Effect of Relative Humidity on pansion and Microstructure of Heat-Cured Mortars
by Luis A. Graf
Effect of Relative Humidity on pansion and Microstructure of Heat-Cured Mortars
by Luis A. Graf
Research and Development Bulletin RD139
2007 Portland Cement Association
Abstract: The research presented in this report studies the effect of moisture exposure on long-
term expansion in heat-cured mortars due to delayed ettringite formation (DEF). Mortar bar
samples were prepared using standard Ottawa sand and cured at an elevated temperature of
90ºC, and subsequently stored for more than 4 years under several long-term storage conditions,
including: immersed under pure water and above water in moist air at 75%, 80%, 84%, 90%,
92%, 97%, and 100% relative humidity (RH). Weight and length changes were recorded period-
ically, while relative humidity and temperature were constantly monitored. Mortar samples
were also analyzed at different ages for chemical and mineralogical composition and
microstructure characterization. Several techniques were used including X-ray fluorescence,
X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and optical
microscopy. Emphasis was placed on observing the different cracking patterns and expansion
performance between samples stored under water and those stored above water. The study also
looked at the effect of alkali leaching on occurrence and rate of expansion.
The results show that DEF-related expansion is influenced by the level of moisture exposure.
Expansion occurs sooner for mortar bars stored under water. However, once expansion occurs
for mortar bars stored in moist air, the observed maximum expansion exceeds that for samples
stored under water. Expansion of samples stored in moist air occurs sooner for those stored at
higher RH of 100%, 97%, and 92%, in that order. Samples stored at 90% RH and below did not
show expansion, but did expand upon subsequent exposure to 92% RH. Alkali leaching seems
to accelerate the occurrence of expansion for samples immersed under water. The study suggests
that a threshold level of moisture exposure for expansion to occur seems to be between 92% and
90% RH.
|
|
|