Compressive Strength

Compressive strength of a cementitious mixture is one of the most important measures of performance. Since previous literature has found pozzolans to decrease compressive strength (Lilkov et al., 2011; Erdem et al., 2007; Hossain, 2003; Campbell et al., 1982), additional tests like isothermal calorimetry and TGA were performed to better understand the underlying mechanisms of how pozzolans affect the compressive strength of cementitious mixtures.

2.3.1.1 Compressive Strength

Although the SAI test from ASTM C 618 (2012) measures compressive strength, it does so, on the basis of constant flow instead of constant water to cementitious material ratio (w/cm). As such, the results are often unfavorable for mixtures incorporating SCMs with high water demands, like zeolites. To remedy this, compressive strength tests were carried out on pozzolan-containing mortar cubes with a constant w/cm. Other than the fixed w/cm, the cubes were made according to the instructions in ASTM C 109 (2011). The w/cm for the zeolite mixtures was fixed at 0.55, due to their high water demand. The w/cm of the other mixtures, containing unaltered volcanic and sedimentary pozzolans, was fixed at 0.5. For each w/cm, a control mixture (with no SCMs) and a mixture containing fly ash was also made. The SCM replacement dosage used for all the natural pozzolan and fly ash mortars was 20% by weight of cement, as specified in ASTM C 109 (2011).

Additionally, the SAI tests only measure compressive strength at 7 and 28 days. To get a better indication of early age as well as long term strength, the compressive strength of the fixed w/cm mortar mixtures was tested at 1, 3, 7, 28, 90 and 365 days. The average compressive strength was calculated using three mortar cube specimens from

two separate batches that were made consecutively, on the same day. In certain cases, if the range of the compressive strength data from the three mortar cubes was greater than 8.7% of the average, the result that deviates most from the average was discarded, as instructed by ASTM C 109 (2011). The average was then calculated from two mortar specimens instead of three and the final range of the two samples was checked to see it was less than or equal to 7.6% of the average, as required by ASTM C 109 (2011). The results section reports if the 7.6% range was exceeded when the average compressive strength was calculated from two mortar cubes.

2.3.1.2 Isothermal Calorimetry

Isothermal calorimetry was performed with the compressive strength tests to see if the decreased strength of pozzolan mixtures seen in previous literature (Lilkov et al., 2011; Erdem et al., 2007; Hossain, 2003; Campbell et al., 1982) was a result of retardation in cement hydration caused by the addition of pozzolans. The data from the isothermal calorimetry tests, which measure the heat from ongoing hydration reactions of cementitious pastes, can be used to plot heat of hydration curves. Comparing the hydration curves of the pozzolan mixtures to that of the control mixture or the inert quartz mixture can give valuable information on whether the addition of the SCMs accelerates or retards the hydration process of cement.

The control paste mixture was made with 50 g of cement and 22.5 g of water, giving the paste a w/cm of 0.45. For the SCM paste mixtures, 20% of the cement by mass was replaced with the SCM being tested. The water content was kept the same as the control mixture, giving the pozzolan-containing paste a w/cm of 0.45 as well. Prior to each test, the pastes were mixed by hand for 2 minutes. Approximately 10 g of paste was

added to the calorimetry vials after mixing and the heat evolution from hydration was measured at 23°C for 72 hours using a Thermometric TAM Air Isothermal Calorimeter.

2.3.1.3 Thermal Gravimetric Analysis/Differential Scanning Calorimeter (TGA/DSC)

While isothermal calorimetry is a good characterization test to understand the early age effect of pozzolans on cement hydration, it does not provide any information on pozzolanic reactions that typically occur at a later stage. Since a pozzolanic reaction involves the conversion of calcium hydroxide into calcium silicate hydrate (C-S-H), an easy way to track the progress of a pozzolanic reaction is to monitor the decrease of calcium hydroxide in a cementitious mixture over time, using thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC).

For the thermal gravimetry tests, the paste designs were identical to those used for isothermal calorimetry. After mixing the pastes by hand for 2 minutes, they were cured at room temperature and 100% relative humidity, until they reached the desired test age, which was 7, 28 and 90 days for the current study. After curing, the samples were weighed and then broken into small chunks (approximately less than 10 mm in length) that were stored in a vacuum desiccator for 14 ± 1 days. The samples were reweighed after being removed from the desiccator at 14 days, and the change in weight was recorded as the amount of water lost on drying. The samples were then crushed and sieved through a No. 325 sieve (45 µm opening) to ensure uniformity during testing. The sieved samples were stored under vacuum in a desiccator until they were tested on a Mettler Thermogravimetric Analyzer, Model TGA/DSC 1. During the test, the chamber gas used was nitrogen and the samples were contained in alumina crucibles with lids. The weight losses of the samples were recorded as they were heated from 40°C to 1000°C, at

a rate of 20°C/min. The measured weight loss was used to plot the TGA curve. The heat flow during this interval was recorded as well and was used to plot the DSC curves. Figure 2.2 shows a representative TGA/DSC plot. The DSC curve was used to pinpoint the exact temperatures between which the calcium hydroxide in the paste decomposed (labeled points A and B in Figure 2.2). Using those temperatures, the weight loss due to calcium hydroxide decomposition was calculated from the TGA curve. Finally using molecular weights and the recorded weight change from water loss in the desiccator, the weight loss from calcium hydroxide decomposition was converted to the calcium hydroxide content per gram of cement in the initial paste.