Most of the world’s wastewater infrastructure is constructed out of the most versatile material the world has yet to see: Portland cement-based concrete. The material is strong, durable and relatively cost-effective. In harsh environments, however, it has its limitations.
In certain environments, like wastewater systems, conditions are not conducive to Portland cement-based concretes. These environments become breeding grounds for bacteria. Concrete, while displaying great strength and structural integrity, is still a porous material. This porosity becomes the ideal breeding ground for microscopic bacteria.
As the bacteria begins to flourish they create sulfuric acid as a waste by-product. Concrete is at its strongest and most durable when it is at a high pH (Alkali). The sulfuric acid (H2SO4) begins to weaken the concrete and cause it to deteriorate. This acidic environment also results in the corrosion of any steel rebar that might be used in the concrete. Expanding rebar also causes concrete damage.
In order to preserve the concrete structures, the ideal plan of action has been to coat the concrete with a barrier material. These materials have typically been epoxy and urethane-based. These barrier coatings are not magical materials, but simply materials that are not as porous as concrete, thus allowing less bacterial colonization. These materials, however, above all are more resistant to the acidic environment caused by the sulfuric acid (H2SO4).
A number of cementitious materials have been developed for use as protective liners for the traditional Portland cement-based concrete structures. This space has been dominated by the product commonly known as calcium aluminate. This product essentially makes use of a high-fired clinker brick that is ground up and added to the cement mixture in various proportions.
The incorrect use of calcium aluminate cements has led to widespread construction problems, especially during the third quarter of the twentieth century when this type of cement was used because of its faster hardening properties.
After several years, some of the buildings and structures collapsed due to degradation of the cement, and many had to be torn down or reinforced. Heat and humidity accelerate the degradation process called “conversion”; the roof of a swimming pool was one of the first structures to collapse in the United Kingdom.
While the longevity of calcium aluminates is a contentious issue, this topic was not the focus of this study. The goal of this study was to test calcium aluminates and compare their relative chemical resistance to that of the new generation epoxy modified mortar. These tests were conducted in the same acidic environments that would be present in wastewater infrastructure.
There have been many claims made about the chemical resistance of calcium aluminate-based mortars. However, there is scant empirical data to bolster these claims or provide a comparison to other high-performance, new-technology mortar products.
The acidic environment in the wastewater system is caused specifically by sulfuric acid. Sulfuric acid is a highly corrosive, strong mineral acid with the molecular formula H2SO4. It is this acid that was tested in this study.
In order to obtain an accelerated test with noticeable results, the study endeavored to use the strongest concentration of the acid commercially available. Commercial car battery acid is sulfuric acid at concentrations between 31% and 34%. It was decided that this would be the acid and concentration used.
A cured core sample of each material was partially immersed in this sulfuric acid solution. The partial immersion was to help us understand whether evaporative action plays a role in the corrosion.
Each sample started out as close as possible to the same weight and size and then was weighed weekly to determine its progressive weight loss.
Once a week, core samples were removed from acid bath and immersed in a bucket of fresh water for a period of two hours. After the two hour immersion, the cores were oven-dried to make sure all moisture was fully evaporated.
The oven-dried samples were then weighed and photographed and the weight was compared to their original pre-exposure weight.
The weekly observations were done for a period of four weeks, at which point the data was tabulated and compared and the material evaluations concluded.
WEEK 0: INITIAL IMMERSION
WEEK 1: SOAK IN WATER, DRY AND WEIGH
WEEK 2: SOAK IN WATER, DRY AND WEIGH
WEEK 3: SOAK IN WATER, DRY and WEIGH
WEEK 4: SOAK IN WATER, DRY AND WEIGH
SAMPLES AFTER STUDY
Chart Showing % Weight Loss Over Four Week Period for Respective Samples
POST-IMMERSION – CROSS-SECTION VISSUAL
POST-IMMERSION – pH Testing
This study was carefully designed to test the resistance of three types of cementitious products to the acidity associated with highly corrosive wastewater environments. The concentration of the immersion liquid was meant to illustrate an accelerated simulation. The test was conducted on multiple occasions with very similar perimeters to verify the results’ accuracy. Each time the results were nearly identical.
Summary of Observations from Testing:
A measure of how well the sample held up to the exposure. The higher the weight loss, the higher the corrosion.
1.The Calcium Aluminate (Sample #1) lost 56.61% of its original sample weight.
2.The Epoxy Modified Mortar (Sample #2) lost 1.14% of its original sample weight.
3.The Pure Fused Calcium Aluminate (Sample #3) lost 40.28% of its original sample weight.
Measure of how well the samples’ compressive strength held up. The higher the pH, the better for cementitious products. Any pH below 10 to 12 has been shown to affect the protection of the rebar from corrosion.
1.The Calcium Aluminate (Sample #1): pH levels between 7 and 8 .
2.The Epoxy Modified Mortar (Sample #2): pH levels between 12 and 13 .
3.The Pure Fused Calcium Aluminate (Sample #3): pH levels between 6 and 7.
The pure fused calcium aluminate performed better than the regular calcium aluminate in the test conducted.
What is clear, though, is that the epoxy modified mortar was by far the best performer. The weight loss was far less for the epoxy modified mortar than for the calcium aluminate or pure fused calcium aluminate(1.14% loss vs. 56.61% and 40.28%, respectively).
The resulting pH levels also indicated that the epoxy modified mortar was still very high on the pH scale, meaning it was still very strong and will theoretically continue to strengthen. It was also in a pH range high enough to continue protecting the rebar from corrosion. The other samples showed pH levels close to neutral.
It has to be kept in mind that the samples were all about 30 days old at the time of immersion and should still show very high pH levels as they were still in the early stages of cure.
One other conclusion that may be drawn is that the epoxy modified mortar is very well sealed. This seal results in a product that is far more corrosion-resistant than any calcium aluminate, pure fused or other.
Empirical Test Conducted 2014.