Although colloidal silica has been around for years, it is recently gaining traction in the concrete and construction industry due to it’s proven benefit to concrete strength and durability. This article explores the history of colloidal silica and various performance indicators of colloidal silica in concrete.
History
Colloidal silica, originally patented in 1959, was first brought to the United States by Bryan H. Green from the U.S. Army Corps of Engineers at the Engineering Research and Development Center in Vicksburg, Mississippi. In 1989, Green introduced colloidal silica as a viscosity modifying admixture for rock-matching grout. Prior to this, he encountered challenges with a very fluid grout that contained hematite sand, which tended to fall out of solution. To address this issue, Green experimented with dextrose and xanthan gums, but these additives made the grout tacky and difficult to work with. Eventually, Green discovered that colloidal silica not only kept the hematite sand in solution, but also densified it, reduced permeability, increased strength, and reduced wave attenuation, among other benefits.
Definition
The word colloidal means "stays in solution". An example of a colloidal suspension is milk, which contains fat and proteins in a dispersed form. Colloidal silica refers to a universal dispersion of nano silica particles, with nano being defined as particles ranging from 1 to 100 nanometers in diameter. To put it into perspective, a single strand of human hair has a diameter of approximately 100,000 to 150,000 nanometers.
Why Colloidal Silica?
Colloidal silica is utilized to effectively manipulate the molecular kinetics of cement hydration, thereby enhancing the strength and durability of concrete. By consuming calcium hydroxide, colloidal silica minimizes pore sizes, pore solution, and pore connectivity, resulting in reinforced concrete that is stronger and longer-lasting.
Strength
Strength is an important factor to consider. It refers to the stress level at which things fail. The goal is to increase the stress level so that failure is prevented. In our lab, we have conducted numerous tests that demonstrate this concept. To benchmark, we use a standard ASTM C94 mix which consists of water, sand, and cement at approximately 4000psi. We compare this reference mix to a variation where colloidal silica is added. The results are impressive. For compressive strength, we observe a significant increase of 15%-30%, especially during the first week (1-7 days). The significant increase in early strength is ideal for pavements and strip forms. After that time frame, the increase in strength becomes less substantial, particularly at 28, 56, and 90. Nevertheless, the strength is still higher compared to the reference mix.
Strength Regression
Years ago, Intelligent Concrete worked on a fast-track pavement project for the Colorado Department of Transportation. The pavement mix used provided excellent results. Within 4-6 hours of placement, a strength of 3500psi was achieved, allowing for open traffic to flow smoothly on the highway. However, issues were encountered with the accelerators employed, such as calcium nitrite, nitrate, sodium thiocyanides, and chlorides, as they tended to create a porous microstructure. As a result, higher early strengths were attained, but a reduction in strength at 28 days was observed. Moreover, this porous microstructure made the pavement more vulnerable to chemical attack from deicing salts and brine. By implementing colloidal silica, shorter curing times were achieved while simultaneously creating denser microstructures. Additionally, the flexural strength, which is essentially a tension measurement, at 28 days, increased 20% when compared to the reference.
Modulus of Elasticity
Using excessive amounts of colloidal silica in concrete can have various effects on its properties. When conducting fractal and brittle tests, it has been observed that using large particles of colloidal silica can make the concrete more brittle. Additionally, there is a reduction in tensile strength and ductility when higher dosages of colloidal silica are used.
Reasonable amounts of colloidal silica in the mixture can result in a steeper line, indicating a slightly stiffer and stickier mixture. However, in terms of linear elasticity, the proportion limit and strain at the proportion limit are higher, preventing brittleness. Colloidal silica is a remarkable technology that addresses various issues related to concrete durability. Unlike common approaches that involve adding more cement, reducing water, or using accelerators, colloidal silica offers a solution to problems such as alkali-silica reaction (ASR), deicing attack, and steel corrosion. Excessive cement content, microstructure, environmental factors, and other materials contribute to durability challenges, which can be mitigated by incorporating colloidal silica.
Abrasion
Abrasion is a notable example of increased performance with colloidal silica in concrete. Intelligent Concrete has conducted various abrasion tests, including the standard abrasion tests that involve applying pressure from steel balls. These tests simulate scenarios such as warehouses and workforce lifts, where pressure is exerted on concrete surfaces. Comparing the results obtained using the reference and colloidal silica, a significant reduction in abrasive wear ranging from 30 to 60% was observed.