Influence of cement and aggregate type on thaumasite formation in concrete

In this study the influence of binder type on the formation of thaumasite in mortar prisms made with expanded clay lightweight aggregate (LWA) or quartz sand was examined. For this purpose mortar prisms were made, which after 28 days of curing in deionised water were exposed to a sulphate solution or deionised water. The length and weight change of the prisms was recorded in triplicate as a function of time of exposure to dry–wet cycles at 5 ± 1 °C.The influence of the binder type on the expansion in the sulphate solution can be ordered from strong to weak as follows: (1) CEM I + limestone filler, (2) CEM I, (3) CEM I + fly ash, and (4) CEM III/A. Because the porosity of the LWA was able to accommodate the growing sulphate crystals, the mortar prisms made with LWA were still largely intact after 3 years of exposure. The only exception being the mortar prisms containing limestone filler. The mortar prisms made with quartz sand and exposed to the sulphate solution were all bent, broken or disintegrated after 24 weeks. The prisms exposed to deionised water showed minimal expansion. Key factors controlling the formation of thaumasite are discussed.     

Influence of cement type on resistance to attack from two carboxylic acids

In a number of circumstances, concrete may be required to possess resistance to organic acids. These are frequently carboxylic acids. This paper examines the effect of two such acids – acetic and butyric – on hardened cement paste specimens made from three cement types – Portland cement (PC), a combination of PC and fly ash (PC/FA), and a calcium sulfoaluminate cement (CSA). Specimens were exposed to solutions of the acids and deterioration characterized in terms of mass loss and pH measurements, micro-CT scanning, and chemical and mineralogical analysis. Additionally geochemical modelling was used to further examine the mechanisms involved during acid attack. The CSA cement was most resistant to attack, with the PC paste displaying the least resistance. This resistance has been partly attributed to the higher acid neutralization capacity of CSA cement. However, this paper demonstrates that the enhanced performance is most probably the result of a denser microstructure.     

Resistance of blended cement pastes subjected to organic acids: Quantification of anhydrous and hydrated phases

Concrete for agricultural construction is often subject to aggressive environmental conditions. Ground granulated blast furnace slag (GGBFS) or metakaolin (MK) largely improve the chemical resistance of the binder. Anhydrous particles seem particularly resistant to the acid solution. The purpose of this study is to quantify anhydrous particles in blended cement pastes as a function of acid exposition time in order to evaluate their acid resistance.Cement pastes were moist cured for 28 days and then immersed in an acetic acid solution for 2 months. The quantification of the anhydrous phases was carried out using ²⁹Si MAS NMR, selective dissolution and back-scattered electron (BSE) images analysis, while the hydrated phases content was evaluated by TGA. After 28 days of hydration, 60% of OPC, 44% of GGBFS and 76% of MK particles were hydrated. The amount of anhydrous particles drops for all materials during acid immersion. After 2 months of immersion, the amount of anhydrous particles drops by 49%, 23% and 15% for OPC, GGBFS, and MK respectively. This study confirms that GGBFS and MK anhydrous and hydrates phases present higher acid resistance than OPC.     

Performance and properties of mortar mixed with nano-CuO and rice husk ash

This paper presents an experimental study on a new mixture scheme of mortar. Unlike most of existing work, the present study investigates nano-CuO (NC), and its combined effects with cement replacement i.e., rice husk ash (RHA) on durability performance, as well as strength and permeability properties of mortars. Comprehensive observations of both the performance and properties improvements on RHA-containing mortar specimens were determined with the addition of NC. To this end, a series of tests for examining the strength both directly (compressive strength) and indirectly (Ultrasonic Pulse Velocity), electrical resistivity, chloride permeability, water absorption and microstructure characteristics (i.e., SEM micrographs, Mercury intrusion porosimetry (MIP) & capillary analyses) of mortar specimens were performed. A relationship between the Rapid chloride permeability test (RCPT) and electrical resistivity was also studied in order to recommend an alternative method for quality control in the presence of RHA and NC. Finally, a mixture scheme which provides relatively satisfactory properties improvement with positive environment credential is suggested.