By Daniel Furr, PE, Quality Control Manager, Stalite Lightweight Aggregate
Carbon negative refers to a state where an entity removes more carbon dioxide from the atmosphere than it emits. The methods of Carbon Dioxide (CO2) removal are carbon capture, sequestration, or avoidance. So, today’s question is: Does lightweight concrete masonry remove more carbon dioxide from the environment than it emits?
Let’s start with Environmental Product Declarations (EPDs) and how the current cradle-to-gate analysis does not begin to tell the story on CO2 and greenhouse gases. EPDs have become the focus of design for resilient and sustainable structures. Most are aware cement accounts for approximately 7% of all global CO2 emissions. Almost half of cement CO2 emissions occur during the production process when limestone is heated. The chemical reaction that occurs in the kiln to convert the limestone into clinker also creates CO2. Like cement, most lightweight aggregates (LWAs) in the U.S. are produced through heated rotary kilns. The bloating of the raw LWA does not produce CO2, like cement does, however, the fuel used to heat the LWA kiln does. These CO2 emissions are captured in cradle-to-gate EPDs while most benefits for lightweight CMU and concrete products occur after the gate and are ignored in cradle-to-gate analyses.
EPD Stages: Cradle-to-Gate (A1 to A3) vs. Cradle-to-Grave (A1 to C4)
The term “cradle-to-gate” is most commonly used in carbon reduction discussions and includes raw material supply, transportation of raw materials to manufacturing facility, manufacturing and storage (stages A1 to A3). If you see an analysis done using cradle-to-gate information, it is incomplete and can tell a false story. Lightweight masonry has advantages over normalweight masonry and many other construction materials, which all are seen after the material leaves the gate.
Stages A4 and A5 includes transportation of the block to the jobsite and construction of the building. Lightweight masonry requires fewer trucks to deliver and is faster and more efficient to place into structures leading to CO2 reductions in life-cycle analysis in this section.
Stages B1 to B7 include building use stage, building maintenance, building repair, building replacement or refurbishment, and energy and water use or savings. Lightweight masonry has insulating properties and can greatly reduce the amount of energy required to operate a building. See below for more detail. Lightweight masonry and masonry in general are some of the most durable and most resilient materials used in construction and offer many advantages in the B1 to B7 stages that lead to CO2 reductions. Concrete masonry also sequesters CO2 during its life.
C1 to C4 includes end-of-life stages of demolition, waste processing and disposal. Masonry is one of the most environmentally friendly products known when discussing end-of-life processes. It does not decompose to release CO2 and methane into the atmosphere. This leads to CO2 reductions in this phase of a building’s life.
Why do we continue to build with concrete if cement is such a large contributor to global greenhouse gas (GHG) emissions? Several of the reasons include the long-life of concrete structures, low life-cycle cost, resilience, and safety and reliability. For a concrete masonry unit (CMU), most of its CO2 emissions come from the cement used to produce it. However, some of that CO2 is reabsorbed, or sequestered, during its life, thus reducing the CMU’s overall CO2 emissions. Until recently, the amount of CO2 sequestered by dry-cast concrete, such as dry-cast CMU, was not well understood and assumed to be similar to that of wet-cast concrete. Beginning in 2020, research was conducted by the NCMA (CMHA) to determine the rate and amount of CO2 that a CMU sequesters naturally when exposed to atmospheric conditions. The results were astounding and showed that, on average, 22% of all total CO2 emissions associated with the process of CMU manufacturing are sequestered within 2 years.
Given that CMU on average shows significantly greater carbon sequestration than wet-cast concrete, what is the potential for even more savings utilizing lightweight aggregate? Lightweight aggregate within CMU provides insulation and reduces energy usage. Table 4 below from ESCSI’s “Evaluation of Single Wythe Masonry Energy Use with Lightweight (Smart Wall) Systems” shows the total energy use of a typical secondary school.
Electricity has CO2 impact of .9526 lbs CO2 per KWH. The results of the energy savings shown above converted to CO2 reduction are shown below.
The data show that use of lightweight CMU would generate an average net savings of 2.3 million pounds of CO2 compared to use of normalweight CMU in this structure. The lightweight CMU would generate more CO2 in the A1 to A3 phases of analysis, approximately 230,000 pounds more. But, at 5 years, the increased CO2 to produce the Lightweight CMU has already been made up for in energy related CO2 savings. As a bonus, the lightweight CMU used in this design would save an average of $329,000 in energy costs over 30 years. These results are not surprising; most products that offer long-term benefits have a higher cradle-to-gate CO2 input value. Similar to lightweight CMU, insulation, low-e windows, high efficiency HVAC systems all have their primary benefits after the gate and require greater initial CO2 input in A1 to A3, and using cradle-to-gate analysis to determine any of these materials’ environmental qualities is flawed at best.
So, can lightweight concrete masonry be carbon negative? The answer is YES. When you consider carbon sequestration, construction efficiencies, durability, resilience and energy savings, lightweight masonry is carbon negative when compared to normalweight masonry.
References
McGinley, W. Mark, “Evaluation of Single Wythe Masonry Energy Use with Lightweight (Smart Wall) Systems” University of Louisville. (2019).