Performance Of Selected Nano Tubes In Cement Mortar In Presence Of
Nano Particles After Hydration
Cement pastes and mortars are normally prepared with cement and sand. Water is added to prepare a homogeneous mix that can make the mortar workable to adhere to any concrete or brick surface. Cement acts as a bonding material but in case of smooth surface like AAC Blocks, concrete, Fly ash bricks and similar surface, the bonding particles of cement slurry can’t impregnate. Therefore, a lack of bond between cement mortar and smooth surface is noticed. This phenomenon is the root cause of bond failure resulting into crack propagation on smooth surface like AAC Blocks, etc. This is a very common problem as noticed in every such building project. Although there are several other factors which are responsible to develop cracks on AAC Blocks and similar masonry units. In this connection we can make a general study on AAC Blocks.
AAC Block
The composition is changed in different manufacturing unit.
1. Autoclaved Aerated Concrete is a lightweight, great insulating, durable building product, which is manufactured in different sizes. This is also known as Autoclaved Lightweight Concrete (ALC).
2. This is the eco – friendly and green building materials.
3. AAC is a made of cement, fly ash, sand, water and aluminium powder or aerating additives. Aluminium powder reacts with calcium hydroxide and water to form H2. The hydrogen gas increases volume of the raw mix by gas bubbles. After steaming and hardening process and reacting with calcium hydroxide calcium silicate hydrate is produced, which gives AAC its strength and lightweight properties.
Cement Binder - It hardens and binds all material together.
Fly Ash - A by-product of burning pulverised coal. It provides thermal insulation, fire resistance and sound absorption
Sand – An aggregate material, sand with silica content more than 80% is generally used in AAC bricks.
Limestone - A sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3) and it can be directly purchased as limestone powder.
Aluminium Powder - Aluminium is an aerating agent, air bubbles (hydrogen gas) is produced after reaction with calcium hydroxide.
Gypsum - Gypsum is used for fast setting
Preparation Of Mix
1. 53-grade Ordinary Portland Cement (OPC).
2. Fly ash and sand are mixed with water to prepare slurry. The slurry is mixed with other lime powder, cement, gypsum and aluminium powder.
3. Lime powder is another ingredient.
Processing (Dosing, Mixing)
Dosing and mixing is an important operation for AAC blocks manufacturing process. Process of dosing and mixing means the quality of final products. Maintaining the ratio as Fly ash or Sand: Lime: Cement: Gypsum = 69:20:8:3.
Aluminium @0.08% and water used in a ratio of 0.60 to 0.65. At this step the ingredients are mixed homogeneously and finely to produce the appropriate product ready for manufacturing AAC blocks. The control system accumulates all the components in a mixing vessel. A small pot is used for Aluminium powder to add into the vessel.
Churning of the mixture is begun for a standard time and the mix is poured into moulds. Dosing and mixing process is continued because if there is a break between charging and discharging of elements, the left mixture might start hardening and choke up the entire unit.
But, AAC Blocks shows few problems which are important to notice. One important reason is crack development which is due to volumetric changes. These changes appear in the product due to drying/wetting, temperature change or carbonation. Drying shrinkage presents the most important volumetric change during the lifetime of any AAC construction. First, the rate of drying shrinkage depends on the composition of AAC.
It is reported in that drying shrinkage of AAC with only cement as a binder is significantly higher than that produced with lime or lime and cement. Knowledge of the proper composition of AAC is not enough, as long as there is still the common practice of designing AAC constructions that involves empirical processes. These processes usually either underestimate the risk of possible failures due to drying shrinkage, or are not powerful enough to reveal potential weakness. Even the national standard in the Czech Republic CSN EN 680-733 1356:2006 underestimates the risks of possible damage by defining the conventional drying range of moisture content for estimation of drying shrinkage from 30% to 6% by mass.
These thresholds represent the moisture content change from freshly produced AAC to the equilibrium moisture content in the AAC. It has been concluded in recent research that drying shrinkage of different types of AAC is increasing with decreasing moisture content in the AAC pore system and this increase is fastest in the range of very low moisture, much lower than 6%.
This feature of the AAC drying process is commonly not considered by designers and may be responsible for surface cracking. On the other hand, in some research papers there already exist more sophisticated approaches for designing AAC constructions, so far dealing with heat and moisture transport only. In this paper, we present a hygro-thermo-mechanical model, which is used for the description of hygric expansion/shrinkage of AAC. This model is verified by approximation of a laboratory experiment for determination of drying shrinkage. Finally, the usage of verified model settings is demonstrated in the long-term simulation of hygric expansion/ shrinkage of an AAC wall exposed to real weather data. The initial moisture content of the material is set differently, ranging from 30% to 6% by mass and thus describing the state of the material in different time intervals between its journey from the factory and construction on site.
The best performance of AAC was achieved with the lowest initial moisture content of the block. To avoid mechanical damage caused by drying shrinkage, it is strongly recommended to use as much dry AAC as possible.
Introduction Of Nano Tubes And Nano Particles In Cement Matrix Of Plaster
The microstructural investigation of the Nano composites reveals that Nano tubes helps in filling upmicro/nano pores between the calcium silicate hydrate (CSH) and ettringite during the process of hydration, after placing mortar on AAC Blocks or similar surface the Nano tubes acts as an anchoring element which helps mortar to fix with the smooth surface.
The following SEM IMAGES X-ray Diffraction pattern shows the performance, The Nano tubes are along with Nano particles produce the composite which enhance the durability of the structure. Cracks are not developed on the plastered or putty surface.
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