New Tech for Acid Attack — Part 3

by Whitney B. Belkowitz and Jon S. Belkowitz, PhD

Experiments Conducted

Compressive strength

For the grout samples used in a compressive strength and bulk electrical resistivity, the mixing procedure for the grout was followed per ASTM C 305 (ASTM C 305–12 2012) and specimens were cast per ASTM C 192 (ASTM C 192–12 2012) into 50.8 mm (2.0 inch) diameter by 101.6 mm (4.0 inch) tall cylindrical non-absorbent plastic molds. After the 24-hour curing period, the samples were removed from their molds and placed in a temperature controlled lime-water bath. The mass loss samples were cast in 50.8 mm (2.0 inch) by 50.8 mm (2.0 inch) non-absorbent plastic cube molds. On the specimen break date, three cylindrical samples were prepared tested per ASTM C 39 (ASTM C 39–12 2012). As per ASTM C 39, the specimens were centered in the closed loop hydraulic press and crushed under a metered load. The ultimate load in newtons (N) is divided by the cross-sectional area (mm) to calculate the compressive strength (MPa) at failure. Specimens were broken at seven, 28 and, 56 days from the cast date.

Bulk electrical resistivity

The electrical resistivity of the concrete specimens were measured on the same three samples that were measured for compressive strength as shown in Figure 1 as a means to identify how the nano silica size and surface area impacted the permeability of the HCM. A denser concrete relates to lower porosity and pore connectivity, which leads to a lower permeability and a higher resistivity. Shane et al showed through the use of chloride ion permeability and electrical resistivity that as permeability decreased, the electrical resistivity increased (Shane 1999). Table 2 lists a comparison of the rapid chloride permeability test measurements (ASTM C 1202–12 2012) and bulk electrical resistivity values.

Figure 1 — Bulk Electrical Resistivity of concrete specimen (Shane 1999).

Table 2 — Comparison between charge passed in Coulombs during rapid chloride permeability test and bulk electrical resistivity (Shane 1999).

The electrical conductivity test method consists of measuring the bulk resistivity of water-saturated concrete cylinders by the use of two plate electrodes in contact with the end surfaces of the specimen. An alternating electrical current was applied through the concrete specimen and the resistance of a concrete specimen measured. The resistance, measured in ohms (Ohms), is the raw data measured and used to determine the bulk electrical resistivity, rho (p). The resistivity is given in Equation 1, where L is the specimen length (cm), A is the specimen cross-sectional area (cm2) and Ohms is the resistance of the specimen.

Equation 1 — — — — p = Ohms x (A/L)

Mass loss

The mass loss experiment was an adaptation (soaked in a HCL acid bath for seven days) of the experiment employed by Hewadye et al to identify the impact of chemical admixtures on sulfuric acid attack resistance (Hewayde 2007). At the end of seven, 28 and 56 days of curing, two grout specimens were tested for mass loss due to HCl acid attack. The specimens were first placed in an oven and dried at 105 C for 24 hours when the specimens reached a constant mass. The specimens were then allowed to cool to room temperature, the specimens were then weighed and then immersed into the acid bath. Mass loss measurements were performed after the specimens were allowed to soak in the acid bath for seven days. Once the specimens were removed from the acid solution, they were rinsed with acid solution and oven dried at 105 C for 24 hours when the specimens reached a constant mass. The percentage of mass loss at each date was calculated according to Equation 2 (Hewayde 2007):

Equation 2 — — — — Mass Loss (ML) = [(M1 - M2) / M1] x 100%

Where M1 is the specimens mass before immersion and M2 is the specimens mass after immersion.

Modeled and simulated total porosity

The models and simulations used in this research were employed to identify how the change in HCM, Metakaolin content, and nano silica would impact the total porosity of the HCM. The software package used, Virtual Cement and Concrete Testing Laboratory (VCCTL) was developed by the National Institute of Standards and Technology. VCCTL simulates a wide variety of cement paste physical properties where these physical properties have been compared to experimental measurements for degree of hydration, heat of hydration, chemical shrinkage, setting time, compressive strength development, and pore solution concentrations (D. P. Bentz 2005). For the purposes of this research, the change in total porosity was analyzed as a function of OPC replacement with Metakaolin and nano silica. Bentz et al, showed that silica typically reacts with the calcium hydroxide (CH) (define on first use) phase to form a pozzolanic C-S-H gel of a different specific gravity and molar stoichiometry than the C-S-H gel formed from conventional cement hydration. Furthermore, Bentz et al, found in the presence of excess silica, the conventional C-S-H may convert to the pozzolanic C-S-H form (D. J. Bentz 2000). Bentz et al, validated these models using experimental data on the influence of silica fume additions on both the adiabatic temperature rise of concretes and the chloride ion diffusivity of low w/c ratio cement-silica fume pastes (D. J. Bentz 2000, D. W. Bentz 1998).