Sustainability in production of building materials is an extremely important aspect of today’s challenges, as it shapes the future of the construction industry. More environmentally friendly cements, the use of green energy and the reduction of CO₂ emissions are widely discussed in the AAC industry. Simple water reduction by a few percent, made possible through the use of modern AAC additives, can also help to reach environmental goals, as the benefits of water reduction are high and can be calculated into reductions in CO₂ emission and energy usage.

The water-to-solids ratio (W/S) and the usage of water in the production depend predominantly on the technology of the plant and raw materials used. There is no universally optimal value of W/S. However, based on the experience of the authors, there is always room for the reduction of the W/S ratio with positive results. Water has very high heat capacity and it therefore greatly affects the energy balance of the AAC mix. When reduced, it diminishes the heat capacity of the green cake (Fig. 1). Lower heat capacity translates into lower energy required for the autoclaving process. This results directly in a lower gas bill and thus lower overall CO₂ emissions.

Benefits of water reduction

The benefits of water reduction include:

·     Lower energy consumption

·     Diminished consumption of freshwater

·     The autoclaving process is more effective

·     The finished product is lighter and of higher quality

·     Optimised rising time before cutting

Lower energy consumption

Lower water contents in the green cake relate to lower quantities of water that needs to be converted into steam. This is basic thermodynamics. Phase transition from water at 100°C to steam at 100°C requires 2,257 kJ of energy per 1 kg of water used, which needs to be supplied from the boiler (Fig. 2). Less water in the autoclave means less energy is needed to heat it up and convert it into steam. Clients using Poromix Additive experience gas bills reduced by up to 3,5%.

Less freshwater consumption

A lower freshwater consumption provides economic and environmental benefits, as water is an important natural resource. The reduction in water consumption can be as high as 2 - 3%, which represents a substantial amount regardless of the plant’s capacity.

Autoclaving is more effective (less risk for autoclaving damage)

In the autoclaving process the water in the cakes is first heated to reach 100°C and then it changes its state to hot steam. In this process, the water from the middle of the cake migrates to sides of the cake and leaves. Less water in the cake means there is a lower risk for the first vapor to condensate on the side of the cake, creating water pockets. Under a rapid increase of pressure in the autoclave, the water in the pores migrates rapidly, creating thermal cracks. This is why a reduced water content can reduce some of the cracks created in the autoclave.

An additional benefit of lower water contents in the autoclave is that less condensate is created, which means that less of it needs to be removed. Condensate that is not removed from the autoclave on time consumes energy from the fresh steam creating a risk of unfinished reaction in the cakes. This situation results in “soft patches” on AAC products.  

A reduced water content is better for the finished product

Less water in the beginning means less water in the end of the process. After autoclaving, the products have 35 - 40% moisture and do not have time to organically dry. Often, AAC is shipped to customers with moisture contents of up to 20%. Reducing this by even a few percentages will over time contribute to lower fuel consumption of the delivery trucks, further reducing the carbon footprint, and facilitating reaching the desired conditions in the finished building.

Reduced water can decrease the rising time before cutting

A lower water content can result in a faster rising time, as more energy from binders remains in the cake. When the cast-to-cut time is adjusted correctly, opportunities for increasing the capacity of a plant are created.

 Summary

AAC additives such as Poromix can reduce the viscosity of the mix and increase the flow by means of water surface tension reduction. This effect can be summarized as water replacement, where in terms of viscosity the effect of adding the additive is equivalent to the effect of adding water (Fig. 4).

Even though every plant is different in terms of raw materials, recipes and densities produced, it is without a doubt beneficial to reduce any amount of water used in the cake. After decades of development in the concrete industry, now is the time that dedicated additives for AAC are available and offer the possibility to replace a portion of the water used.