Our company has two physical labs to test cement quality. Recently, test results of 2 lab have a big gap of 7D and 28D strength. We already checked the compressive machines of 2 Labs based on the same cubic specimens. Results showed that there is no problem with the machine. We also elimiated human error by proficiency test. But the gap still there. At the movement, we could not find the solution for this problem :-(
If you have experience related to this issue, I would appreciate to recieve your sharing.
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Re: Cement Strength
The possible reasons for the compressive strength result variation between two laboratories are
- Cement water ratio.
- Cement sand ratio.
- Gauging time and gauging style.
- Variation in Vibration machine RPM and number of taping (POCKING) for Cement compaction in mould.
- Abnormality in compressive strength mould size.
- Temperature and Humidity status in humidity chamber/curing tank/ Testing room.
- Curing tank water quality (may be more alkaline).
- Status of Compressive Testing Machine (calibration and rate of loading).
Re: Cement Strength
During hydration, clinker silicate minerals in the cement react with water to form calcium silicate hydrates and Ca(OH)2. eg
2 (3CaO.SiO2) + 7 H2O --> 3CaO.2SiO2.4H2O + 3 Ca(OH)2
If the water in the curing tank initially contains no Ca(OH)2, the Ca(OH)2 released during the cement hydration reactions will diffuse out of the cement paste due to osmotic pressure. This loss of Ca(OH)2 represents a loss of solids content in the final sample, resulting in strength loss.
As the Ca(OH)2 concentration in the curing tank water increases, with the addition of subsequent samples, the effect would progressively get less and less on later samples until the Ca(OH)2 concentration reached saturation equilibrium.
At that point there would be no influence on measured strength because the Ca(OH)2 concentration within the pores of the sample would be the same as that in the surrounding liquid, preventing any leaching of Ca(OH)2.
Therfore to remove this possible variable from the test, the curing tank water should be pre-saturated with Ca(OH)2 before the samples are placed in it. In fact, most standard methods call for the curing tank water to be adjusted to, and maintained at, a specific pH using Ca(OH)2 before use... usually pH 12-13.
Hope that helps.