[Image above] Representative backscattered electron images of concrete specimens created with ordinary Portland cement (OPC), crushed soda–lime silicate glass aggregate, and varying amounts of calcium nitrate. (A) Specimens with no calcium nitrate showed widespread alkali–silica reaction gel formation and microcracking. (B) Specimens with 20 mass% calcium nitrate showed marginal ASR gel formation and no cracking. Credit: Xiao et al., Journal of the American Ceramic Society
When I started to notice excessive condensation forming in my fridge last year, I tried to pull back on the number of items stored inside. But once it got to the point that anything more than a jug of water and leftover pizza would result in pools of water collecting in the veggie drawers, my property manager finally ordered a replacement.
When the maintenance team came to haul away the leaking fridge, I learned that at least a dozen other residents had required replacements this year as well. Apparently, all the fridges came from the same manufacturer, and they all had faulty defrost systems. The property owner likely thought it was a good deal getting so many fridges for cheap. But the consecutive failings demonstrated they should have invested in a reliable brand the first time rather than wasting time and money replacing the fridges so soon.
The decision to save money in the short term at the expense of long-term upkeep is a situation we see play out time and again in the corporate world. And it is one the Nebraska Department of Transportation is currently wrestling with regarding roads in the city of Omaha.
While many roads and highways use asphalt, concrete is used often as well in urban areas thanks to its durability. But when constructing with concrete, it is important to consider the possible dangers of alkali-silica reactions (ASR).
ASR is a common cause of concrete deterioration. It occurs when aggregates containing certain forms of silica react with hydroxyl ions in the alkaline cement pore solution. The reaction produces a gel, which absorbs water from the surrounding environment and expands. Expansion of the gel exerts pressure on the concrete, causing it to crack and fail.
Using aggregates low in silica is an obvious way to prevent ASR. But in Nebraska and surrounding states, high silica levels are common in the sand and gravel used as aggregates. Rather than importing low-silica aggregates from farther away, which would be expensive, the Nebraska Department of Transportation relies on adding coal residue called fly ash to the concrete because it is known to slow the progress of ASR.
In the 1990s, there was debate that some forms of fly ash seemed to make ASR worse. The city of Omaha, instead of using the fly ash generally regarded as safe, decided to eliminate the use of fly ash entirely in 1996.
Omaha city authorities started using fly ash again in 2015, but almost 20 years without this ASR-mitigation strategy means that the newer concrete is in worse shape than pavement poured decades earlier. The city is now grappling with replacing all the crumbling roads, as reported by Omaha World-Herald staff.
Even though bringing fly ash back helps prevent ASR for now, the supply of fly ash is declining due to the decreased production of coal in North America. That means Omaha would need to either import fly ash from farther away, which, like low-silica aggregates, would be expensive, or find another cost-effective admixture to mitigate ASR.
Calcium nitrate is a globally available commodity chemical commonly used as an agricultural fertilizer. Previous studies found it can effectively mitigate ASR in cementitious systems, but the exact mechanisms behind this mitigation remain unclear.
In a recent paper, researchers from the University of California, Los Angeles aimed to clarify exactly how calcium nitrate helps mitigate ASR in concrete. To do so, they combined calcium nitrate with two types of cement (Type I/II and Portland limestone). They then added aggregates of varying reactivity as well as different types and dosages of supplementary cementitious materials, such as amorphous steel slag and Class C and Class F fly ashes.
Using microstructural examinations, dissolution studies, and thermodynamic calculations, the researchers determined that calcium nitrate induces the formation of calcium silicate hydrate, portlandite, and calcite precipitate mixtures, which form on the aggregates’ surfaces at the expense of typical ASR gels. Such precipitates create a dissolution barrier and inhibit ASR in formulations both with and without supplementary cementitious materials.
“The outcomes indicate that CN [calcium nitrate] is an efficient and cost-effective ASR mitigation additive (∼$250–$600 per tonne), particularly in a time of dwindling fly ash supplies,” they conclude.
The paper, published in Journal of the American Ceramic Society, is “Calcium nitrate effectively mitigates alkali–silica reaction by surface passivation of reactive aggregates” (DOI: 10.1111/jace.20004).
