Influence of Moisture- and CO_2-Induced Ageing of Cement on the Performance of Chemical Admixtures Used in Concrete and Mortar

【Author】

Dubina E;Plank J;

【Abstract】

<正>Ageing is the reaction of anhydrous cement with water vapour in air,leading to partial hydration and carbonation at the surface of the cement grains.Industrial cements undergo some early surface hydration already during their manufacturing process or during transportation and storage.In the cement plant a first contact with water can occur from the dehydration of gypsum which is ground together with the clinker in a ball mill at elevated temperature,or later during storage in a silo.This may lead to the formation of lumps which along with hydration of the cement can cause a phenomenon known as "silo set".In dry mix mortar formulations it is also possible that powdery additives which possess a residual moisture content of up to 5%release water to the cement In such case,local annular layers of partially hydrated cement are formed around the additive particles. Alteration of cement properties during storage resulting from ageing is highly undesirable,but has been noticed repeatedly by users.The principal consequences of this ageing phenomenon may include decreased compressive strength and heat of hydration,altered rheological properties and poor response to addition of chemical admixtures.Moreover,applicators have reported that cement which was stored for longer periods in humid air may fail entirely in the field The purpose of this study was to obtain a more in-depth understanding of the influence of ageing on the interaction of cement with chemical admixtures.The exposure conditions tested were 35℃and 90%relative humidity over I and 3 days respectively. Although these conditions do not reflect climate conditions in central Europe,they do simulate the uptake of considerable quantities of moisture and CO_2 over an extended period of time and therefore present an accelerated ageing scheme. As admixtures,three superplasticters(NSF,PCE and casein),one water-retention agent(MHEC) and two accelerators(Ca formate and amorphous Al_2O_3) were investigated. The results confirm that ageing of cement can lead to a significant change of the flow, water retention and setting behaviour patterns.As a consequence of this partial surface hydration,the specific surface area and surface charge of cement are altered,which substantially impacts the interaction with chemical admixtures.The following correlations were found: In aged cement the effectiveness of superplasticizers generally is less than in fresh cement.The dispersing power of casein superplasticizer is least strongly decreased in aged cement,while polycarboxylate is strongly and NSF dispersant particularly strongly affected. Aged cement exhibits a significantly improved water retention capacity with,at the same time,a higher water demand to achieve the workability required in practice. Consequently,in aged cement the dosage of methylcellulose to reach high water retention is lower than in fresh cement. In aged cement,the accelerating effect of Ca formate and amorphous Al_2O_3 is significantly decreased.After ageing for 3 d,they may even retard cement hydration. Originality For the first time,the interaction between chemical admixtures and aged cement which has been partially hydrated and carbonated as a result of exposure to moist air was studied under controlled experimental conditions.As admixtures,several superplasticizer,retarder and accelerator samples were tested.It is shown that the effects of aged cement mostly are antagonistic,and that some admixtures are more seriously affected by the ageing phenomenon than others.A second finding of the study is that the specific surface area and surface charge of cement are altered during ageing. This mechanistically explains the change in the behaviour of admixtures.

【Keywords】

cement;;ageing;;moisture;;carbonation;;admixtures

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