Sulfate Attack on Portland Limestone Cements Manufactured with Low C3A Portland Clinker

摘要

Environmental aspect of building materials has become a public concern for this century. Among the strategies to reduce the energy consumption and CO2 emission, the production of Portland limestone cement (PLC) is described as a fast and economical technical solution for cement industry around the world. Developed in France, PLC was adopted by European standard allowing up to 35% of limestone filler (LF). Today, the production of these cements is extendedly around the world. Negative impacts of incorporation of large amount of LF in cement are centered on the durability aspects, especially due to increase the susceptibility to sulfate attack and thaumasite formation. The conjecture is if this problem could be reduced using a denser mixtures (low w/cm) and a low C3A Portland clinker as code requirements proposes. The objective of this paper is to study the sulfate resistance of PLC containing up to 33% of LF and low C3A Portland clinker to determine the evolution of attack at different temperatures and the influence of LF content and w/cm on the sulfate performance. The experimental work was divided in two phases: the first correspond to paste specimens (w/cm = 0.50) exposed to sulfate (33.8 g/l SO4 2-and 4.3 g/l Mg2+) at 5 and 20 oC for 36 months. The second one, correspond to mortar specimens where the w/cm was adjusted according to LF content in PLC in order to obtain the same effective w/c (0,50) and exposed to the same conditions during 12 months. After 36 months, paste prisms either with or without LF presented a large surface damage at 5 oC, while the same specimens remain undamaged at 20 oC. But, this damage is only limited to 2-3 millimeter depth. The surface damage increases when increase the proportion of limestone filler. According to XRD results, damage is attributed to ettringite formation at expanse of AFm phases, which is promoted by low temperature. At 20 oC, Mg2+ and CO2 dissolved in solution form a surface protective layer of brucite-carbonate that delays sulfate attack. Deposition of carbonate is firstly in calcite form and later in aragonite form depending on the [[Ca2+]/[Mg2+] into the pore surface. For the mortar specimens, visual aspect and compressive strength show that reduction of w/cm according to the level of LF replacement contributes to mitigate the penetration of sulfate ions and degradation of surface. The depth of altered zone determined using optical microscopic techniques on thin sections is large at low temperature.