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NFB curing area using solar energy at Dai Dung green material JSC.
Reporter: How does curing process in the production and use of NFBs affect the building structure?
Associate Professor PhD Bach Dinh Thien: The purpose of curing is to create favourable conditions for the solidification of concrete and to ensure the required humidity of the environment. To do so, several methods are commonly applied such as: Covering with polymer films, filling with sand, regularly moistening, using cotton fabric from synthetic materials, arranging water tanks for soaking, etc. When concrete dries quickly at an early age, significant shrinkage deformation appears and micro cracks appear as well resulting in deteriorating the concrete structure, reducing the concrete quality. Repairing the structure by creating favorable conditions that could not be done later, so proper concrete curing at an early age is necessary to achieve good quality of concrete, especially NFBs are belonging to products having a large surface module.
According to our research results, in the cold season in the North and the Central, the dry season in the South Viet Nam during the first 7 days NFBs are much dehydrated (On average, up to 88.6% of moisture loss compared to the moisture loss in 28 days). In the hot season in the North and the Central, the rainy season in the South, the moisture loss is not much higher than in the cold season in the North and the dry season in the South (An average of 89% of the moisture loss compared to the moisture loss in 28 days). This is very dangerous because this dehydration is still potential in the NFB structure with many capillary pores connected together, so it is necessary to regularly cure NFBs at an early age of 7 days. It is then necessary to water but less frequently, mainly moisturizing.
Thus, considering the volumetric stability of NFBs under the current curing condition, the time can be 2 months (95-96% shrinkage compared to the shrinkage at 6 months of age).
For the NFBs of companies in the South, in the dry season the shrinkage at the age of 28 days is about 0.79mm/m (About 65% of shrinkage at the age of 2 months), in 2 months (56 days) it is about 1.15mm/m. From the age of 2 months onwards, NFBs still have shrinkage but negligible (At the age of 6 months, there is an additional shrinkage of 0.15mm/m, about 11% of the total shrinkage at 6 months - the shrinkage at 6 months as 1.30mm/m). In the rainy season, the weather impacts in the South are not as significant as in the dry season.
Reporter: Why is it necessary to study curing process? Is there any difference in curing technology in Viet Nam compared to other countries?
Associate Professor PhD Bach Dinh Thien: Viet Nam is located in the interior of the Northern Hemisphere with a tropical monsoon climate having the basic characteristics of heat, humidity and distinct seasonal differentiation. Due to the characteristics of the country stretching from the North to the South, the climate also varies by region: The Northern provinces are tropical monsoon with hot summer and cold winter, 4 construction climate sub-regions are: B1, B2, B3 and B4. The Southern provinces have typical monsoon climate characteristics which are basically characterized by high temperature, little change in the year, the wet and rain are distinctly seasonal, divided into 3 construction climate sub-regions as N1, N2 and N3.
Climate factors including humidity, air temperature, wind speed, solar radiation, rainfall in each region and each season are also very different, so the curing process depending on technology equipment, moisture for shaping of the materials for NFB production is very different. With equipment lines made by mechanical producers in the Viet Nam, the moisture of concrete mixture used for NFB production is usually in the range of 9-10% while the lines imported from G7 countries have moisture in concrete mixture as 7-9%.
For firms in some countries that have transferred concrete brick production equipment and technology to a number of production facilities in Viet Nam, they have not considered all climate factors in Viet Nam, so they cannot yet have instruction in curing technology suitable to the conditions of each region. For concrete brick curing technology for tropical climates, we have to deal with by ourselves.
Reporter: What is the meaning and effectiveness of science and technology, economics, and environment of curing process?
Associate Professor PhD Bach Dinh Thien: In outdoor condition, hot and humid weather occurs in case of dehydration with a large extent of concrete samples, so the structural stresses of concrete appear with great value, leading to the loss of mass in concrete so its strength decreases with changes in mechanical properties. At the later age of curing due to the impact of repeated temperature effects, the heat stress will adversely affect the structure of the concrete leading to a reduction in the strength of concrete.
Therefore, to ensure the quality of NFBs in hot and humid climate condition during the production process, it is necessary to apply a reasonable moisturizing curing mode. There are two stages of NFB curing in the production process: Shaped products are stored on pallets; Curing mode when bricks are packed into bales should be maintained with adequate moisture required for continued hydration of cement clinker minerals.
Good implementation of the NFB curing process both ensures the NFB quality, increases the life of the masonry and will eventually reduce the investment costs of the construction works by extending the required maintenance period.
Reporter: So, are there any basic differences in the application of curing for NFB plants in the 3 regions today?
Associate Professor PhD Bach Dinh Thien: The natural moisturizing curing process is divided into 2 stages: The initial curing is when the NFBs have just been formed, are still on the shelves and cured when they have been put into bales, these two stages are successive to each other without interruption.
In the first stage, it is necessary to ensure the NFB’s evaporation of water under the influence of external climatic factors, while not allowing mechanical forces to affect the surface of concrete bricks.
The climatic differences between the 3 regions of Viet Nam lead to different evaporation of NFBs. However, it is necessary to cure NFBs for at least the first 7 days in a humid environment to avoid dehydration, then continue to spray water against moisture loss less frequently up to 14 days.
Reporter: So, what is the role of NFBs in building materials today?
Associate Professor PhD Bach Dinh Thien: In construction, building materials play an important role as food in social life. Building materials are indispensable materials and often account for around 60% of the value in construction of technical and social infrastructure works. Among types of materials used in construction, building materials account for the most amount, from which they will form the covering structure of buildings and construction works, separating spaces between construction works with different uses.
Non-fired building materials and components are essentially materials of concrete, which are indispensable materials in modern construction. Currently, more than 2 billion cubic meters of concrete in the world is used every year, concrete is one of the building materials used in the largest amount, partly determining the degree of civilization development.
The most popular NFBs in Viet Nam market are aggregated cement blocks - concrete bricks, followed by autoclaved aerated concrete building blocks, acotec panels and AAC panels, pressed soil cement bricks, laterite, plasterboard wall panels, ... Statistics released by the Department of Construction Materials (Ministry of Construction) show that in 2018, Viet Nam produced 7 billion NFBs. In 2020, it is expected to produce 12.5 billion NFBs (SBUs).
In developed countries, NFBs account for about 60% of total amount of building materials, while fired clay bricks (Red bricks) only account for about 10-15%. In Viet Nam, the demand for building materials is very high because it is in the development stage. Demand for 2020 is estimated at about 30 - 32 billion bricks but current production of building materials accounts for about 30% of total amount of building materials. Therefore, in order for NFBs to be used more and more in construction works, we need to continue promoting propaganda so that NFBs are more well used in combination with modern production lines having high productivity, good curing process and lower costs that will create a stronger foothold for NFBs in the future.
Reporter: Sincerely thank you!