site technical person.  Mortar remodeling can only be done once.

7. What are the transportation and storage requirements of wet mixed mortar?  

 The transportation and storage of wet mixed mortar have the following requirements:  

(1) Transport vehicles with stirring devices should be used for transport, and the transport process should be operated according to regulations.  The size of the transporter should follow the economic principle.  The charging port should be kept clean, there should be no water in the body of the cylinder, and water should not be added at will during transportation and unloading to ensure that the mix ratio of mortar meets the design requirements, so as to ensure the quality of mortar.

(2) After wet mixed mortar is transported to the site, such as brick or block masonry ash pool, need to use waterproof mortar (water absorption rate is less than 5%) plastering.  The ground of the ash storage pond should be leveled with a certain slope for easy cleaning.  Ash pool should be covered with sufficient area to prevent rain and sun protection.  The mortar is stored in the ash pond, and the surface of the ash pond is completely covered with plastic sheets to ensure that the mortar is in a closed state.  

(3) Pay attention to the operational time design of wet mortar, and construction should be carried out within the operational time range.

8. wet mortar admixture if peculiar smell, caking, can continue to use?  

Cannot be used.  Peculiar smell may be the admixture is contaminated by microorganisms, lost the function of regulation;  Clumping is often caused by the reaction of the water-retaining agent and other incompatibilities, resulting in the loss of its effectiveness.