(365c) CO2 Utilization in Concrete Block Production

Concrete blocks have been widely utilized as load-bearing and non-load- bearing partition walls in construction. The US market for concrete blocks and bricks is projected to increase to 4.3 billion units per year in 2014. In comparison to cast-in-place concrete, concrete block structures not only exhibit superior performance due to precast quality, but also represent a low environmental impact construction system. Concrete blocks are made of calcium silicate cement, porous in nature and mass produced. The concrete block plants are also demographically distributed and strategically located close to CO₂ sources such as power plants and cement plants. Concrete blocks are reactive with carbon dioxide. It is the calcium silicate cement in concrete blocks that captures carbon dioxide and converts it into calcium carbonate, a thermodynamically stable solid that contributes to strength gain and durability improvement of the final products. Currently concrete blocks are produced by steam curing. If this process can be replaced by CO₂ curing, concrete blocks can serve as CO₂ sink for carbon storage.

This paper presents a study on the possibility of replacing steam curing by CO₂ curing and the CO₂ utilization capacity in concrete block production. The effect of initial curing on CO₂ curing of lightweight concrete blocks was examined with both slab samples representing web sections and full size blocks. Initial curing was performed for 0, 4, 6, 8, and 18 hours. The subsequent CO₂ curing was then carried out in a chamber under a pressure of 0.1 MPa. The early-age and 28-day compressive strength of steam cured, carbonated, and hydrated samples were compared. Due to loss of water caused by initial curing, the carbonated concretes exhibited lower 28-day strength in comparison to hydrated and steam-cured samples. A water spray mechanism was devised to restore the lost water, and ultimately, the late compressive strength was comparable among samples. The optimized process parameters were applied to full-size block production in laboratory. The following conclusions can be drawn.

Initial curing is beneficial for carbon uptake in an early CO₂ curing process for an accelerated hydration and carbon dioxide storage. However, initial curing could be detrimental to late strength development because of the water loss due to the process. With water compensation immediately after carbonation, high early strength, equivalent late strength, and superior carbon uptake can be achieved.

For early carbonation targeted in the first 24 hours after casting, it is not possible to reduce the internal relative humidity to ideal 50-60%. No matter what drying process is used, internal RH remains higher than 80% within 24 hours. Water content is a better parameter to justify the condition for carbonation reaction.

Carbon uptake capacity by concrete blocks is dependent on initial curing. Taking cement binder in blocks as reference, the carbon uptake in 4-hour carbonation treatment reached approximately 8.5% by zero initial curing, 22% by 4 to 8 hours initial curing and 24% by 18 hours initial curing. Longer carbonation time of 96 hours could promote carbon uptake to 35%. It is corresponding to a degree of carbonation of 70%. For commercial application, it is suggested initial curing of 18 hours followed by 4-hour carbonation at 0.1 MPa and subsequent water spray to balance the carbon uptake and strength gain. Initial curing of 18 hours can be implemented through overnight curing. Carbon uptake of 22-24% by concrete in a process of less than 24 hours is considered as superior. It is obtained from lightweight concrete with expanded slag aggregates.

The process parameters obtained from slab samples are successfully applied to full size block production in laboratory. Both carbon uptake and strength gain in blocks are of close value to slab tests. Therefore, laboratory work can be scaled up to commercial production. XRD patterns illustrate that the carbonation products are stable calcite with the consumption of C₂S, C₃S, CH and possibly CSH.

Annual cement production in US is 100 million tones and its corresponding CO₂ emission is about 80 million tones. If all blocks produced in US are processed by CO₂ curing for accelerated hydration and carbon storage, the block production could consume 0.77 or 2.0 million tones of CO₂ per year based on an uptake rate at 10 or 24%, respectively. The corresponding emission reduction in cement industry can reach 1.0% or 2.5% by concrete block production alone. 

CO₂ curing process can replace steam to reduce embodied energy in concrete blocks, utilize sufficient amount of carbon dioxide in the vicinity of CO₂ sources, and produce the final products with equivalent performance.