**Your sand may absorb some water, but much of the “thirst” is probably the extra paste needed to make the grains move past one another.** Dryness, fineness, and the amount of sand all matter.

First, **0.42 is not a universally optimal water:cement ratio**. That number is associated with theoretical water requirements for complete cement hydration under particular conditions; it does not guarantee a workable mortar. Use ratios **by mass**, and choose the mix around both its hardened properties and its ability to be placed and consolidated. [ACI curing guide](https://www.concrete.org/portals/0/files/pdf/previews/308r_16_preview.pdf)

Several effects explain what you’re seeing:

- **Dry sand really can absorb water into pores within its grains.** Mix designs normally account for this using *saturated surface-dry* sand: its internal pores are full, but there is no excess water on the grain surfaces. Sand drier than that requires additional absorption water; wetter sand contributes water to the batch. Water added to satisfy absorption is treated differently from extra free mixing water when calculating the effective water:cement ratio. [ACI moisture corrections](https://www.concrete.org/frequentlyaskedquestions.aspx?faqid=757)
- **Fine sand increases water demand even if it absorbs very little.** It has more surface area to wet and lubricate. Grain shape and size distribution also affect packing and friction. Your fine paint-texturing sand performing worst fits this explanation, although its actual composition is unknown. [Mortar grading study](https://doi.org/10.1016/j.conbuildmat.2010.11.011)
- **You may have too little cement paste for the amount of sand.** A water:cement ratio specifies the composition of the paste, but says nothing about how much paste surrounds the aggregate. Too little paste leaves a harsh, gritty mix. A superplasticizer can make the paste flow better, but cannot supply missing paste volume. [American Cement Association](https://www.cement.org/cement-concrete/applications-of-cement/)

For scale, suppose you use **1 kg cement, 3 kg completely dry sand, and 420 g water**. If that particular sand has 1% absorption, it needs approximately **30 g additional water** to reach its saturated surface-dry condition. That is an illustrative calculation, not an assumed absorption value for your sand.

**Hand mixing can contribute**, particularly with a stiff mix and a PCE that needs thorough distribution. But it need not be the main cause. Also, “0.5% by weight” needs a denominator: admixture dosages are commonly based on cement or total cementitious material, and liquid products and powders have different concentrations. Follow your particular product’s instructions for dose, addition sequence, and mixing time. For example, one Sika PCE specifies thorough wet mixing for at least 60 seconds in a mixer; that does not translate directly into 60 seconds of hand stirring. [Sika product instructions](https://gcc.sika.com/en/construction/concrete/water-reducers/pce-based-super-plasticizers/sikaplast-550.html)

Clay contamination is another possible reason for poor PCE response, especially with backyard sand. Certain clays adsorb the admixture and greatly reduce its effectiveness. Sieving alone does not establish that sand is clean. [PCE–clay research](https://doi.org/10.1016/j.cemconres.2012.03.005)

For your next small trial:

1. Use clean, washed sand intended for mortar or concrete, with a range of grain sizes.
2. Weigh everything and record the sand:cement ratio as well as water:cement.
3. Try presoaked sand, drained and brought approximately to surface-dry condition. Simply draining it can leave surface water, so this is a diagnostic trial rather than precise moisture control.
4. Use the PCE exactly as its instructions specify, mixing small batches thoroughly before deciding more water is necessary.
5. If it remains gritty, test a lower sand:cement ratio while keeping water:cement fixed.

Yes, other people’s damp sand can supply water that your desert-dry sand lacks. Hot, dry conditions can also increase evaporation and accelerate workability loss, so mix in shade with cool materials and protect the finished material during curing. [NRMCA hot-weather guidance](https://www.nrmca.org/wp-content/uploads/2021/01/12pr.pdf)

A pourable, self-consolidating mixture needs a suitable overall formulation. Holding water:cement at 0.42 and adding PCE to an arbitrary mortar recipe does not necessarily produce one.