**Cat diesel fuel pressure tester**, EHV Power Technology, Pueblo, CO, USA), then placed them in a desiccator for at least 24 h to remove any residual moisture and then weighed. No corrections were made for moisture. The desiccated samples were then compressed to the required density using a controlled hydraulic press (Universal Press, Model-H-300).

Characterisation of materials {#s3c}

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Microstructural and chemical characterisations of all cement pastes were carried out on the same samples. These included X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Detls of these characterisations are given in [table ,3](#RSOS150805TB3){ref-type="table"}.

Characterisation of samples for gas diffusion test was carried out using a Perkin Elmer Optima 7000 CIR system. Samples were characterised for density and apparent porosity by volumetric methods. The apparent porosity (*ε*) was calculated using $$varepsilon = frac{ ext{Volume~of~sample~in~r~} - ext{~Volume~of~sample~in~saturated~solution~}}{ ext{Total~volume~of~sample~}}.$$ where 'Volume of sample in r' is the volume of cement paste immersed in r, and 'Volume of sample in saturated solution' is the volume of the cement paste immersed in saturated solution (pure deionized water).

The density (*ρ*) was obtned by measuring the mass of the cement paste samples placed in a sample holder and subtracting the residual mass after drying. The apparent porosity of the sample was calculated from the density measurement by dividing the porosity by the density.

The apparent density was used as a measure of the sample density, and the apparent porosity was used to calculate the apparent permeability as follows: $$k = frac{ ext{d}P}{Delta ext{D}},$$ where *k* is the permeability (cm s^−1^), *d* is the thickness of sample (cm), Δ*D* is the difference in the saturated solution and dry density, *i.e.* Δ*D* = dry density − saturated solution density, and *P* is the pressure difference across the thickness of sample, calculated using [[@RSOS150805C32]] $$P = frac{Delta P}{k},$$ where Δ*P* is the pressure difference across the thickness of the sample. The pressure drop across the thickness of the sample was measured with a digital pressure gauge.

In addition to the apparent permeability, the permeability (*K*~m~) is calculated as the ratio of Darcy's permeability to the area of the pore system. For the given permeability *k* and *K*~m~, the resistance is calculated as follows: $$R = frac{P}{k}.$$

The resistance of the sample was obtned from the measured pressure drop Δ*P* and the apparent permeability *k*. The permeability of the sample was determined using Eqn (2.1). The resistance of the sample was calculated using Eqn (2.2).

3.. Results {#s3}

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To study the effect of the addition of the polysaccharide gellan gum on the performance of concrete, the permeability and apparent permeability of a cement paste with different concentrations of gellan gum (0, 1, 3, 5 and 10%) were measured. The influence of gellan gum on the hydration time of the cement paste was also investigated. The cement slurry of 2.0 kg cement (i.e. 15 kg of dry cement) with 0, 1, 3, 5 and 10% gellan gum was mixed with 1.0 kg sand and 1.0 kg coarse and fine aggregate, and poured into a mould with dimensions of 200 mm × 180 mm × 140 mm. The mixture was then mixed using a vibrating plate shaker. The mixture was then cured in a plastic box for 3, 7, 10, 14 and 28 days. The compressive strength of the samples was measured and used to evaluate the effect of the addition of gellan gum on the hydration process and the compressive strength of the concrete.

3.1.. Hydration process {#s3a}

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The water absorption and hydration time of the cement paste with different concentrations of gellan gum were shown in [figure ,4](#RSOS191211F4){ref-type="fig"}. The water absorption of the cement paste increased with the addition of gellan gum. The addition of gellan gum shortened the hydration time and increased the hydration rate. The hydration time was 2.4, 3.8, 5.3, 6.1 and 7.5 h when the proportion of gellan gum was 0, 1, 3, 5 and 10%, respectively. Figure 4.Influence of the percentage of gellan gum on the water absorption and hydration time of the cement paste.

3.2.. Compressive strength {#s3b}

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[Figure ,5](#RSOS191211F5){ref-type="fig"} shows the effects of the addition of gellan gum on the compressive strength of the cement paste. The results showed that the compressive strength of the cement paste increased with the addition of gellan gum. The influence of the addition of gellan gum increased with the increase of the proportion. The compressive strength was 2.7, 3.3, 3.9, 5.2 and 6.4 MPa when the proportion of gellan gum was 0, 1, 3, 5 and 10%, respectively. Figure 5.Influence of the percentage of gellan gum on the compressive strength of the cement paste.

4.. Discussion {#s4}

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The water absorption and hydration time of the cement paste with different concentrations of gellan gum were shown in [figure ,4](#RSOS191211F4){ref-type="fig"}. The water absorption of the cement paste increased with the addition of gellan gum. The addition of gellan gum greatly increased the hydration time of the cement paste, which was related to the viscosity of the cement paste. According to the results, the hydration of the cement paste with the addition of gellan gum was delayed. As a gelling agent, the hydration of the cement paste with the addition of gellan gum could slow down the hydration of the cement paste, which was similar to the situation of using methylcellulose.

[Figure ,5](#RSOS191211F5){ref-type="fig"} shows the effects of the addition of gellan gum on the compressive strength of the cement paste. With the increase of the addition of gellan gum, the compressive strength of the cement paste increased. At the same gellan gum content, the addition of gellan gum greatly influenced the compressive strength of the cement paste. The relationship between the change of the compressive strength and the changes of the

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