Your sand probably causes **both water absorption and a higher demand for cement paste**. Desert dryness can contribute, and hand mixing can make the problem harder to overcome.

First, **0.42 is not a universally optimal water:cement ratio**. It is a plausible target for some mixes, measured **by mass**, but the right ratio depends on the required strength, durability, workability, and ingredients. The American Cement Association recommends lowering it as far as practical while retaining enough workability for proper placement and consolidation. [ACA guidance](https://www.cement.org/cement-concrete/applications-of-cement/)

There are two distinct effects behind the “thirst”:

- **Absorption:** Dry sand can take water into pores within its grains. Mix calculations normally account for this using the *saturated surface-dry* condition, or SSD: the internal pores are full, and the grain surfaces carry no extra water. Water needed to bring dry aggregate to SSD is excluded from the effective water:cement ratio. Wet sand’s surface water counts toward that ratio. [NRMCA moisture guidance](https://www.nrmca.org/wp-content/uploads/2020/04/Tip6w.pdf)
- **Paste demand:** Cement paste must fill the spaces between grains and provide enough coating for them to move past one another. Adding sand while holding cement and water constant leaves progressively less paste available. Fine sand has more surface area per kilogram; angular grains and poor grading also increase demand. This produces exactly the harsh, gritty consistency you describe. [FHWA guidance](https://highways.dot.gov/sites/fhwa.dot.gov/files/FHWA-RD-00-025.pdf)

The very fine texture sand performing worst is consistent with increased surface area. Its actual composition and suitability for cement mixtures would need checking.

For an **illustrative calculation**, suppose you use 1 kg cement and 3 kg completely dry sand with a measured absorption of 1%. The sand needs 30 g water to reach SSD. You would then add **450 g total water** to supply 420 g effective mixing water and maintain an effective ratio of 0.42. That 1% is an example; your sand’s absorption must be established. Simply soaking and draining sand leaves an uncertain amount of surface water.

Your PCE result also needs a little investigation. **“0.5%” alone does not establish whether the dose is suitable**: the product’s concentration, whether it is liquid or powder, and the specified dosing basis matter. Addition sequence and thorough mixing matter too. For example, one Sika PCE specifies 0.8–1.0% of cementitious mass for ordinary workability, higher doses for flowing concrete, and at least 90 seconds of wet mixing. Those instructions apply to that product; follow yours. [Sika product instructions](https://mng.sika.com/en/construction/concrete-admixtures/precast-concrete/superplasticizers/sika-viscocrete-5510.html)

Backyard sand could also contain clay. Some clays adsorb PCE strongly, reducing the amount available to disperse cement. Sieving does little to remove clay coating the grains. [Experimental study](https://doi.org/10.1016/j.cemconres.2012.03.005)

For your next small trial:

1. Use clean, washed sand intended for the mixture, with a suitable range of grain sizes.
2. Weigh everything and account for aggregate moisture.
3. Follow the PCE’s dosing and addition instructions. Mix thoroughly, scraping and folding the whole batch before judging its consistency.
4. If it remains gritty, test a lower sand:cement ratio while keeping the effective water:cement ratio fixed. Improvement would point toward insufficient paste.
5. Mix in shade and protect the placed material during curing. Low humidity, heat, and wind increase moisture loss. [NRMCA hot-weather guidance](https://www.nrmca.org/wp-content/uploads/2021/01/12pr.pdf)

A pourable, self-consolidating mix needs a deliberately balanced paste volume, aggregate grading, and admixture system. [NRMCA SCC guidance](https://www.nrmca.org/wp-content/uploads/2021/01/37pr.pdf)

What cement:sand proportions are you using, by mass, and what is the exact PCE product? Those details would help distinguish absorption, insufficient paste, and admixture problems.